Calixarene-Based Surfactants: Conformational-Dependent Solvation Shells for the Alkyl Chains

Thermodynamic parameters obtained from studying the micellization of amphiphilic p-sulfonatocalix[n]arenes were correlated with the alkyl chain length and with the number of monomeric units (n) in the calix[n]arene structure. The micellization Gibbs free energy (?${G{{{\rm o}\hfill \atop {\rm M}\hfill}}}$) becomes more negative upon increasing the alkyl chain length of the p-sulfonatocalix[4]arene. This is in agreement with the trend generally observed for other surfactants. However, the ?${G{{{\rm o}\hfill \atop {\rm M}\hfill}}}$ value for transferring one CH2 group from the bulk aqueous medium to the micelle [?${G{{{\rm o}\hfill \atop {\rm M}\hfill}}}$(CH2)] is lower than the value generally observed for single-chain surfactants, suggesting the existence of intramolecular interactions between the alkyl chains of the free unimers. On the other hand, the critical micelle concentration (cmc; per alkyl chain unit) increased with the increasing number of monomeric units. These results are explained on the basis of the conformation adopted by the calixarene in the bulk solution. The calix[4]arene derivatives are preorganized into the cone conformation, which is favorable for the formation of globular aggregates. The calix[6]arene and calix[8]arene derivatives do not adopt cone conformations. Changing these conformations to the more favorable cone conformer in the aggregates implies an energetic cost that contributes to making ?${G{{{\rm o}\hfill \atop {\rm M}\hfill}}}$ less efficient. In the case of the calix[6]arene derivative this energetic cost is enthalpic, whereas in the case of the octamer it is both enthalpic and entropic. Both the ?${G{{{\rm o}\hfill \atop {\rm M}\hfill}}}$(CH2) value and the change in heat capacity (?C${{\rm p}{{{\rm o}\hfill \atop {\rm M}\hfill}}}$) seem to indicate that for the cone calix[4]arene derivatives all alkyl chains are solvated by the same hydration shell, whereas in the case of the highly flexible calix[8]arene derivative each alkyl chain is individually hydrated.