CORRELATION BETWEEN OBSERVED CRYSTALLINE SELF-ASSEMBLY OF FLUOROCARBON AND HYDROCARBON AMPHIPHILES AT THE AIR-WATER-INTERFACE AND CALCULATED LATTICE ENERGY - DETERMINATION OF ELECTROSTATIC PROPERTIES OF THE CF2 GROUP FROM A LOW-TEMPERATURE X-RAY-DIFFRACTION STUDY OF PERFLUOROGLUTARAMIDE

Recently, grazing-incidence X-ray diffraction studies of insoluble amphiphilic molecules have shown that molecules possessing fluorocarbon chains crystallize more efficiently on the surface of water that those possessing hydrocarbon chains. Here, we perform lattice energy calculations involving atomic electrostatic and van der Waals parameters on model two-dimensional crystals of hydrocarbon chains and fluorocarbon chains which possess crystalline arrangements similar to those of the corresponding amphiphilic films on water. The electrostatic parameters of CF2 groups were determined from an X-ray study of the deformation electron density of perfluoroglutaramide, using single-crystal low-temperature (approximately 100 K) X-ray diffraction data. The net charge on the fluorine atoms q = -0.14 e is almost twice that on hydrogen atoms q = +0.06 e, suggesting that intramolecular repulsion between fluorines will limit the possibility for conformational disorder in fluorocarbon chains. The calculated lattice energy of the model 2-D crystalline films of vertical fluorocarbon chains containing 20 carbons is lower by about 3.0 kcal per CF2 group than the lattice energy of the model 2-D crystalline films of vertical hydrocarbon chains with the same number of carbons. We conclude that the crystallization behavior of amphiphilic molecules with fluorocarbon chains is determined by a higher backbone stiffness and higher interchain attractive van der Waals forces than for molecules with hydrocarbon chains. In addition, we show that it is possible to simply correlate the calculated lattice energies and the observed crystalline self-assembly at the air-water interface in various amphiphilic systems, in particular the homologous series of carboxylic acid molecules CnH2n+1CO2H (n = 13, 19, 20, 21, 29): for more attractive lattice energies, the extent of crystalline order is larger.