Vol 79, No 3 (2017)

An analytical expression has been derived for the quasi-stationary size distribution of surfactant aggregates in a micellar system approaching the final equilibrium state. In contrast to previously known relations, the derived expression takes into account variations in the concentration of monomers during the slow relaxation and enables one to determine the previously unknown fine structure of the linearized mode of slow relaxation, i.e., its dependence on the aggregation numbers in the range between the maximum and minimum of the work of aggregation. This dependence has been reliably confirmed by the numerical solution of the set of linearized Becker–Döering difference equations, which describe the molecular mechanism of the kinetics of micellization and micellar relaxation. In turn, the expression found for the relaxation mode makes it possible to refine the analogous “fine structure” of aggregation rates at different points of the same range between the maximum and minimum of the work of aggregation, in which the aggregation rates appear to be low but exhibit a nonmonotonic behavior. This behavior is also confirmed by the numerical solution of the Becker–Döering difference kinetic equations.

A spherical micelle structure has been studied for cationic (n-dodecyltrimethylammonium chloride) and nonionic (hexaethylene glycol mono-n-hexyl ether) surfactants in pure water and a sodium chloride solution. The molecular-dynamics has been used to simulate the self-assembly of aggregates from an initially homogeneous mixture of water and surfactant molecules and to gain insight into the structure of micelles and their surface layers. The radial distribution functions obtained for charged components have been employed to calculate the local electric potentials of the micelles and the contributions from the charges of water atoms, ions, and a surfactant to it. It has been shown that, similarly to previously studied ionic micelles, in nonionic surfactant micelles, the contributions from water molecules and polar groups (and ions in the case of the salt solution) to the electric potential are mutually compensated in the region of the electrical double layer. Therefore, the resultant electric potential of the surface layer rapidly tends to zero.

A sodium 1,4-bis[(2-ethylhexyl)oxy]-1,4-dioxybutane-2-sulfonate (NaАОТ)–water–isooctane three-component system is calculated by the molecular-dynamics method. In a wide range of relative water contents w₀, reverse micelles are obtained with different morphologies: single spherical and cylindrical micelles and their spatial networks. It is shown that w₀ and surfactant concentration are the main shape-generating factors. The data obtained are in good agreement with previous results of simulations and experimental data.

It has been shown that, in spite of the success achieved in describing atomic-scale friction, serious problems concerning friction force microscopy remain unsolved. The traditional theoretical approach, which, on default, ignores possible effects of nonlocality and memory, leads to a number of contradictions. Proceeding from the general concepts of nonequilibrium statistical mechanics, rigorous analysis, and simple atomistic simulation, it has been shown that the phonon mechanism of energy dissipation upon friction is essentially nonlocal with the prevalence of memory effects.

Three consecutive stages of evaporation of a sessile water microdroplet are studied both theoretically and experimentally under the conditions of contact angle hysteresis. The influence of thermal effects on the dynamics of droplet evaporation is quantitatively investigated on the basis of the obtained experimental results. The features of droplet evolution are analyzed at the final stage, when both the contact angle and the radius of the droplet base decrease with time. It is shown that evaporation at this stage also occurs in a steady-state regime, but the average droplet temperature approaches the ambient temperature.

Diffusion coefficients of monoalkyl ethers of poly(ethylene glycols), С₈Е₄ and С₁₂Е₅, and octyl-β-D-glucopyranoside have been measured in aqueous micellar solutions within wide ranges of temperatures and concentrations by dynamic light scattering. The study of the С₁₂Е₅–water system containing cylindrical micelles has been supplemented with viscosity measurements, which make it possible to better monitor structural transformations and phase transformations.

Data available on the viscosity of micellar solutions of surfactants belonging to the homologous series of alkyltrimethylammonium bromides containing 10, 12, 14, and 16 carbon atoms in their alkyl chains, as well as sodium dodecyl sulfate solutions, have been analyzed. It has been shown that the systems under consideration may be adequately described by the Einstein equation. The results obtained have made it possible to identify the concentration ranges in which the intermicellar interaction does not affect significantly the flow of the micellar solutions. Data on the relative viscosity of the studied micellar solutions have been employed to calculate the hydrodynamic radii of micelles formed by the surfactants with different lengths of hydrocarbon radicals in their molecules.

The Monte Carlo method has been used to calculate the potential of mean force for Na⁺ and Cl⁻ ions interacting in model planar nanopores with structureless walls under the conditions of the material contact with water vapor at room temperature and above water boiling point. The interactions have been described using a detailed many-body model calibrated with respect to experimental data on the free energy of attachment reactions and the results of quantum-chemical calculations. Dissociation becomes possible when the vapor density increases as a sufficient number of molecules are pulled into the field of the ions. The dissociation proceeds sooner under the conditions of the nanopore than in bulk water vapor. Hydration decreases the energy of the dissociated state; however, the entropy component of the free energy partly compensates for the decrease in the internal energy, thereby increasing the stability of a contact ion pair. After the dissociation of a contact ion pair (CIP), ions are retained within a cluster in the state of a solvent-separated ion pair (SSIP). Fluctuations in the number of pulled-in vapor molecules, which are correlated with fluctuations in the interionic distance, stabilize the SSIP states with respect to recombination, while a decrease in the screening of the field of ions under the conditions of the nanopore stabilize the SSIP states with respect to cluster decay. The conditions of the nanopore stimulate the passage of an ion pair from the CIP to the SSIP state due to the rearrangement of the statistical weights in favor of molecules being located in the interionic gap. Thus, under the conditions of the nanopore, the stability of the SSIP states increases with respect to both the recombination of the ions and the decay of the ion-molecular associate.

Spreading of a droplet of a concentrated oil-in-water emulsion over a water surface has been monitored. At the initial moment, this process proceeds at a high rate like an explosion. The stationary spreading is soon followed by the development of instability with the formation of a flower-shaped structure. At the final stage, the spot of the spread liquid may somewhat shrink. The characteristic times of these three stages are about 0.1, 1, and 10 s. The explosive character of the initial stage of spreading may be explained by the loss of dimensionality (3D → 2D) and the passage of the dispersion medium into the aqueous substrate with the release of a high free energy, while the subsequent spreading of the oil spot is explained by the Marangoni effect. A decrease in the energy of the interaction between dispersed phase (“oil”) particles in the emulsion leads to the phase separation. The final shrinkage of the oil spot may be due to the surface wave disturbance, which arises upon the explosive phase separation in the emulsion.