WO2025109069A1 - W/o emulsion of chitosan(s), comprising a liquid fatty phase and at least one nonionic surfactant, process for the preparation thereof, and cosmetic composition containing same - Google Patents

Abstract

The invention relates to a water-in-oil (W/O) emulsion comprising: - from 0.5% to 20% by weight of one or more chitosans relative to the total weight of the emulsion, and one or more acids with a pKa < 5, dissolved in an aqueous phase with a pH < 6, - one or more nonionic surfactants, the HLB of said surfactant or of the mixture of said surfactants ranging from 2 to 8, and - a fatty phase which is liquid at room temperature. It also relates to the process for the preparation thereof and to a cosmetic composition comprising same in the presence of a washing base.

Description

DESCRIPTION TITLE: W/O EMULSION OF CHITOSAN(S), COMPRISING A LIQUID FATTY PHASE AND AT LEAST ONE NONIONIC SURFACTANT, 5 PROCESS FOR THE PREPARATION THEREOF, AND COSMETIC COMPOSITION CONTAINING SAME The present invention relates to a water-in-oil emulsion comprising an aqueous phase based on chitosan(s), a liquid fatty phase, and one or more nonionic 10 surfactants, the HLB of said nonionic surfactant or mixture of nonionic surfactants ranging from 2 to 8. The invention also relates to a cosmetic composition containing same, and to a process for preparing such an emulsion. The invention also relates to the use of said emulsion for the cosmetic 15 treatment, preferably the cleansing and/or conditioning, of keratin fibres, in particular of human keratin fibres such as the hair, for instance the head hair. The formulation of environmentally friendly cosmetic products, that is to say products whose design and development take account of environmental issues, is becoming a major preoccupation for contributing towards meeting the global 20 challenges. In this context, it is important to develop new cosmetic compositions with a better carbon footprint, in particular by promoting the use of starting materials which are renewable and/or which have a good index of naturalness and/or of natural origin and more particularly of plant origin, while reducing the use of compounds of 25 petrochemical origin. It is therefore essential to propose more sustainable compositions and/or preparation processes and/or ingredients, thus enabling these environmental challenges to be met. In the field of cleansing and conditioning keratin fibres, in particular human30 keratin fibres such as the hair, patent application WO 2021/013546 describes a water- in-oil emulsion composition which allows improved administration of a skin benefit agent having a solubility in water at 25°C of at least 0.001%. It comprises an aqueous phase comprising the benefit agent, a hydrophobic phase based on petrolatum or wax, and an emulsifier. The skin benefit agent is selected from antimicrobial cationic 35 polymers, soluble vitamins, soluble sunscreens, alpha-hydroxy acids (AHAs), dihydroxyacetone (DHA), caffeine, peptides and hyaluronic acid. Chitosan can be used as an antimicrobial polymeric active agent. The fatty substances used in this document, such as beeswax, rice bran wax and carnauba wax, are solid at room temperature, or else those such as petrolatum are 5 semi-solid. However, these solid or semi-solid fatty substances have the drawback of adding heaviness to the hair, which can lead to an unwanted greasy feel, in particular when they are incorporated into shampoo compositions. Moreover, chitosan is a polysaccharide of interest due to its chemical functionalities and its natural origin. For cosmetic uses, it makes it possible in 10 particular to obtain hair fibre conditioning effects, especially on damaged hair (or head hair). However, the incorporation of chitosan into cosmetic compositions containing anionic species remains a challenge. For example, in a shampoo, the presence of anionic surfactants leads to the formation of aggregates by electrostatic 15 interaction with chitosan. The compositions obtained become inhomogeneous and/or unstable over time, which makes them unusable for cosmetic applications. There is therefore a need to provide formulations which make it possible to convey chitosan in cosmetic products, while avoiding the formation of aggregates in the presence of anionic species such as surfactants present in washing bases, and which 20 make it possible to obtain a better foam quality (start of foaming and/or foam abundance) and/or speed of hair (or head hair) drying. The applicant has thus discovered, surprisingly, that the combination of 5% to 20% by weight, preferably 5% to 15% by weight, of at least one chitosan and at least one acid with a pKa < 5, which is preferably organic, solubilized in an aqueous phase 25 with a pH < 6, of at least one nonionic surfactant, the HLB of said surfactant or mixture of said surfactants ranging from 2 to 8, and of a fatty phase which is liquid at room temperature, in a water-in-oil (W/O) emulsion, made it possible to meet this need. Thus, one subject of the present invention is a water-in-oil (W/O) emulsion comprising: 30 - from 5% to 20% by weight, preferably from 5% to 15% by weight, of one or more chitosans, relative to the total weight of the emulsion, and one or more acids with a pKa < 5, which are preferably organic, solubilized in an aqueous phase with a pH < 6, - one or more nonionic surfactants, the HLB of said nonionic surfactant or of the mixture of said nonionic surfactants ranging from 2 to 8, and 35 - a fatty phase which is liquid at room temperature (25°C). The emulsion can be incorporated into cosmetic compositions containing anionic species such as washing bases without causing destabilization of the composition by formation of aggregates. The emulsion is homogeneous, i.e. does not have aggregates. It is storage 5 stable and in particular exhibits no phase separation over time, in particular over 1 month at room temperature. Furthermore, the cosmetic compositions comprising said emulsion exhibit improved cosmetic properties such as the foam qualities (start of foaming and/or foam abundance) and/or the speed of hair drying. 10 Another subject of the invention is a cosmetic composition comprising a water-in-oil emulsion according to the invention, and one or more anionic surfactants. Another subject of the invention is a process for preparing said emulsion. A further subject of the invention is the use of said emulsion in cosmetics. Yet another subject of the invention is a method for the cosmetic treatment of 15 keratin fibres, in particular human keratin fibres, such as the hair (or head hair), comprising the application of said cosmetic composition to said fibres. Other subjects, features, aspects and advantages of the invention will emerge even more clearly on reading the description and the examples that follow. In that which follows and unless otherwise indicated, the limits of a range of 20 values are included within that range, notably in the expressions “between...” and “ranging from ... to ...”. The expression “at least one” used in the present description is equivalent to the expression “one or more”. 