A molecular dynamics study of local pressures and interfacial tensions of SDS micelles and dodecane droplets in water

胶束 表面张力 拉普拉斯压力 十二烷 化学 十二烷基硫酸钠 分子动力学 热力学 分析化学(期刊) 色谱法 水溶液 物理化学 有机化学 计算化学 物理
作者
Masahiro Kitabata,Kazushi Fujimoto,Noriyuki Yoshii,Susumu Okazaki
出处
期刊:Journal of Chemical Physics [American Institute of Physics]
卷期号:144 (22): 224701-224701 被引量:11
标识
DOI:10.1063/1.4953405
摘要

To obtain the radial (normal) and lateral (transverse) components of the local pressure tensor, PN(R) and PT(R), respectively, and the interfacial tension of micelles, molecular dynamics (MD) calculations were performed for spherical sodium dodecyl sulfate (SDS) micelles. The local pressure tensor was calculated as a function of radial distance R using the Irving-Kirkwood formula. Similar MD calculations were also carried out for an n-dodecane droplet in water to compare the differences in the local pressure and interfacial tension values with those of the micelles. The calculated interfacial tensions were 20 ± 5 and 44 ± 10 mN/m for the SDS micelles and dodecane droplets, respectively. The excess free energies due to the interfacial tension were 340 and 1331 kJ/mol for the SDS micelle and dodecane droplet, respectively. The micelles are stabilized by 991 kJ/mol by covering their hydrophobic cores with hydrophilic groups. The dodecane droplet has a large interfacial tension caused by the zero or positive values of PN(R) − PT(R) at all values of R. In contrast, the small interfacial tension in the SDS micelles comes from the negative PN(R) − PT(R) values over a wide range of R. The pressure difference between the inside and outside of the oil droplet and its interfacial tension well satisfies the Laplace equation. However, the hydrophobic core of the SDS micelle is quite different from the liquid alkane, and the SDS micelles do not follow Laplace’s picture. Decomposing the interfacial tension into contributions from various interactions, it is found that those between charged and polar groups dominate the interfacial tension of the SDS micelles. The positive electrostatic potential (1.3 V) on the micelle surface and the negative potential (−0.15 V) on the oil droplet contribute to the interfacial tensions by 19 and 0.5 mN/m, respectively. Thus, the interfacial tension of the SDS micelles is produced by electrostatic interactions, in contrast to the dodecane droplet.

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