Soluble monolayers of n-decyl glucopyranoside and n-decyl maltopyranoside. Phase changes in the gaseous to the liquid-expanded range

To examine the transition from the gaseous to the liquid-expanded monolayer state, surface tension data were recorded for n-decyl beta-d-glucopyranoside (Glu) and n-decyl beta-d-maltopyranoside (Mal) solutions at low concentrations and at different temperatures. Comparisons were also made with n-decyl beta-d-thiomaltopyranoside (S-Mal) solutions at room temperature. The transitions observed occur at very low concentrations and surface pressures, about 0.5% of the critical micelle concentration (cmc) and between 0.8 and 1 mN/m for Glu and Mal at 22 degrees C. For S-Mal the transition is recorded for a concentration of 0.5% of the cmc as well, but the surface pressure is lower, about 0.4 mN/m. The gradual change in molecular area about the transition is from about 500 to 200 A(2) and 400 to 150 A(2) for Mal and Glu, respectively, and from about 800 to 250 A(2) for S-Mal. The comparatively large molecular areas after the transitions are incompatible with the notion that a coherent hydrocarbon film would cover the entire surface already at this stage. Standard surface thermodynamics was applied to elucidate the nature of these transitions in combination with two model concepts: The formation of an infinite network of surfactant molecules and, second, the formation of surface micelles. Hard-disk simulation results were employed to quantify the additional surface pressure after the transition attributed to the formation of surface micelles. In conclusion the formation of surface micelles is plausible as the hard-disk model is capable of accounting for the additional surface pressure increase with acceptable accuracy. Further, vibrational sum frequency spectroscopy was used to investigate the transition for Mal. Using the distinct feature of the non-hydrogen-bonded OH ("free OH") at 3700 cm(-)(1) for probing the surface water state, it could be determined that the surface holds a sizable fraction of unperturbed surface water even after the transition from the Henry range. The decrease in the free OH signal was found to correlate with the increase in surface density of surface micelles.