25 The water-in-oil emulsion or W/O emulsion comprises a continuous liquid fatty phase (or oily phase) and an aqueous dispersed phase. Aqueous phase The aqueous phase comprises water, one or more chitosans and one or more acids, 30 preferably organic acids, with a pKa of less than 5. The amount of aqueous phase varies preferably from 50% to 90% by weight, better still from 50% to 80% by weight, and more preferentially from 65% to 75% by weight, relative to the total weight of the W/O emulsion. The W/O emulsion contains a total amount of water ranging preferably from 20% to 80% by weight, better still from 30% to 70% by weight, and more preferentially from 40% to 50% by weight, relative to the total weight of the W/O emulsion. The aqueous phase has a pH of less than 6, preferably ranging from 2 to 6, 5 better still from 3 to 5. Chitosan The chitosan(s) is (are) present in the aqueous phase of the W/O emulsion. The chitosan(s) may have a degree of acetylation of less than or equal to 40%, preferably 10 less than or equal to 30%, more preferentially less than or equal to 15%, better still between 1% and 10%. The chitosan(s) used preferably has (have) a molecular weight of less than 200 kDa, preferably between 1000 and 150000 Da. The chitosan(s) is (are) present in an amount ranging from 5% to 20% by weight, preferably from 5% to 15% by weight, more preferentially from 6% to 14% by 15 weight, better still from 7% to 13% by weight relative to the total weight of the W/O emulsion. Acid with a pKa < 5 The W/O emulsion according to the present invention comprises one or more acids 20 with a pKa of less than 5. This or these acid(s) may be organic or mineral, preferably organic. As examples of mineral acids, mention may be made of hydrochloric acid. Preferably, the acid(s) is (are) organic. The acid(s) with a pKa of less than 5 is (are) in the aqueous phase. 25 Preferably, the acid(s) with a pKa of less than 5 is (are) organic. It is (they are) preferentially chosen from carboxylic acids of formula (I) below:

in which: A represents a monovalent group when n is 0, or a polyvalent group when n is greater 30 than or equal to 1; A represents a saturated or unsaturated, cyclic or acyclic, aromatic or non-aromatic hydrocarbon group comprising from 1 to 6 carbon atoms, optionally interrupted with one or more heteroatoms such as O or NH, preferably O, and/or substituted with one or more hydroxy groups; preferably, A represents a monovalent C1-C6 alkyl or phenyl group, or a polyvalent C1-C6 alkylene or phenylene group optionally substituted with one or more hydroxy groups; n represents an integer ranging from 0 to 10, preferably from 0 to 5, better still from 0 to 2. More particularly, the acid(s) with a pKa of less than 5 is (are) chosen from ^-hydroxy acids. More particularly, the acid(s) with a pKa of less than 5 is (are) chosen from the compounds of formula (I) in which A represents a phenyl group or a C1-C6, in particular C1-C4 alkyl or alkylene group, optionally substituted with one or more hydroxy groups, preferably with one hydroxy group, or a 6-membered heterocyclic group substituted with a plurality of OH groups, one of the ring members of which denotes an oxygen atom; and n ranges from 0 to 2. More particularly, the acid(s) with a pKa of less than 5 is or are chosen from those of formula (I) in which: n=0 and A represents a C1-C6, in particular C1-C4 alkyl group, optionally substituted with an OH group; or n=0 and A is a phenyl group, or a phenyl group substituted with an OH group; or n=0 and A represents a 6-membered heterocyclic group substituted with a plurality of OH groups, one of the ring members of which denotes an oxygen atom; or n = 1 or 2, and A represents a divalent or trivalent C1-C6, in particular C2-C4 alkyl group, optionally substituted with an OH group. Even more preferentially, the acid(s) with a pKa of less than 5 is or are chosen from acetic acid, citric acid, glutaric acid, lactic acid, gluconic acid, maleic acid and mixtures thereof, better still from citric acid and lactic acid, and even better still it is lactic acid. The W/O emulsion according to the invention comprises a total amount of the acid(s) with a pKa of less than 5, ranging preferably from 0.5% to 20% by weight relative to its total weight. Preferably, the weight ratio of the total amount of acid(s) with a pKa of less than 5 to the amount of chitosan(s) (acid(s) with a pKa of less than 5)/chitosan(s)) ranges from 0.1 to 5, even more preferentially from 0.5 to 2. Nonionic surfactant(s) The W/O emulsion according to the present invention comprises one or more nonionic surfactants, the HLB of said nonionic surfactant or of the mixture of said nonionic surfactants ranging from 2 to 8. When a mixture of nonionic surfactants is used, the mixture shall have an HLB ranging from 2 to 8. The HLB of a surfactant mixture containing a% of A and b% of B (weight percentage relative to the total weight of the mixture) is then calculated as follows: HLB (A + B) = a% × HLB (A) + b% × HLB (B). The term “HLB” (hydrophilic-lipophilic balance) is well known to those skilled in the art, and denotes the hydrophilic-lipophilic balance of a surfactant at 25°C in the Griffin sense. The term “hydrophilic-lipophilic balance (HLB)” means the equilibrium between the size and strength of the hydrophilic group and the size and strength of the lipophilic group of the surfactant. The HLB value according to Griffin is defined in J. Soc. Cosm. Chem., 1954 (Volume 5), pages 249-256. Use may notably be made of the nonionic surfactants having an HLB ranging from 2 to 8 which are mentioned in the reference handbook McCutcheon’s Emulsifiers & Detergents, International Edition from 1998 et seq. Reference may also be made to Kirk-Othmer’s Encyclopedia of Chemical Technology, volume 22, pages 333-432, 3rd edition, 1979, Wiley, for the definition of the emulsifying properties and functions of surfactants, in particular pages 347-377 of this reference, for nonionic surfactants. The nonionic surfactants suitable for the invention may in particular be chosen, alone or in mixtures, from: · oxyalkylenated, linear or branched, saturated or unsaturated C16-C30 fatty alcohols; · optionally oxyalkylenated, linear or branched, saturated or unsaturated C16-C30 fatty amides; · poly(ethylene oxide) esters of linear or branched, saturated or unsaturated C16-C30 fatty acids; · sorbitan esters of linear or branched, saturated or unsaturated C16-C30 fatty acids, which are optionally polyoxyethylenated; · glycerol or polyglycerol esters of linear or branched, saturated or unsaturated C10- C30, preferably C10-C24 fatty acids, or glycerol or polyglycerol esters of polymers of these fatty acids; · saturated or unsaturated, oxyethylenated plant oils; · (C16-C30) alkyl glucosides, preferably (C16-C22) alkyl glucosides; · condensates of ethylene oxide and/or of propylene oxide; · oxyethylenated and/or oxypropylenated silicones; and · phospholipids such as lecithins. The term “oxyalkylenated” is understood to mean compounds which comprise a number of ethylene oxide and/or propylene oxide groups which may range from 1 to 15, preferably from 1 to 10. The term “polyglycerolated” is understood to mean compounds which comprise a number of glycerol groups which may range from 2 to 10, preferably from 2 to 6. The oxyalkylenated, linear or branched, saturated or unsaturated C16-C30, preferably C16-C24 fatty alcohols are notably oxyethylenated and/or oxypropylenated. They comprise in particular 1 to 10, and more preferentially 2 to 5 oxyethylene and/or oxypropylene units. As examples of oxyalkylenated, linear or branched, saturated or unsaturated C16-C30 fatty alcohols, mention may notably be made of cetyl alcohol ethoxylated with 2 oxyethylene units (CTFA name “Ceteth-2”), stearyl alcohol ethoxylated with 2 oxyethylene units (CTFA name “Steareth-2”), steareth 4, beheneth-2 and oleth-2. The poly(ethylene oxide) esters of linear or branched, saturated or unsaturated C16-C30 fatty acids may be those having a number of ethylene oxide (EO) units ranging from 2 to 10, and more preferentially from 2 to 6. Mention may, for example, be made of diethylene glycol monostearate (2 EO), and polyoxyethylenated monostearate comprising 5 EO. As sorbitan esters of linear or branched, saturated or unsaturated C16-C30 fatty acids, which are optionally polyoxyethylenated, use may be made of those having a number of ethylene oxide (EO) units ranging from 0 to 10, and more preferentially from 0 to 5, for example sorbitan monostearate, sorbitan monoisostearate and sorbitan tristearate. The