by Keyword: Steric repulsion

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Obiols-Rabasa, M., Oncins, G., Sanz, F., Tadros, T. F., Solans, C., Levecke, B., Booten, K., Esquena, J., (2017). Investigation of the elastic and adhesion properties of adsorbed hydrophobically modified inulin films on latex particles using Atomic Force Microscopy (AFM) Colloids and Surfaces A: Physicochemical and Engineering Aspects 524, 185-192

Graft polymer surfactants provide very good colloidal stability because of strong steric repulsions between adsorbed surfactant films. The elastic and adhesion properties of adsorbed hydrophobically modified inulin polymer surfactant (INUTEC NRA) have been directly measured using Atomic Force Microscopy (AFM) measurements. For this purpose, poly(methyl methacrylate/butyl acrylate), P(MMA/BuA), latexes prepared in the presence of the hydrophobically modified inulin (INUTEC NRA) were used. These latexes (diameter 118 nm and polydispersity index of 1.05) showed a very high colloidal stability in water and in the presence of electrolyte (up to 0.2 mol dm−3 KBr). The latexes were deposited on mica, which was silanated to enhance the adhesion of the latex particles to the surface. A silicon nitride tip with approximately 10 nm diameter that also contained an adsorbed layer of surfactant was used in the AFM apparatus. The tip was allowed to approach, contact thereafter the particles with an applied force of 12.5 nN, and finally detach from the film. Both elastic (Young’s) modulus of the film and adhesion force were studied. The results showed that the adsorbed surfactant films are highly elastic and their elastic modulus and adhesion force did not change significantly with the presence of Na2SO4 up to 0.05 mol dm−3. The high elastic contribution to the steric interaction ensures strong repulsion between the latex particles both in water and at high electrolyte concentrations. In addition, the lack of dependence of adhesion force on electrolyte concentration ensures uniform deposition of the latex particles on a flat substrate as for example in coating applications. These results show the advantages of using a graft polymer surfactant for enhancing the stability of particle suspensions, as illustrated in previous investigations.

Keywords: AFM, Colloidal stability, Interaction forces, Steric repulsion