ENTROPIC FORCES BETWEEN AMPHIPHILIC SURFACES IN LIQUIDS
ENTROPIC FORCES BETWEEN AMPHIPHILIC SURFACES IN LIQUIDS
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DOI：
10.1021/j100181a007
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发表时间：
1992-01-23
影响因子：
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通讯作者：
WENNERSTROM, H
中科院分区：
文献类型：
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作者：
ISRAELACHVILI, JN;WENNERSTROM, H
液体中的表面活性剂胶束、脂质双层和微乳液液滴等流体两亲表面之间的力包括预期的吸引范德华力和排斥双电层力（两个 DLVO 力）。 然而，由于这些界面的动态（类流体）性质，还存在因热激发表面模式的重叠而产生的额外熵涨落力。 其中三种力是波动力、蠕动力和突出力。 前两个源于双层或膜的集体运动，可以分别用它们的连续弹性模量来描述。 最后一个是由烃链和突出表面的分子其他部分的分子尺度波动（“分子突出”力）以及重叠的移动头基之间类似的渗透排斥力（“头基重叠”力）引起的。 结果表明，这两种力预计会随着距离大致呈指数衰减，水中的特征衰减长度约为 0.2 nm。 长期以来，人们一直认为这些力是由水结构引起的（通常称为“水合”力）。 通过回顾该领域最近的实验和理论进展，得出的结论是，这种力并非主要归因于水结构（它发生在除水之外的其他液体中），并且更类似于聚合物覆盖表面之间的空间排斥力。 真正的水合或溶剂化效应可能仅在两亲表面之间的相互作用中发挥间接作用，主要是确定突出基团的水合尺寸（排除体积）以及其他相互作用势的起源平面的位置。 对水中不带电两亲表面之间的 DLVO 力、熵力和真正水合力的相对贡献进行了定量评估。 结论是，在自由双层之间，大间距（> 3 nm）时波动排斥占主导地位，中间间距（1.5-3 nm）时范德华吸引力占主导地位，而较小间距（< 1.5 nm）时突出和重叠排斥占主导地位。 然而，在具有很长头基的双层之间，头基重叠排斥可能在所有分离中占主导地位。 通过这种新的解释，现在可以更好地理解双层相互作用产生的许多现象。
The forces between fluid amphiphilic surfaces such as surfactant micelles, lipid bilayers, and microemulsion droplets in liquids include the expected attractive van der Waals and repulsive electric double-layer forces (the two DLVO forces). However, because of the dynamic (fluid-like) nature of these interfaces, additional entropic fluctuation forces are also present which arise from the overlap of thermally excited surface modes. Three of these forces are the undulation, peristaltic, and protrusion forces. The first two arise from collective motions of bilayers or membranes and can be described in terms of their continuum elastic moduli, respectively. The last arises from molecular-scale fluctuations of hydrocarbon chains and other parts of the molecules protruding out of the surfaces ("molecular protrusion" force), and a similar osmotic repulsion between overlapping mobile headgroups ("headgroup overlap" force). It is shown that both these two forces are expected to decay roughly exponentially with distance with characteristic decay lengths in water of about 0.2 nm. These forces have long been believed to be due to water structure (and are commonly called the "hydration" force). By a review of recent experimental and theoretical progress in this area, it is concluded that this force is not primarily due to water structure (it occurs in other liquids than water) and that it is more akin to the steric repulsion between polymer-covered surfaces. Genuine hydration or solvation effects probably play only an indirect role in the interactions between amphiphilic surfaces, mainly in determining the hydrated sizes (excluded volumes) of the protruding groups and the positions of the planes of origin of other interaction potentials. A quantitative assessment is made of the relative contributions of DLVO forces, entropic forces, and genuine hydration forces between uncharged amphiphilic surfaces in water. It is concluded that between free bilayers the undulation repulsion dominates at large separations (> 3 nm), the van der Waals attraction at intermediate separations (1.5-3 nm), and the protrusion and overlap repulsions at smaller separations (< 1.5 nm). However, between bilayers with very long headgroups, the headgroup overlap repulsion may dominate at all separations. With this new interpretation many phenomena arising from bilayer interactions may now be better understood.