glycerol or polyglycerol esters of linear or branched, saturated or unsaturated C10-C30, preferably C10-C24 fatty acids, or polymers of these fatty acids, comprise in particular a number of glycerol units ranging from 1 to 8, better still from 1 to 6. As examples of such esters, mention may notably be made of tetraglyceryl (or polyglyceryl-4) sebacate, hexaglyceryl monolaurate, glyceryl stearate, glyceryl isostearate, diglyceryl monostearate, diglyceryl distearate, triglyceryl (or polyglyceryl- 3) diisostearate, tetraglyceryl (or polyglyceryl-4) diisostearate, tetraglyceryl tristearate, decaglyceryl decastearate, hexaglyceryl tristearate, decaglyceryl pentastearate, polyglyceryl-4 polyhydroxystearate, glyceryl monooleate, triglyceryl (or polyglyceryl- 3) polyricinoleate, hexaglyceryl (or polyglyceryl-6) polyricinoleate, glyceryl monobehenate, glyceryl dibehenate and the glycerol ester of palmitic and stearic acids. Preferably, the nonionic surfactant(s) according to the invention is (are) chosen from glycerol or polyglycerol esters of linear or branched, saturated or unsaturated C10-C30 fatty acids, or polymers of these fatty acids, sorbitan esters of linear or branched, saturated or unsaturated C16-C30 fatty acids, lecithins and mixtures thereof. More preferentially, they are chosen from polyglyceryl-4 diisostearate/polyhydroxystearate/sebacate, polyglyceryl-3 or polyglyceryl-6 polyricinoleate, polyglyceryl-3 diisostearate, sorbitan isostearate, and mixtures thereof. The total amount of the nonionic surfactant(s), said nonionic surfactant or the mixture of said nonionic surfactants having an HLB ranging from 2 to 8, preferably ranges from 2% to 10% by weight relative to the total weight of the W/O emulsion. Liquid fatty phase The liquid fatty phase is preferably contained in an amount ranging from 3% to 35% by weight relative to the total weight of the W/O emulsion. For the purposes of the present invention, a “liquid fatty phase” is understood to mean a fatty phase which is liquid at 25°C and at atmospheric pressure (1.013×10⁵ Pa). The liquid fatty phase contains one or more fatty substances chosen from alkanes, triglyceride oils of plant or synthetic origin, fatty acids, fatty alcohols, fatty esters other than triglyceride oils, fatty ethers, and mixtures thereof. For the purposes of the present invention, an “oil” denotes a water-immiscible compound which is liquid at 25°C and atmospheric pressure (1.013×10⁵ Pa). The term “immiscible” is understood to mean that the mixture of the same amount of water and of oil, after stirring, does not result in a stable solution comprising only a single phase, under the above-mentioned temperature and pressure conditions. Observation is made by naked eye or using a phase-contrast microscope, if necessary, on 100 g of mixture obtained after sufficient stirring with a Rayneri blender to produce a vortex within the mixture (as a guide, 200 to 1000 rpm), the resulting mixture being left to stand, in a closed flask, for 24 hours at room temperature before observation. Fatty alcohols, fatty esters, fatty ethers and fatty acids more particularly have at least one saturated or unsaturated, linear or branched hydrocarbon group comprising from 6 to 40, better still from 8 to 30, carbon atoms, even better still from 10 to 22 carbon atoms, which is optionally substituted, in particular with one or more hydroxy groups (in particular 1 to 4). If they are unsaturated, these compounds may comprise one to three conjugated or non-conjugated carbon-carbon double bonds. The alkanes may be chosen from hydrocarbon oils containing from 8 to 16 carbon atoms, and notably: - branched C10-C16 alkanes, for instance isoalkanes (also known as isoparaffins) such as C13-C16 Isoparaffin, isododecane, isodecane, isohexadecane, and for example oils sold under the trade names Isopar or Permethyl, alone or as mixtures, preferably isododecane (also known as 2,2,4,4,6-pentamethylheptane), for example sold by Ineos, more preferentially isododecane; - linear alkanes, for example C10-C16 alkanes, alone or as mixtures, for instance hexane, decane, undecane, tridecane or n-dodecane (C12) and n-tetradecane C14) sold by Sasol under the respective references Parafol 12-97 and Parafol 14-97, the undecane- tridecane mixture, the mixtures of n-undecane (C11) and n-tridecane (C13) obtained in Examples 1 and 2 of patent application WO 2008/155059 by the company Cognis, and mixtures thereof, and also mixtures of n-undecane (C11) and n-tridecane (C13) Cetiol Ultimate® from the company BASF; - non-aromatic, cyclic C5-C12 alkanes. The alkanes may also be chosen from hydrocarbon oils chosen from linear or branched compounds of mineral or synthetic origin, for instance: i) liquid paraffin, ii) squalane such as the reference Neossance Squalane sold by Amyris, isoeicosane, iii) mixtures of linear, saturated hydrocarbons, particularly C14-C30 and more particularly C15-C28 hydrocarbons, such as mixtures whose INCI names are, for example, the following: C15-C19 alkane, C18-C21 alkane, C21-C28 alkane, for instance the products Gemseal 40, Gemseal 60 and Gemseal 120 sold by Total, Emogreen L19 sold by SEPPIC, Emogreen L15 sold by SEPPIC, Emogreen HP40 sold by SEPPIC, iv) hydrogenated or non-hydrogenated polybutenes, for instance the products of the Indopol range sold by INEOS Oligomers, products having the INCI name Hydrogenated Polyisobutene; v) hydrogenated or non-hydrogenated polyisobutenes, preferably hydrogenated polyisobutenes, for instance the non-volatile compounds of the Parleam® range sold by the company Nippon Oil Fats, vi) hydrogenated or non- hydrogenated polydecenes, for instance compounds of the Puresyn® range sold by the company ExxonMobil, vii) decene/butene copolymers, butene/isobutene copolymers, and viii) mixtures thereof. The triglyceride oils of plant or synthetic origin are preferably triglycerides constituted of fatty acid esters of glycerol, in particular the fatty acids of which may have chain lengths ranging from C7 to C22. These oils may be linear or branched, and saturated or unsaturated. By way of example, mention may notably be made of heptanoic or octanoic triglycerides, caprylic/capric acid triglycerides, caprylic/capric/succinic acid triglycerides, plant oils such as wheat germ oil, sunflower oil, grapeseed oil, sesame oil, cumin oil, corn oil, apricot kernel oil, castor oil, shea oil, avocado oil, olive oil, soybean oil, sweet almond oil, palm oil, rapeseed oil, cottonseed oil, hazelnut oil, macadamia oil, jojoba oil, alfalfa oil, poppy oil, pumpkin oil, marrow oil, blackcurrant oil, evening primrose oil, millet oil, barley oil, quinoa oil, rye oil, safflower oil, candlenut oil, passionflower oil, musk rose oil, groundnut oil, coconut oil, argan oil, kaya oil; the liquid fraction of shea butter, and the liquid fraction of cocoa butter; and also mixtures thereof. The fatty acids which can be used in the invention are preferably C6-40, better still C8-30 and even more preferentially C10-22 carboxylic acids. As examples of fatty acids, mention may notably be made of lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, isostearic acid, arachidic acid, and mixtures thereof. The fatty alcohols are in particular saturated or unsaturated, linear or branched, and notably include from 10 to 26 carbon atoms. They are preferably monoalcohols. Advantageously, the C10-C26 alcohols are fatty alcohols, which are preferably branched or unsaturated when they comprise at least 14 carbon atoms. Preferably, the fatty alcohol comprises from 10 to 24 carbon atoms, and more preferentially from 12 to 22 carbon atoms, notably such as lauryl alcohol, isostearyl alcohol, oleyl alcohol, 2-butyloctanol, 2-undecylpentadecanol, 2-hexyldecyl alcohol, isocetyl alcohol, octyldodecanol and mixtures thereof. The fatty esters other than the triglyceride oils mentioned