Radomir I. Slavchov

Hofmeister effect on micellization, thin films and emulsion stability

The Hofmeister effect on the critical micelle concentration (CMC), the thin liquid film electrostatic disjoining pressure (Π(el)) and the critical coalescence pressure of emulsion drops (P(CR)) were investigated. For CMC literature data were used, but Π and P(CR) were measured by us. The essence of the theoretical approach was to modify existing theories of CMC and Π(el) by using generalized Gouy equation and dimensionless surface potential (Φ(S)), involving the counterion specific adsorption energy (u(0)). The computational procedure of u(0) does not involve any adjustable parameters. Linear dependences of ln(CMC), Φ(S) and P(CR) on u(0) were found in conformity with Hofmeister series. The experimental slopes of ln(CMC) and Φ(S) vs. -u(0)/k(B)T were negative and very close to the theoretical ones. A hypothesis was put forward for explanation of the positive slopes of P(CR) on u(0). The obtained results suggest that the counterion specific adsorption energy u(0) encompasses all major factors, involved in the Hofmeister effect for the studied phenomena. If this is confirmed by analysis of more phenomena, revealing Hofmeister effect, one could claim that u(0) is the factor controlling the Hofmeister effect and a powerful tool for its study.

Surface tension of concentrated electrolyte solutions

The surface tension σ of inorganic electrolyte aqueous solutions at a given concentration c follows the Hofmeister series. The explanation of this phenomenon was sought in the increased adsorption of certain ions due to specific ion-surface interactions. However, the ion-specific dependence of the activity coefficient γ(±) on c also influences σ, and its contribution to the ion-specificity of σ prevails. Thus, the surface tension of potassium salts follows the order σ(KOH)>σ(KCl)>σ(KNO3), which turns out to be a direct corollary of the corresponding activity coefficients series: γ(KOH)>γ(KCl)>γ(KNO3). In fact, the adsorption of NO(3)(-) at the water surface is lower than that of OH(-) and Cl(-)! If the bulk ion-specific effects are correctly evaluated, Schmutzers classical model predicts accurately the surface tension of a large number of inorganic salt solutions in a wide concentration range, without adjustable parameters. This model accounts for image and hydration forces. Comparison with tensiometric data shows that other ion-surface interactions play a role only in the adsorption of ions of bare radius larger than a threshold value of about 1.95 Å (e.g. HCOO(-), I(-), SCN(-)).

Quadrupole terms in the Maxwell equations: Born energy, partial molar volume, and entropy of ions.

A new equation of state relating the macroscopic quadrupole moment density Q to the gradient of the field ∇E in an isotropic fluid is derived: Q = αQ(∇E - U∇·E/3), where the quadrupolarizability αQ is proportional to the squared molecular quadrupole moment. Using this equation of state, a generalized expression for the Born energy of an ion dissolved in quadrupolar solvent is obtained. It turns out that the potential and the energy of a point charge in a quadrupolar medium are finite. From the obtained Born energy, the partial molar volume and the partial molar entropy of a dissolved ion follow. Both are compared to experimental data for a large number of simple ions in aqueous solutions. From the comparison the value of the quadrupolar length LQ is determined, LQ = (αQ/3ɛ)(1/2) = 1-4 Å. Data for ion transfer from aqueous to polar oil solution are analyzed, which allowed for the determination of the quadrupolarizability of nitrobenzene.

Streaming potential effect on the drainage of thin liquid films stabilized by ionic surfactants.

Dynamic effects originating from the electric double layers (EDL) are studied in thin liquid films (TLF) containing ionic and nonionic surfactants. To account for such effects, the EDL are to be incorporated into the differential equations describing the TLF drainage. Numerical simulations in the literature have shown that foam films containing ionic surfactants can drain at a slower rate than that predicted by the Reynolds equation (V(Re)) which postulates rigid planar film surfaces. However, the physical reason of the trend has remained unclarified, and the numerical results have not been validated by any experimental data. In the present study, experiments on the drainage of planar foam films were conducted with the anionic surfactant sodium dodecylsulfate (SDS) in the presence of additional electrolyte (0.02 M NaCl) and with the cationic tetrapentylammonium bromide (TPAB). The obtained results are in accord with the numerical simulations from the literature (V/V(Re) < 1). Such behavior was observed already in our preceding experiments on planar TLF with SDS without added electrolyte. These results were compared to the data of the experiments with TLF containing nonionic surfactant, and differences in the drainage pattern between ionics and nonionics were established. A new theoretical model was developed to account for the dynamic effects arising from EDL. According to the present model, the liquid outflow drags the bulk charges of EDL toward the film border, thus generating streaming potential (as in capillary tubes), which in turn brings the charges back toward the center to maintain the state of zero total electrical current. This creates reverse convection of the liquid near the surfaces, resulting in a velocity of film drainage smaller than V(Re). The present theory predicts kinetic dependence closer to the experiment than the Reynolds equation. The limitations of this new model are specified: it is valid for high ionic strength or low value of the surface potential.

