RU2541759C1 - Method of determining concentration of grafted amino groups on surface of mineral fillers - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: method of determining concentration of grafted amino groups on the surface of mineral filling agents includes preparation of acetylating solution by mixing initial components, its addition to weighed portion of modified mineral filler sample, exposure for quantitative realisation of reaction, titration with alkali solution in presence of indicator, calculation of concentration of amino groups by difference of results of idle titration and sample titration. As acetylating solution used is 0.5-0.6 M solution of acetic anhydride in mixed solvent dichloroethane-pyridine in ratio from 0.5:1-2:1, which contains 0.025-0.15 mol/l of chloric acid as catalyst. 0.5-0.6 M of acetylating solution in mixed solvent, containing chloric acid, is added to weighed portion of modified mineral filler, ratio of weighed portion of sample weight to volume of acetylating solution constitutes 1:4-1:5, after which it is exposed and hydrolysing mixture, consisting of dimethylformamide, pyridine, water, taken in ratio 6:3:1 respectively, is added. After that, obtained mixture is centrifuged to separate residue.EFFECT: simplification and extension of assortment of analysed materials, containing grafted amino groups on the surface of mineral fillers.13 ex, 1 tbl

Description

The invention relates to analytical chemistry, namely to determining the concentration of grafted amino groups on the surface of mineral fillers, which can be used in the production of composite materials, modified mineral fillers and various sorbents based on them.

In the production of composite materials, the modification of mineral fillers is carried out in order to increase wettability, adhesion, as well as create a chemical bond between the polymer and the filler. Moreover, the number of functional groups grafted after modification will affect the properties of the resulting material.

A known method for determining the concentration of grafted amino groups on the surface of modified silica gel by reverse acid-base titration [Chemistry of grafted surface compounds. / ed. G.V. Lisichkina. - M .: FIZMATLIT, 2003. - 592 p., S.282]. In this case, the contact of the modified silica gel with hydrochloric acid occurs, as a result of which the grafted amino groups react with the latter to form the corresponding ammonium cation. Then the excess hydrochloric acid is titrated with alkali. This method is simple to implement, does not require the use of expensive reagents, but cannot be used to determine the concentration of grafted amino groups on the surface of many mineral fillers, such as basalt, mica, talc, etc. The reason for this is the presence in them of basic oxides, which also react with hydrochloric acid. In this case, the consumption of hydrochloric acid for basic oxides in a blank experiment can be much larger than with titration of a modified filler. Therefore, it is impossible to determine the concentration of grafted amino groups on the surface of many mineral fillers by this method.

A known method for determining the concentration of grafted amino groups on the surface of mineral fillers, which consists in the quantitative determination of carbon, hydrogen and nitrogen by analyzing the combustion products of the sample. The concentration of amino groups is calculated according to elemental analysis [Liu, Y.-H., Lin, H.-P., & Mou, C.-Y. (2004). Direct method for surface silyl functionalization of mesoporous silica. / Langmuir: the ACS journal of surfaces and colloids, 20 (8), 3231-3239].

There is also a method for determining the concentration of grafted amino groups on the surface of mineral fillers by X-ray photoelectron spectrometry [Okusa, H., Kurihara, K., & Kunitake, T. (1994). Chemical modification of molecularly smooth mica surface and protein attachment. Langmuir, 10 (10), 3577-3581].

However, these methods require the use of expensive technically sophisticated equipment, and therefore cannot be implemented in many enterprises producing modified fillers and composite materials.

Known methods for determining amino groups by the method of acetylation, used to determine amino groups in liquid or solid soluble samples, for example, by the method of Magnuson and Cherry [Siggia S., Hannah J.G. Quantitative organic analysis by functional groups: Per. from English - M .: Chemistry, 1983. P.25]. Acetylation is carried out with a 1 M solution of acetic anhydride in dichloroethane containing 0.15 mol / L perchloric acid as a catalyst. To a sample of a liquid sample containing 4-5 mmol of amino groups, add 10 ml of an acetylating solution, hold for 5 minutes and add 35-40 ml of a hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1). Stand for 5-10 minutes, then add a few drops of the indicator and titrate with an alcoholic solution of potassium hydroxide. The content of amino groups is calculated by the difference between the results of blank titration and titration of the sample. The advantages of this method are the ease of preparation of the acetylating solution, since it does not need to be cooled during preparation. A solution of anhydride in dichloroethane is practically colorless and stored for at least 2 months. In addition, the highest rate of acetylation reaction in this solvent is noted. However, this method is not applicable for the analysis of modified mineral fillers. The concentration of functional groups on their surface is usually 0.4-1.5 mmol / g, i.e. for analysis, on average, a sample weight of 5 g is required. At the same time, 10 ml of an acetylating solution is not enough for good wetting of the sample, and, consequently, for a quantitative reaction. When diluting the acetylating solution with dichloroethane for better wetting of the sample, the amino group acetylation reaction does not proceed completely, giving underestimated results, and when the amount of acetylating solution of the same concentration increases with respect to the sample weight, the experimental error greatly increases, including due to an increase in the amount of perchloric acid relative to the number groups.