above comprise a total number of carbon atoms of greater than or equal to 8, more advantageously greater than or equal to 10. They may be chosen from: (i) linear aliphatic esters of formula R-C(O)-OR’ in which R-C(O)-O- represents a carboxylic acid residue comprising from 2 to 40 carbon atoms and R’ represents a hydrocarbon chain containing from 1 to 40 carbon atoms, aliphatic hydrocarbon esters of alkylene glycol, in particular ethylene glycol or propylene glycol, the total number of carbon atoms advantageously being at least 10. As examples of such esters, mention may for example be made of isoamyl laurate, cetostearyl octanoate, isopropyl myristate, isopropyl palmitate, isopropyl stearate or isostearate, ethyl palmitate, 2- ethylhexyl palmitate, isostearyl isostearate, octyl stearate, isostearyl heptanoate, cetyl octanoate, cocoyl caprylate/caprate, tridecyl octanoate, hexyl laurate, neopentanoic acid esters, for instance isodecyl neopentanoate, isotridecyl neopentanoate, isostearyl neopentanoate and 2-octyldodecyl neopentanoate, isononanoic acid esters, for instance isononyl isononanoate, isotridecyl isononanoate and octyl isononanoate, oleyl erucate, isocetyl stearate, isostearyl behenate and myristyl myristate; ii) fatty acid esters of alcohols or polyols, such as polyglyceryl-2 triisostearate, glyceryl triisostearate; glyceryl tris(2-decyltetradecanoate); propylene glycol dioctanoate, polyethylene glycol diheptanoate, propylene glycol bis(2-ethylhexanoate), pentaerythrityl tetraisostearate, pentaerythrityl tetrapelargonate, pentaerythrityl tetrakis(2-decyltetradecanoate), pentaerythrityl tetraisostearate, and pentaerythrityl tetraisononanoate; iii) aromatic esters such as tridecyl trimellitate, C12-C15 alcohol benzoate, the 2- phenylethyl ester of benzoic acid and butyloctyl salicylate; iv) fatty esters of alpha hydroxy acids, polyacids or amino acids such as triisoarachidyl citrate, isopropyl lauroyl sarcosinate, and diisopropyl sebacate; v) polyesters obtained by condensation of dimer and/or trimer of unsaturated fatty acid and of diol such as those with the INCI name dilinoleic acid/butanediol copolymer, dilinoleic acid/propanediol copolymer; polyesters obtained by condensation of fatty acid dimer and of diol dimer, such as dimer dilinoleyl dimer dilinoleate; vi) carbonates of formula R1-O-C(O)-O-R2, where R1 and R2, which may be identical or different, represent a linear or branched C4 to C12, and preferentially C6 to C10 alkyl chain; the total number of carbon atoms in the carbonate advantageously being at least 10. The carbonate oils may be dicaprylyl carbonate (or dioctyl carbonate), sold under the name Cetiol CC® by the company BASF, bis(2-ethylhexyl) carbonate, sold under the name Tegosoft DEC® by the company Evonik, dipropylheptyl carbonate (Cetiol 4 AII from BASF), dibutyl carbonate, dineopentyl carbonate, dipentyl carbonate, dineoheptyl carbonate, diheptyl carbonate, diisononyl carbonate or dinonyl carbonate, and preferably dioctyl carbonate; vii) and mixtures thereof. The fatty ethers include the oils of formula R3-O-R4 in which R3 and R4 independently denote a linear, branched or cyclic C6-C24 alkyl group, preferably a C6- C18 alkyl group, and preferably a C8-C12 alkyl group. It may be preferable for R3 and R4 to be identical. Linear alkyl groups that may be mentioned include a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an eicosyl group, a behenyl group, a docosyl group, a tricosyl group and a tetracosyl group. Branched alkyl groups that may be mentioned include a 1,1-dimethylpropyl group, a 3-methylhexyl group, a 5 5-methylhexyl group, an ethylhexyl group, a 2-ethylhexyl group, a 5-methyloctyl group, a 1-ethylhexyl group, a 1-butylpentyl group, a 2-butyloctyl group, an isotridecyl group, a 2-pentylnonyl group, a 2-hexyldecyl group, an isostearyl group, a 2- heptylundecyl group, a 2-octyldodecyl group, a 1,3-dimethylbutyl group, a 1-(1- methylethyl)-2-methylpropyl group, a 1,1,3,3-tetramethylbutyl group, a 3,5,5- 10 trimethylhexyl group, a 1-(2-methylpropyl)-3-methylbutyl group, a 3,7-dimethyloctyl group and a 2-(1,3,3-trimethylbutyl)-5,7,7-trimethyloctyl group. Cyclic alkyl groups that may be mentioned include a cyclohexyl group, a 3-methylcyclohexyl group and a 3,3,5-trimethylcyclohexyl group. As examples of fatty ethers, mention may be made of caprylyl ether, dilauryl 15 ether, diisostearyl ether, dioctyl ether, nonylphenyl ether, dodecyl dimethylbutyl ether, cetyl dimethylbutyl ether, cetyl isobutyl ether and mixtures thereof, better still caprylyl ether, such as the reference Cetiol OE sold by BASF. Preferably, the liquid fatty phase contains one or more fatty substances chosen from liquid alkanes, triglyceride oils of plant or synthetic origin, liquid fatty acids, 20 liquid fatty alcohols, liquid fatty esters other than triglyceride oils, liquid fatty ethers, and mixtures thereof, better still from liquid alkanes, triglyceride oils of plant or synthetic origin, liquid fatty acids, liquid fatty esters other than triglyceride oils, liquid fatty ethers and mixtures thereof, even better still from mixtures of saturated, linear hydrocarbons, particularly C14-C30, and more particularly C15-C28 hydrocarbons, 25 capric/caprylic/succinic acid triglycerides, sesame oil, oleic acid, oleyl erucate and dicaprylyl carbonate, and mixtures thereof. A liquid fatty substance is understood to mean a liquid alkane, a liquid fatty acid, a liquid fatty alcohol, a liquid fatty ester, a liquid fatty ether, a fatty substance having a melting point below or equal to 25°C at atmospheric pressure (1.013×10⁵ Pa). 30 The W/O emulsion may be prepared by a process known to those skilled in the art. More particularly, the process for preparing the W/O emulsion of the invention comprises the following steps: (a) preparation of the aqueous phase consisting in mixing, in a first container, one or more chitosans with water and one or more acids with a pKa of less than 5, so as to 35 obtain an aqueous phase with a pH of less than 6, (b) preparation of the fatty phase consisting in mixing, in a second container separate from the first, one or more fatty substances and a nonionic surfactant having an HLB ranging from 2 to 8 or a plurality of nonionic surfactants, said mixture of nonionic surfactants having an HLB ranging from 2 to 8, and 5 (c) addition of the mixture obtained in step (a) to the mixture obtained in step (b) with stirring; it being possible for step (b) and/or (c) to optionally comprise a heating step. An example of a process consists firstly in solubilizing at least one chitosan in water in the presence of at least one acid with a pKa of less than 5, with stirring at a 10 temperature ranging from 20°C to 45°C. A chitosan gel is then obtained. In a thermostatically controlled vessel with controlled mechanical stirring, for example by means of a rotor/stator, at least one fatty substance, preferably at least one oil, and at least one nonionic surfactant with an HLB ranging from 2 to 8 or a mixture of nonionic surfactants such that the HLB of said mixture ranges from 2 to 8, are mixed, 15 optionally with a supply of heat, preferably at room temperature (25°C) with stirring, until homogenized. The chitosan gel is then introduced into the liquid fatty phase with stirring, optionally with a supply of heat, preferably at room temperature (25°C), preferentially for a period of time ranging from 10 to 20 minutes. 20 If heat has been supplied during the introduction of the aqueous phase into the fatty phase, stirring is maintained until the mixture has cooled to room temperature. A homogeneous water-in-oil emulsion of variable viscosity is then obtained. The invention also relates to a cosmetic composition comprising the W/O emulsion as described above. In addition to the W/O emulsion described above, the 25 composition may comprise one or more surfactants chosen from anionic, cationic, nonionic, amphoteric or zwitterionic surfactants, and mixtures thereof, preferably from anionic, amphoteric or zwitterionic surfactants, and mixtures thereof. According to a preferred embodiment, the cosmetic composition comprises the W/O emulsion as described above and one or more anionic surfactants. 