Colloid Journal

Aqueous biphasic systems containing non-ionic ethoxylated surfactants are perspective as media for some biochemical reactions. A comprehensive study of phase and aggregation behavior of these micellar systems, as well as consideration of the effects of biocomponents, their nature and concentration, pH and temperature are evident. In this work, the influence of salt medium (phosphate buffer), pH and temperature, as well as the presence of different bioorganic additives on cloud point and structural-thermodynamic properties (CMC and aggregation number) were studied for micellar solutions of non-ionic surfactants Triton X-114 and Tergitol NP-7 in water and buffers with pH 6.8, 4.3, 2.7. As additives were used the components of a biocatalytic hydrolysis reaction of penicillin G, namely penicillin G potassium salt, phenylacetic acid, and 6-aminopenicillanic acid. The different effects of the biocomponents on the cloud point and CMC value of Triton X-114 in the buffer with pH 4.3 are discussed. In addition, it was shown that the CMC value and the aggregation number of Triton X-114 decrease in buffer solutions (salting-out effect) and the CMC value increase at growing the temperature to 37°C (what is typical for ethoxylated non-ionic surfactants). The addition of phenylacetic acid (10 mM) leads to a reduction in the aggregation number of Triton X-114 at pH 4.3; other bio-additives have practically no effect. Thus, the studied additives have different effects on the structure and phase behavior of investigated micellar solutions, and this is an important aspect for choosing an extraction system to carry out a bioreaction.

Using the modified Poisson–Boltzmann (PB) theory, which includes restrictions on the maximum attainable concentration of ionic species in a solution C_max determined by their effective sizes, the distributions of electrostatic potential φ(r) and ion concentration near a spherical nanoparticle with radius a immersed in a 1 : 1 electrolyte solution have been studied under the conditions of constant surface charge density σ_s. For weakly charged particles, the φ(r) profiles are almost independent of C_max and coincide with the profile obtained in terms of the classical PB model. Surface potential |φ_s| gradually increases with a rise in |σ_s|. Far from the particle, when the potential becomes lower than its thermal value \({{\varphi }_{{\text{T}}}} = \frac{{kT}}{e}\) (k is the Boltzmann constant, T is the temperature, and e is the elementary charge), the potential decreases exponentially irrespective of the counterion sizes: \(\psi \left( r \right) = \frac{{\varphi \left( r \right)}}{{{{\varphi }_{{\text{T}}}}}} = {{\psi }_{{{\text{eff}}}}}\frac{a}{r}\exp \left( { - \kappa \left( {r - a} \right)} \right),\) where r is the distance from the particle center and κ is the reciprocal Debye radius. According to the classical PB theory, the growth of the surface charge leads to the saturation of \(\left| {{{\psi }_{{{\text{eff}}}}}} \right|~\left( { \to 4} \right),\) with the value of \(\left| {{{\psi }_{{\text{s}}}}} \right|\) obtained by solving the nonlinear PB equation being higher than \(\left| {{{\psi }_{{{\text{eff}}}}}} \right|.\) In the modified PB theory, which takes into account the size of electrolyte ions in the simplest form, this effect of saturation is absent. Now, \(\left| {{{\psi }_{{{\text{eff}}}}}} \right|\) depends on both the value of the surface charge and the sizes of counterions. Moreover, at a large size of counterions, \(\left| {{{\psi }_{{{\text{eff}}}}}} \right|\) substantially exceeds the corresponding value obtained by solving the nonlinear modified PB equation. The difference between the electrical double layer properties obtained by solving the classical and modified PB equations directly follows from the fact that the modified theory predicts the appearance of a condensed layer at a particle surface, with the concentration of counterions in this layer being equal to C_max. Therewith, the thickness of the layer grows with increasing |σ_s| (at a constant size of the ions) and the size of the ions (at constant σ_s).

