Effect of short non-ionic amphiphiles derived from ethylene and propylene glycol alkyl ethers on the CMC of SDS

The effect of the short non-ionic amphiphiles: n-alcohol (n = 3-6), polypropylene glycol alkyl ether (CnPOm, for n = 3, 4; m = 1-3) and polyethylene glycol (CnEOm, for n = 3; m = 1 and n = 4; m = 1-3) on the critical micellar concentration (CMC) of sodium dodecyl sulfate (SDS) in aqueous solutions at 25 °C has been determined experimentally by measuring the electrical conductance. The results show that there is a linear relation between the number of propylene glycol and ethylene glycol groups and the slope of the CMC decrease with amphiphile concentration. The propylene glycol group appears to be more efficient in lowering the CMC than the ethylene glycol group which is less hydrophobic. Considering the polypropylene glycol alkyl ethers as the condensation of an n-alcohol and a certain number of propylene glycol groups it was possible to classify these amphiphiles according to their efficiency to decrease the CMC of SDS: C4PO3 > C6PO0 > C4PO2 > C3PO3 > C4PO1 > C3PO2 > C5PO0 > C3PO1 > C4PO0 > C3PO0. This trend is different from the series of hydrophobicity of these compounds, inferred recently from water/CnPOm binary phase diagrams: C6PO0 > C5PO0 > C4PO4 > C4PO3 > C4PO1 > C4PO0 > C3PO3 > C3PO2 > C3PO1 > C3PO0. The hydrophobicity of these compounds increases much more with the addition of one carbon in the alkyl chain than with the addition of one propylene glycol group. The co-surfactant behavior of some of these molecules is also discussed and compared in terms of the topology of their ternary phase diagrams SDS/water/co-surfactant. Some striking differences in the co-surfactant behavior of the studied amphiphiles are found between their CMC decrease property and the topology of their corresponding ternary systems.