Calculating the surface potential of unionized monolayers.
Calculating the surface potential of unionized monolayers.
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计算未电离单层的表面电势。
DOI：
10.1103/physreve.49.1439
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发表时间：
1994
期刊：
影响因子：
--
通讯作者：
Bayes
中科院分区：
文献类型：
--
作者：
Taylor;Bayes
提出了一个定量模型，该模型可以根据群偶极矩的已知值确定单分子膜在空气-水界面的表面电势的贡献。基于Lennard-Jones和Dent方法，该模型表明，只要形成单分子膜的分子长度大于约0.5 nm，凝聚的长链正构烷酸单分子膜的头部和尾部基团的力矩就可以被假定为不相互作用，从而证实了Demchak和Fort[J胶体界面科学]模型的主要原理之一。46,191(1974)]。同样地，结果表明，对于长度大于0.5 nm的分子，亚相的成像效应只对头部基团重要，而对尾部基团可以忽略不计。该模型表明，作用于单层中偶极子的局域场可以用单层的相对介电常数来描述。确定了烷酸疏水链区局部相对介电常数的上界为1.98~2.24。导致这些值的局域场的主要贡献来自形成单分子层疏水区的亚甲基的“层”。对于烷酸的远端甲基的情况，仅考虑面内甲基偶极的相互诱导去极化导致局部相对介电常数低至1.18。结果表明，尽管水平方向的亚甲基基矩不能对凝聚单分子膜的表面电势产生直接贡献，但它们确实通过其局域场产生了显著的间接贡献。
A quantitative model is presented which enables contributions to the surface potential of a monolayer at the air-water interface to be determined from known values of group dipole moments. Based on the Lennard-Jones and Dent approach, the model shows that the head- and tail-group moments in a condensed, long-chain, n-alkanoic acid monolayer may be assumed to be noninteracting so long as the monolayer-forming molecule is more than about 0.5 nm in length, thus confirming one of the main tenets of the model by Demchak and Fort [J. Colloid Interface Sci. 46, 191 (1974)]. By the same token, it is shown that imaging effects in the subphase are only important for the head group and negligible for the tail group of a molecule more than about 0.5 nm long. The present model shows that the local field acting on dipoles in the monolayer can be described in terms of a relative permittivity for the monolayer. An upper bound of between 1.98 and 2.24 is established for the local relative permittivity of the hydrophobic chain region of alkanoic acids. The major contribution to the local field which leads to these values is from the ``layers'' of methylene groups forming the hydrophobic region of the monolayer. For the case of the distal methyl groups of an alkanoic acid, consideration of only the mutual induced depolarization of in-plane methyl dipoles leads to a local relative permittivity as low as 1.18. Thus, it is shown that even though horizontally directed methylene-group moments cannot make a direct contribution to the surface potential of a condensed monolayer, they do make a significant indirect contribution via their local field.