CONTINUUM ELECTROSTATICS INVESTIGATION OF IONIC MICELLES USING ATOMISTIC MODELS

The key parameter related to the structure of the electric double layer of ionic surfactant micelles – electrostatic potential – is considered. A brief overview of experimental methods and theoretical models for estimating electrostatic potential- is given. The calculating method for the electrostatic potential based on a numerical solution of the Poisson-Boltzmann equation using an atomistic model of anionic surfactant micelle - is proposed. The parameters necessary for the construction of atomistic models - are obtained from molecular dynamic modeling. The electrostatic potentials for the micelles of sodium dodecyl sulfate and cetyltrimethylammonium bromide at different ionic strengths - were calculated by this method. The results are discussed in comparison with the values calculated in the simplified model, the Ohshima – Healy – White equation.

Aggregation of surfactants in aqueous systems

Surfactants form micelles in aqueous solution above the critical micelle concentration (cmc); micelles are dynamic structures. Micellization is driven by the transfer of surfactant chains from water to the micelle core. There is an optimum size (aggregation number) and shape for micelles (dependent on surfactant molecular shape and packing within micelles) for which the standard free energy of micellization, Δ‎micμ‎o, is minimum. Inert electrolyte influences the degree of dissociation (α‎) of ionic micelles and hence micellar shape and aggregation number. Micellization in mixed surfactant solutions, and in mixed polymer + surfactant systems is also discussed. Micelles can dissolve (solubilize) other amphiphilic materials and nonpolar oils, which changes the cmc. If sufficient oil is solubilized, microemulsion droplets result (see Chapter 10). As the concentration of micelles in solution rises, intermicellar interactions lead to the formation of a variety of lyotropic liquid crystalline phases (mesophases).

Ionic micelles and aromatic additives: a closer look at the molecular packing parameter

Tuning mixture of ionic surfactants with aromatic additives results in micelles with impressive viscoelastic properties.