In this article we describe a novel molecular dynamics simulation of a single mechanically stable water-containing reverse micelle in an apolar solvent. It takes explicit account of the forces between component molecules and achieves a degree of realism hitherto unobtained but without attempting to reproduce the detailed properties of any specific surfactant system. Steric stabilization was achieved by using a model cationic surfactant with single Lennard-Jones (LJ) interaction centers to represent the hydrophobic tails, each being attached to its cation head group by a harmonic bond. Specific chain structures are not necessary. Neutrality is maintained by mobile anions. The simple point charge (SPC) model is used for the water molecules, and single-site LJ interaction centers are used to represent the apolar solvent molecules. The numbers of surfactant, water, and solvent molecules are 36, 72, and 1079, respectively, and these are contained in a spherical cavity of a solvent continuum. Periodic boundary conditions are not used. The radial density profiles of the equilibrated assembly show that the surfactant forms a "coat" but that there is significant roughening of the interface. Its mean shape and fluctuations are analyzed by using a spherical harmonic expansion. The anions are strongly adsorbed at the cation head group surface, and there is significant penetration of water into the hydrophobic region. The outermost water molecules are not H-bonded but are strongly coordinated to anions in the ionic layer. We believe that this latter effect may have some relevance to the role of cosurfactants in stabilizing micelle formation.
The size and aggregation number of reversed micelles formed by the system aerosol-OT+H20+ organic solvent have been determined by viscosity and dynamic light scattering methods. For the viscosity method, a procedure for deriving values of the aggregation number from particles of variable density is described. Measurements were made in cyclohexane, toluene and chlorobenzene. The dynamic light scattering method, based on photon correlation spectroscopy, yields single exponential correlation functions from which values of the translational diffusion coefficient and the micelle radius can be derived. The droplet size was found to depend primarily on the ratio of surfactant to water concentrations, but was essentially independent of solvent and concentration at a fixed surfactant to water concentration ratio. Satisfactory agreement was obtained among the two methods discussed in this paper and one (sedimentation ultracentrifugation) described previously.Aerosol-OT (sodium bis-2-ethylhexylsulphosuccinate) or AOT ( fig. 1) is an anionic surfactant capable of solubilising very large amounts of water in organic solvents. For example, in n-heptane, a 0.1 mol dm-3 solution of AOT can solubilize up to 10 % water. We have determined the size and aggregation number of the reversed micelles (or water-in-oil microemulsion droplets) formed by the three component system ( fig. l), by the application of viscosity and dynamic light scattering methods. Such systems are of considerable topical interest, which, from our point of view, include the study of the properties of the heterogeneous water present in the aqueous core of the droplets and the mechanism of reversed micellar catalysis and of novel synthesis at interfaces.The nature of the water in reversed micelles has been recently investigated by n.m.r. spectroscopy.1° These studies indicate that when only small amounts of water are present, the solubilised water is highly immobilised. Bulk water properties are not observed until the water content of the system exceeds 1 %.As a prelude to the detailed study of the kinetics of reactions in reversed micellar media, it is necessary to determine the size and aggregation number of the droplets as a function of the concentrations of added AOT and water. Viscosity measurements have the advantage of being quick and easy to perform and the system is only mildly perturbed during measurement. Dynamic light scattering is a promising technique for measuring size parameters, and it has already been demonstrated ' 9 that by analysing scattered light intensity fluctuations using photon correlation spectroscopy (PCS) it is possible to study translational diffusive motions of macromolecules and micelles suspended in aqueous s~l u t i o n .~ It is of interest, therefore, t Present Address :
Computer simulation of the (100) face of crystalline potassium chloride has been achieved using the method of molecular dynamics and a lamina model of thickness 10 ionic layers. The basic cell contained 360 ions, interacting via a Tosi-Fumi potential. A solution has been obtained for the long range correction to the Coulomb potential for an ionic lamina. This differs significantly from the solution for a 3-dimensional lattice.Surface distortion, surface energy and stress have been evaluated and agree well with experiment and previous model calculations. The ratios of surface mean square amplitudes of vibration perpendicular to the surface divided by the bulk values show an approximately 25 % anharmonic enhancement over previous calculations. The layerwise velocity autocorrelation functions yield power spectra featuring a peak for the surface mono-layer at 75 cm-' which compares favourably with the frequency of surface acoustic modes predicted by previous calculations.
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