The effect of hydrophobic-lipophilic interactions on chemical reactivity: 3. Contributions of lipophilic interactions to the binding of hydrocarbon substrates by amylose-type hosts and of hydrophobic interactions to the binding of fluorocarbon surfactants

Abstract

With cyclodextrins (α- and β-CD) and sodium carboxymethylamylose (Na-CMA) as hosts, and with H (CF2)12CO3K (1), CH2 (CH2)10CO2K (2), CI (CF2)8CH2CH2N+Me3I- (3), CH3CH2)6-CH2N+Me3I- (4), Cl(CF2)10CH2CH2N+Me3I- (5) and CH3 (CH2)10 CH2N+Me3I- (6) as guest substrates, the different behaviors of fluorocarbon and hydrocarbon surfactants have been studied by surface tension measurements. Limited cavity size prevents the inclusion of fluorocarbon surfactants by α-CD, but the binding by β-CD is stronger for the fluorocarbon substrate 3 than that for its hydrocarbon analog 4. A comparison of the thermodynamic parameters of the β-CD binding process for 3 and 6 reveals that for the former the binding is driven by entropy or hydrophobic forces, but for the latter the process is enthalpy-favored. Notably, Na-CMA fails to bind the fluorocarbon substrates. A crucial difference between the cyclodextrins and the amylose-type hosts lies in the fact that the former hosts possess pre-organized cavities whereas the latter have to readjust their conformations from loose and extended helices with random coils to interrupted helices during the process of binding. Apparently, this extra energy requirement demands contributions from lipophilic interactions which do not exist between fluorocarbon chains and the hosts. Thus lipophilic forces are significant in hydrophobic-lipophilic interactions.