Electrostatic forces are typically produced in low polarity solvents by the addition of surfactants or charge-control additives. Although widely used, there is no consensus on the mechanism by which surfactants control the level of particle charge. We report an investigation using highly sensitive, single particle optical microelectrophoresis measurements combined with a small-angle neutron scattering study to establish the mechanism of charging by the surfactant AOT in the nonpolar solvent n-dodecane. We show that polymer-grafted particles with no chemically bound surface charges only charge above the critical micellar concentration of the surfactant. The surface potential increases gradually with increasing surfactant concentration c, before finally saturating at high c. The increase in the surface potential is correlated to the amount of surfactant adsorbed onto the surface of the particle. Using deuterated AOT and contrast variation techniques, we demonstrate that the surfactant is adsorbed within the polymer layer surrounding the particle core, probably as individual molecules rather than surfactant aggregates. A simple thermodynamic model accounts for the concentration dependence of the observed surface potential.
The history dependence of the glasses formed from flow-melted steady states by a sudden cessation of the shear rateγ is studied in colloidal suspensions, by molecular dynamics simulations, and modecoupling theory. In an ideal glass, stresses relax only partially, leaving behind a finite persistent residual stress. For intermediate times, relaxation curves scale as a function ofγt, even though no flow is present. The macroscopic stress evolution is connected to a length scale of residual liquefaction displayed by microscopic mean-squared displacements. The theory describes this history dependence of glasses sharing the same thermodynamic state variables, but differing static properties.PACS numbers: 64.70. P-83.50.-v Materials are often produced by solidification from the melt, involving nonequilibrium quenches. This imprints a history-dependent microstructure that strongly affects macroscopic material properties. One example is residual stresses [1,2]: if particle configurations cannot fully relax to equilibrium, some of the stresses, arising in the presence of flow in the melt, persist in the solid.Small glass droplets (known as Prince Rupert's drops or Dutch tears since the 17th century) vividly display the effects of residual stresses [3]: they withstand the blow of a hammer onto their main body, but explode when the slightest damage is inflicted upon their tail (releasing the frozen-in stress network). Today, safety glass and "Gorilla glass" covers for smartphones are deliberately pre-stressed during production to strengthen them. A theoretical understanding of residual stresses and their microscopic origins is however still not achieved.We seek to understand generic mechanisms by which residual stresses arise. A convenient starting point is to investigate the stress relaxation σ(t) following the cessation of shear flow of rateγ, from a well-defined nonequilibrium stationary state (NESS). Such "mechanical quenches" are ubiquitous in soft matter, where pre-shear is applied to "rejuvenate" the otherwise ill-defined glassy state [4][5][6][7]. For these systems, the soft-glassy rheology model (SGR) [8] predicts asymptotic power laws that imply the relaxation of stresses to zero [9]. In the following, we will reserve the term residual stress to describe a finite, persistent stress remaining in the (ideal) glass even at arbitrarily large times after the cessation of flow.In addition to macroscopic rheology, we investigate the evolution of the microscopic dynamics as characterized by the waiting-time dependent mean-squared displacements (MSD). The latter reveal the dynamical shrinkage of shear-fluidized regions after cessation, and phenomena akin to, yet different from the intensely studied aging dynamics after thermal quenches [10,11].Experiments on a variety of colloidal suspensions, together with molecular-dynamics (MD) simulations, provide a coherent qualitative picture that can be rationalized by mode-coupling theory of the glass transition (MCT) [12] within the integration-through-transients (ITT) formalis...
Aggregate structures of two model surfactants, AOT and C12E5 are studied in pure solvents D2O, dioxane-d8 (d-diox) and cyclohexane-d12 (C6D12) as well as in formulated D2O/d-diox and d-diox/C6D12 mixtures. As such these solvents and mixtures span a wide and continuous range of polarities. Small-angle neutron scattering (SANS) has been employed to follow an evolution of the preferred aggregate curvature, from normal micelles in high polarity solvents, through to reversed micelles in low polarity media. SANS has also been used to elucidate the micellar size, shape as well as to highlight intermicellar interactions. The results shed new light on the nature of aggregation structures in intermediate polarity solvents, and point to a region of solvent quality (as characterized by Hildebrand Solubility Parameter, Snyder polarity parameter or dielectric constant) in which aggregation is not favored. Finally these observed trends in aggregation as a function of solvent quality are successfully used to predict the self-assembly behavior of C12E5 in a different solvent, hexane-d14 (C6D14).
A new approach to thicken dense liquid CO(2) is described using the principles of self-assembly of custom-made CO(2) compatible fluorinated dichain surfactants. Solutions of surfactants in CO(2) have been investigated by high-pressure phase behavior, small-angle neutron scattering (HP-SANS) and falling cylinder viscosity experiments. The results show that it is possible to control surfactant aggregation to generate long, thin reversed micellar rods in dense CO(2), which at 10 wt % can lead to viscosity enhancements of up to 90% compared to pure CO(2). This represents the first example of CO(2) viscosity modifiers based on anisotropic reversed micelles.
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