Micromechanics and contact forces of colloidal aggregates in the presence of surfactants.
We report measurements of the bending mechanics of colloidal aggregates consisting of poly(methyl methacrylate) (PMMA) flocculated with 250 mM MgCl2 in the presence of either pentaethylene glycol monododecyl ether (C(12)E(5)), a nonionic surfactant, or sodium dodecyl sulfate (SDS), an anionic surfactant. In the absence of surfactant, singly bonded aggregates exhibit a substantial bond rigidity, kappa(0), in the linear bending regime. With the addition of surfactant, the tangential restoring force between particles becomes weaker; aggregates exhibit nonlinear mechanics at a lower critical bending moment, M(c), and the bond rigidity decreases. The decrease in kappa(0) is related to the reduction of the surface energy of adhesion between particles, W(SL). We find that W(SL) decreases with increasing surfactant concentration below the critical micelle concentration (cmc). However, above the cmc, W(SL) remains constant within experimental error. These results confirm the relation between the bond rigidity and the surface energy of adhesion and clearly demonstrate that, on the basis of this relationship, surface-active agents provide a means of tuning the macroscopic elasticity and yield stress of colloidal gels. Last, the mechanics of the critical moment is consistent with the surfactant lowering the stress at which the contact line between the particles de-pins.
University of Delaware, Department of Chemical Engineering, Newark, DE 19716, USA.
John P
JP
Pantina
Eric M
EM
Furst