The closest analogue to the proposed technical solution is a method for determining amino groups by the method of acetylation according to the method of Fritz and Schenk [Siggia S., Hannah J.G. Quantitative organic analysis by functional groups: Per. from English - M .: Chemistry, 1983. P.21]. Acetylation is carried out with a 2 M solution of acetic anhydride in ethyl acetate or pyridine containing 0.15 mol / L perchloric acid as a catalyst. To a sample of a liquid sample containing 3-4 mmol of amino groups, add 5 ml of an acetylating solution in ethyl acetate or pyridine, hold for about 5 minutes and add 2 ml of water and 10 ml of a solution of pyridine in water (3: 1). Stand for 5-10 minutes, then add a few drops of the indicator and titrate with an alkali solution, such as potassium hydroxide.

The preparation of an acetylating solution in ethyl acetate is carried out according to the following procedure: 150 ml of ethyl acetate, 4 g of 72% perchloric acid and 8 ml of acetic anhydride are pipetted into a flask with a ground stopper. The mixture was kept at room temperature for at least 30 minutes, then cooled to 5 ° C, 42 ml of cold acetic anhydride was added and the mixture was kept for another 1 hour at 5 ° C. The prepared reagent is suitable for no more than 2 weeks. From the description of the method shows that the preparation of acetylating solution is very time-consuming.

When preparing an acetylating solution in pyridine, 0.8 g of perchloric acid are carefully added dropwise into a flask containing 30 ml of pyridine, and 10 ml of acetic anhydride are added with stirring. The prepared solution deteriorates within a few hours, as a result of which it should be used freshly prepared. At the same time, the preparation of small amounts of a solution (1-5 ml) with exact concentrations of reagents is difficult because of the need to dose the components in small quantities. This method is not applicable for the analysis of modified mineral fillers. The concentration of functional groups on their surface is usually 0.4-1.5 mmol / g, i.e. for analysis, on average, a sample weight of 5 g is required. At the same time, 5 ml of an acetylating solution is not sufficient for good wetting of the sample, and, consequently, for a quantitative reaction. Also, when analyzing solid samples, it is necessary to separate the analyzed solution from the solid phase, which cannot be done with the given ratios of the amount of the analyzed sample and the solution. With an increase in the amount of an acetylating solution of the same concentration relative to the sample weight, the experimental error greatly increases, including due to an increase in the amount of perchloric acid relative to the number of determined groups. Dilution of the solution or a decrease in the amount of perchloric acid in these acetylating solutions is impossible, since at its concentration less than 0.15 mol / L, acetylation does not proceed completely.

Thus, this method is laborious and cannot be applied to the analysis of modified mineral fillers, in contrast to the known analogues.

The technical result is to simplify the method and expand the assortment of materials under study containing grafted amino groups on the surface of mineral fillers.

To achieve a technical result, it is proposed to use a 0.5-0.6 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine in a ratio of 0.5: 1 to 2: 1 containing 0.025-0.15 mol / l perchloric acid as an acetylating solution. acid as a catalyst. An acetylating solution is prepared by mixing the necessary proportions of a solution of acetic anhydride in dichloroethane and pyridine. A 0.5-0.6 M acetylating solution in a mixed solvent containing perchloric acid is added to a sample of the modified mineral filler. The ratio of the sample to the volume of the acetylating solution is 1: 4-1: 5. Maintain for quantitative reaction for at least 5 minutes, after which add a hydrolysis mixture consisting of dimethylformamide, pyridine, water, taken in a ratio of 6: 3: 1, respectively. The resulting mixture was centrifuged to separate the precipitate and an aliquot of the titration solution was taken. Titration is carried out with an aqueous alkali solution, for example, potassium hydroxide or sodium hydroxide in the presence of an indicator. As an indicator, you can take, for example, a solution of thymol blue, titration is carried out before the color changes from yellow to blue. The content of amino groups is calculated by the difference between the results of blank titration and titration of the sample.

