de Vos Wiebe M, Cattoz Beatrice, Avery Michael P, Cosgrove Terence, Prescott Stuart W
School of Chemistry, University of Bristol , Cantock's Close, Bristol BS8 1TS, U.K.
Langmuir. 2014 Jul 22;30(28):8425-31. doi: 10.1021/la501877v. Epub 2014 Jul 9.
Optical flow cell reflectometry was used to study the adsorption of poly(vinylpyrrolidone) (PVP) to a silica surface and the subsequent surfactant adsorption and polymer desorption upon exposure to the anionic surfactant sodium dodecyl sulfate (SDS). We have studied these effects as a function of pH and surfactant concentration, but also for two different methods of silica preparation, O2 plasma and piranha cleaning. As a function of pH, a plateau in the amount adsorbed of ∼0.6 mg/m(2) is observed below a critical pH, above which the adsorption decreases to zero within 2-3 pH units. An increase in pH leads to dissociation of surface OH groups and a decreased potential for hydrogen bonding between the polymer and surface. For the plasma- and piranha-cleaned silica, the critical pH differs by 1-2 pH units, a reflection of the much larger amount of surface OH groups on piranha-cleaned silica (for a given pH). Subsequent rinsing of the adsorbed layer of PVP with an SDS solution leads to total or partial desorption of the PVP layer. Any remaining adsorbed PVP then acts as an adsorption site for SDS. A large difference between plasma- and piranha-cleaned silica is observed, with the PVP layer adsorbed to plasma-cleaned silica being much more susceptible to desorption by SDS. For a plasma-cleaned surface at pH 5.5, only 30% of the originally adsorbed PVP is remaining, while for piranha-cleaned silica, the pH can be increased to 10 before a similar reduction in the amount of adsorbed PVP is seen. For a given pH, piranha-cleaned silica has a higher surface charge, leading to a smaller amount of adsorbed SDS per PVP chain on a piranha-cleaned surface compared to a plasma-cleaned surface under identical conditions. In that way, the high negative surface charge makes desorption by negatively charged SDS more difficult. The high surface charge thus protects the neutral polymer from surfactant-mediated desorption.
采用光流通池反射法研究了聚乙烯吡咯烷酮（PVP）在二氧化硅表面的吸附，以及随后在暴露于阴离子表面活性剂十二烷基硫酸钠（SDS）时表面活性剂的吸附和聚合物的解吸。我们研究了这些效应与pH值和表面活性剂浓度的关系，还研究了两种不同的二氧化硅制备方法，即O2等离子体处理和浓硫酸-过氧化氢混合溶液清洗。作为pH值的函数，在临界pH值以下观察到吸附量的平台期约为0.6 mg/m(2)，高于该临界pH值时，吸附量在2-3个pH单位内降至零。pH值升高会导致表面羟基解离，聚合物与表面之间氢键形成的可能性降低。对于经等离子体处理和浓硫酸-过氧化氢混合溶液清洗的二氧化硅，临界pH值相差1-2个pH单位，这反映了在给定pH值下，经浓硫酸-过氧化氢混合溶液清洗的二氧化硅表面羟基数量要多得多。随后用SDS溶液冲洗吸附的PVP层会导致PVP层全部或部分解吸。任何残留的吸附PVP随后都充当SDS的吸附位点。观察到经等离子体处理和浓硫酸-过氧化氢混合溶液清洗的二氧化硅之间存在很大差异，吸附在经等离子体处理的二氧化硅上的PVP层更容易被SDS解吸。对于pH值为5.5的经等离子体处理的表面，仅剩下30%最初吸附的PVP，而对于经浓硫酸-过氧化氢混合溶液清洗的二氧化硅，在吸附的PVP量出现类似减少之前，pH值可升高至10。在给定的pH值下，经浓硫酸-过氧化氢混合溶液清洗的二氧化硅具有更高的表面电荷，导致在相同条件下，与经等离子体处理的表面相比，经浓硫酸-过氧化氢混合溶液清洗的表面上每个PVP链吸附的SDS量更少。这样，高负表面电荷使得带负电荷的SDS更难实现解吸。因此，高表面电荷保护中性聚合物免受表面活性剂介导的解吸作用。
