Intermediate ordering in a liquid supported monolayer: A molecular dynamics study

We report the results of molecular dynamics simulations of a water supported monolayer of F(CF2)6(CH2)6OC6H4COOH. A grazing incidence x-ray diffraction study of this system showed, unexpectedly, the absence of any strongly ordered condensed phase. We find that the monolayer has a partially ordered structure with a range of order intermediate between that typically found in monolayers of, e.g., arachidic acid, and that expected for a two-dimensional liquid. The calculated width of the first order diffraction peak is about ten times larger than for arachidic acid, yet is ten times smaller than expected for a two-dimensional liquid. We argue that monolayers with intermediate order represent a different class of systems that those heretofore studied, and that these systems may have some unusual properties.