Amphiphilic polyoxometalate (POM) surfactants were prepared by covalently grafting double hydrophobic tails with chain lengths C₁₂H₂₅, C₁₄H₂₉, C₁₆H₃₃ or C₁₈H₃₇ onto the lacunary Wells–Dawson {P₂W₁₇O₆₁} headgroup (K₁₀[P₂W₁₇O₆₁OSi₂(C_nH_2n+1)₂]). The critical micelle concentrations (CMCs) of these novel surfactants in aqueous solutions were determined by conductivity, and micelle formation was studied by small angle neutron scattering (SANS). Surprisingly, the amphiphiles with longer hydrophobic tails tend to form less elongated and more globular micelles in water.
A systematic study was performed on mixed surfactant systems containing P₂W₁₇-2C_n, (n = 12, 14 or 16) and hexaethylene glycol monododecyl ether (C₁₂EO₆). Critical micelle concentrations (CMCs) of these mixtures were measured and used to calculate the interaction parameters based on regular solution theory, enabling prediction of micellar composition. The CMCs of these mixtures suggest unusual unfavourable interactions between the two species, despite formation of mixed micelles. Micellar compositions obtained from SANS concurred with those calculated using the averaged interaction parameters for P₂W₁₇-2C_n/C₁₂EO₆ (n = 12 and 14). We attribute the unfavourable interactions to a combination of different phenomena: counterion-mediated interactions between P₂W₁₇ units and the unfolding of the ethylene oxide headgroups of the nonionic surfactant, yet micelles still form in these systems due to the hydrophobic interactions between surfactant tails.

Karen Edler

University of Bath

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