Wang Hongfang, Evans Drew, Voelcker Nicolas H, Griesser Hans J, Meagher Laurence
Future Industries Institute , University of South Australia , Mawson Lakes , South Australia 5095 , Australia.
Department of Materials Science and Engineering , Monash University , Clayton , Victoria 3800 , Australia.
Langmuir. 2019 Sep 10;35(36):11679-11689. doi: 10.1021/acs.langmuir.9b02176. Epub 2019 Aug 26.
Manipulating the surface properties of materials via the application of coatings is a widely used strategy to achieve desired interfacial interactions, implicitly assuming that the interfacial forces of coated samples are determined exclusively by the surface properties of the coatings. However, interfacial interactions between materials and their environments operate over finite length scales. Thus, the question addressed in this study is whether interactions associated with bulk substrate materials could act through thin coatings or, conversely, how thick a coating needs to be to completely screen subsurface forces contributed by underlying substrates. Plasma polymer layers were deposited on silicon wafer substrates from ethanol vapor, with identical chemical composition, ultrasmooth surfaces, and varying thicknesses. Using colloid-probe atomic force microscopy, electrical double-layer forces were determined in solutions of various ionic strengths and fitted using the Derjaguin-Landau-Verwey-Overbeek theory. For the thicker ethanol plasma polymers, the fitted surface potentials reflected the presence of surface carboxylate groups and were invariant with thickness. In contrast, for coatings <18 nm thick, the surface potentials increased steadily with decreasing film thickness; the measured electrical double-layer forces contained contributions from both the coating and the substrate. Theoretical calculations were in agreement with this model. Thus, our observations indicate that the higher surface potential of the underlying SiO surface can influence the interactions between a colloid particle and the multilayer structure if coatings are sufficiently thin. Such superposition needs to be factored into the design of coatings aimed at the control of material interactions via surface forces.
通过施加涂层来操纵材料的表面性质是一种广泛使用的策略，以实现所需的界面相互作用，这隐含地假设涂层样品的界面力完全由涂层的表面性质决定。然而，材料与其环境之间的界面相互作用在有限的长度尺度上起作用。因此，本研究解决的问题是，与块状基底材料相关的相互作用是否可以通过薄涂层起作用，或者相反，涂层需要多厚才能完全屏蔽下层基底贡献的次表面力。从乙醇蒸汽中在硅片基底上沉积等离子体聚合物层，这些聚合物层具有相同的化学成分、超光滑表面和不同的厚度。使用胶体探针原子力显微镜，在各种离子强度的溶液中测定双电层力，并使用德亚金-朗道-韦弗伊-奥弗贝克理论进行拟合。对于较厚的乙醇等离子体聚合物，拟合的表面电位反映了表面羧酸盐基团的存在，并且不随厚度变化。相比之下，对于厚度小于18 nm的涂层，表面电位随着膜厚度的减小而稳步增加；测得的双电层力包含涂层和基底的贡献。理论计算与该模型一致。因此，我们的观察结果表明，如果涂层足够薄，下层SiO表面较高的表面电位会影响胶体颗粒与多层结构之间的相互作用。在旨在通过表面力控制材料相互作用的涂层设计中，需要考虑这种叠加。
