RU2805767C1 - Method of obtaining standard samples for measurement of zeta potential values of particles of colloidal systems using optical methods - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the methods of preparing standard samples for measuring the zeta potential of particles in colloidal systems and can be used to improve the efficiency of laboratory control services in chemical, petrochemical, clinical and pharmaceutical laboratories, as well as water purification and water treatment services. A method of obtaining standard samples for measuring the zeta potential of particles of colloidal systems by optical methods includes the preparation of low-concentrated aqueous solutions of sulfur-containing amino acids, such as L-cysteine, to obtain standards with a value in the positive region of the electrokinetic (zeta) potential, and N-acetyl-L-cysteine for obtaining standards with a value in the negative region of the electrokinetic (zeta) potential; mixing the resulting solutions of sulfur-containing amino acids with a solution of silver acetate or nitrate, respectively, in a molar ratio of 1.25 or equimolar, respectively, so that the concentration of sulfur-containing amino acids in the mixture is in the range of 0.75–3.00 mM, depending on the required value of the zeta potential of the particles; keeping the mixture in a place protected from light at a temperature of 18–28° C for 48–72 hours to form standard samples with the required values of the zeta potential of particles. Additionally, low-concentration aqueous solutions of sulfur-containing amino acids are mixed with a solution of oxalic acid based on deionized water so that the concentration of oxalic acid is in the range of 0.08–0.19 mM. Immediately before measurements, a solution of sodium chloride in an amount equimolar with oxalic acid is added to the standard sample for additional stabilization.

EFFECT: development of a method of producing standard samples characterized by a number of values above 70 mV in absolute value with a relative standard deviation of zeta potential values of not more than 10%.

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Description

The invention relates to methods for preparing standard samples, with stable and reproducible values, for measuring the zeta potential values of particles of colloidal systems by optical methods. The proposed supramolecular systems are intended to be used as standard samples of particle zeta potential values.

There is a known method for obtaining reference standards for measuring zeta potential based on goethite nanoparticles https://www-s.nist.gov/srmors/certificates/1980.pdf.

The disadvantage of this method is the multi-stage synthesis, which results in a suspension of a precursor that requires additional preparation for further measurements, which increases the error in the absolute value of the measured electrophoretic mobility. Also, the known comparison standard does not provide the ability to calculate the zeta potential using the instrumental method, and to implement it it is necessary to carry out a manual calculation of the measured parameter based on the obtained experimental data.

There is a known method for obtaining comparison standards by regulating the values of the electrophoretic mobility of aggregates and, as a consequence, their zeta potential due to the introduction of electrolytes US 5108568, publ. 04/28/1992 The known method involves the separation of biological systems by electrophoresis - a method of reducing the electrophoretic mobility of macromolecules, particles, cells and other substances so that the macromolecules, particles or cells interact with an affinity compound consisting of a hydrophilic neutral polymer, such as polyethylene glycol, modified with a macromolecule that provides specific binding.

The disadvantage of the known method for the synthesis of reference standards is the increase in the viscosity of the systems, which leads to a decrease in electrophoretic mobility and, accordingly, zeta potential.

Known methods for obtaining reference standards based on monodisperse polystyrene latexes SU 1816764 A1, publ. 05/23/1993 and Kim, S., Kim, C. A., Choi, Y. H., & Jung, M. Y. Synthesis of Polystyrene Nanoparticles with Different Surface Modification by Emulsion Polymerization and Measurement of IgG Adsorption and Stability for the Application in Latex-Protein Complex Based Solid-Phase Immunoassay. Polymer Bulletin, 2008, 62(1). These known methods are used as the basis for the synthesis of metrological standards by Malvern, UK. Well-known standards from Malvern, UK have a zeta potential value of -42 mV with a standard deviation of the measurement results of 10%.

The disadvantage of the known methods for producing reference standards for measuring zeta potential, based on the use of polystyrene latexes, is due to the technology of synthesis of spherical aggregates of polystyrene latex, namely the need for a copolymerization operation with a monomer, the result of which is the creation of a potential-forming layer on the surface of these aggregates, which limits the range of possible values comparison standards. Based on NIST (USA) polystyrene latex spheres, standard samples SRM 1992 and 1993 were developed, registered as ERM-FD305 and ERM-FD306, the zeta potential values of the particles are - 58 mV and - 56 mV, respectively.

