Disorder in Langmuir monolayers:: 2.: Relation between disordered alkyl chain packing and the loss of long-range tilt orientational order

A loss of long-range tilt orientational order in 1,2-hexadecandiol monolayers is observed by Brewster angle microscopy. It coincides with a coexistence of two different lattices in the monolayer over a large surface pressure range detected by grazing incidence X-ray diffraction. In one lattice the alkyl chains are tilted toward nearest neighbors and in the other toward next-nearest neighbors. In monolayers of 2-hydroxypalmitic acid, 2-palmitoylglycerol, and 2-hexadecanol a condensed phase with short-range tilt orientational order is formed. The diffraction patterns show a superposition of the peaks from lattices which differ with respect to the tilt azimuth of the alkyl chains. The behavior of 1,2-hexadecandiol monolayers is related to that of 2-hydroxypalmitic acid, 2-palmitoylglycerol, and 2-hexadecanol. In monolayers of 2-hydroxypalmitic acid/palmitic acid mixtures a crossover from one behavior to the other is observed. The loss of order in Langmuir monolayers is initiated by a misfit of alkyl chains and headgroups. Chains can only adapt to headgroups which are additionally too large perpendicular to the tilt direction by a distortion of their packing. If the size of the headgroup perpendicular to the tilt direction exceeds a particular value, a disordered packing results. If substances with smaller headgroups are added to such monolayers, initially the spacing perpendicular to the chain tilt direction is optimized and then second the polar tilt angle is reduced.