Influence of temperature and headgroup size on condensed-phase patterns in langmuir monolayers of some oxyethylenated nonionic surfactants

The surface phase behavior in Langmuir monolayers of some oxyethylenated nonionic surfactants of the general formula C16En, with n = 1, 2, 3, and 4, at the air-water interface has been studied by film balance and Brewster angle microscopy (BAM) over a wide range of temperatures. The C16E4 monolayers cannot show any indicative features of phase transition because of strong dipolar as well as hydration-induced repulsive interactions between the bulky headgroups. On the other hand, the monolayers of C16E1, C16E2, and C16E3 show a sharp cusp point followed by a pronounced plateau region in their respective isotherms with subsequent formation of a variety of structures in the two-phase coexistence region between the liquid expanded (LE) and liquid condensed (LC) phases at different temperatures. As usually observed, the domains of C16E1, which bears only one ethylene oxide (EO) unit in the headgroup, are circular at lower temperatures while fractal at higher temperatures. On the other hand, those for C16E2 and C16E3 are initially found to be irregular structures, which attain increasingly compact shape with increasing temperature, and finally become circular when the subphase temperature is 26 and 15 degrees C for C16E2 and C16E3, respectively. It is concluded that a higher degree of dehydration around the headgroup region appreciably reduces the headgroup size, which imparts to the molecules an increase in hydrophobicity, thereby a closer molecular packing. Consequently, the line tension of the interface increases, showing compact structures at higher temperatures. Since C16E1 bears only one EO unit in its headgroup, the dehydration effect cannot appreciably raise its hydrophobicity to overcome the increases in thermal motion and chain flexibility of the molecules. Rather, increases in subphase temperature result in a decrease in the line tension of the interface, giving fractal structures at higher temperatures.