30 The W/O emulsion according to the invention is contained preferably in an amount ranging from 0.1% to 10% by weight, preferably from 0.1% to 5% by weight, more preferentially from 0.5% to 3% by weight, relative to the total weight of the cosmetic composition. The W/O emulsion according to the invention can be incorporated into 35 cosmetic compositions, notably compositions containing anionic species, in particular anionic surfactants such as washing bases. Anionic surfactant(s) The cosmetic composition may comprise one or more anionic surfactants. An anionic surfactant is understood to mean a surfactant comprising, as ionic or ionizable groups, only anionic groups. In the present description, a species is termed as being “anionic” when it bears at least one permanent negative charge or when it can be ionized to a negatively charged species, under the conditions of use of the composition of the invention (for example the medium or the pH) and not comprising any cationic charge. The anionic surfactants may be sulfate, sulfonate and/or carboxylic (or carboxylate) surfactants. Needless to say, a mixture of these surfactants may be used. It is understood in the present description that: - the carboxylate anionic surfactants comprise at least one carboxylic or carboxylate function (-COOH or -COO-) and may optionally also comprise one or more sulfate and/or sulfonate functions; - the sulfonate anionic surfactants comprise at least one sulfonate function (-SO3H or -SO3^–), and may optionally also comprise one or more sulfate functions, but do not comprise any carboxylate functions; and - the sulfate anionic surfactants comprise at least one sulfate function but do not comprise any carboxylate or sulfonate functions. The carboxylic anionic surfactants that may be used thus include at least one carboxylic or carboxylate function (-COOH or -COO-). They may be chosen from the following compounds: acyl glycinates, acyl lactylates, acyl sarcosinates, acyl glutamates; alkyl-D-galactosideuronic acids, alkyl ether carboxylic acids, alkyl(C6-30 aryl) ether carboxylic acids, alkylamido ether carboxylic acids; and also the salts of these compounds; the alkyl and/or acyl groups of these compounds comprising from 6 to 30 carbon atoms, notably from 12 to 28, even better still from 14 to 24 or even from 16 to 22 carbon atoms; the aryl group preferably denoting a phenyl or benzyl group; it being possible for these compounds to be polyoxyalkylenated, notably polyoxyethylenated, and then preferably comprising from 1 to 50 ethylene oxide units, better still from 2 to 10 ethylene oxide units. Use may also be made of C6-C24 alkyl monoesters of polyglycoside- polycarboxylic acids such as C6-C24 alkyl polyglycoside-citrates, C6-C24 alkyl polyglycoside-tartrates and C6-C24 alkyl polyglycoside-sulfosuccinates, and salts thereof. Preferentially, the carboxylic anionic surfactants are chosen, alone or as a mixture, from: - acyl glutamates, notably C6-C24, or even C12-C20 acyl glutamates, such as stearoyl glutamates, and in particular disodium stearoyl glutamate; - acyl sarcosinates, notably C6-C24 or even C12-C20 acyl sarcosinates, such as lauroyl sarcosinates, and in particular sodium lauroyl sarcosinate; - acyl lactylates, notably C12-C28 or even C14-C24 acyl lactylates, such as behenoyl lactylates, and in particular sodium behenoyl lactylate; - C6-C24 and notably C12-C20 acyl glycinates; - (C6-C24)alkyl ether carboxylates, and notably (C12-C20)alkyl ether carboxylates; - polyoxyalkylenated (C6-C24)alkyl(amido) ether carboxylic acids, in particular those comprising from 2 to 50 ethylene oxide groups; in particular in the form of alkali metal or alkaline-earth metal salts, ammonium salts or aminoalcohol salts. The sulfonate anionic surfactants that may be used include at least one sulfonate function (-SO3H or -SO3^–). They may be chosen from the following compounds: alkyl sulfonates, alkylamide sulfonates, alkylaryl sulfonates, α-olefin sulfonates, paraffin sulfonates, alkyl sulfosuccinates, alkyl ether sulfosuccinates, alkylamide sulfosuccinates, alkyl sulfoacetates, N-acyl taurates, acyl isethionates; alkyl sulfolaurates; and the salts of these compounds; the alkyl groups of these compounds comprising from 6 to 30 carbon atoms, notably from 12 to 28, even better still from 14 to 24 or even from 16 to 22 carbon atoms; the aryl group preferably denoting a phenyl or benzyl group; it being possible for these compounds to be polyoxyalkylenated, notably polyoxyethylenated, and then preferably comprising from 1 to 50 ethylene oxide units, better still from 2 to 10 ethylene oxide units. Preferentially, the sulfonate anionic surfactants are chosen, alone or as a mixture, from: - C6-C24, notably C12-C20 alkyl sulfosuccinates, notably lauryl sulfosuccinates; - C6-C24, notably C12-C20 alkyl ether sulfosuccinates; - alpha-(C6-C24)olefin sulfonates, preferably alpha-(C12-C18)olefin sulfonates, such as C14-16 olefin sulfonates, in particular in the form of alkali metal or alkaline-earth metal salts, ammonium salts or aminoalcohol salts. The sulfate anionic surfactants that may be used include at least one sulfate function (-OSO3H or -OSO3-). They may be chosen from the following compounds: alkyl sulfates, alkyl ether sulfates, alkylamido ether sulfates, alkylaryl polyether sulfates, monoglyceride sulfates; and the salts of these compounds; the alkyl groups of these compounds comprising from 6 to 30 carbon atoms, notably from 12 to 28, even better still from 14 to 24 or even from 16 to 22 carbon atoms; the aryl group preferably denoting a phenyl or benzyl group; it being possible for these compounds to be polyoxyalkylenated, notably polyoxyethylenated, and then preferably comprising from 1 to 50 ethylene oxide units, better still from 2 to 10 ethylene oxide units. Preferentially, the sulfate anionic surfactants are chosen, alone or as a mixture, from: - alkyl sulfates, notably C6-C24, or even C12-C20 alkyl sulfates; - alkyl ether sulfates, notably C6-C24, or even C12-C20 alkyl ether sulfates, preferably comprising from 2 to 20 ethylene oxide units; in particular in the form of alkali metal or alkaline-earth metal salts, ammonium salts or aminoalcohol salts. When the anionic surfactant is in salt form, said salt may be chosen from alkali metal salts, such as the sodium or potassium salt, ammonium salts, amine salts and in particular amino alcohol salts, and alkaline-earth metal salts, such as the magnesium salt. Examples of amino alcohol salts that may be mentioned include monoethanolamine, diethanolamine and triethanolamine salts, monoisopropanolamine, diisopropanolamine or triisopropanolamine salts, 2-amino-2- methyl-1-propanol salts, 2-amino-2-methyl-1,3-propanediol salts and tris(hydroxymethyl)aminomethane salts. Alkali metal or alkaline-earth metal salts and in particular sodium or magnesium salts are preferably used. Preferentially, the anionic surfactants are chosen, alone or as a mixture, from: - C6-C24, notably C12-C20 alkyl sulfates, - C6-C24, notably C12-C20 alkyl ether sulfates, preferably comprising from 1 to 20 ethylene oxide units, - C6-C24, notably C12-C20 alkyl sulfosuccinates, notably lauryl sulfosuccinates, - C6-C24, notably C12-C20 acyl sarcosinates, such as lauroyl sarcosinates, - alpha-(C6-C24)olefin sulfonates, preferably alpha-(C12-C18)olefin sulfonates, such as C14-16 olefin sulfonates, in particular in the form of alkali metal or alkaline-earth metal salts, ammonium salts or aminoalcohol salts. The anionic surfactant(s) is (are) preferably present in the composition in an amount ranging from 1% to 20% by weight, notably from 5% to 15% by weight, relative to the total weight of the composition. Amphoteric or zwitterionic surfactant(s) The cosmetic composition may comprise one or more amphoteric or zwitterionic surfactants. In particular, the amphoteric or zwitterionic surfactant(s), which are preferably non-silicone, may notably be optionally quaternized secondary or tertiary aliphatic amine derivatives, in which the aliphatic group is a linear or branched chain including from 8 to 22 carbon atoms, said amine derivatives containing at least one anionic group, for instance a carboxylate, sulfonate, sulfate, phosphate or phosphonate group. Mention may in particular be made of (C8-C20)alkylbetaines, (C8- C20)alkylsulfobetaines, (C8-C20)alkylamido(C1-C6)alkylbetaines and (C8- C20)alkylamido(C1-C6)alkylsulfobetaines, and mixtures thereof. Among the optionally quaternized secondary or tertiary aliphatic amine derivatives that can be used, as defined above, mention may also be made of the compounds having the respective structures (II) and (III) below: Ra-CONHCH2CH2-N⁺(Rb)(Rc)-CH2COO-, M⁺, X- (II) formula (II), in which: - Ra represents a C10 to C30 alkyl or alkenyl group derived from an acid RaCOOH preferably present in hydrolysed coconut kernel oil; preferably, Ra represents a heptyl, nonyl or undecyl group; - Rb represents a β-hydroxyethyl group; - Rc represents a carboxymethyl group; - M⁺ represents a cationic counterion derived from an alkali metal or alkaline-earth metal, such as sodium, an ammonium ion or an ion derived from an organic amine; and - X- represents an organic or inorganic anionic counterion, such as that chosen from halides, acetates, phosphates, nitrates, (C1-C4)alkyl sulfates, (C1-C4)alkyl sulfonates or (C1-C4)alkylaryl sulfonates, in particular methyl sulfate and ethyl sulfate; or alternatively M⁺ and X- are absent; Ra’-CONHCH2CH2-N(B)(B’) (III) formula (III), in which: - B represents the group -CH2CH2OX’; - B’ represents the group -(CH2)zY’, with z = 1 or 2; - X’ represents the group -CH2COOH, -CH2-COOZ’, -CH2CH2COOH or CH2CH2- COOZ’, or a hydrogen atom; - Y’ represents the group -COOH, -COOZ’ or -CH2CH(OH)SO3H or the group CH2CH(OH)SO3-Z’; - Z’ represents a cationic counterion derived from an alkali metal or alkaline-earth metal, such as sodium, an ammonium ion or an ion derived from an organic amine; - Ra’ represents a C10 to C30 alkyl or alkenyl group of an acid Ra’-COOH which is preferably present in coconut kernel oil or in hydrolysed linseed oil, preferably Ra’ is an alkyl group, notably a C17 group, and its iso form, or an unsaturated C17 group. These compounds are classified in the CTFA dictionary, 5th edition, 1993, under the names disodium cocoamphodiacetate, disodium lauroamphodiacetate, disodium caprylamphodiacetate, disodium capryloamphodiacetate, disodium cocoamphodipropionate, disodium lauroamphodipropionate, disodium caprylamphodipropionate, disodium capryloamphodipropionate, lauroamphodipropionic acid and cocoamphodipropionic acid. By way of example, mention may be made of the cocoamphodiacetate sold by the company Rhodia under the trade name Miranol^® C2M Concentrate. Use may also be made of the compounds of formula (IV): Ra’’-NHCH(Y’’)-(CH2)nCONH(CH2)n’-N(Rd)(Re) (IV) formula (IV), in which: - Y’’ represents the group -COOH, -COOZ’’ or -CH2-CH(OH)SO3H or the group CH2CH(OH)SO3-Z’’; - Rd and Re, independently of each other, represent a C1 to C4 alkyl or hydroxyalkyl radical; - Z’’ represents a cationic counterion derived from an alkali metal or alkaline-earth metal, such as sodium, an ammonium ion or an ion derived from an organic amine; - Ra’’ represents a C10 to C30 alkyl or alkenyl group of an acid Ra’’-COOH which is preferably present in coconut kernel oil or in hydrolysed linseed oil; and - n and n’ denote, independently of each other, an integer ranging from 1 to 3. Among the compounds of formula (IV), mention may be made of the compound classified in the CTFA dictionary under the name sodium diethylaminopropyl cocoaspartamide and sold by Chimex under the name Chimexane HB. These compounds may be used alone or as mixtures. Among the amphoteric or zwitterionic surfactants mentioned above, use is advantageously made of (C8-C20)alkyl betaines, such as cocoyl betaine, (C8- C20)alkylamido(C3-C8)alkylbetaines, such as cocamidopropyl betaine, (C8-C20)alkyl amphoacetates, (C8-C20)alkyl amphodiacetates and mixtures thereof; preferably (C8- C20)alkyl betaines, (C8-C20)alkylamido(C3-C8)alkyl betaines and mixtures thereof; and better still (C8-C20)alkylamido(C3-C8)alkyl betaines, and mixtures thereof. Preferably, the total amount of the amphoteric or zwitterionic surfactant(s) ranges from 0.1% to 10% by weight, better still from 0.1% to 5% by weight, relative to the total weight of the composition. The cosmetic composition according to the invention may further comprise one or more additives chosen from salts, emollients, moisturizers, fragrances, peptizers, vitamins, thickeners, suspending agents, conditioning agents, preservatives, anionic polymers, cationic polymers other than the chitosan(s) used in the W/O emulsion, nonionic polymers and amphoteric polymers, pigments, dyes, pearlescent agents and mixtures thereof. A person skilled in the art will take care to select the optional additives and the amount thereof such that they do not harm the properties of the cosmetic composition of the present invention. These additives may be present in the composition according to the invention in an amount ranging from 0 to 20% by weight, relative to the total weight of the composition. Another subject of the present invention is a method for cosmetically treating, notably cleansing and/or conditioning keratin fibres, in particular human keratin fibres such as the hair, comprising a step of applying a cosmetic composition as described above to said fibres. The composition may be applied directly to damp or dry hair, preferably to damp hair. The step of applying the composition according to the invention may optionally be followed by a rinsing step. Preferably, the step of applying the composition according to the invention is followed by a rinsing step. Moreover, this step of applying the composition according to the invention 5 may optionally be followed by a drying step. Another subject of the present invention is a use of the composition as described previously, for the cosmetic treatment, in particular the cleansing and/or conditioning, of keratin fibres, in particular of human keratin fibres such as the hair. More particularly, the composition according to the invention is used as a hair 10 shampoo. The examples that follow serve to illustrate the invention without, however, being limiting in nature. Examples 15 Examples 1 to 6 Preparation of the aqueous phase of the W/O emulsion Two chitosan gels were prepared beforehand by solubilizing a chitosan in an aqueous solution containing lactic acid. The proportions are indicated in Table 1 below as % by weight of active material relative to the total weight of the gel. 20 Table 1 Gel 1 2 Chitosan (KiOsmetine P from KitoZyme) 14.3 - Chitosan (KiOsmetine CsG from - 14.3 KitoZyme) Lactic acid 14.3 14.3 Water qs 100 100 The chitosan in solution in water and lactic acid were mixed with stirring using a rotor/stator between 20°C and 45°C. Two gels having a pH between 3.5 and 4 were obtained. 