MOLECULAR-DYNAMICS STUDIES OF THE STRUCTURE AND PROPERTIES OF MONOLAYERS OF PERFLUORINATED AMPHIPHILES

We report the results of molecular-dynamics simulations of monolayers of CF3(CF2)nCOOH supported on water, using a pseudoatom model for the amphiphile and a continuum model for the water surface. The calculations explore the influence of temperature, chain length, and surface density on the packing structure of the monolayer. These fluorinated amphiphiles support a much lower gauche configuration concentration than do their corresponding hydrocarbon analogs and, of course, have different intermolecular interactions. The most striking results of the simulations are as follows. (i) Even and odd members of the series CF3(CF2)nCOOH, under the same external constraints (T = 300 K, pi = 0.16 dyn cm-1), pack in monolayers in which the tilt is, respectively, towards next-nearest neighbors and towards nearest neighbors. (ii) At constant surface pressure a monolayer of CF3(CF2)10COOH is predicted to undergo a transition between 325 and 350 K, in which the direction of the tilt changes from towards next-nearest neighbors to towards nearest neighbors, not unlike a pseudorotation. (iii) Between 275 and 325 K a monolayer of CF3(CF2)10COOH, held at constant surface pressure (pi = 0.16 dyn cm-1), is predicted to have negative thermal expansion. A number of other aspects of the structures of these monolayers, and the correspondence with the structures of monolayers of hydrocarbon amphiphiles, are briefly discussed.