Two-dimensional phase transition of styrene adsolubilized in cetyltrimethylammonium bromide admicelles on mica

The phase behavior of styrene adsolubilized in cetyltrimethylammonium bromide (C16TAB) aggregates adsorbed on mica was studied in situ using repulsive force atomic force microscopy (AFM) at bulk surfactant concentrations below the critical micelle concentration and without added electrolytes. Two critical phase transitions were observed for the styrene/adsorbed C16TAB system by studying the deflection images and force curves. The first critical phase transition occurred when the phase of the two-component admicelle shifted from a lamellar structure, with no discernable surface structure, to patches of swollen admicelle. Force curve analysis shows that the thickness of the C16TAB admicelle increases as does force required to penetrate the patches. The strong short-range repulsive force (rupture force) is thought to be attributable to the mechanical deformation of the liquid-like droplet within the admicelle by the AFM tip, described by fitting the data to the Hertz and JKR models. The second critical phase transition occurs when the bilayer with swollen patches transforms to a bilayer containing solute droplets with a thickness of 20 nm or more, a transition explainable by existing solubilization theories. Styrene molecules, being slightly polar, solubilize primarily at the admicelle-water interface. At low concentrations the adsolubilization of styrene results in a reduction of the head–head repulsion which causes the lamella to change to short hemi-cylinders and hemi-spherical structures. At high concentrations, styrene molecules begin to preferentially adsolubilize into the hydrocarbon core of the bilayer causing the lamella-droplet transformation.