Measuring the adsorption, desorption and complexation of surfactants, polymers and nanoparticles is key to understanding the stability and behaviour of real formulations with different behaviours being observed depending on the specific materials involved. In combination with traditional colloid characterisation techniques, we will illustrate the use of solvent relaxation nuclear magnetic resonance (NMR) to characterise nanoparticle surfaces in detail. Solvent relaxation NMR can be used to study the adsorption and desorption of polymer onto surfaces in quite complex samples, since each environment has a different contribution to the overall average relaxation rate that is measured. The availability of a bench-top NMR with which to perform these measurements permits detailed studies such as the construction of a pseudo-isotherm for the adsorption of polymers to nanoparticles.

Analysis of the surfactant-mediated desorption of polymer from the silica–water interface illustrates the technique; the binding of charged micelles to the adsorbed polymer leads first to an elongation of the polymer chains and a reduction in the near-surface polymer concentration followed, at higher concentrations of surfactant, by desorption of the polymer. The addition of a non-ionic surfactant leads to the formation of mixed micelles and reverses this process.

The competition for stabiliser between different types of particles (silica, alumina, titania) can also be monitored. For the case where poly(vinyl pyrrolidone) (PVP) is adsorbed onto an alumina-modified silica particle, the NMR technique shows that the addition of bare silica particles causes the PVP to desorb from the aluminate-modified silica and instead adsorb to the pure silica.

Keywords: polymer, surfactant, colloid, adsorption, competition, NMR.

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Last edited: Friday September 10, 2010

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