Measurement of the kinetic rate constants for the adsorption of superspreading trisiloxanes to an air/aqueous interface and the relevance of these measurements to the mechanism of superspreading
Measurement of the kinetic rate constants for the adsorption of superspreading trisiloxanes to an air/aqueous interface and the relevance of these measurements to the mechanism of superspreading
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DOI：
10.1016/s0021-9797(03)00530-7
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发表时间：
2003-11-15
影响因子：
9.9
通讯作者：
Maldarelli, C
中科院分区：
文献类型：
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作者：
Kumar, N;Couzis, A;Maldarelli, C
超铺展三硅氧烷表面活性剂是一类两亲物，由非极性三硅氧烷头基 (((CH3)(3)-Si-O)(2)-Si(CH3)(CH2)(3)-) 和由 4 至 8 个环氧乙烷组成的极性部分（乙氧基化物，-OCH2CH2-）组成。浓度远高于临界聚集体浓度的毫米大小的三硅氧烷水溶液滴在非常疏水的表面上迅速扩散，在平衡时完全润湿。润湿可以理解为液滴前进周边处的三硅氧烷能够吸附在空气/水和水/疏水固体界面上并显着降低这些界面的张力，从而产生正铺展系数的结果。快速扩散可能是由于液滴扩散时在周边保持正扩散系数。然而，当液滴扩散时，由于界面膨胀，周边的空气/水和固体/水界面上的表面活性剂被耗尽。如果表面活性剂从周边铺展的水膜吸附到固体和流体表面上的速率超过由于面积膨胀而导致的稀释效应，则铺展系数可以保持正值。由于薄膜中的表面活性剂储量因吸附到膨胀的界面而不断耗尽，因此这项任务变得更加困难。如果吸附不能跟上周边的面积膨胀，并且接触线处的表面浓度降低，则可能会产生阻碍液滴运动的负扩散系数。然而，在这种情况下，如果假设液滴顶点处的表面浓度与周边相比保持较高，则马兰戈尼机制可以解释快速扩散，从而使周边处比顶点处更高的张力将液滴拉出。为了保持高的顶点浓度，表面活性剂的吸附必须超过由于向外流动而在顶点处的界面膨胀速率。这是可以想象的，因为与接触线处不同，液滴中心液体中的表面活性剂储量不会因吸附到膨胀的固体表面上而持续耗尽。为了了解快速扩散，我们测量了未聚集的三硅氧烷表面活性剂从底层到空气/水表面的吸附动力学速率常数。假设单体的本体浓度均匀且未消耗，计算出的吸附动力学速率代表在没有直接吸附聚集体的情况下表面活性剂单体可以吸附到空气/水表面上的最快速度。通过测量三硅氧烷吸附到干净的悬垂气泡界面上时的动态张力松弛来获得动力学常数。我们发现，动力学吸附速率仅与超级扩散中观察到的面积膨胀速率处于同一数量级，因此未聚集的通量不能在空气/水界面（无论是在顶点还是在周边）保持非常高的表面浓度。因此，为了保持正铺展系数或马兰戈尼梯度，需要增加表面活性剂吸附通量，并且建议直接吸附聚集体（在三硅氧烷的情况下是双层和囊泡）作为一种可能性。 (C) 2003 Elsevier Inc. 保留所有权利。
Super-spreading trisiloxane surfactants are a class of amphiphiles which consist of nonpolar trisiloxane headgroups (((CH3)(3)-Si-O)(2)-Si(CH3)(CH2)(3)-) and polar parts composed of between four and eight ethylene oxides (etboxylates, -OCH2CH2-). Millimeter-sized aqueous drops of trisiloxane solutions at concentrations well above the critical aggregate concentration spread rapidly on very hydrophobic surfaces, completely wetting out at equilibrium. The wetting out can be understood as a consequence of the ability of the trisiloxanes at the advancing perimeter of the drop to adsorb at the air/aqueous and aqueous/hydrophobic solid inter-faces and to reduce considerably the tensions of these interfaces, creating a positive spreading coefficient. The rapid spreading can be due to maintaining a positive spreading coefficient at the perimeter as the drop spreads. However, the air/aqueous and solid/aqueous interfaces at the perimeter are depleted of surfactant by interfacial expansion as the drop spreads. The spreading coefficient can remain positive if the rate of surfactant adsorption onto the solid and fluid surfaces from the spreading aqueous film at the perimeter exceeds the diluting effect due to the area expansion. This task is made more difficult by the fact that the reservoir of surfactant in the film is continually depleted by adsorption to the expanding interfaces. If the adsorption cannot keep pace with the area expansion at the perimeter, and the surface concentrations become reduced at the contact line, a negative spreading coefficient which retards the drop movement can develop. In this case, however, a Marangoni mechanism can account for the rapid spreading if the surface concentrations at the drop apex are assumed to remain high compared to the perimeter so that the drop is pulled out by the higher tension at the perimeter than at the apex. To maintain a high apex concentration, surfactant adsorption must exceed the rate of interfacial dilation at the apex due to the outward flow. This is conceivable because, unlike that at the contact line, the surfactant reservoir in the liquid at the drop center is not continually depleted by adsorption onto an expanding solid surface. In an effort to understand the rapid spreading, we measure the kinetic rate constants for adsorption of unaggregated trisiloxane surfactant from the sublayer to the air/aqueous surface. The kinetic rate of adsorption, computed assuming the bulk concentration of monomer to be uniform and undepleted, represents the fastest that surfactant monomer can adsorb onto the air/aqueous surface in the absence of direct adsorption of aggregates. The kinetic constants are obtained by measuring the dynamic tension relaxation as trisiloxanes adsorb onto a clean pendant bubble interface. We find that the rate of kinetic adsorption is only of the same order as the area expansion rates observed in superspreading, and therefore the unaggregated flux cannot maintain very high surface concentrations at the air/aqueous interface, either at the apex or at the perimeter. Hence in order to maintain either a positive spreading coefficient or a Marangoni gradient, the surfactant adsorptive flux needs to be augmented, and the direct adsorption of aggregates (which in the case of the trisiloxanes are bilayers and vesicles) is suggested as one possibility. (C) 2003 Elsevier Inc. All rights reserved.