Intramolecular and intermolecular ion-dipole interactions in sodium lauryl ether sulfates (SLES) self-aggregation and mixed micellization with Triton X-100

Micellization of a series of sodium lauryl ether sulfate (SLES), C₁₂H₂₅(OCH₂CH₂)_xO SO₃Na, where x = 1 (CS130), 2 (CS230), and 3 (CS330) and their interaction with Triton X-100 in aqueous media were investigated by fluorescence quenching of (Ru(bipy)₃²⁺, 2Cl^-) by 9-methylanthracene, conductivity, and surface tension measurements at room temperature. A similar critical micelle concentration (CMC) value of 0.80 mM was found for the three ethoxylated surfactants, well below the CMC of sodium dodecyl sulfate (SDS) (8.2 mM), although the micelle ionization degree of SLES micelles were significantly higher (β_ion = 0.70) compared to the reported value in SDS micelles (β_ion = 0.18). This unexpected result was attributed to intramolecular and intermolecular attractive ion-dipole interactions between the sulfate ion (SO₄^-) and the O → CH₂ dipole of the oxyethylene (OE) group in SLES micelles. Also, the observed invariance of the CMC with the degree of ethoxylation was attributed to the fact that this ion-dipole interaction is effective only with the OE group linked to SO₄^- (intramolecular attractive interaction) and the sulfate group of a given surfactant in SLES micelles with the OE group adjacent to the sulfate group of a nearby surfactant (intermolecular attractive interaction). Changes in CMCs with the composition of the mixture of SLES (CS130, CS230, and CS330) and Triton X-100 so determined were analyzed by applying Rubingh's regular solution theory to obtain a similar interaction parameter β^M = -1.39, lower than the well-known corresponding interaction parameter in SDS/Triton X-100 reported in the literature (β^M = -3.80). This difference in surfactant-surfactant interaction in the two mixed systems was attributed to the intramolecular and intermolecular ion-dipole attractive interactions in single SLES micelles, whereas such interactions are not occurring in SDS single micelles. This will be reflected by a relatively stronger self-repulsion between the sulfate groups in SDS micelles. In mixed micelles, the intramolecular ion-dipole attraction in SLES surfactants may have the dominant factor in the overall interactions between SLES surfactants and Triton X-100. Consequently, this may be reflected by a lower β^M as determined from our results. Interestingly, the decrease of the CMCs for SLES/Triton X-100 systems was also independent of the degree of ethoxylation, suggesting that both intramolecular and intermolecular ion-dipole interaction in mixed micelles involves only the first OE group linked to the sulfate moiety.