Markov chain model for the critical micelle concentration of surfactant mixtures

An extension of the Markov chain model (MC) for micellization is proposed, which allows the distribution of the surfactants between the monomer solution and the micelles in a mixed surfactant system to be predicted. The dependence of the critical micelle concentration (cmc) on the composition of the solution is investigated. The equilibrium thermodynamic relation between cmc and micelle composition is discussed. The case of ternary mixtures is analyzed, and theoretical triangular diagram is constructed according to MC. Available experimental data for binary and ternary mixtures agree well with the new MC theory. The dependence of MC parameters on the structure of the surfactants is discussed. Comparison of MC with the simple mixture (“regular solution”) model is presented. The parameters of the MC theory are related to the interaction parameter βSM of the simple mixture model.

Quantum hydrodynamics of electron gases.

Electron gases in metals are described as quantum charged Newtonian viscous fluids experiencing Ohmic Darcy friction on the solid lattice ions as well. The dispersion relation of the electron acoustic waves is derived, which shows the existence of new quantum diffusion processes. The electric double layer near a metal surface is studied, which exhibits a new quantum oscillatory-decaying behavior different from the Friedel oscillations.

Effect of the surface polarizability on electrostatic screening in semiconductors

Following the Gibbs approach a general electrostatic model of heterogeneous systems with non-homogeneous interfaces is proposed. The intrinsic surface polarization is taken into account through the introduction of a surface dielectric constant and the electrostatic boundary conditions are generalized as two-dimensional Poisson equations. This model is applied to analysis of the electrostatic potential of charged defects on a semiconductor surface. As a result, a good theoretical fit of the experimental data is obtained. The fitting value of the surface dielectric constant is in good agreement with its theoretical estimation in the framework of the Gibbs approach.

Fully atomistic molecular-mechanical model of liquid alkane oils: Computational validation

Fully atomistic molecular dynamics simulations were performed on liquid n‐pentane, n‐hexane, and n‐heptane to derive an atomistic model for middle‐chain‐length alkanes. All simulations were based on existing molecular‐mechanical parameters for alkanes. The computational protocol was optimized, for example, in terms of thermo‐ and barostat, to reproduce properly the properties of the liquids. The model was validated by comparison of thermal, structural, and dynamic properties of the normal alkane liquids to experimental data. Two different combinations of temperature and pressure coupling algorithms were tested. A simple differential approach was applied to evaluate fluctuation‐related properties with sufficient accuracy. Analysis of the data reveals a satisfactory representation of the hydrophobic systems behavior. Thermodynamic parameters are close to the experimental values and exhibit correct temperature dependence. The observed intramolecular geometry corresponds to extended conformations domination, whereas the intermolecular structure demonstrates all characteristics of liquid systems. Cavity size distribution function was calculated from coordinates analysis and was applied to study the solubility of gases in hexane and heptane oils. This study provides a platform for further in‐depth research on hydrophobic solutions and multicomponent systems.

Cohesive and Non-cohesive Adsorption of Surfactants at Liquid Interfaces

The ability of soluble nonionic surfactants to form liquid expanded (LE) phase is investigated. It is found that the adsorption behavior of nonionic surfactants at liquid interfaces falls into one of two well-defined types: cohesive (with liquid expanded phase) and non-cohesive behavior (without liquid expanded phase). The effect of surfactant structure and properties of the medium on the adsorption parameters of both adsorption types are analyzed. A model of the adsorption constant is proposed, which compares well with experimentally observed values.

Adsorption of Ions at Uncharged Insoluble Monolayers

A method is proposed for the experimental determination of the adsorption of inorganic electrolytes at a surface covered with insoluble surfactant monolayer. This task is complicated by the fact that the change of the salt concentration alters both chemical potentials of the electrolyte and the surfactant. Our method resolves the question by combining data for the surface pressure versus area of the monolayer at several salt concentrations with data for the equilibrium spreading pressure of crystals of the surfactant (used to fix a standard state). We applied the method to alcohols spread at the surface of concentrated halide solutions. The measured salt adsorption is positive and has nonmonotonic dependence on the area per surfactant molecule. For the liquid expanded film, depending on the concentration, there is one couple of ions adsorbed per each 3-30 surfactant molecules. We analyzed which ion, the positive or the negative, stands closer to the surface, by measuring the effect of NaCl on the Volta potential of the monolayer. The potentiometric data suggest that Na(+) is specifically adsorbed, while Cl(-) remains in the diffuse layer, i.e., the surface is positively charged. The observed reverse Hofmeister series of the adsorptions of NaF, NaCl, and NaBr suggests the same conclusion holds for all these salts. The force that causes the adsorption of Na(+) seems to be the interaction of the ion with the dipole moment of the monolayer.