Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland.
Langmuir. 2011 Jun 7;27(11):7168-75. doi: 10.1021/la200644b. Epub 2011 May 4.
The role of surfactant type in the aggregation and gelation of strawberry-like particles induced by intense shear without any electrolyte addition is investigated. The particles are composed of a rubbery core, partially covered by a plastic shell, and well stabilized by fixed (sulfate) charges in the end group of the polymer chains originating from the initiator. In the absence of any surfactant, after the system passes through a microchannel at a Peclet number equal to 220 and a particle volume fraction equal to 0.15, not only shear-induced gelation but also partial coalescence among the particles occurs. The same shear-induced aggregation/gelation process has been carried out in the presence of an ionic (sulfonate) surfactant or a nonionic (Tween 20) steric surfactant. It is found that for both surfactants shear-induced gelation does occur at low surfactant surface density but the conversion of the primary particles to the clusters constituting the gel decreases as the surfactant surface density increases. When the surfactant surface density increases above certain critical values, shear-induced gelation and eventually even aggregation do not occur any longer. For the sulfonate surfactant, this was explained in the literature by the non-DLVO, short-range repulsive hydration forces generated by the adsorbed surfactant layer. In this work, it is shown that the steric repulsion generated by the adsorbed Tween 20 layer can also protect particles from aggregation under intense shear. Moreover, the nonionic steric surfactant can also protect the strawberry-like particles from coalescence. This implies a decrease in the fractal dimension of the clusters constituting the gel from 2.76 to 2.45, which cannot be achieved using the ionic sulfonate surfactant.
研究了在没有添加任何电解质的情况下，强烈剪切作用下表面活性剂类型对草莓状颗粒聚集和凝胶化的影响。这些颗粒由橡胶状核心组成，部分被塑料壳覆盖，并通过聚合物链末端的固定（硫酸盐）电荷稳定，这些电荷来自引发剂。在没有任何表面活性剂的情况下，当系统在 Peclet 数等于 220 和颗粒体积分数等于 0.15 的条件下通过微通道时，不仅会发生剪切诱导凝胶化，而且颗粒之间也会发生部分聚结。在存在离子（磺酸盐）表面活性剂或非离子（吐温 20）空间位阻表面活性剂的情况下，进行了相同的剪切诱导聚集/凝胶化过程。结果发现，对于两种表面活性剂，在低表面活性剂表面密度下都会发生剪切诱导凝胶化，但随着表面活性剂表面密度的增加，初级颗粒转化为构成凝胶的团簇的转化率降低。当表面活性剂表面密度增加到一定临界值以上时，剪切诱导凝胶化甚至聚集就不再发生。对于磺酸盐表面活性剂，文献中解释这是由于吸附的表面活性剂层产生的非-DLVO、短程排斥水合力所致。在这项工作中，表明吸附的吐温 20 层产生的空间排斥也可以在强烈剪切下保护颗粒不聚集。此外，非离子空间位阻表面活性剂还可以防止草莓状颗粒聚结。这意味着构成凝胶的团簇的分形维数从 2.76 降低到 2.45，而使用离子磺酸盐表面活性剂则无法实现这一降低。