The disadvantage of the proposed standards is that in fact there is only one value of the particle zeta potential, which does not exceed 70 mV modulo value.

It is known to use colloidal silica particles to create standard samples of the zeta potential of particles, as described in the review “Hyde, EDER, Seyfaee, A., Neville, F., & Moreno-Atanasio, R. (2016). Colloidal Silica Particle Synthesis and Future Industrial Manufacturing Pathways: A Review. Industrial & Engineering Chemistry Research, 55(33), 8891-8913. " Thus, silicon dioxide with a diameter of 20 nm is used to obtain the reference standard ERM-FD100, EU for measuring zeta potential.

The disadvantage of the known ERM-FD100 comparison standards is the high value of the standard deviation (more than 50%) at a zeta potential value of 43 mV.

There is a known method for obtaining reference standards based on the synthesis of colloidal particles of silicon dioxide, described in the article by A.D. Levin, M.K. Alenichev, V.M. Masalov, N.S. Sukhinina, G.A. Emelchenko. Development of standard samples of electrokinetic (zeta) potential of nanoparticles // Russian nanotechnologies. 2018. Volume 13. No. 1-2. 93-99c. In the known method for obtaining standard samples of zeta potential, the synthesis of a colloidal solution of monodisperse nanoparticles of silicon dioxide is used using heterogeneous hydrolysis of tetraethoxysilane and the catalyst L-arginine. Known reference standards have additional stabilization, which makes it possible to reduce the measurement error, however, due to the features of the known synthesis scheme, the zeta potential value lies in the range from -30 mV to -50 mV, which sharply limits the scope of application of the known reference standards and is a disadvantage of the method.

The closest method for preparing standard samples with a value in the positive region of the electrokinetic (zeta) potential for measuring the zeta potential values of particles of colloidal systems by optical methods is described in the article by A. N. ADAMYAN. and others. Gel formation in low-concentrated aqueous solutions containing L-cysteine and silver acetate. Vestnik TvGU, Series "Chemistry", 2016, N2, pp. 89-98, including the preparation of a low-concentrated aqueous solution of L-cysteine (sulfur-containing amino acid); mixing the resulting solution with a solution of silver acetate in a molar ratio of Ag+/L-Cys=1.25 to obtain solutions with a stable electrokinetic (zeta) potential in the positive region, in particular +64 mV, so that the concentration of the sulfur-containing amino acid in the mixture is 3 mM ; keeping the mixture in a place protected from light at room temperature for 6-12 hours to form supramolecular solutions with the required electrokinetic (zeta) potential values.

The closest way to prepare standard samples with a value in the negative region of the electrokinetic (zeta) potential for measuring the zeta potential values of particles of colloidal systems by optical methods is the method of obtaining supramolecular systems with a stable value of the zeta potential of particles RU 2746992 “Averkin D.V., Mezheumov I N., Belenkiy D.I., Khizhnyak S.D., Pakhomov P.M. // Method for obtaining reference standards for measuring electrokinetic (zeta) potential from 04/23/2021.” In the known method, to obtain the claimed standard samples of the zeta potential of particles, supramolecular systems are used, prepared on the basis of low-concentrated aqueous solutions of sulfur-containing amino acids. Samples are prepared by mixing a solution of the amino acid N-acetyl-L-cysteine with a solution of silver acetate in an equimolar ratio at a concentration of sulfur-containing amino acids in the mixture ranging from 0.75 to 2.5 mM, depending on the value of the electrokinetic (zeta) potential; Next, the mixture is kept in a place protected from light at a temperature of 18-28°C for 24-48 hours to form supramolecular solutions with the required values of electrokinetic (zeta) potential.

The disadvantage of these systems is that they reproduce particle zeta potential values in the range of only up to 70 mV modulo value.

The purpose of the claimed invention is to develop a method for obtaining standard samples for measuring the zeta potential of particles of colloidal systems by optical methods based on sulfur-containing amino acids, for example, L-cysteine or N-acetyl-L-cysteine, and various silver salts (silver acetate or nitrate), differing from the prototype with a number of values above 70 mV in absolute value: from 70 to 75 mV and a relative standard deviation of the zeta potential values of no more than 10% (Table 1).