Distinctive features of the prototype are:

- the use of a 0.5-0.6 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine, taken in a ratio of from 0.5: 1 to 2: 1;

- the concentration of perchloric acid used as a catalyst is 0.025-0.15 mol / l;

- the use of the ratio of the mass of the sample to the volume of the acetylating solution, comprising from 1: 4 to 1: 5;

- centrifuging the sample to separate the sediment of the modified mineral filler;

- the use of a hydrolyzing mixture of the composition: dimethylformamide: pyridine: water in a ratio of 6: 3: 1, respectively, known by the method of Magnuson and Cherry.

The combination of these distinguishing features allows us to simplify the method for determining amino groups in various, including solid, insoluble materials, because the acetylating solution is prepared by mixing 2 solutions, and not three, as in the prototype, which are stable for at least 2 months, which allows the preparation of small volumes of acetylating solution with exact concentrations of reagents. Such advantages compared to the prototype can reduce the consumption of starting reagents in the analysis of a single sample and expand the assortment of materials under study.

Example 1. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine (1: 1) is added. ) containing 0.15 mol / L perchloric acid. The reaction mixture is kept for 5 minutes, after which 6 ml of a hydrolysis mixture containing dimethylformamide: pyridine: water are added in a ratio of 6: 3: 1, respectively. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.42 ± 0.03 mmol / g.

Example 2. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine (1: 1) is added. ) containing 0.075 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.40 ± 0.03 mmol / g.

Example 3. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine (1: 1) is added. ) containing 0.025 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.42 ± 0.03 mmol / g.

Example 4. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine (1: 1) is added. ) containing 0.010 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.36 ± 0.03 mmol / g.

Example 5. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine (1: 1) is added. ), not containing perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.02 ± 0.03 mmol / g.

Example 6. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 0.8 ml of a 0.6 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine is added (2 : 1) containing 0.075 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.41 ± 0.03 mmol / g.

Example 7. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.4 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine (1: 1) is added. ) containing 0.050 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The found number of amino groups was 0.85 ± 0.03 mmol / g.

Example 8. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 0.2 ml of a 2 M solution of acetic anhydride in pyridine containing 0.15 mol / l is added. perchloric acid. The acetylating solution does not completely wet the mineral filler, as a result of which quantitative analysis is impossible.

Example 9. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. The acetylation solution of Example 8 is diluted 4 times with pyridine. Add 1 ml of a 0.5 M solution of acetic anhydride in pyridine containing 0.037 mol / l perchloric acid. The reaction mixture is kept for 5 minutes, after which 0.4 ml of water and 2 ml of a solution of pyridine in water are added (3: 1). The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.17 ± 0.03 mmol / g.

Example 10. A sample of a modified silica gel containing 1.40 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in pyrdine containing 0.075 mol / l of perchloric acid is added. and another 0.5 ml of dichloroethane. The reaction mixture is aged for 30 minutes, after which 0.4 ml of water and 2 ml of a solution of pyridine in water are added (3: 1). The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 1.24 ± 0.03 mmol / g.

Example 11. A sample of modified micromica containing 0.28 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine (1: 1) is added. ) containing 0.025 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 0.29 ± 0.03 mmol / g.

Example 12. A sample of a modified basalt microplate filler containing 0.18 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine is added (1 : 2) containing 0.025 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 0.17 ± 0.03 mmol / g.

Example 13. A sample of a modified chopped basalt fiber filler containing 0.14 ± 0.03 mmol / g of grafted amino groups weighing 0.2000 g is placed in a container. 1 ml of a 0.5 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine ( 1: 1) containing 0.025 mol / L perchloric acid. The reaction mixture is aged for 5 minutes, after which 6 ml of hydrolysis mixture (dimethylformamide: pyridine: water in a ratio of 6: 3: 1) is added. The mixture is then centrifuged to separate the precipitate and an aliquot of a 1 ml solution is taken for titration. Titration is carried out in the presence of a thymol blue indicator until the color changes from yellow to blue with an aqueous solution of potassium hydroxide. The amount of amino groups found was 0.13 ± 0.03 mmol / g.

The content of amino groups in the samples taken for analysis by the proposed method was determined for silica gel by reverse acid-base titration, for other mineral fillers - by the method of thermogravimetric analysis.

The table summarizes the information from examples No. 1-13 for the proposed method and the prototype method.

