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 monolaye rs is observed by Brewster angle microscopy. It coincides with a coexistenc e of two different lattices in the monolayer over a large surface pressure range detected by grazing incidence X-ray diffraction. In one lattice the a lkyl chains are tilted toward nearest neighbors and in the other toward nex t-nearest neighbors. In monolayers of 2-hydroxypalmitic acid, 2-palmitoylgl ycerol, and 2-hexadecanol a condensed phase with short-range tilt orientati onal order is formed. The diffraction patterns show a superposition of the peaks from lattices which differ with respect to the tilt azimuth of the al kyl chains. The behavior of 1,2-hexadecandiol monolayers is related to that of 2-hydroxypalmitic acid, 2-palmitoylglycerol, and 2-hexadecanol. In mono layers of 2-hydroxypalmitic acid/palmitic acid mixtures a crossover from on e behavior to the other is observed. The loss of order in Langmuir monolaye rs 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 headgro up perpendicular to the tilt direction exceeds a particular value, a disord ered packing results. If substances with smaller headgroups are added to su ch monolayers, initially the spacing perpendicular to the chain tilt direct ion is optimized and then second the polar tilt angle is reduced.