This goal is achieved by adding a solution of oxalic acid to the supramolecular systems at the time of mixing and additional stabilization with sodium chloride at the time of mixing or immediately before measurement in such a way that in all cases the standard deviations of the zeta potential values of the particles are no more than 10%.

The technical result of the present invention is: to develop a method for obtaining standard samples with a number of values above 70 mV modulo with a relative standard deviation of zeta potential values of no more than 10%, used when calibrating equipment using electrophoretic light scattering methods ELS and PALS to determine the zeta potential of particles colloidal systems.

The technical result is achieved by creating supramolecular systems based on sulfur-containing amino acids and salts of silver acetate or nitrate with a regular spatial structure of stabilized aggregates.

Supramolecular systems are prepared by mixing low-concentrated aqueous solutions of sulfur-containing amino acids with a solution of silver acetate or nitrate in an equimolar or molar ratio of 1.25 (to obtain precursors with a value in the negative region of the electrokinetic (zeta) potential) and at a molar ratio of 1.25 (to obtain precursors with a value in the positive region of the electrokinetic (zeta) potential and the negative region of the electrokinetic (zeta) potential) so that the concentration of sulfur-containing amino acids in the mixture will range from 0.75 to 3 mM. First, a solution of oxalic acid is added to the solution of the sulfur-containing amino acid in such an amount that the final concentration of oxalic acid in the system will be 0.08-0.19 mM. Next, the mixture is kept in a place protected from light at a temperature of 18-28°C for 48-72 hours to form supramolecular solutions. For additional stabilization, it is necessary to add sodium chloride in an equimolar ratio to the added oxalic acid solution immediately before measurement.

A study of the state of the art has established the absence of methods for obtaining standard samples for measuring the zeta potential of particles based on sulfur-containing amino acids, for example, L-cysteine and N-acetyl-L-cysteine and silver acetate by adding to them a calculated amount of oxalic acid solution and additional stabilization with sodium chloride in an amount equimolar to oxalic acid.

The invention is illustrated by graphic materials Table 1, Fig. 1-4.

Table 1. Values of zeta potential, pH, relative electrical conductivity and electrophoretic mobility of aggregates in the claimed supramolecular reference solutions.

Fig.1. Results of testing samples in the negative range of particle zeta potential values based on stability over time.

Rice. 2. Results of testing samples in the positive range of particle zeta potential values based on stability over time.

Rice. 3. Values of the sizes of aggregates in systems with a positive value of the zeta potential of particles at rest and during the measurement process: 1 - sample with a value of the zeta potential of particles plus 71 mV, a - at rest, b - during the measurement process; 2 - sample with a particle zeta potential value plus 75 mV, a - at rest, b - during measurement.

Rice. 4. Values of aggregate sizes in systems with a positive zeta potential of particles at rest and during measurements: 1 - sample with a zeta potential of particles minus 73 mV based on N-acetyl-L-cysteine and silver acetate in a ratio of 1 to 1.25, _Cam-ty = 1.5 mM, with the addition of oxalic acid and sodium chloride, a - at rest, b - during measurement; 2 - sample with a zeta potential of particles minus 73 mV based on N-acetyl-L-cysteine and silver acetate in a ratio of 1 to 1, _Cam-ty = 3 mM, with the addition of oxalic acid and sodium chloride, and - in the state rest, b - in the process of measurement; 3 - sample with a particle zeta potential value of minus 75 mV based on N-acetyl-L-cysteine and silver acetate in a ratio of 1 to 1.25, _Cam-ty = 3 mM, with the addition of oxalic acid and sodium chloride.

The essence of the invention is as follows.

It has been experimentally established that standard samples prepared from supramolecular solutions based on L-cysteine and N-acetyl-L-cysteine and silver acetate with the addition of calculated amounts of oxalic acid solution and stabilization with sodium chloride satisfy qualitative metrological requirements - discrepancy (standard deviation) of values electrophoretic mobility and zeta potential of charged particles in the claimed standards is no more than 10% of the absolute value for the results of one series of measurements and no more than 15% of the absolute value for the results of different series of measurements (Table 1). The dynamic viscosity of supramolecular solutions of the obtained standard samples, measured on an SV-10 vibration viscometer, does not exceed 3 mPa⋅s.

