Dependence of phase behavior of some non-ionic surfactants at the air-water interface on micellization in the bulk

The temperature-dependent surface phase behavior of two sparingly soluble surfactants, namely, ethylene glycol n-dodecyl ether (EGDE) and ethylene glycol n-tetradecyl ether (EGTE), at the air-water interface was investigated by film balance and Brewster angle microscopy (BAM). A cusp point followed by a pronounce plateau region in the surface pressure-time (pi-t) adsorption isotherms of the amphiphiles measured by film balance indicates the first-order phase transition. Bright two-dimensional condensed phase domains in a dark background are observed by BAM just after the phase transition. In both cases the critical surface pressure necessary for the phase transition increases with increasing temperature. The domains are found to be circular up to 5 and 27degreesC for EGDE and EGTE, respectively, above which they show a fingering pattern. Condensed domains are observed up to 23 and 37degreesC for EGDE and EGTE, respectively. The surface properties of the amphiphiles are found to be markedly affected by their tendency to aggregate in the bulk as micelles. The CMC values of both the amphiphiles sa maximum at a definite temperature, T-max, that corresponds ell to their respective maximum temperatures of domain formation. An increase in temperature beyond T-max results in an increasing trend for the formation of micelles. Consequently the system suffers from a shortage of two-dimensional surface concentration of the molecules to attain the surface pressure necessary for phase transition. With increasing temperature, the enthalpy, DeltaH(m)degrees, and entropy, DeltaS(m)degrees, of micellization change from negative to positive in both cases. An enthalpy-entropy compensation effect is found to hold for both the amphiphiles over the entire temperature range. The thermodynamic quantities reveal that the increase in temperature is favorable for micellization when the temperature exceeds the corresponding T-max of the amphiphiles. (C) 2002 Elsevier Science (USA).