Aqueous Suspensions of Poly(ethylene Glycol)/Pyrocarbon/Fumed Silica

Aqueous suspensions of fumed silica and pyrocarbon/silica (CS) in the presence of dissolved poly(ethylene glycol) (PEG) were studied using (1)H NMR spectroscopy with freezing-out of bulk water and quantum chemical computations of the chemical shifts. The freezing effect for PEG/water is akin to that for low-molecular organics, as formation of solid phases of water (ice) and PEG occurs, and their mixture forms at the eutectic temperature. In the aqueous suspensions of fumed silica or CS, PEG molecules are localized at the solid-liquid interfaces and do not form the bulk solution even at large concentrations; however, the amount of bulk undisturbed water rises due to formation of the immobilized PEG layer. For such suspensions of silica or CS at a low amount of pyrocarbon (C(C)=4 wt%), there is a portion of the graph of the surface free energy (gamma(S)) increasing nearly linearly with the PEG concentration (C(PEG)); however, in the case of large C(C)=40 wt% in CS, a similar effect is not observed, as gamma(S) is maximal at low C(PEG)=0.1 wt%. Copyright 2001 Academic Press.