Phosphatidylcholine Monolayer Structure at a Liquid−Liquid Interface

Vibrational sum frequency spectroscopy has been used to examine the vibrational structure of phosphatidylcholine monolayers adsorbed to a water−carbon tetrachloride interface. The surfactants employed in this study belong to a family of saturated, symmetric phosphatidylcholines with acyl chain lengths ranging from C 12 to C 18 in increments of two methylene units. Vibrational spectra provide direct information about the orientation and degree of order among the acyl chains of the adsorbed phosphatidylcholines. Differences among spectra recorded under various polarization conditions show that acyl chains do not exhibit long range order or preferred tilt angle. Rather, acyl chains within a tightly packed monolayer stand up with their methyl C 3 axes aligned perpendicular to the interface. Relative methyl and methylene symmetric stretch band intensities show that order within the monolayer increases with increasing surface coverage. Temperature-dependent studies of monolayer order suggest that a barrier exists to organic solvent penetration of the acyl chain network of a tightly packed, adsorbed monolayer. At a liquid−liquid interface, shorter chain phosphatidylcholine species form monolayers more ordered than those of longer chain species, although the dependence of monolayer order on acyl chain length is small. This trend reverses in monolayers at the air−water interface where longer chain phosphatidylcholines form monolayers dramatically more ordered than those of their shorter chain counterparts. The disparity in behavior between monolayers at the liquid−liquid and air−water interfaces is interpreted as evidence of acyl chain solvation by the organic CCl 4 solvent.