Table The dependence of the degree of acetylation on the interaction conditions No. of Example Way Mineral filler The concentration of anhydride, (mol / l) The concentration of perchloric acid, (mol / l) Acetylation time, min. The ratio of the weight of the sample to the volume of acetylating solution Amine groups introduced (mmol / g) Found amino groups, (mmol / g) one declared silica gel 0.5 0.150 5 1: 5 1.40 ± 0.03 1.42 ± 0.03 2 declared silica gel 0.5 0,075 5 1: 5 1.40 ± 0.03 1.40 ± 0.03 3 declared silica gel 0.5 0,025 5 1: 5 1.40 ± 0.03 1.42 ± 0.03 four declared silica gel 0.5 0.010 5 1: 5 1.40 ± 0.03 1.36 ± 0.03 5 declared silica gel 0.5 0,000 5 1: 5 1.40 ± 0.03 1,02 ± 0,03 6 declared silica gel 0.6 0,075 5 1: 4 1.40 ± 0.03 1.41 ± 0.03 7 declared silica gel 0.4 0,050 5 1: 5 1.40 ± 0.03 0.85 ± 0.03 8 prototype silica gel 2 0.15 5 1: 1 1.40 ± 0.03 It is not possible to completely wet the sample. 9 prototype silica gel 0.5 0,037 5 1: 5 1.40 ± 0.03 1.17 ± 0.03 10 prototype silica gel 0.5 0,075 thirty 1: 5 1.40 ± 0.03 1.24 ± 0.03 eleven declared micromica 0.5 0,025 5 1: 5 0.28 ± 0.03 0.29 ± 0.03 12 declared Basalt Micro Plate Filler 0.5 0,025 5 1: 5 0.18 ± 0.03 0.17 ± 0.03 13 declared Chopped basalt fiber 0.5 0,025 5 1: 5 0.14 ± 0.03 0.13 ± 0.03

As can be seen from the above examples, the use of a 0.5-0.6 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine allows us to apply the ratio of the weight of the sample to the volume of the acetylating solution from 1: 4 to 1: 5, depending on the specific surface and density of the analyzed material . When using a solution with a concentration of less than 0.5 M, the acetylation reaction proceeds quantitatively (example 7), and using a solution with a concentration of more than 0.6 M leads to a too small ratio of the weight of the sample to the volume of the acetylating solution while maintaining the necessary equimolar ratios, and therefore the inability to properly moisten the test sample (example 8).

The ratio of dichloroethane - pyridine in an acetylating solution can vary from 0.5: 1 to 2: 1. Studies have shown that using a ratio of less than 0.5: 1 makes it necessary to use a more concentrated solution of acetic anhydride in dichloroethane, which causes an exothermic effect that is too strong when mixed with pyridine in the preparation of the acetylating solution and reduces its quality. Using a ratio of more than 2: 1 (example 6) leads to a hanging concentration of dichloroethane in the analyzed mixture, which complicates mixing with an aqueous solution of potassium hydroxide during titration. The concentration of perchloric acid used as a catalyst, less than 0.025 mol / l does not allow the quantification of the reaction (example 4, 5).

So, in comparison with the prototype, we see that the range of materials studied has expanded. The preparation of an acetylating solution containing pyridine is simpler than that of the prototype, and therefore the method itself becomes simpler. In addition, the proposed method allows to reduce the concentration of perchloric acid in an acetylating solution. This helps to reduce the occurrence of side reactions, such as the interaction of perchloric acid with the amino groups or basic oxides that make up the fillers, which increases the accuracy of determination and reduces the consumption of the reagent. For titration, an aqueous solution of potassium hydroxide is used, which is more stable during storage and cheaper than the alcohol solution used in the titration of an acetylating mixture in pure dichloroethane.

Claims (1)

A method for determining the concentration of grafted amino groups on the surface of mineral fillers, including the preparation of an acetylating solution by mixing the starting components, adding it to a sample of the modified mineral filler, keeping it for quantitative reaction, titration with an alkali solution in the presence of an indicator, calculating the concentration of amino groups from the difference in the results of blank titration and titration of a sample, characterized in that 0.5- 0.6 M solution of acetic anhydride in a mixed solvent of dichloroethane - pyridine in a ratio of 0.5: 1-2: 1, containing 0.025-0.15 mol / L perchloric acid as a catalyst, add 0 to a sample sample of a modified mineral filler, 5-0.6 M acetylating solution in a mixed solvent containing perchloric acid, the ratio of the weight of the sample to the volume of the acetylating solution is 1: 4-1: 5, after which they stand and add a hydrolysis mixture consisting of dimethylformamide, pyridine, water taken in a ratio of 6: 3: 1, respectively the resulting mixture is centrifuged to separate the precipitate.