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

Abstract

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 (ΔG(M)(o)) 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 ΔG(M)(o) value for transferring one CH(2) group from the bulk aqueous medium to the micelle [ΔG(M)(o)(CH(2))] 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 ΔG(M)(o) 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 ΔG(M)(o)(CH(2)) value and the change in heat capacity (ΔCp(M)(o)) 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.

Aggregation of p-Sulfonatocalixarene-Based Amphiphiles and Supra-Amphiphiles

“…While the first correlation is in line with the generally observed behavior for conventional surfactants, the increase in the CMC (in mole of alkyl chain units) with the number of monomeric units is contrary to what was expected on the basis of the results reported for oligomeric surfactants [26–36]. To get more insight into this particular observation, the micellization of these compounds was further studied by isothermal titration calorimetry (ITC) [60]. …”

“…Besides Δ H M , the free energy of micellization (Δ G M ) can also be determined, and consequently, knowing these two thermodynamic functions, the entropy of micellization (Δ S M ) can be also calculated. As can be easily demonstrated, Δ G M (in kJ·mol −1 of alkyl chain) is given by Equation 1 [60,64]:…”

“…Amphiphilic p -sulfonatocalix[ n ]arenes used to establish structure aggregation relationships in references [60,61]. …”

Counterion‐Controlled Self‐Sorting in an Amphiphilic Calixarene Micellar System

“…[4] This special class of amphiphiles was shown to aggregate into globularm icelles with critical micelle concentrations (CMC) that decrease with increasing the length of the alkyl chains and are little affected by the ring size. [4,[21][22][23][24] Despite not being extensively investigated, the potential of amphiphilic SCn to be appliedi nt he formulation of hydrogels and multifunctional drug delivery vehicles is currently being explored. [25][26][27][28] Some of the most important applicationso fm icelles arise from their ability to solubilise and compartmentalize both apolar, polar and ionic compounds.…”

“…These observations are supported by the coincidenceo ft his concentration value with the reported CMC for SC4TB (3 mm). [23,24] It is worth noting that the CMC corresponds to am ore or less narrower concentration range rather than ap oint and therefore more sensible properties (such as the emission of fluorescence probes fluorescence) can be affected by the presence of micellar aggregates at concentrations slightly below the CMC. [37] It should be remarked that critical micelle concentration for SC4TB is mainlyu naffected by the presence of DMSI (see the Supporting Information).…”