Attractions in sterically stabilized silica dispersions

Computer simulations of self-assembly of anisotropic colloids

Computer simulations have played a significant role in understanding the physics of colloidal self-assembly, interpreting experimental observations, and predicting novel mesoscopic and crystalline structures. Recent advances in computer simulations of colloidal self-assembly driven by anisotropic or orientation-dependent inter-particle interactions are highlighted in this review. These interactions are broadly classified into two classes: entropic and enthalpic interactions. They mainly arise due to shape anisotropy, surface heterogeneity, compositional heterogeneity, external field, interfaces, and confinements. Key challenges and opportunities in the field are discussed.

Stabilizing Colloidal Particles against Salting-out by Shortening Surface Grafts

A dramatic improvement is reported in the stability of colloidal particles when stabilizing surface grafts are systematically shortened from small polymers to single monomers. The colloidal dispersions consist of fluorinated latex particles, exhibiting a weak van der Waals attraction, with grafted steric layers of poly(ethylene glycol) (PEG) of different chain lengths. Using an effective salting-out electrolyte, Na₂CO₃, particle aggregates are detected above a threshold salt concentration that is independent of the particle concentration. The results are interpreted in terms of a sudden onset of nondispersibility of single particles, triggered by the solvent not completely wetting particle surfaces. By decreasing the PEG chain length, the threshold salt concentration is found to increase sharply. For grafts with just a single ethylene glycol group, dispersions remain stable up to exceedingly high concentrations of Na₂CO₃. However, on removal of the surface coverage altogether, the classical stability behavior of charge-stabilized dispersions is recovered. The behavior can be captured by a simple model that incorporates effective polymer-solvent interactions in the presence of an electrolyte.

Quantification of nanoparticle interactions in pure solvents and a concentrated PDMS solution as a function of solvent quality

Using turbidity measurements, we quantified the interactions between PDMS-grafted silica nanoparticles (PDMS-g-silica) in pure solvents and a concentrated polymer solution with a focus on detecting the impact of solvent quality on graft layer stretching. This work is an extension of our previous work where we showed that interfacial wetting of the grafted polymer leads to depletion restabilization in semidilute and concentrated polymer solutions in good solvents (Dutta, N.; Green, D. Langmuir 2008, 24, 5260-5269). Subsequently, we showed that the criterion for depletion restabilization holds for both good and marginally poor solvents (Dutta, N.; Green, D. Langmuir 2010, 26, 16737-16744). In this work, we quantified nanoparticle interactions in terms of the second virial coefficient (B2), which captures the stretching of the brush in a good solvent in comparison to compression in a poor solvent. The transition from stretching to compression of the graft layer as a function of solvent quality was also supported by self-consistent mean-field (SCF) calculations. The PDMS-g-silica nanoparticles in a concentrated polymer solution in a good solvent within the complete wetting region behaved as though they were in a good solvent rather than in a polymer melt where on the basis of the SCF calculations the graft layers were expected to behave ideally. Overall, our results indicate that turbidity measurements can be used to determine the second virial coefficients for polymer-grafted nanoparticles in solvents and concentrated polymer solutions, and the relative values of the coefficients correspond well to those from theoretical calculations.

Impact of solvent quality on nanoparticle dispersion in semidilute and concentrated polymer solutions

We investigated how solvent quality affects the stability of polymer-grafted nanoparticles in semidilute and concentrated polymer solutions, which extends our previous studies on these types of dispersions in good solvents [Langmuir 2008, 24, 5260-5269]. As discussed in the current article, dynamic light scattering (DLS) was used to quantify the diffusion of polydimethylsiloxane-grafted silica nanoparticles, or PDMS-g-silica, in bromocyclohexane as well as in PDMS/bromocyclohexane solutions. We established that bromocyclohexane is a theta solvent for PDMS by varying the temperature of the solutions with PDMS-g-silica nanoparticles and detecting their aggregation at a theta temperature of T(Θ) = 19.6 °C. Using this temperature as a benchmark for the transition between good and bad solvent conditions, further stability tests were carried out in semidilute and concentrated polymer solutions of PDMS in bromocyclohexane at T = 10-60 °C. Irrespective of temperature, i.e., solvent quality, we found that the nanoparticles dispersed uniformly when molecular weight of the graft polymer was greater than that of the free polymer. However, when the free polymer molecular weight was greater than that of the graft polymer, the nanoparticles aggregated. Visual studies were also used to confirm the correspondence between nanoparticle stability and graft and free polymer molecular weights in a wide range of marginally poor solvents with PDMS. Further, the correspondence between nanoparticle stability and instability with graft and free polymer molecular weight and solvent quality was also supported with self-consistent mean-field calculations. Thus, by relating experiment and theory, our results indicate that nanoparticle stability in semidilute and concentrated polymer solutions is governed by interactions between the graft and free polymers under conditions of variable solvency.

