MICELLIZATION AND ADSORPTION OF FLUORINATED AMPHIPHILES - QUESTIONINGTHE 1CF(2)APPROXIMATE-TO-1.5CH(2) RULE

The surface activity of a series of partially fluorinated amphiphiles with a dimorpholinophosphate polar head, a perfluoroalkyl terminal and a hydrocarbon spacer, CnF(2n+1)(CH2)(m)OP(O)[N(CH2CH2)(2)O](2) (FnCmD MP, m = 1-11, n = 4-10), was investigated, and the contributions of th e CF2 and CH2 groups to the energies of adsorption and micellization o f the amphiphiles were determined. In the literature, such data are on ly available for amphiphiles with either totally fluorinated or totall y hydrogenated hydrophobes. We determined the impact of the fluorocarb on segment on the contribution of the hydrocarbon spacer to the adsorp tion and micellization processes. Delta G(mic) and Delta G(ads) were e valuated as -4.2 +/- 0.5 and -4.1 +/- 0.6 kJ mol(-1) per CF2 group, co nsistent with results reported on totally fluorinated surfactants (-3. 3 to -5.2 kJ mol(-1)). In contrast, the values for Delta G(mic) and De lta G(ads) per CH2 group (-1.0 +/- 0.4 and -1.0 +/- 0.6 kJ mol(-1), re spectively) were substantially lower than those measured for hydrocarb on analogues of the FnCmDMPs (-2.4 +/- 0.4 and -2.5 +/- 0.5 kJ mol(-1) ), which fall in the range observed for standard hydrocarbon amphiphil es (-2.4 to -3.05 kJ mol(-1)). These results show that a hydrocarbon c hain grafted to a fluorinated chain does not fully participate in the micellization and adsorption processes, and behaves as if it were shor ter by a factor of about three. Thus it is primarily the length of the fluorinated chain that controls micellization and adsorption of such surfactants; the spacer plays only a minor role, and great caution mus t be exercised when applying the 1 CF2 approximate to 1.5 CH2 rule. It is proposed that the hydrocarbon spacer adopts a folded conformation in order to better occupy the void volume which results from the diffe rence in cross-sections between fluorocarbon and hydrocarbon chains (c a. 30 vs. 20 Angstrom(2), respectively).