ABSTRACT
Interaction forces between mesoscopic objects are fundamental to soft-condensed matter and are among the prime targets of investigation in colloidal systems. Surfactant molecules are often used to tailor these interactions. The forces are experimentally accessible and for a first theoretical analysis one can make use of a parallel-plate geometry. We present molecularly realistic self-consistent field calculations for an aqueous nonionic surfactant solution near the critical micellization concentration, in contact with two hydrophobic surfaces. The surfactants adsorb cooperatively, and form a monolayer onto each surface. At weak overlap the force increases with increasing compression of the monolayers until suddenly a symmetry braking takes place. One of the monolayers is removed jump-like and as the remaining monolayer can relax, some attraction is observed, which gives way to repulsion at further confinement. The restoring of symmetry at strong confinement occurs as a second-order transition and the force jumps once again from repulsion to attraction. It is anticipated that the metastable branch of the interaction curve will be probed in a typical force experiment. Under normal conditions pronounced hysteresis in the surface force is predicted, without the need to change the adsorbed amount jump-like.
ABSTRACT
Dilute emulsions of dodecane in water were prepared under constant flow rate conditions with binary surfactant systems. The droplet size distribution was measured as a function of the mixed surfactant composition in solution. The systems studied were (a) the mixture of anionic sodium dodecyl sulfate (SDS) with nonionic hexa(ethyleneglycol) mono n-dodecylether (C12E6) and (b) the mixture of cationic dodecyl pyridinium chloride (DPC) with C12E6. At a constant concentration of SDS or DPC surfactant in solution (below the CMC) the mean emulsion droplet size decreases with the increase in the amount of C12E6 added to the solution. However, a sharp break of this droplet size occurs at a critical concentration and beyond this point the mean droplet size did not significantly change upon further increase of the C12E6. This point was found to corresponded to the CMC of the mixed surfactant systems (as previously determined from microcalorimetry measurements) and this result suggested the mixed adsorption layer on the emulsion droplet was similar to the surfactant composition on the mixed micelles. The emulsion droplet size as a function of composition at the interface was also studied. The mean emulsion droplet size in SDS-C12E6 solution was found to be lower than that in DPC-C12E6 system at the equivalent mole fraction of ionic surfactant at interface. This was explained by the stronger interactions between sulphate and polyoxyethylene head groups at the interface, which facilitate the droplet break-up. Counterion binding parameter (beta) was also determined from zeta-potential of dodecane droplets under the same conditions and it was found that (beta) was independent of the type of the head group and the mole fraction of ionic surfactant at interface.
