Adsorption and Surfactant-Mediated Desorption of Poly(vinylpyrrolidone) on Plasma- and Piranha-Cleaned Silica Surfaces

Abstract

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.

Adsorption of charged and neutral polymer chains on silica surfaces: The role of electrostatics, volume exclusion, and hydrogen bonding

“…Our model correctly describes the adsorption to both types of silica surfaces with only the density of ionizable groups as an adjustable parameter. This is in accordance with the hypothesis derived from contact angle measurements that the difference in cleaning procedure leads to different densities of SiOH groups on the surface [25]: a plasma-cleaned surface will have mostly siloxane groups (Si-O-Si) at the silicon-air interface, whereas a piranha-cleaned surface has (up to four times) more silanol groups (SiOH).…”

“…Here we apply this equation of state for the first time to the case of polymer adsorption. Our model based on hydrogen bonding between Si-OH surface groups and the polymer chains correctly predicts the experimentally measured pH-dependent adsorption-desorption isotherm of a neutral model polymer, poly(vinyl pyrrolidone) (PVP) to silica [25]. The adsorbed amount of PVP decays sharply for pH pK a,s , in good agreement with experimental data, due to the combined deprotonation of Si-OH groups and excluded volume interactions between polymer chains and ions that accumulate close to the charging silica surface.…”

“…The experimental procedures are described in detail in Ref. [25]. Plasma-cleaned surfaces exhibited a static water (Milli-Q) contact angle θ ≈ 30 • , whereas piranha-cleaned surfaces had a static contact angle θ < 10 • .…”

“…Van der Beek et al showed that, as a consequence, the adsorption to silica of the two model polymers, PVP and PEO, depends on the pH as well, with the adsorbed amount dropping sharply between pH 9 and 11 [16]. Others showed that an increase in ionic strength leads to a decrease in the adsorbed amount for both PEO [24] and PVP [25]. Hydrogen bonding with Si-OH groups is the driving force for the adsorption of both PEO and PVP to silica [13,16].…”

“…Recently, De Vos et al showed that the adsorption of PVP to a silica surface also depends on the method used to clean and prepare the silica surface [25]. For an O 2 plasma-cleaned silica surface, a drop in polymer adsorption was observed between pH 7 and 10, while for piranha-cleaned silica surface the drop was observed between pH 9 and 11.…”