Article pubs.acs.org/JPCB
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Dynamics of Molecules Physically Adsorbed onto Metal Oxide Nanoparticles: Similarities between Water and a Flexible Polymer Panagiotis A. Klonos* Department of Physics, National Technical University of Athens, Zografou Campus,15780, Athens, Greece
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ABSTRACT: Water at interfaces is considered present and important within almost all physical processes. Interfacial water shows nonbulk properties, however, its structure/organization and dynamics are on a still open debate. In parallel to that, the interfacial polymer in nanocomposites exhibits modified structure and dynamics, the latter characteristic being widely considered responsible for the improved nanocomposite properties. One of the most suitable tools for studying dynamics of a variety of small molecules and macromolecules is broadband dielectric spectroscopy (BDS), mainly due to its high resolving power. BDS is employed here in combination with isothermal hydration and has proved quite illuminating in recording for the first time striking similarities in the dynamics behavior between water molecules and flexible polydimethylsiloxane macromolecules, when the latter molecules are individually adsorbed physically on the attractive surfaces of metal oxide nanoparticle. The qualitative similarities between water and polymer interfacial dynamics, for example, in their exhibiting timescale and the elevation of the respective activation energy upon increasing of interfacial molecules fraction, lay most probably at the origins of the same main surface-adsorbed molecule interaction, i.e.. the hydrogen bonding. Whereas, the quantitative differences in molecular dynamics, e.g., slower, more energetically demanding, and cooperative interfacial dynamics for the polymer than for water, arise from the different scale of the two molecular motions (200 m2/g) based PNCs. The overall results suggest that the increase in hint and φint does not result in clear acceleration/deceleration of the respective dynamics but most probably in an altering in the interfacial layer structure (insets to Figure 3). For example, the increase in Eact without tremendous time-scale acceleration could correlate with an increasing in the thickness or mass density of the interfacial molecules layer. Most possibly, the recorded Eact elevation is due to the increase in the interfacial density, as upon increasing of hint/φint the interfacial molecules, water/chains (Scheme 1), are expected to approach with each other (insets to Figure 3). The latter is also the main reason for observing an additional contribution of φint to the cooperativity (fragility) of αint, also,in agreement with the Adam−Gibbs theory.12 The larger Eact and slower dynamics for interfacial polymer as compared to interfacial water can be rationalized also in terms of the interfacial layer thickness, i.e., ≥1 nm for the polymer9,12,21 and
