Surface properties of perfluoro acids as affected by terminal branching and chlorine substitution

Abstract

Abstract : When an otherwise fully fluorinated carboxylic acid molecule was altered by terminal branching and terminal chlorine atom substitution, significant increases resulted in the wettability of its adsorbed monolayer. Two new homologous series of acids (CF3)2CF(CF2)nCOOH and CF2C1(CF3)CF(CF2)nCOOH, where n ranged from 11 to 1 and 9 to 1, respectively, were adsorbed as condensed monolayers on highly polished clean disks of chromium and platinum. The critical surface tension of wetting (gamma sub c) of each resulting surface was determined by contact angle measurements on homologous series of n-alkanes, open-chain polydimethylsiloxanes, and miscellaneous other liquids. In every case the wettability, expressed as gamma sub c, was increased by branching or by terminal chlorine substitution. Thus, in the series (CF3)2CF(CF2)nCOOH, gamma sub c on chromium ranged from 13.3 dynes/cm (when n = 11) to a maximum of 15.2 (when n = 1), whereas in the series CF2 C1(CF3)CF(CF2)nCOOH, gamma sub c ranged from 17.2 (when n = 9) to around 20 (when n = 1).9 These results are excellent examples of the effect on gamma sub c on substituting one terminal chlorine atom in the surface for a fluorine atom. For a given acid monolayer, almost the same gamma sub c values were obtained on each of the two metals except that gamma sub c became larger on platinum than on chromium when the acid was a lower homolog in each series. The causes of this divergence relate to a smaller adsorption site spacing and the consequent tighter molecular packing and better chain adlineation of the adsorbed compound on chromium. (Author)