25 Preparation of the fatty phase of the W/O emulsion In a thermostatically controlled vessel with controlled mechanical stirring, for example by means of a rotor/stator, the fatty substance(s) and the nonionic surfactant(s) specified in Tables 2 and 3 were mixed at 500 rpm and at room temperature (25°C), until homogenized. A fatty phase was thus obtained. Preparation of the W/O emulsion 5 A chitosan gel as prepared above is then introduced into the fatty phase with stirring at 2000 rpm with the rotor/stator for 10 min at room temperature. A homogeneous water-in-oil emulsion of variable viscosity is then obtained. In Tables 2 and 3 which follow, all the proportions are expressed as % by weight of active material (% AM) relative to the total weight of the W/O emulsion. Table 2 Example 1 2 3 Sesame oil (Refined CT sesame oil from Etico) 22.5 22.5 22.5 Polyglyceryl-3 polyricinoleate (AKOLINE PGPR from AarhusKarlshamn) 7.5 7.5 - HLB: 3-4 Polyglyceryl-4 diisostearate/polyhydroxystearate/ sebacate (Isolan GPS from Evonik - - 7.5 Goldschmidt) HLB: 5 Gel 1 from Table 1 70 - - Gel 2 from Table 1 - 70 70 Stability over 1 month at room Fluid Fluid Fluid temperature (25°C) - macroscopic homogeneous homogeneous homogeneous appearance cream cream cream Table 3 Example 4 5 6 Sesame oil (Refined CT sesame oil from Etico) 22.5 - 22.5 Oleyl Erucate (CETIOL J 600 from BASF) - 22.5 - Polyglyceryl-6 polyricinoleate (SY-GLYSTER CRS-75 from Sakamoto Yakuhin) 7.5 - - HLB: 3 Polyglyceryl-3 diisostearate (Lameform TGI from BASF) - 7.5 - HLB: 3 Polyglyceryl-3 polyricinoleate HLB: 4 (and) sorbitan isostearate HLB: 4.7 (Arlacel 1690-LQ-(MV) - - 7.5 from Croda) Gel 2 from Table 1 70 70 70 Stability over 1 month at room Fluid Viscous Fluid temperature (25°C) - macroscopic homogeneous homogeneous homogeneous appearance cream paste cream The W/O emulsions based on chitosan were evaluated visually according to the criteria of homogeneity (phase separation) and consistency (thicker or thinner product, which flows more or less easily) after storage for 1 month at room 5 temperature. The results are indicated above in Tables 2 and 3. Examples 7-11 The same preparation process as in Examples 1-6 was applied to prepare W/O emulsions with various oils in Examples 7-11. The emulsions obtained from the10 ingredients of Tables 4 and 5 were also evaluated in the same way as in Examples 1- 6. The proportions indicated in Tables 4 and 5 are in % by weight of active material relative to the total weight of the W/O emulsion. Table 4 Example 7 8 Oleyl Erucate (CETIOL J 600 from BASF) 22.5 - Caprylic/Capric/Succinic Triglycerides (MIGLYOL 829 - 22.5 from CREMER OLEO) Polyglyceryl-3 polyricinoleate (AKOLINE PGPR from AarhusKarlshamn) 7.5 7.5 HLB: 3-4 Gel 1 from Table 1 70 70 Stability over 1 month at room Fluid Thick temperature (25°C) - homogeneous homogeneous macroscopic appearance cream cream Table 5 Example 9 10 11 C15-C19 Alkane & Hydrogenated Polyfarnesene (EMOGREEN HP40 from 22.5 - - SEPPIC) Dicaprylyl carbonate (Cetiol CC from BASF) - 22.5 - Oleic acid (EDENOR OL 75 MY from EMERY - - 22.5 OLEOCHEMICALS) Polyglyceryl-3 polyricinoleate (AKOLINE PGPR from 7.5 7.5 7.5 AarhusKarlshamn) HLB: 3-4 Gel 1 from Table 1 70 70 70 Stability over 1 month at Fluid Fl ture (25°C) - Homogeneo uid room tempera us homogeneous hom ppearance crea ogeneous macroscopic a m cream cream Examples 12-15 5 The same preparation process as in Examples 1-6 was applied to prepare W/O emulsions with various proportions of aqueous phase in Examples 12-15. The emulsions obtained from the ingredients of Table 6 were also evaluated in the same way as in Examples 1-6. The proportions indicated in Table 6 are in % by weight of active material relative to the total weight of the W/O emulsion. Table 6 Example 12 13 14 15 Oleyl Erucate (CETIOL J 600 from BASF) 35 28 8 3 Polyglyceryl-3 polyricinoleate (AKOLINE PGPR from 5 2 2 2 AarhusKarlshamn) HLB: 3-4 Gel 1 from Table 1 60 70 90 95 Stability over 1 month at Fluid Fluid room temperature (25°C) - homogeneous homogeneous Slight Slight macroscopic appearance cream cream creaming creaming Example 16 Two washing bases A and B were prepared comprising the ingredients below in Tables 7 and 8 respectively, in proportions expressed as % by weight of active material relative to the total weight of each washing base. Table 7 Washing base A % by weight SODIUM LAURETH SULFATE 9.8 COCAMIDOPROPYL BETAINE 1.2 WATER qs 100 Table 8 Washing base B % by weight SODIUM C14-16 OLEFIN SULFONATE 10 SODIUM LAUROYL SARCOSINATE 1.52 COCAMIDOPROPYL BETAINE 2.0 WATER qs 100 99% by weight of each washing base A or B were then mixed with 1% by weight of the W/O emulsion from Example 1, with stirring with a rotor/stator at 1500 rpm for 10 min. The compositions A1 and B1 were thus obtained. 5 These compositions A1 and B1 were compared respectively with compositions A0 and B0 in which the ingredients of the W/O emulsion were introduced directly and successively, that is to say without separate prior preparation of the W/O emulsion. 10 Table 9 Composition A0 A1 (comparative) (invention) Washing base A 99 99 Sesame oil Refined CT sesame oil from Etico 0.225 - Polyglyceryl-3 polyricinoleate AKOLINE PGPR from 0.075 - AarhusKarlshamn Chitosan KiOsmetine CsG from KitoZyme 0.1 - Lactic acid 0.1 - Water 0.50 - W/O emulsion from Example 1 - 1 Visual appearance of the final Precipitation in the Dispersion of composition form of aggregates chitosan in the form of an emulsion Table 10 Composition B0 B1 (comparative) (invention) Washing base B 99 99 Sesame oil Refined CT sesame oil from Etico 0.225 - Polyglyceryl-3 polyricinoleate AKOLINE PGPR from 0.075 - AarhusKarlshamn Chitosan KiOsmetine CsG from KitoZyme 0.1 - Lactic acid 0.1 - Water 0.50 - W/O emulsion from Example 1 - 1 Visual appearance of the final Precipitation in the Dispersion of composition form of aggregates chitosan in the form of an emulsion The visual appearance of the compositions was evaluated macroscopically and microscopically immediately after mixing. The results are indicated in Tables 9 5 and 10. Compositions A1 and B1 according to the invention, obtained by incorporating the chitosan-based water-in-oil emulsion into the washing bases A and B, have a homogeneous appearance with good dispersion of the chitosan. On the other hand, the comparative compositions A0 and B0, obtained by direct incorporation of 10 the same ingredients, do not allow a good dispersion of the chitosan, which forms aggregates. Example 17 The following compositions were prepared comprising the washing base A or 15 B from Example 16 and at least one of the W/O emulsions from Examples 2-13. The visual appearance of the compositions was evaluated macroscopically and microscopically immediately after mixing. The results are indicated in Tables 11-13. Table 11 Composition A2 A3 A4 A5 A6 Washing base A 99 99 99 99 99 W/O emulsion of Ex.2 1 - - - - W/O emulsion of Ex.3 - 1 - - - W/O emulsion of Ex.4 - - 1 - - W/O emulsion of Ex.5 - - - 1 - W/O emulsion of Ex.6 - - - - 1 Visual Dispersion Dispersion Dispersion Dispersion Dispersion appearance of the of the of the of the of the chitosan chitosan chitosan chitosan chitosan Table 12 Composition A7 A8 A9 A10 Washing base A 99 99 99 99 W/O emulsion of Ex.7 1 - - - W/O emulsion of Ex.8 - 1 - - W/O emulsion of Ex.9 - - 1 - W/O emulsion of Ex.10 - - - 1 Dispersion Dispersion Dispersion Dispersion Visual appearance of the of the of the of the chitosan chitosan chitosan chitosan 5 Table 13 Composition A11 B11 A12 A13 Washing base A 99 - 99 99 Washing base B - 99 - - W/O emulsion of Ex.11 1 1 - - W/O emulsion of Ex.12 - - 1 - W/O emulsion of Ex.13 - - - 1 Dispersion Dispersion Dispersion Dispersion Visual appearance of the of the of the of the chitosan chitosan chitosan chitosan

Claims

