Accounting for effective interactions among charged microgels.

We introduce a theoretical approach to describe structural correlations among charged permeable spheres at finite particle concentrations. This theory explicitly accounts for correlations among microions and between microions and macroions and allows for the proposal of an effective interaction among macroions that successfully captures structural correlations observed in poly-N-isopropyl acrylamide microgel systems. In our description the bare charge is fixed and independent of the microgel size, the microgel concentration, and the ionic strength, which contrasts with results obtained using linear response approximations, where the bare charge needs to be adapted to properly account for microgel correlations obtained at different conditions.

P-NIPAM in water-acetone mixtures: experiments and simulations.

The lower critical solution temperature (LCST) of poly-N-isopropylacrylamide (p-NIPAM) diminishes when a small volume of acetone is added to the aqueous polymer solution, and then increases for further additions, producing a minimum at a certain acetone concentration. Here this behavior is observed through the variation of the hydrodynamic radius RH of p-NIPAM microgels with temperature, measured by dynamic light scattering (DLS), when adding increasing amounts of acetone in the molar fraction range of 0.00 to 0.25. This size trend of microgels with temperature is well captured by all-atom molecular dynamic simulations, which are implemented for a single 30-mer, at similar solvent and temperature conditions. Both DLS measurements and simulations indicate that the shrunken state continuously augments its size with increasing acetone content. This, in turn, leads to a minimum of the globule-to-coil transition temperature, which should correspond to the minimum of the LCST. Furthermore, density profiles, as obtained by considering a membrane arrangement of oligomers, reveal a preferential interaction of the polymer with acetone to the detriment of water. We observe how the membrane loses water content as the temperature is increased while keeping a similar amount of acetone in its interior. This competition between water and acetone for the polymer surface plays a major role in the enthalpy driven dependence of the critical temperature with acetone concentration.

Evidence of electrostatic-enhanced depletion attraction in the structural properties and phase behavior of binary charged colloidal suspensions.

In this paper we study the structure and phase behavior of binary mixtures of charged particles at low ionic strength. Due to the large size asymmetry between both species, light scattering measurements give us access only to the partial static structure factor that corresponds to the big particles. We observe that the addition of small charged colloids produces a decrease of the main peak of the measured static structure factor and a shift to larger scattering vector values. This finding is in agreement with theory based on integral equations with the Hypernetted-Chain Closure (HNC) relation. The effective interaction between two big particles due to the presence of small particles is obtained by a HNC inversion scheme and used in numerical simulations that adequately reproduce the experiments. We find that the presence of small particles induces an electrostatic depletion screening among the big colloids, creating around them an exclusion zone for the small charged colloids greater than that caused in the case of neutral small colloids, which in turn augments the depletion effect.

Impact of volume transition on the net charge of poly-N-isopropyl acrylamide microgels.

We explore the electrostatic properties of poly-N-isopropyl acrylamide microgels in dilute, quasi-de-ionized dispersions and show that the apparent net charge of these thermosensitive microgels is an increasing function of their size, the size being conveniently varied by temperature. Our experimental results obtained in a combination of light scattering, conductivity, and mobility experiments are consistent with those obtained in Poisson-Boltzmann cell model calculations, effectively indicating that upon shrinking the number of counterions entrapped within the microgels increases. Remarkably, this behavior shows that the electrostatic energy per particle remains constant upon swelling or deswelling the microgel, resulting in a square root dependence of the net charge on the particle radius.

Three dimensional cross-correlation dynamic light scattering by non-ergodic turbid media.

We investigate dynamic light scattering by non-ergodic turbid media with an adapted version of the method proposed by Pusey and van Megen [Physica A 157, 705 (1989)]. Our formulation follows the derivation of the original method by extending it to the three dimensional cross-correlation scheme (3DDLS). The main finding is an expression to obtain the dynamic structure factor from light scattering that takes into account the system turbidity and the peculiarities of the 3D geometry. From 3DDLS measurements in well-controlled solid-like systems of different turbidity, we confirm that our results can be interpreted reasonably well by the theoretical approach described here. Good agreement is found with earlier reported results on similar systems.

Dynamic arrest in charged colloidal systems exhibiting large-scale structural heterogeneities.

Suspensions of charged liposomes are found to exhibit typical features of strongly repulsive fluid systems at short length scales, while exhibiting structural heterogeneities at larger length scales that are characteristic of attractive systems. We model the static structure factor of these systems using effective pair interaction potentials composed of a long-range attraction and a shorter range repulsion. Our modeling of the static structure yields conditions for dynamically arrested states at larger volume fractions, which we find to agree with the experimentally observed dynamics.

Renormalization in charged colloids: non-monotonic behaviour with the surface charge.

The static structure factor S(q) is measured for a set of deionized latex dispersions with different numbers of ionizable surface groups per particle and similar diameters. For a given volume fraction, the height of the main peak of S(q), which is a direct measure of the spatial ordering of latex particles, does not increase monotonically with the number of ionizable groups. This behaviour cannot be described using the classical renormalization scheme based on the cell model. We analyse our experimental data using a renormalization model based on the jellium approximation, which predicts the weakening of the spatial order for moderate and large particle charges.

Effective charges of colloidal particles obtained from collective diffusion experiments.

In this work, the collective diffusion coefficient of highly charged colloidal particles in dilute dispersions has been measured by means of dynamic light scattering. The possibility of obtaining valuable information about the particle charge from these data is looked into with the help of electrophoresis experiments. Our results suggest that this is possible in the case of slight or moderately interacting particles as long as experimental data are properly treated. For highly interacting colloids, however, such information could not be so reliable, presumably due to certain shortcomings of the experimental technique at low angle. The role of charge renormalization is also discussed in this work.