ELLIPSOMETRY STUDIES OF THE SELF-ASSEMBLY OF NONIONIC SURFACTANTS AT THE SILICA WATER INTERFACE - EQUILIBRIUM ASPECTS

The nature of layers of a series of poly(ethylene glycol) monoalkyl et hers (C(n)E(m)) adsorbed on silica surfaces has been systematically in vestigated by means of null ellipsometry. The results show that adsorp tion remains low until a well-defined concentration ((almost-equal-to 0.6-0.9)cmc) is exceeded. Then, as surfactants in the interfacial regi on start to self-assemble, it increases abruptly and plateau adsorptio n is generally observed prior to the cmc. The normal extension of the interfacial aggregates is relatively constant from intermediate to hig h surface coverage. Increasing the ethylene oxide to hydrocarbon ratio results in a decreased adsorption. The mean optical thickness, on the other hand, is relatively independent of the number of ethylene oxide groups in the surfactant but almost linearly dependent on the length of the hydrocarbon tail. The values obtained for these parameters sugg est that the adsorbed layer is built up of discrete surface aggregates , or micelles, with dimensions resembling those observed in bulk solut ion. A more refined optical model of the adsorbed layer confirms the n otion of surface micelles growing with increasing hydrocarbon content. It also points out that the extension of the surface micelles is slig htly larger than the measured mean optical thickness. In addition to s tudies of neat C(n)E(m) surfactants, we also examine the adsorption of mixed surfactant systems. Changes observed in adsorption on altering the bulk ratio of two surfactants are well correlated to the bulk mice llar surfactant ratio calculated by ideal solution theory.