Unusual Excess Free Energies of Mixing in Mixtures of Partially Fluorinated and Hydrocarbon Surfactants at the Air鈥揥ater Interface: Correlation with the Structure of the Layer
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- 作者：Andrew J. Jackson ; Chu Chuan Dong ; Robert K. Thomas ; Jeffrey Penfold
- 刊名：Langmuir
- 出版年：2015
- 出版时间：January 13, 2015
- 年：2015
- 卷：31
- 期：1
- 页码：272-282
- 全文大小：593K
- ISSN：1520-5827
文摘
The air鈥搘ater interface of three mixtures of partially fluorinated surfactants and hydrocarbon surfactants, C4F9C11H22N(CH3)3Br (fC4hC11TAB) with hexadecyltrimethylammonium bromide (C16TAB), (CF3)2C3F6C10H20N(CH3)3Br (fC5hC10TAB) with C16TAB, and C8F17C6H12N(CH3)3Br (fC8hC6TAB) with C18TAB, have been investigated using surface tension (ST) and neutron reflection (NR). Using the composition of the layer determined by NR, the pseudophase separation model was used to fit the variation of concentration for a specific ST to a free energy of mixing, GE, that included adjustable quadratic, cubic and quartic terms. In all three cases, GE was found to be highly unsymmetrical, being approximately ideal at low surface fractions of hydrocarbon surfactant and repulsive at high fractions with a maximum value of 0.2鈥?.3RT. The corresponding structure of the layer was also determined by NR and showed that the initial ideal behavior of GE probably results from a balance of a gain in energy from a reduced immersion of the fluorocarbon chain, brought about by screening of the fluorocarbon from water by the hydrocarbon surfactant, and a loss from increased fluorocarbon鈥揾ydrocarbon repulsion. At higher concentration, there is no space in the layer for further screening and the fluorocarbon鈥揾ydrocarbon repulsion leads to the expected positive GE. The calculated GE also indicated that there should be phase separation of the two components in the interface over a bulk composition range of about 60鈥?5% hydrocarbon surfactant. However, experiment indicates no phase separation. It is suggested that there are a number of possible additional negative contributions to GE close to a phase transition, which are not possible for a true bulk phase separation, and which prevent surface phase separation unless it is strongly favored.