Anakhov Mikhail V, Gumerov Rustam A, Richtering Walter, Pich Andrij, Potemkin Igor I
Physics Department, Lomonosov Moscow State University, Leninskie Gory 1-2, Moscow 119991, Russian Federation.
DWI - Leibniz Institute for Interactive Materials e.V., Forckenbeckstraße 50, Aachen 52056, Germany.
ACS Macro Lett. 2020 May 19;9(5):736-742. doi: 10.1021/acsmacrolett.0c00191. Epub 2020 May 4.
It is known that microgels can serve as soft, permeable and stimuli-responsive alternative of solid colloidal particles to stabilize oil-water emulsions. The driving force for the adsorption of the microgels on interface of two immiscible liquids is a shielding of unfavorable oil-water contacts by adsorbed subchains, that is, the decrease of the surface tension between the liquids. Such phenomenon usually proceeds if volume fractions of the two liquids are comparable with each other and the microgel concentration is not high enough. The natural question arises: what is going on with the system in the opposite case of strongly asymmetric mixture (one of the liquids (oil) has a very small fraction) or high microgel concentration (the overall volume of the microgels exceeds the volume of the minor oil component)? Here we demonstrate that the microgels uptake the oil whose concentration within the microgels can be orders of magnitude higher than outside, leading to the additional microgel swelling (in comparison with the swelling in water). Thus, the microgels can serve as scavengers and concentrators of liquids dissolved in water. At first glance, this effect seems counterintuitive. However, it has a clear physical reason related to the incompatibility of oil and water. Absorption of the oil by microgels reduces unfavorable oil-water contacts by microgel segments: the microgels have a higher concentration of the segments at the periphery, forming a shell. The microgels with uptaken oil are stable toward aggregation at very small oil concentration in the mixture. However, an increase in the oil concentration can lead to aggregation of the microgels into dimers, trimers, and so on. The increasing concentration of oil mediates the attraction between the microgels: the oil in the aggregates appears to be localized in-between the microgels instead of their interior, which is accompanied by the release of the elastic stress of the microgels. A further increase in the oil concentration results in a growth of the size of the oil droplets between the microgels and the number of the microgels at the droplet's periphery, that is, the emulsion is formed.
已知微凝胶可作为固体胶体颗粒的柔软、可渗透且对刺激有响应的替代品，用于稳定油水乳液。微凝胶在两种不混溶液体界面上吸附的驱动力是被吸附的亚链对不利的油水接触的屏蔽作用，即液体之间表面张力的降低。这种现象通常在两种液体的体积分数彼此相当且微凝胶浓度不够高时发生。自然而然会产生这样一个问题：在强烈不对称混合物（两种液体之一（油）的比例非常小）或高微凝胶浓度（微凝胶的总体积超过次要油成分的体积）的相反情况下，系统会发生什么？在此我们证明，微凝胶会吸收油，微凝胶内部油的浓度可能比外部高几个数量级，从而导致微凝胶额外膨胀（与在水中的膨胀相比）。因此，微凝胶可作为溶解在水中的液体的清除剂和浓缩剂。乍一看，这种效应似乎有违直觉。然而，它有一个与油和水不相容相关的明确物理原因。微凝胶对油的吸收通过微凝胶链段减少了不利的油水接触：微凝胶在外围链段浓度较高，形成一个壳层。在混合物中油浓度非常低时，吸收了油的微凝胶对聚集是稳定的。然而，油浓度的增加会导致微凝胶聚集成二聚体、三聚体等。油浓度的增加介导了微凝胶之间的吸引力：聚集体中的油似乎位于微凝胶之间而不是其内部，这伴随着微凝胶弹性应力的释放。油浓度的进一步增加会导致微凝胶之间油滴尺寸的增大以及油滴周边微凝胶数量的增加，即形成乳液。
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