Temperature and Pressure Based NMR Studies of Detergent Micelle Phase Equilibria

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• 作者：Rohan Alvares ; Shaan Gupta ; Peter M. Macdonald ; R. Scott Prosser
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：May 29, 2014
• 年：2014
• 卷：118
• 期：21
• 页码：5698-5706
• 全文大小：404K
• 年卷期：v.118,no.21(May 29, 2014)
• ISSN：1520-5207

文摘

Bulk thermodynamic and volumetric parameters (螖G_mic^掳, 螖H_mic^掳, 螖S_mic^掳, 螖C_p,mic^掳, 螖V_mic^掳, and 螖魏_mic^掳) associated with the monomer鈥搈icelle equilibrium, were directly determined for a variety of common detergents [sodium n-dodecyl sulfate (SDS), n-dodecyl phosphocholine (DPC), n-dodecyl-尾-d-maltoside (DDM), and 7-cyclohexyl-1-heptyl phosphocholine (CyF)] via ¹H NMR spectroscopy. For each temperature and pressure point, the critical micelle concentration (cmc) was obtained from a single ¹H NMR spectrum at a single intermediate concentration by referencing the observed chemical shift to those of pure monomer and pure micellar phases. This permitted rapid measurements of the cmc over a range of temperatures and pressures. In all cases, micelle formation was strongly entropically favored, while enthalpy changes were all positive, with the exception of SDS, which exhibited a modestly negative enthalpy of micellization. Heat capacity changes were also characteristically negative, while partial molar volume changes were uniformly positive, as expected for an aggregation process dictated by hydrophobic effects. Isothermal compressibility changes were found to be consistent with previous measurements using other techniques. Thermodynamic measurements were also related to spectroscopic studies of topology and micelle structure. For example, paramagnetic effects resulting from the addition of dioxygen provided microscopic topological details concerning the hydrophobicity gradient along the detergent chains within their respective micelles as detected by ¹H NMR. In a second example, combined ¹³C and ¹H NMR chemical shift changes arising from application of high pressure, or upon micellization, of CyF provided site-specific details regarding micelle topology. In this fashion, bulk thermodynamics could be related to microscopic topological details within the detergent micelle.