Self-consistent field modeling of poly(ethylene oxide) adsorption onto silica: the multiple roles of electrolytes.
In technological applications, it is increasingly important to understand and predict interfacial phenomena. Using a self-consistent field model within the Scheutjens-Fleer discretization scheme, we have developed a molecularly realistic model of the adsorption of poly(ethylene oxide) (PEO) onto silica from an aqueous solution. The bulk solution consists of water, PEO, 1:1 electrolyte, protons, and hydroxyl ions. The solvent quality is good below and becomes poor above a threshold ionic strength (of around 1 M). The silica surface features a number of silanol groups that have an affinity for PEO (by means of H bonding) when these groups are not dissociated. In line with experimental data, the surface changes from adsorbing to nonadsorbing at a sufficiently high pH. Even though PEO is uncharged, there is a complex effect of the ionic strength on the interfacial characteristics. For example, we report a non-monotonic behavior of the adsorbed amount as a function of ionic strength. Going from a low to a high ionic strength at a neutral or slightly basic pH, the adsorbed amount initially decreases as the surface affinity decreases (caused by the reduction of adsorption sites when, as a result of screening, the surface is increasingly charged) but then increases as a result of a reduction in solvent quality. These results indicate that molecularly realistic models can reveal much richer interfacial behavior than anticipated from generic models. The predictions follow many experimental findings.
Postmus
Bart R
BR
Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands. bart.postmus@wur.nl
Leermakers
Frans A M
FA
Stuart
Martien A Cohen
MA