Adsorption Properties of Surface Chemically Pure Sodium Perfluoro-n-alkanoates at the Air/Water Interface: Counterion Effects within Homologous Series of 1:1 Ionic Surfactants
Adsorption Properties of Surface Chemically Pure Sodium Perfluoro-n-alkanoates at the Air/Water Interface: Counterion Effects within Homologous Series of 1:1 Ionic Surfactants
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DOI：
10.1021/la503450k
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发表时间：
2015-01-27
期刊：
影响因子：
3.9
通讯作者：
Geggel, Katrina
中科院分区：
文献类型：
--
作者：
Lunkenheimer, Klaus;Prescher, Dietrich;Geggel, Katrina
从实验平衡表面张力（σ（e））与浓度对数（c）等温线内的同源系列的全氟-正-链烷酸钠水溶液计算的饱和吸附的不寻常的行为表示在流体界面处的同源离子1：1两亲物的吸附中的一个特殊问题。采取了特殊的预防措施，以保证所有解决方案的表面化学纯度，避免表面活性微量杂质的伪造效果。令人惊讶的是，所有同系物的吸附等温线显示理想的表面行为。短链同系物的最小表面积需求每分子吸附略有下降，随着链长的增加，但然后急剧上升后，已通过最低。类似的特征已被观察到与化学上完全不同的烷基硫酸钠烃类表面活性剂的同源系列。比较边界层和本体中的相应3D饱和浓度，变得明显的是，在高本体浓度下，当边界层和本体浓度具有相同的数量级时，吸附行为可以被视为阴离子表面活性剂的吸附行为。然而，在同系物最强的表面活性的条件下，吸附似乎变得越来越受静电排斥，导致越来越大的横截面积。当德拜长度明显大于饱和吸附剂的直径时，偏离了离子行为。以前可用的两亲离子吸附理论要么处理表面活性剂离子和抗衡离子在吸附边界层的电条件，或者完全忽略了以前的理论与双离子行为。Warszynski等人的表面准二维电解质的新理论模型似乎能够描述离子两亲物在流体界面上的吸附。我们的结论是，两种替代方法的条件可以满足内的同源系列的离子两亲物作为限制情况下。
The unusual behavior of saturation adsorption calculated from experimental equilibrium surface tension (sigma(e)) versus logarithm of concentration (c) isotherms within the homologous series of aqueous sodium perfluoro-n-alkanoate solutions represents a particular problem in the adsorption of homologous ionic 1:1 amphiphiles at fluid interfaces. Special precautions were taken to guarantee surface-chemical purity for all solutions, avoiding falsifying effects by surface-active trace impurities. Surprisingly, all homologues adsorption isotherms reveal ideal surface behavior. The minimal surface area demand per molecule adsorbed for shorter-chain homologues slightly decreases with increasing chain lengths but then goes up steeply after having passed a minimum. A similar feature has been observed with the chemically quite different homologous series of the hydrocarbon surfactants of sodium-n-alkylsulfates. Comparing the corresponding 3D saturation concentrations in the boundary layer and in the bulk, it becomes evident that at high bulk concentrations when boundary layer and bulk concentrations are of the same order of magnitude the adsorption behavior may be treated as that of a pseudononionic surfactant. However, under conditions of the homologues strongest surface activity, adsorption seems to become increasingly governed by electrostatic repulsion, resulting in increasingly greater cross-sectional areas. Deviation from pseudononionic behavior sets in when the Debye length becomes distinctly greater than the adsorbents diameter at saturation. Formerly available theories on ionic amphiphiles adsorption deal either with electrical conditions of surfactant ions and counterions in the adsorption boundary layer or alternatively with pseudononionic behavior neglecting the former theories completely. Warszynski et al.s novel theoretical model of the surface quasi-two-dimensional electrolyte seems to be capable of describing the adsorption of ionic amphiphiles at fluid interfaces in general. We conclude that the conditions of the two alternative approaches may be met within homologous series of ionic amphiphiles as limiting cases only.