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.

We probe the roto-translational Brownian motion of optically anisotropic particles suspended in water with a simple and straightforward optical microscopy experiment that does not require positional or rotational particle tracking. We acquire a movie of the suspension placed between two polarizing elements and we extract the translational diffusion coefficient D T and the rotational diffusion coefficient D R from the analysis of the temporal correlation properties of the spatial Fourier modes of the intensity fluctuations in the movie. Our method is successfully tested with a dilute suspension of birefringent spherical colloidal particles obtained by polymerizing an emulsion of droplets of liquid crystal in a nematic phase, whose roto-translational dynamics is found to be well described by theory. The simplicity of our approach makes our method a viable alternative to particle tracking and depolarized dynamic light scattering.

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.

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.

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.Charged colloids have been extensively used as model systems to study the static and dynamic properties of strongly interacting particles in both in-and out-ofequilibrium states [1]. In many cases their phase behavior could be described by the well established DerjaguinLandau-Verwey-Overbeek theory [2]; a theory that considers the interactions between charged colloids to be determined by repulsive screened Coulomb interactions. More recently, however, experimental and theoretical investigations of highly charged colloidal suspensions showed evidence for the existence of effective long-range attractions between like-charged particles (for a review see [3]). In particular, structural investigations reporting gas-crystal and gas-liquid coexistence associated to the formation of voids [4] led to controversial discussions on the limitation of the Derjaguin-Landau-Verwey-Overbeek theory. Despite the enhanced efforts in understanding the origin of these phenomena, our comprehension of the phase behavior of highly charged colloidal systems remains far from complete. In particular, implications of effective attractions on the dynamical properties of charged colloids remain to be addressed.In this Letter we report on an experimental investigation of the volume fraction dependent structural and dynamic properties of charged liposome suspensions at quasideionized conditions. For all volume fractions investigated, the suspensions are characterized by structural heterogeneities at large length scales, while exhibiting typical features of strongly repulsive, disordered fluid systems at shorter length scales. We successfully model the static structure factor by effective pair interaction potentials that comprise a long-range attraction and a shorter range repulsion. This modeling reveals an unusual development of the effective particle-particle interactions with volume fraction. In particular, the position of the repulsive barrier ceases to decrease at large volume fractions, which causes a dynamic arrest of the system. Our findings indicate that in highly charged colloidal systems the formation of structural heterogeneities and the dynamic arrest have a common origin.Our liposomes are composed of phosphatidylserine (PS) and egg phosphatidylcholine (PC) at a ratio of PS=PC $ 1. They are prepared by using the technique described in Ref. [5], yielding unilamellar vesicles with a mean diameter of d ¼ 120 AE 12 nm as determined by static and dynamic li...

The wide range of applications of poly­(N-isopropylacrylamide) (PNIPAM) is based on the temperature dependence of its coil-to-globule transition, which strongly relies on the solvent. Here, we focus on the cononsolvency effect of PNIPAM oligomers in aqueous 1-propanol mixtures that is studied by molecular dynamics simulations of single chains and membrane-like arrangements. The complete phase diagram is sketched from the radius of gyration of the simulated oligomers, and it is compared to that obtained from the hydrodynamic radius of PNIPAM microgels, finding a good agreement. At the water-rich region, the decrease of the lower critical solution temperature (LCST) with increasing cosolvent concentration is independent of the polymer length and concentration. In this region, the radius of gyration of our simulated oligomer is strongly temperature dependent and a coil–globule transition temperature is easily captured. Conversely, at the alcohol-rich region, simulations show a monotonically increasing radius of gyration of the oligomer with alcohol concentration, a radius that is practically independent of temperature. This finding is in line with a polymer phase separation, showing an upper critical solution temperature (UCST) for a small cosolvent concentration window, which depends on the polymer length and concentration. Hence, in this case, polymer–polymer effective interactions are key to phase separation instead of its single chain conformation, contrasting with the coil–globule transition. Indeed, we find a soft-coil like structure for the simulated oligomer around a propanol molar fraction of 0.24, which is close to the mixture composition where the UCST phase transition is detected. Finally, in line with an UCST scenario, our simulated membrane turns unstable at high temperature and alcohol concentration.

This work deals with the short-time effective diffusion coefficient of charged and uncharged liposomes, measured ͑as a function of the volume fraction͒ using fiber optic dynamic light scattering. Particularly, we are interested in the interplay between electrostatic and hydrodynamic interactions on the diffusion of these lipid vesicles. Regarding the charged liposome, it has been found that the effective diffusion coefficient can be theoretically justified for volume fractions not exceeding certain critical value. In applying the theoretical approach, a surface charge has been obtained which is consistent with the electrokinetic characterization of the liposome. Regarding the uncharged liposome, the hard-sphere model seems to account for reasonably well the self-diffusion data. In addition, comparing the measurements of the short-time self-diffusion coefficient for both liposomes, we conclude that strong electrostatic forces ͑direct interactions͒ slow down diffusion processes.

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