Purification, Surface Tensions, and Miscibility Gaps of Alkyldimethyl and Alkyldiethylphosphine Oxides

Alkyldimethyl (C n DMPO) with chain lengths of n = 8 (octyl), 10 (decyl), 12 (dodecyl), and 14 (tetradecyl) as well as alkyldiethyl (C n DEPO) phosphine oxides with chain lengths of n = 10, 12, and 14 were synthesized and purified to study how the adsorption properties and the location of the miscibility gap of these surfactants depend on the size of the head group and on the length of the alkyl chain. After surfactant purification, the surface tension isotherms were determined from which the cmc, the minimum surface tension σcmc, the maximum surface concentration Γmax, and the minimum surface area A min were obtained. As expected, for one homologous series, a decrease in the cmc and an increase in Γmax was observed with increasing alkyl chain length. For two surfactants of the same alkyl chain length, the cmc values of the C n DEPO surfactants are approximately two times lower than those of the C n DMPO surfactants. However, the Γmax values of C n DEPO are lower than those of C n DMPO as two ethyl chains are sterically more demanding than two methyl chains. In addition to the adsorption properties, the location of the miscibility gap as a function of the alkyl chain length and the head group size was studied. Its location depends on the total number of carbon atoms and not primarily on the length of the main alkyl chain. This observation reflects the decreasing water solubility which can be tuned by increasing the length of either the main alkyl chain or of the shorter head group chains.