Adsorption of mixed surfactants at the oil-water interface

The adsorption of the mixed nonionic/anionic surfactants of hexaethylene gl ycol monododecyl ether (C12E6) and sodium dodecyl sulfate (SDS) at the hexa decane/water interface has been measured. Small-angle neutron scattering (S ANS) in combination with hydrogen/deuterium isotopic substitution, has been used to obtain the structure and composition of the adsorbed mixed surfact ant layer at the hexadecane/water interface of a hexadecane in water emulsi on, and of the mixed micelles in equilibrium with the emulsion. Measurement s of the adsorption of SDS alone have established the experimental approach , and the analysis procedures used. The adsorption isotherm obtained for th e SDS from the SANS data is in good agreement with that obtained from surfa ctant electrode measurements. The SANS data show clearly the point at which , with increasing surfactant concentration, free micelles are formed in equ ilibrium with emulsion dl droplets. The variation of the composition of the adsorbed layer for the C12E6/SDS mixture at the hexadecane/water interface is not consistent with that previously obtained at the air-water interface , or in mixed micelles. Furthermore. it is not consistent with the straight forward application of theories, such as regular solution theory. The evolu tion of composition with concentration can be explained if it is assumed th at there is partitioning of the C12E6 into the oil phase, and that some sol ubilization of hexadecane into the mixed micelles takes place. Analysis of the scattering data for the micelles in equilibrium with the emulsion confi rms the latter of these two assumptions. On the basis of the data obtained, we have also been able to estimate directly the monomer concentrations of the surfactants in both the oil and water phases.