Micelle Formation and Surface Adsorption of Octaethylene Glycol Monoalkyl Ether (CnE8)

Abstract

Abstract The surface tension was measured against the concentration of CnE8 (n = 14, 16, and 18) at temperatures of 298.2, 303.2, 308.2, 313.2, and 318.2 K, from which the critical micelle concentration (CMC) was determined. The CMC decreased with increasing carbon number at lower temperatures, where the decreasing rate became much smaller at higher temperatures. The CMC values were examined by pyrene fluorescence, and the I1/I3 ratio of the pyrene spectrum steeply decreased below the CMC, and finally reached a plateau at higher surfactant concentrations. The I1/I3 ratios indicate that the microenvironment of pyrene in the micellar region becomes more hydrophobic with increasing the alkyl chain of CnE8. The aggregation number by a static light-scattering method increased with increasing the alkyl chain at a definite temperature and with raising the temperature for all of the surfactants. The thermodynamic parameters (ΔG°, ΔH°, TΔS°) of the micelle formation were calculated from the temperature dependence of CMC and the aggregation number, and the micellization was found to be entropy-driven. The surface excess concentration (Γ) was also determined from the change in the surface tension with the concentration from which the molecular surface area (A) below the CMC was evaluated. The molecular surface areas suggest that longer monoalkyl ethers form a bi-molecular layer with the hydrophobic tail intruding inwards. The positive entropy change (Δs) for the surface adsorption decreased and stayed almost constant with increasing concentration for C14E8 and C16E8, while the change remained almost zero for C18E8 at whole concentrations below the CMC. These results suggest that the non-fully extended alkyl chain in the bulk could not well contribute to a positive entropy change upon adsorption, which results in a smaller decreasing rate in CMC with increasing carbon number of alkylchain for C16E8 and C18E8 compared with the decreasing rate for CnE8 with n less than 14.