Dependence of Phase Behavior of Some Non-ionic Surfactants at the Air-Water Interface on Micellization in the Bulk

摘要

The temperature-dependent 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 pronounced plateau region in the surface pressure-time (pi-t) adsorption isotherms of the amphiphiles measured by film balance indicates the first-order phase transition. Bvright 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 transitio increases with increasing temperature. The domains are found to be circular u to 5 and 27 deg C for EGDE and EGTE, respectively, above which they show a fingering pattern. Condensed domains are observed up to 23 and 37 deg C for EGDE and EGTE, respectively. The surface properties of he amphiphiles are found to be markedly affected by their tendency to aggregate in the bulk as micelles. The CMC values of both the amphiphilles show a maximum at a definite temperature, T_max, that corresponds wel 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~deg, and entropy, DELTAS_m~deg, 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-ature is favorable for micellization when the temperature exceeds the corresponding T_max of the amphiphiles.