Molecular ordering and phase transitions in alkanol monolayers at the water-hexane interface.
Molecular ordering and phase transitions in alkanol monolayers at the water-hexane interface.
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水-己烷界面处烷醇单层的分子排序和相变。
DOI：
10.1063/1.1752888
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发表时间：
2004
期刊：
影响因子：
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通讯作者：
M. Schlossman
中科院分区：
文献类型：
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作者：
A. Tikhonov;S. Pingali;M. Schlossman
用x射线反射率、x射线非镜面漫射散射和界面张力测量研究了正烷醇(H(CH(2))(m)OH （m =20、22、24或30）的散装水和散装己烷溶液之间的界面。烷醇吸附在界面上形成单层。密度最高、温度最低的单分子层含有从-CH(2)OH到-CH(3)基团的链式无序烷烃分子。在链的末端，包括-CH(3)基团，其链密度与刚高于其冻结温度的散装液态烷烃中所观察到的相似。烷基醇头团区域的密度比水蒸气界面上的单体水或有序头团区域的密度大10%。我们推测，这种较高的密度是水渗透到无序单层的头群区域的结果。水与醇分子1:3的比例与我们的数据一致。我们还设置了一个上限，即一个己烷到五个或六个烷醇分子混合到单层的烷基链区域。相反，在水蒸气界面处的H(CH(2))(30)OH形成一个紧密排列、有序的几乎刚性棒相。界面张力随温度的变化表明，水-己烷界面发生相变，界面过剩熵发生显著变化。这种转变发生在几乎完全被烷醇覆盖的低温界面到具有低得多烷醇密度的高温界面之间。短链烷烃的过渡似乎是一级的，而长链烷烃的过渡似乎是弱一级或二级的。x射线数据与界面上存在单层结构域一致，并确定了结构域覆盖率（被烷醇结构域覆盖的界面比例）作为温度的函数。这种温度依赖性与二阶相变的理论模型是一致的，该模型解释了作为线张力和远程偶极子力之间平衡的畴稳定。我们测量的几个方面表明，域的存在代表了空间非均匀相的外观，而不是两个均匀相的共存。
The interface between bulk water and bulk hexane solutions of n-alkanols (H(CH(2))(m)OH, where m=20, 22, 24, or 30) is studied with x-ray reflectivity, x-ray off-specular diffuse scattering, and interfacial tension measurements. The alkanols adsorb to the interface to form a monolayer. The highest density, lowest temperature monolayers contain alkanol molecules with progressive disordering of the chain from the -CH(2)OH to the -CH(3) group. In the terminal half of the chain that includes the -CH(3) group the chain density is similar to that observed in bulk liquid alkanes just above their freezing temperature. The density in the alkanol headgroup region is 10% greater than either bulk water or the ordered headgroup region found in alkanol monolayers at the water-vapor interface. We conjecture that this higher density is a result of water penetration into the headgroup region of the disordered monolayer. A ratio of 1:3 water to alkanol molecules is consistent with our data. We also place an upper limit of one hexane to five or six alkanol molecules mixed into the alkyl chain region of the monolayer. In contrast, H(CH(2))(30)OH at the water-vapor interface forms a close-packed, ordered phase of nearly rigid rods. Interfacial tension measurements as a function of temperature reveal a phase transition at the water-hexane interface with a significant change in interfacial excess entropy. This transition is between a low temperature interface that is nearly fully covered with alkanols to a higher temperature interface with a much lower density of alkanols. The transition for the shorter alkanols appears to be first order whereas the transition for the longer alkanols appears to be weakly first order or second order. The x-ray data are consistent with the presence of monolayer domains at the interface and determine the domain coverage (fraction of interface covered by alkanol domains) as a function of temperature. This temperature dependence is consistent with a theoretical model for a second order phase transition that accounts for the domain stabilization as a balance between line tension and long range dipole forces. Several aspects of our measurements indicate that the presence of domains represents the appearance of a spatially inhomogeneous phase rather than the coexistence of two homogeneous phases.