Dynamic scattering, pH measurements, and viscometry methods were used to determine the self-organization conditions to obtain the reported particle zeta potential values. These methods were used because they make it possible to assess the degree of organization of systems without destroying the structure of supramolecular systems.

The introduction of oxalic acid makes it possible to increase the number of bonds between nanoparticles and the charged aggregates formed by them in the supramolecular system through the formation of several anionic bonds by each acid molecule. According to measurements of pH and electrical conductivity of the systems, pH values for test samples in the positive range of particle zeta potential values range from 2.5 to 4, and conductivity values range from 0.18 to 1.5 μS/cm. The pH values for the test samples in the negative range of particle zeta potential values are in the range from 3 to 3.5, and the electrical conductivity values are in the range from 0.3 to 0.4 μS/cm. The results obtained confirm the compliance of the prototypes with the requirements for standard samples of zeta potential of particles.

It has been established that the introduction of oxalic acid during the process of self-organization leads to an increase in the electrical conductivity of the systems and a decrease in pH below 4. There is an increase in the degree of protonation of the amino acid and an improvement in the dissociation of the silver salts used in the preparation, which makes it possible to strengthen the intermolecular interactions of the initial solutions of the amino acid and the silver salt. The presence of additional binding anions increases the density of DES particles at the time of self-organization. As a result, the particles remain connected to each other at rest (Fig. 3.a and Fig. 4.a), but it is easier for them to “scatter” when an electric field is applied, which improves their detection at the time of measurements.

It has been established that when oxalic acid is introduced with a concentration of 20 mM in an amount exceeding 0.03 ml per 1 ml of a supramolecular system, the pH decreases below 2.5 in systems with a positive zeta potential, which initiates the precipitation of silver mercaptide in all studied systems . The introduction of oxalic acid in an amount of less than 0.005 ml per 1 ml of a supramolecular system is not enough to stabilize the DES of charged particles, which leads to their partial aggregation during the measurement process, a decrease in the zeta potential of particles less than 70 mV in absolute value and the formation of bi- and trimodal distribution of particles according to zeta potential values.

The introduction of an electrolyte in the form of sodium chloride neutralizes the excess charge on the surface of the potential-determining layer of charged particles in the supramolecular systems under study, further stabilizing them. The introduction of sodium chloride should be equimolar to the amount of introduced oxalic acid for the reason that at the time of measurement, sodium cations neutralize the excess surface charge of particles caused by the adsorption of oxolate anions into the moving layer of DES. Chloride anions, in turn, bind silver cations released during the destruction of submicron particles. Thus, when silver chloride is introduced in an amount less than equimolar, it leads to the retention of some large particles in supramolecular systems and the formation of a polymodal distribution of particles according to the zeta potential value. Excess sodium chloride reduces the zeta potential values of the particles due to the formation of hydrogels.

The inventive method for obtaining reference standards is characterized by a single stage, as well as the absence of the need for additional equipment and reagents.

The proposed standard samples were examined for long-term stability of particle zeta potential values. The data in Table 1 are averaged values for 10 systems synthesized at different times. To check the reproducibility of the results, multiple measurements of the zeta potential values of particles in samples obtained by the claimed method were used. It has been experimentally established that the comparison standards obtained by the claimed method are stable over time for up to 6 months. In Fig. Figures 1-2 show curves of changes in the zeta potential values of particles of standard samples depending on the storage time of the prepared reference standard. It has been established that all standard samples based on sulfur-containing amino acids and silver salts are quite stable and retain the average zeta potential unchanged during storage for up to 6 months. Each point on the graph is the average of a series of 10 measurements.

Standard samples were studied by dynamic light scattering. In Fig. Figure 3 presents the results of measurements of the hydrodynamic diameter of particles in standard samples in the positive range. Measurements were carried out in a U-shaped capillary cell before the zeta potential measurements were taken and immediately after the series of measurements was completed. It has been established that the introduction of oxalic acid leads to an increase in the average diameter of aggregates in systems up to 600-700 nm, however, at the time of measuring the zeta potential, the average particle diameter decreases and amounts to 30-50 nm - 38 nm in a standard sample with a particle zeta potential value of plus 71 mV and 43 nm in a standard sample with a particle zeta potential value plus 75 mV. The true size of stable charged particles in the system is less than 200 nm; the additional introduction of electrolytes in the form of sodium chloride leads to an increase in the ionic atmosphere and additional stabilization of EDL aggregates during measurements due to the neutralization of excess charge on the carboxyl groups of oxalic acid.

