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Temperature Dependence of the Surface Phase Behavior and Micelle Formation of Some Nonionic Surfactants

2003; American Chemical Society; Volume: 107; Issue: 4 Linguagem: Inglês

10.1021/jp021212g

1520-6106

Md. Nazrul Islam, Teiji Kato,

Spectroscopy and Quantum Chemical Studies

The surface phase behavior and micellar properties of two nonionic surfactants, namely, ethylene glycol mono n-dodecyl ether (EGDE) and ethylene glycol mono n-tetradecyl ether (EGTE), have been investigated at different temperatures. From the study of film balance and Brewster angle microscopy (BAM), it has been observed that both of the amphiphiles show a first-order phase transition indicating conspicuous cusp points in their respective adsorption isotherms. This is further confirmed by the observation of bright 2-D condensed domains at the solution surfaces just after the appearance of the cusp points in the adsorption isotherms. Each of the above amphiphiles has a definite temperature above which they cannot show any indicative feature of phase transition, even with solutions of concentration above the critical micelle concentration (CMC) of the amphiphiles. These temperatures are found to be 23 and 37 °C for EGDE and EGTE, respectively. The CMC values of the amphiphiles increase with increasing temperature, showing a maximum value at a definite temperature (Tmax), and then decrease with further increases in temperature, which is just the opposite trend of the usual behavior of ionic surfactants. Interestingly, Tmax values of both of the amphiphiles correspond well to their respective maximum temperature of the appearance of the phase transition in the adsorbed monolayers. The standard thermodynamic parameters for micelle formation were calculated to determine the nature of thermodynamic process involved in micellization. Both enthalpy, ΔH°m, and entropy, ΔS°m, terms change their sign from negative to positive and increase monotonically with increasing temperature. The enthalpy term ΔH°m is found to be zero at Tmax, which suggests that micellization is entirely an entropy-driven process at this temperature. A linear relation in the ΔHm° versus ΔSm° plots is observed with slopes 304 and 296 for EGDE and EGTE, respectively, having dimensions of the Kelvin temperature. Both of the thermodynamic quantities compensate each other, leading the free energy to be a negative value.