CLAIMS 1. Water-in-oil (W/O) emulsion comprising: - from 5% to 20% by weight, preferably from 5% to 15% by weight, of one or more 5 chitosans relative to the total weight of the emulsion, and one or more acids with a pKa < 5, solubilized in an aqueous phase with a pH < 6, - one or more nonionic surfactants, the HLB of said nonionic surfactant or of the mixture of said nonionic surfactants ranging from 2 to 8, and - a fatty phase which is liquid at room temperature. 10 2. Water-in-oil emulsion according to Claim 1, characterized in that the chitosan(s) has (have) a degree of acetylation of less than or equal to 40%, preferably less than or equal to 30%, more preferentially less than or equal to 15%, better still between 1% and 10%, and a molecular weight preferably between 5000 and 150000 Da. 15 3. Water-in-oil emulsion according to Claim 1 or 2, characterized in that the acid(s) with a pKa < 5 is (are) organic, preferentially chosen from the carboxylic acids of formula:

in which: 20 A represents a monovalent group when n is 0, or a polyvalent group when n is greater than or equal to 1; A represents a saturated or unsaturated, cyclic or acyclic, aromatic or non-aromatic hydrocarbon group comprising from 1 to 6 carbon atoms, optionally interrupted with one or more heteroatoms such as O or NH, and/or substituted with one or more hydroxy groups; preferably, A represents a monovalent C1-C6 alkyl or phenyl 25 group, or a polyvalent C1-C6 alkylene or phenylene group optionally substituted with one or more hydroxy groups; n represents an integer ranging from 0 to 10, preferably from 0 to 5, better still from 0 to 2; better still from acetic acid, citric acid, glutaric acid, lactic acid, gluconic acid, maleic 30 acid and mixtures thereof. 4. Water-in-oil emulsion according to any one of the preceding claims, characterized in that it comprises a total amount of the acid(s) with a pKa of less than 5, ranging from 0.5% to 20% by weight relative to its total weight. 5. Water-in-oil emulsion according to any one of the preceding claims, characterized in that the weight ratio of the total amount of acid(s) with a pKa of less than 5 to the amount of chitosan(s) (acid(s) with a pKa of less than 5)/chitosan(s)) ranges from 0.1 to 5, preferably from 0.5 to 2. 6. Water-in-oil emulsion according to any one of the preceding claims, characterized in that the nonionic surfactant(s) is (are) chosen from: · oxyalkylenated, linear or branched, saturated or unsaturated C16-C30 fatty alcohols; · optionally oxyalkylenated, linear or branched, saturated or unsaturated C16-C30 fatty amides; · poly(ethylene oxide) esters of linear or branched, saturated or unsaturated C16-C30 fatty acids; · sorbitan esters of linear or branched, saturated or unsaturated C16-C30 fatty acids, which are optionally polyoxyethylenated; · glycerol or polyglycerol esters of linear or branched, saturated or unsaturated C10- C30, preferably C10-C24 fatty acids, or polymers of these fatty acids; · saturated or unsaturated, oxyethylenated plant oils; · (C16-C30) alkyl glucosides, preferably (C16-C22) alkyl glucosides; · condensates of ethylene oxide and/or of propylene oxide; · oxyethylenated and/or oxypropylenated silicones; and · phospholipids such as lecithins; preferably from glycerol or polyglycerol esters of linear or branched, saturated or unsaturated C10-C30 fatty acids, or polymers of these fatty acids; sorbitan esters of linear or branched, saturated or unsaturated C16-C30 fatty acids, lecithins and mixtures thereof; better still from polyglyceryl-4 diisostearate/polyhydroxystearate/ sebacate, polyglyceryl-3 or polyglyceryl-6 polyricinoleate, polyglyceryl-3 diisostearate, sorbitan isostearate, and mixtures thereof. 7. Water-in-oil emulsion according to any one of the preceding claims, characterized in that the total amount of the nonionic surfactant(s), the HLB of said nonionic surfactant or mixture of said nonionic surfactants ranging from 2 to 8, ranges from 2% to 10% by weight relative to the total weight of the W/O emulsion. 8. Water-in-oil emulsion according to any one of the preceding claims, characterized in that the liquid fatty phase is contained in an amount ranging from 3% to 35% by weight relative to the total weight of the W/O emulsion. 9. Water-in-oil emulsion according to any one of the preceding claims, characterized in that the liquid fatty phase comprises one or more fatty substances chosen from alkanes, triglyceride oils of plant or synthetic origin, fatty acids, fatty alcohols, fatty esters other than triglyceride oils, fatty ethers, and mixtures thereof, preferably from liquid alkanes, triglyceride oils of plant or synthetic origin, liquid fatty acids, liquid fatty alcohols, liquid fatty esters other than triglyceride oils, liquid fatty ethers and mixtures thereof, and more preferably from mixtures of saturated, linear hydrocarbons, particularly C14-C30, and more particularly C15-C28 hydrocarbons, capric/caprylic/succinic acid triglycerides, sesame oil, oleic acid, oleyl erucate and dicaprylyl carbonate, and mixtures thereof. 10. Process for preparing the water-in-oil emulsion according to any one of the preceding claims, comprising the following steps: (a) preparation of the aqueous phase consisting in mixing, in a first container, one or more chitosans with water and one or more acids with a pKa of less than 5, so as to obtain an aqueous phase with a pH of less than 6, (b) preparation of the fatty phase consisting in mixing, in a second container separate from the first, one or more fatty substances and a nonionic surfactant having an HLB ranging from 2 to 8 or a plurality of nonionic surfactants, said mixture of nonionic surfactants having an HLB ranging from 2 to 8, and (c) addition of the mixture obtained in step (a) to the mixture obtained in step (b) with stirring; it being possible for step (b) and/or (c) to optionally comprise a heating step. 11. Cosmetic composition comprising the water-in-oil emulsion according to any one of Claims 1 to 9, preferably in an amount ranging from 0.1% to 10% by weight, preferably from 0.1% to 5% by weight, relative to the total weight of the composition. 12. Cosmetic composition according to Claim 11, further comprising one or more surfactants chosen from anionic, cationic, nonionic, amphoteric or zwitterionic surfactants, and mixtures thereof, preferably from anionic, amphoteric or zwitterionic surfactants, and mixtures thereof. 13. Cosmetic composition according to Claim 11 or 12, comprising one or more anionic surfactants, preferably chosen, alone or as a mixture, from: - C6-C24, notably C12-C20 alkyl sulfates, - C6-C24, notably C12-C20 alkyl ether sulfates, preferably comprising from 1 to 20 ethylene oxide units, - C6-C24, notably C12-C20 alkyl sulfosuccinates, notably lauryl sulfosuccinates, - C6-C24, notably C12-C20 acyl sarcosinates, notably lauroyl sarcosinates, - alpha-(C6-C24)olefin sulfonates, preferably alpha-(C12-C18)olefin sulfonates, such as C14-16 olefin sulfonates, in particular in the form of alkali metal or alkaline-earth metal salts, ammonium salts or aminoalcohol salts. 14. Cosmetic composition according to Claim 12 or 13, characterized in that the total amount of anionic surfactant(s) ranges from 1% to 20% by weight, better still from 5% to 15% by weight, relative to the total weight of the composition. 15. Cosmetic composition according to any one of Claims 11 to 14, comprising one or more amphoteric or zwitterionic surfactants, preferably chosen from (C8-C20)alkylbetaines, (C8-C20)alkylamido(C3-C8)alkylbetaines and mixtures thereof, better still from (C8-C20)alkylamido(C3-C8)alkylbetaines and mixtures thereof. 16. Cosmetic composition according to any one of Claims 12 to 15, characterized in that the total amount of amphoteric or zwitterionic surfactant(s) ranges from 0.1% to 10% by weight, preferably from 0.1% to 5% by weight, relative to the total weight of the composition. 17. Method for cosmetically treating, preferably cleansing and/or conditioning, keratin fibres, better still the hair, comprising the application of the cosmetic composition according to any one of Claims 11 to 16, to said fibres. 18. Use of the composition according to any one of Claims 11 to 16, for the cosmetic treatment, preferably the cleansing and/or conditioning, of keratin fibres, better still of the hair.