Particles of standard samples in the negative range before the start of measurements of the zeta potential of particles are characterized by a wide polydisperse particle size distribution after stabilization with oxalic acid (Fig. 4). A feature of the structure of systems based on N-acetyl-L-cysteine is the absence of a pronounced crystalline phase in silver nanoparticles. Nanoparticles in such systems are a composite particle of complex composition. It was found that at the time of measurements, the particle size decreases to 30-50 nm, then the particles quickly stick together to form stabilized aggregates. The role of the introduced electrolytes, as in samples with a positive zeta potential, is to neutralize the excess charge on the carboxyl groups of oxalic acid. The introduction of sodium chloride electrolyte at the time of mixing the system leads to blocking of the protonated carboxyl groups of the amino acid, which impairs the stability of the systems and does not allow the formation of stable aggregates. It is advisable to introduce an electrolyte in the form of sodium chloride immediately before starting measurements, which simplifies the destruction of large particles and further stabilizes charged particles with a size of 30-50 nm.

The introduction of electrolytes into supramolecular systems stabilized by oxolate anions does not lead to a significant change in pH in the systems. This confirms that the introduction of such small amounts of sodium chloride leads to stabilization of the potential-determining layer of aggregates in the original solution, without causing changes in the structure of the potential-determining aggregates.

The method is carried out as follows.

Low-concentrated aqueous solutions of sulfur-containing amino acids, such as L-cysteine, are prepared to obtain standards with a value in the positive region of the electrokinetic (zeta) potential, and N-acetyl-L-cysteine, to obtain standards with a value in the negative region of the electrokinetic (zeta) potential. The resulting solutions of sulfur-containing amino acids are mixed with a solution of oxalic acid based on deionized water so that the concentration of oxalic acid is in the range of 0.08-0.19 mM with a solution of silver acetate or nitrate or only silver acetate, respectively, in a molar ratio of 1.25 or equimolar , respectively; so that the concentration of sulfur-containing amino acids in the mixture is in the range of 0.75-3.00 mM, depending on the desired zeta potential of the particles. The mixture is kept in a place protected from light at a temperature of 18-28°C for 48-72 hours to form standard samples with the required values of the zeta potential of the particles. In addition, immediately before measurements, a solution of sodium chloride in an amount equimolar with oxalic acid is added to the standard sample for additional stabilization. The mixing of N-acetyl-L-cysteine with silver acetate in the claimed method can also be done in a molar ratio of 1.25.

To produce the claimed comparison standards, no special equipment is required; they can be produced using standard equipment in a chemical laboratory. Standard samples obtained by the claimed method have zeta potential values above 70 mV modulo: minus 73 mV, minus 75 mV, plus 71 mV, plus 75 mV, standard deviation of values is not more than 10% and have a viscosity of less than 3 mPa⋅s at temperatures in the range of 18-28°C.

Examples of implementation of the proposed method.

Example 1. Preparation of a supramolecular solution of a standard sample with a zeta potential value of plus 75 mV.

Prepare a solution of oxalic acid with a concentration of 0.4 mM based on deionized water. To 0.6 ml of a 10 mM aqueous solution of L-cysteine, add 0.65 ml of a solution of oxalic acid with a concentration of 0.4 mM, the mixture is stirred and 0.75 ml of a 10 mM aqueous solution of silver nitrate is added. The mixture is a pale yellow opalescent solution, which, when matured for 48 hours in the dark at 23°C, becomes transparent, acquiring a faint yellow tint. After this, 0.02 ml of sodium chloride solution at a concentration of 13 mM is added immediately before measurements.

Example 2. Preparation of a supramolecular solution of a standard sample with a zeta potential value equal to plus 71 mV.

Prepare a solution of oxalic acid with a concentration of 0.1 mM based on deionized water. To 0.6 ml of a 10 mM aqueous solution of L-cysteine, add 5.0 ml of an oxalic acid solution with a concentration of 0.1 mM, the mixture is stirred and 0.75 ml of a 10 mM aqueous solution of silver acetate is added. The mixture is a pale yellow opalescent solution, which, when matured for 48 hours in the dark at 23°C, becomes transparent, acquiring a faint yellow tint. After this, 0.01 ml of sodium chloride solution at a concentration of 50 mM is added immediately before measurements.

