A. A. Vanin

Self-assembly in aqueous solutions of imidazolium ionic liquids and their mixtures with an anionic surfactant

Experimental data on micellization in aqueous solutions of 1-alkyl-3-methylimidazolium salts [C(n)mim]X and their mixtures with sodium dodecyl sulfate (NaDS) are reviewed. New results (the critical micelle concentration and enthalpy of micellization) are presented for mixtures of [C(4)mim]PF(6), [C(6)mim]BF(4), [C(6)mim]Br and [C(10)mim]Br with NaDS. Our data cover a wide range (from 0 to 0.9) of solvent-free based mole fractions of ionic liquid (IL). Even very small addition of ILs substantially decreases the cmc of NaDS due to the combined effect of electrostatic and hydrophobic interactions, and formation of mixed micelles. It is shown that the quasichemical aggregation model by Nagarajan and Ruckenstein may be successfully applied to aqueous solutions of long-chain ILs and their mixtures with NaDS. The local structure of micelles is obtained from all-atom MD simulations for [C(n)mim]Br and [C(n)mim]X+NaDS in aqueous medium.

The effect of water on the shape of aggregates in water-in-oil microemulsions according to data of computer simulation

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.

Determination of the role of edge effects in Lennard-Jones fluid adsorption in finite slits by computer simulation

Lennard-Jones fluid adsorption in bounded slits has been calculated by the Monte Carlo method. Isotherms of methane adsorption in circular finite carbon slits with different widths have been calculated in a wide pressure range. For finite slits, the contribution of edge effects to the adsorption behavior of methane has been estimated as compared with infinite slits. Qualitative agreement with the conclusions of the asymptotic theory has been reached for the behavior of local density in finite slits.

The influence of electronic polarizability of components on the electric field of an ionic micelle according to molecular simulation data

Molecular dynamics simulation of a cationic micelle having a rigid hydrophobic core and mobile cationic head groups has been performed taking into account the electronic contribution into solution polarization. As compared with an analogous micelle previously considered with no regard to the polarization effects, the latter manifest themselves as a weaker structuring of micelle crown and greater concentrating of counterions in it. Analysis of the local electric potential has indicated that the discrepancy between the data of the continual and atomistic descriptions of water remains preserved. Thus, agreement between the theory and numerical experiment cannot be achieved when describing the local electric potential with no allowance for the local character of polarization.

Computer Simulation of Luminophore Solubilization in Reverse Micelles

The solubilization of ionic (sodium naphthalene-2,6-disulfonate) and nonionic (diethyl 2,5-dihydroxyterephthalate) organic luminophores in water–isooctane–NaАОТ (sodium 1,4-bis[(2-ethylhexyl) oxy]-1,4-dioxybutane-2-sulfonate) reverse micelles is simulated by the molecular dynamics method. In a stationary state, the localization of luminophore molecules in a micelle appears to be the same irrespective of their initial positions in the system. The position and orientation of solubilized luminophores relative to a reverse micelle depend on the hydrophobicity and the capability for dissociation of the functional groups of their molecules, the size of the reverse micelle, and the structure of its electrical double layer.

The Dipole Moment of Reverse Micelles according to Computer Simulation Data

Sodium 1,4-bis[(2-ethylhexyl)oxy]-1,4-dioxybutane-2-sulfonate (Aerosol OT) reverse micelles in isooctane have been simulated, and the mean-square dipole moment has been calculated. The formed isolated micelles have been classified according to aggregate radius and surface area per one surfactant molecule. It has been shown that, for micelles with a constant surface density of surfactant anion charges, the meansquare dipole moment rises with the aggregate size faster than the squared radius does. Dipole moment values obtained within the atomistic model for a reverse micelle are much higher than the values presented in the literature for the primitive model.

The Effect of Hydroxyl Groups on Solubilization of Pyridine Derivatives in Span 80–Water–n-Decane Reverse Micelles

Computer simulation of pyridine, pyridine-2-ol, and pyridine-2,5-diol solubilization by Span 80–water reverse micelles in n-decane has been performed. All solubilized compounds are polar (their polarity increases in a series pyridine, pyridine-2,5-diol, and pyridine-2-ol) and have different numbers of donors/acceptors forming hydrogen bonds. The most probable positions of pyridine molecules relative to a reverse micelle change fundamentally with a rise in the number of hydroxyl groups in their structure. Pyridine, pyridine-2-ol, and pyridine-2,5-diol are located in the nonpolar medium, on the micelle surface between the head groups of surfactant molecules, and on the inside surface of the aqueous core, respectively. Thus, the number and arrangement of hydrophilic groups in the structure of a molecule, rather than its polarity, have the strongest effect on the ability to solubilization in the reverse micelles.

Molecular-dynamics simulation of the surface layer of a nonionic micelle

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.