Adsorption of the lamellar phase of aerosol-OT at the solid/liquid and air/liquid interfaces

The adsorption of concentrated solutions of di-2-ethylhexylsodium sulfosucc inate (Aerosol-OT or AOT) at hydrophilic solid/aqueous and air/aqueous solu tion interfaces was followed by neutron specular reflection. Reflectivity p rofiles were recorded of protonated AOT in D2O at concentrations of 2 and 5 wt % and over a range of temperatures. Sharp diffraction peaks characteris tic of a lamellar phase were observed, and the repeat spacing of the peaks was very sensitive to temperature. The bulk phase diagram in this concentra tion range over the temperature range 0-65 degrees C was also studied using neutron small angle scattering. For the bulk solution containing 5 wt % AO T, the lamellar phase at 5 degrees C had a repeat distance of 370 +/- 30 An gstrom, which dropped to about half that value at 35 degrees C, and to only 115 Angstrom at 65 degrees C. The lamellar spacing decreased as the AOT co ncentration was lowered; for example, from 325 Angstrom at 5 wt % to 225 An gstrom at 1 wt %, at 15 degrees C. Over some of the concentration and tempe rature range narrow diffraction peaks were observed for the lamellar phase adsorbed at both interfaces, indicating long-range order in the direction n ormal to the surface. The repeat spacing in the adsorbed lamellar phase was generally 10-20% smaller than in the bulk solution. Although addition of s mall amounts of electrolyte (0.1 and 1 mM NaBr) had only a small effect on the small angle scattering from the bulk lamellar phase, it caused the disa ppearance of the adsorbed lamellar phase. These observations are consistent with an overall attraction of the lamellar phase to the surface in the abs ence of added electrolyte.