Rheological and Light Scattering Studies of Cationic Fluorocarbon Surfactant Solutions at Low Ionic Strength

Abstract

The structural and dynamic properties of low ionic strength micellar solutions of the cationic surfactant perfluorooctylbutane trimethylammonium bromide have been investigated by means of dynamic and static light scattering and rheological experiments. In the range of concentration extending from the critical micelle concentration to about the overlap concentration, the surfactant self-assembles into micellar aggregates with sizes of the order of 100 nm. At higher concentrations, the system is characterized by a single correlation length consistent with the formation of a transient network of wormlike micelles. In the dilute regime, the solutions exhibit a very large shear thickening effect (up to 50) and a very slow recovery (hours) after a thermal or mechanical perturbation. The extent of consistency of the dynamical observations with the various possible structures inferred from the scattering data has been discussed. Some arguments are in favor of the presence of interlinked micellar rings in the quiescent state, which was suggested in a recent theoretical model.

Ring-driven shear thickening in wormlike micelles?

“…Importantly, recent work shows that much of the slow relaxation behavior observed in response to rheological disturbances can be reproduced by studying temperature changes (or even concentration changes [19]) instead [18]. This confirms that some important timescales for rheology stem directly from the micellar kinetics in the quiescent state, as probed in T -jump (or sudden dilution).…”

“…This effort has partly been initiated already [18], but a goal of this paper is to stimulate such an effort across a wider community, addressing a broader class of materials. Currently we are hopeful at least that some elements of our ring-driven shear thickening scenario may indeed apply to some classes of micellar materials.…”

“…We argue that this is not through statics, but through the micellar kinetics. While destabilizing end-caps relative to the central cylinder, adding salt should also reduce the activation energy for all three reaction schemes, both through moderation of the the Coulomb barrier, and through relative stabilization of the inverted curvature required for the transition state; there is some preliminary evidence for this trend [18]. As soon as one of the reactions becomes fast enough that τ link approaches τ z (milliseconds or less) the interlinking of rings will become effectively invisible and our shear thickening scenario should fade away.…”

“…After this time, the sample is strongly and (in many cases) uniformly birefringent, with a near-zero extinction angle (and, when measured, a large first normal stress difference) implying the presence of a condensate of extremely aligned material. The latency time τ lat gets larger as one approachesγ c from above [18], but in any case is usually much bigger thanγ −1 c , so that a large total strain γ c needs to be applied before the thickening occurs. (For example, τ lat ≃ 100s in [17].)…”

“…(For example, τ lat ≃ 100s in [17].) The latency period increases with the gap width of the shear cell [18] and in some cases, before it is over, there is clear spatial inhomogeneity [17,13].…”