The features of (γ-mercaptopropyl)trimethoxysilane (MPTMS) hydrolytic condensation in alkaline aqueous solutions have been studied. The interrelation between the process time, MPTMS concentration, and structure-related morphological characteristics of the obtained organosilica (silsesquioxane) particles has been determined. It has been shown that toroidal particles are mainly formed at low precursor concentrations (≤2.5 mM) in a solution. The formation kinetics of such nanotoroids has been analyzed, and preliminary data have been obtained on the mechanism of this process. These data have been discussed taking into account the information available from the literature on the structure of silsesquioxanes and the role of disulfide bonds in the formation of anisotropic particles of these compounds.

The viscoelastic characteristics (frequency dependences of the components of the two-dimensional complex dynamic modulus) of interfacial layers formed by polymer solutions on liquid surfaces are measured using a newly-designed instrument. The operation of the instrument is based on the use of a Langmuir trough and a new fast-response system for precise measurement of forces in a surface plane at Boussinesq numbers smaller than ten. Examples are presented to illustrate the abilities of the instrument: the measurement of dilatational dynamic elasticity moduli, the determination of the kinetic parameters of polymer solution spreading at interfaces, and the measurement of the moduli for layers of polymer solutions with different concentrations on solvent surfaces in compression–expansion cycles.

The kinetics of spreading of aqueous hydroxypropyl cellulose (HPC) solutions with different concentrations over a water surface and the rheological properties of films formed at the water–air interface have been studied. The performed experiments are distinguished by the fact that the solvent in the polymer solutions is identical to the liquid subphase, while, beginning from a certain concentration, the aqueous HPC solutions are capable of forming a liquid-crystalline phase. The chosen initial polymer solution concentrations correspond to different regions of the phase diagram for the HPC–water system. The contact of a solution droplet with water yields a time-stable spot of an HPC solution layer. In spite of the fact that some solution concentrations correspond to the existence range of a lyotropic liquid-crystalline phase, an isotropic layer is formed on the water surface irrespective of the concentration and phase state of the solution in the droplet. The dynamic dilatational storage modulus of such an interfacial layer at a frequency of 0.03 Hz is equal to 43 ± 2 mN/m; i.e., it almost coincides with the surface tension of the formed interfacial layer.

The aggregation of zinc tetra(4-carboxyphenoxy) phthalocyaninate, ZnPc(COOH)₄, in organic solvents and water at different pH values has been analyzed using electronic absorption and luminescence spectroscopies. It has been found that, in aqueous systems at pH 6–9, ZnPc(COOH)₄ exists in an aggregated state, while it forms molecular solutions in dimethylformamide and dimethyl sulfoxide. The use of cationic surfactants (dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromides) causes the disaggregation of ZnPc(COOH)₄ into individual molecules in aqueous solutions at pH ≥ 6. This opens up possibilities of applying ZnPc(COOH)₄ as a preparation for photodynamic therapy in physiological solutions, because each molecule of this compound may participate in the generation of singlet molecular oxygen.

The aim of this study was to investigate the influence the magnetic field on the adsorption of polyacrylic acid (PAA) on the silica (SiO₂) surface as well as on the zeta potential of the PAA/SiO₂ system. The magnetic field influences the amount of PAA adsorbed on silica. The highest adsorption of PAA was observed at pH 9 and the lowest at pH 3. This tendency is opposite to that for the adsorption of PAA on SiO₂ in the absence of the magnetic field. This discrepancy occurs because magnetic field generates dipoles in the polymer macromolecules due to the presence of delocalized electrons in the ionized carboxylic groups. Consequently, the PAA molecules undergo polarization, which implies their adsorption on the negatively charged surface of silica. It was also observed that the magnetic field influences the zeta potential of the PAA/SiO₂ system by changing the conformation of the adsorbed macromolecules accompanied by changes in the position of the slipping plane.