Example 3. Preparation of a supramolecular solution of a standard sample with a zeta potential value equal to minus 73 mV.

Prepare a solution of oxalic acid with a concentration of 0.4 mM based on deionized water. Add 0.8 ml or 1.25 ml of the prepared oxalic acid solution to 0.6 ml of a solution of N-acetyl-L-cysteine with a concentration of 10 mM or 5 mM and mix. Next, 0.6 ml or 0.75 ml of silver acetate solution with a concentration of 10 mM, respectively, is added to the mixture. We obtain a clear solution, which, when matured for 48 hours in the dark at a temperature of 23°C. After this, 0.02 or 0.01 ml of sodium chloride solution at a concentration of 16 or 50 mM is added immediately before measurements.

Example 4. Preparation of a supramolecular solution of a standard sample with a zeta potential value equal to minus 75 mV.

Prepare a solution of oxalic acid with a concentration of 0.4 mM based on deionized water. To 0.6 ml of a solution of N-acetyl-L-cysteine with a concentration of 10 mM, add 1.25 ml of the prepared oxalic acid solution and mix. Next, 0.75 ml of silver acetate solution with a concentration of 10 mM is added to the mixture, respectively. We obtain a clear solution, which, when matured for 48 hours in the dark at a temperature of 23°C. After this, 0.02 ml of sodium chloride solution at a concentration of 26 mM is added immediately before measurements.

The declared standard samples can be used to improve the efficiency of laboratory control services in chemical, petrochemical, clinical and pharmaceutical laboratories, as well as water purification and water treatment services.

Table 1 Standard sample Zeta potential value mV Standard deviation based on the results of a series of 10 measurements, mV pH Specific electrical conductivity value, μS/cm Electrophoretic mobility, m ² /V⋅s L-cysteine and silver nitrate 1 to 1.25, [3 mM] + 0.02 ml of oxalic acid and 0.02 sodium chloride, + 75 ± 6 2.7 1.475 5.7⋅10 ^-8 L-cysteine and silver acetate 1 to 1.25, [0.75 mM] + 0.01 ml oxalic acid and 0.01 ml sodium chloride + 71 ± 5 3.7 0.144 5.0⋅10 ^-8 N-acetyl-L-cysteine and silver acetate 1 to 1.25, [1.5] mM, 0.02 ml oxalic acid and - 73 ± 6 3.3 0.333 -5.2⋅10 ^-8 N-acetyl-L-cysteine and silver acetate 1 to 1, [3] mM, 0.02 ml oxalic acid and - 73 ± 6 3.1 0.310 -5.1⋅10 ^-8 N-acetyl-L-cysteine and silver acetate 1 to 1.25, [3] mM, 0.02 ml oxalic acid and - 75 ± 7 3.2 0.328 -5.7⋅10 ^-8

Claims (2)

1. A method for obtaining standard samples for measuring the zeta potential of particles of colloidal systems by optical methods, including the preparation of low-concentrated aqueous solutions of sulfur-containing amino acids, such as L-cysteine to obtain standards with a value in the positive region of the electrokinetic (zeta) potential and N-acetyl-L-cysteine to obtain standards with a value in the negative region of the electrokinetic (zeta) potential; mixing the resulting solutions of sulfur-containing amino acids with a solution of silver acetate or nitrate, respectively, in a molar ratio of 1.25 or equimolar, respectively; so that the concentration of sulfur-containing amino acids in the mixture is in the range of 0.75-3.00 mM depending on the desired zeta potential of the particles; keeping the mixture in a place protected from light at a temperature of 18-28°C for 48-72 hours to form standard samples with the required values of the zeta potential of particles, characterized in that additionally low-concentrated aqueous solutions of sulfur-containing amino acids are mixed with a solution of oxalic acid based on deionized water so that the concentration of oxalic acid is in the range of 0.08-0.19 mM; in addition, immediately before measurements, a solution of sodium chloride in an amount equimolar with oxalic acid is added to the standard sample for additional stabilization. 2. The method according to claim 1, characterized in that N-acetyl-L-cysteine is mixed with silver acetate in a molar ratio of 1.25.