低离子强度水溶液中十二烷纳米液滴的表面电位
Electric surface potential of dodecane nano-drops in aqueous solutions of low ioinic strength
摘要：
虽然固体颗粒的表面电荷是其合成的直接结果，但悬浮油滴的表面电荷取决于周围分子的吸附平衡。盐的存在提高了水相的极性，有利于表面活性剂从水溶液中盐析出来并增加其表面过量。然而，电解质也屏蔽了液滴的表面电荷[Debye-Hückel，1923]。结果表明，静电表面电位随着离子强度的增加而增加，直到界面饱和，然后下降。这种行为产生了之前在水包十六烷纳米乳液中观察到的最大值[Urbina-Villalba，2013; 2015]。在这里，作为氯化钠浓度的函数的两个十二烷在水中（d/w）的纳米乳液的表面电位的变化进行评估。正如预期，浓度为0.5和7.5 mM十二烷基硫酸钠（SDS）时获得最大值。然而，表面过量的十二烷滴显示的宏观吸附等温线预测的吸附平衡之间的中间行为，和以前发现的十六烷滴。通过在5分钟内监测分散体的平均半径的变化来跟踪所制备的乳液的稳定性，这是聚集速率的典型评价中所用时间的6倍。在7.5 mM SDS的情况下，最小的尺寸变化与发现的最大表面电位（40 mM NaCl）一致。对于0.5 mM SDS未观察到这一点。三个制度的不稳定被发现存在取决于水相的离子强度。它们对应于以下的普遍性：1）溶解/成熟，2）聚集，和3）晶体沉淀。
英文摘要：
While the surface charge of solid particles is a direct consequence of their synthesis, the one of suspended oil drops depends on the adsorption equilibrium of the surrounding molecules. The presence of salt raises the polarity of the water phase, favoring the salting out of the surfactant from the aqueous solution and increasing its surface excess. Yet, the electrolyte also screens the resulting surface charge of the drops [Debye-Hückel, 1923]. As a result, the electrostatic surface potential increases with the ionic strength until the saturation of the interface and then decreases. This behavior produces a maximum previously observed in hexadecane-in-water nanoemulsions [Urbina-Villalba, 2013; 2015]. Here, the variation of the surface potential of two dodecane-in-water (d/w) nanoemulsions is evaluated as a function of the sodium chloride concentration. As expected, maximum values are obtained for concentrations of 0.5 and 7.5 mM sodium dodecylsulfate (SDS). However, the surface excess of dodecane drops shows an intermediate behavior between the adsorption equilibrium predicted by macroscopic adsorption isotherms, and the one previously found on hexadecane drops. The stability of the prepared emulsions was followed monitoring the change in the average radius of the dispersions during five minutes, six times the lapse of time employed in a typical evaluation of the aggregation rate. In the case of 7.5 mM SDS, the smallest change in size coincides with the maximum surface potential found (40 mM NaCl). This is not observed for 0.5 mM SDS. Three regimes of destabilization are found to exist depending on the ionic strength of the aqueous phase. They correspond to the prevalence of: 1) solubilization/ripening, 2) aggregation, and 3) crystal precipitation.
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