The Frumkin equation—a fundamental law of the thermodynamics of thin films—relates the surface tension of an interfacial region, which includes a thin film, to its disjoining pressure and thickness. The study of the original work has shown that the first derivation of this relation published by A.N. Frumkin in 1938 is thermodynamically inconsistent. In this article, the way is shown in which the Frumkin equation should be correctly derived, interpreted, and used. The following approaches are discussed: the finite-thickness layer method; the Gibbs methods with one, two, and three dividing surfaces; and the gravitational-field method.

The thermodynamics of adsorption of some substituted 1,3,4-oxadiazoles and 1,2,4,5-tetrazines from water–acetonitrile solutions with acetonitrile volume fraction varying within a range of 0.35–0.90 on chemically modified Luna C₁₈ and Discovery C₁₈ silica gels has been studied within a temperature range of 313.15–333.15 K. The thermodynamic characteristics of the adsorption of these compounds on Luna C₁₈ and Discovery C₁₈ silica gels have been determined as depending on solution composition. In the case of Discovery C₁₈, the character of the dependences of different thermodynamic characteristics of the adsorption on liquid phase composition is similar to that of the corresponding dependences previously observed for modified Ascentis®Phenyl silica gel. A similarity has also been found for the dependences of the thermodynamic characteristics of the adsorption on Luna C₁₈ and modified Diasfer-110-C₁₈ silica gel studied previously. The mechanism of the liquid-phase adsorption of 1,3,4-oxadiazoles and 1,2,4,5-tetrazines on Luna C₁₈ is changed within a range of acetonitrile volume fractions of 0.60–0.75, as it is in the case of Diasfer-110-C₁₈. At acetonitrile volume fractions of higher than 0.75, the studied heterocycles are adsorbed on Luna C₁₈ surface dynamically modified with acetonitrile without a direct interaction with the grafted layer of the chemically modified silica gel. At the same time, the mechanism of their adsorption on Discovery C₁₈ remains unchanged while acetonitrile concentration is increased in the liquid phase.

The catalytic activity of a palladium hydrosol has been studied in the reaction of hexacyanoferrate(III) ion reduction with hydrogen. The reaction rate has been found to increase with the degree of palladium nanoparticle saturation with hydrogen. An equation has been derived to describe the dependence of the reaction rate constant on the time of preliminary saturation of the hydrosol with hydrogen. The observed effect seems to be due to the discharge–ionization of hydrogen dissolved in the nanoparticles and an increase in their electron density, which enhance the catalytic activity of the particles.

The Du Nouy ring detachment and pendant drop methods have been employed to study the tensiometric (dynamic and equilibrium surface tensions) and surface rheological (viscoelasticity modulus and phase angle) characteristics of aqueous solutions of fractions of humic and hymatomelanic acids at a solution–air interface. It has been found that the fraction of low-molecular-weight hymatomelanic acids has high surface activity, while its surface layers are characterized by high viscoelasticity and storage elastisity moduli. The experimental dependences of the equilibrium surface tension and viscoelasticity modulus on the concentration of hymatomelanic acid salt solutions are adequately described in terms of the model of real two-dimensional solutions for polymolecular adsorption of polyelectrolytes.

Relative linear adsorption-induced deformation of granules of AR-V microporous carbon adsorbent has been measured upon n-octane adsorption at pressures p of 1 Pa to 1.6 kPa and temperatures T = 273, 293, 313, 353, and 393 K. At p > 200 Pa, the adsorption-induced deformation is positive and grows with p throughout the considered temperature range. Under isobaric conditions, the adsorption-induced deformation and the slope of deformation isotherms increase as T decreases. At p < 200 Pa and T < 350 K, the adsorption-induced deformation is negative. The range of adsorbent contraction narrows with an increase in T, and, at T > 350 K, the adsorbent expands throughout the pressure range. Molecular dynamics simulation has been employed to study the structure of n-octane adsorption associates in model slit-like micropores having a width corresponding to the effective size of micropores in AR-V adsorbent calculated in terms of the theory of volume filling of micropores. The structure of the adsorption associates has been related to the characteristic features of the adsorption-induced deformation of AR-V adsorbent.