Monte Carlo simulation of water/1-butanol/2-ethoxyethan-1-ol mixtures at vapor/liquid interfaces

Configurational-bias Monte Carlo simulations for the transferable potential for phase equilibria force field were carried out to investigate the composition, structure, and hydrogen-bonding at the liquid/vapor interface for saturated solutions of water + 1-butanol + 2-ethoxyethan-1-ol. Both 1-butanol and 2-ethoxyethan-1-ol showed strong enrichment at the liquid/vapor of the water-rich phase. The 1-butanol molecules were mostly found in the outermost layer with their alkyl tails pointing preferentially toward the vapor phase, i.e., their long axis is aligned perpendicular to the surface. In contrast, many of the 2-ethoxyethan-1-ol molecules were found in a subsurface layer, i.e. sandwiched between the 1-butanol layer and the bulk aqueous phase. The great excess of hydrogen bond acceptor sites allowed the 2-ethoxyethan-1-ol to form this bridging layer, and molecules in this layer were preferentially found with their backbone parallel to the surface. Despite forming an energetically, weaker hydrogen bond, the ether oxygen in 2-ethoxyethan-1-ol was not found to accept any fewer hydrogen bonds than the hydroxyl oxygen. As the overall concentration of 2-ethoxyethan-1-ol was increased, its concentration in the bulk region of the aqueous phase increased as well as the water content in the organic phase. Similar to the aqueous phase, the outer surface of the organic phase was mainly occupied by 1-butanol molecules with 2-ethoxyethan-1-ol molecules populating the subsurface layer. This is an abstract of a paper presented at the 229th ACS National Meeting (San Diego, CA 3/13-17/2005).