Connecting nanoscale motion and rheology of gel-forming colloidal suspensions

We report a combined x-ray photon correlation spectroscopy and rheometry study of moderately concentrated suspensions of silica colloids that form a gel on cooling. During gel formation, the suspensions acquire a shear modulus that increases with time, while the thermal motion of the colloids becomes localized over an increasingly restricted range. The nanometer-scale localization length characterizing this motion obeys an exact relationship with the shear modulus predicted theoretically from mode coupling calculations [K. S. Schweizer and G. Yatsenko, J. Chem. Phys. 127, 164505 (2007)]. This scaling thus demonstrates a direct quantitative connection between the microscopic dynamics and macroscopic rheology. It further indicates the importance of local structure over longer-range correlations in dictating the dynamical and mechanical properties of such gels.

Sedimentation in nano-colloidal dispersions: effects of collective interactions and particle charge

This is a review paper summarizing the progress of the development of colloidal sedimentation models for monodisperse, bidisperse, and polydisperse nanoparticle dispersions. This topic is of considerable interest because the sedimentation behavior of nanoparticles plays an important role in many practical systems, such as industrial coatings, optical products, ceramics, paints, dyes, and cosmetics. The limitations of various models are discussed. Multi-particle systems are highlighted, with a focus on the collective thermodynamic interactions resulting in the attractive depletion and repulsive structural forces. The effects of the particle concentration, particle charge, polydispersity in size, and electrolyte concentration on the sedimentation process are briefly summarized. Our contributions to this subject are reviewed.

Thermal diffusion behavior of hard-sphere suspensions

We studied the thermal diffusion behavior of octadecyl coated silica particles (R(h)=27 nm) in toluene between 15.0 and 50.0 degrees C in a volume fraction range of 1%-30% by means of thermal diffusion forced Rayleigh scattering. The colloidal particles behave like hard spheres at high temperatures and as sticky spheres at low temperatures. With increasing temperature, the obtained Soret coefficient S(T) of the silica particles changed sign from negative to positive, which implies that the colloidal particles move to the warm side at low temperatures, whereas they move to the cold side at high temperatures. Additionally, we observed also a sign change of the Soret coefficient from positive to negative with increasing volume fraction. This is the first colloidal system for which a sign change with temperature and volume fraction has been observed. The concentration dependence of the thermal diffusion coefficient of the colloidal spheres is related to the colloid-colloid interactions, and will be compared with an existing theoretical description for interacting spherical particles. To characterize the particle-particle interaction parameters, we performed static and dynamic light scattering experiments. The temperature dependence of the thermal diffusion coefficient is predominantly determined by single colloidal particle properties, which are related to colloid-solvent molecule interactions.

Rheological properties of nanopowder alumina coated with adsorbed fatty acids

The rheological properties of a nanosized alumina powder coated with fatty acid steric stabilizers of varying chain length were investigated. The storage and loss moduli of the complex modulus were measured to characterize the behavior of the flocculated systems. As chain length increased, there was a transition from an elastic response to fluid behavior. However, the fluid system developed elastic characteristics at relatively low volume fractions of 22%. The length of the steric barrier required to produce the fluid dispersion was estimated to be approximately 2 nm and correlates with attractive interactions on the order of the system thermal energy. Moreover, in the flocculated systems, the storage modulus was found to be higher than reported previously in the literature. These higher values were related to the additional attractive forces due to van der Waals attractions between the hydrocarbon tails of the adsorbed fatty acid layers.

Assessment of Small-Angle and Angle-Averaged Structure Factor for Monitoring Electrostatic Colloidal Interactions Using Multiply Scattered Light

The isotropic scattering coefficients of 143-nm diameter polystyrene latex suspensions were measured using frequency-domain photon migration (FDPM) at 687 and 828 nm as a function of volume fraction (0.05-0.3) and ionic strength (1.0 to 120 mM NaCl equivalents) in order to derive the angle-integrated structure factor, S(q), and structure factor at zero wave vector, S(0). The effective surface charges of the dispersions were estimated by fitting the measured isotropic scattering coefficients at each wavelength as a function of volume fraction to the solution of the Orstein-Zernike integral equation using the hard sphere Yukawa potential model and mean spherical approximation as a closure relation. The estimates of surface charges were comparable at both wavelengths, but decreased with ionic strength. At 120 mM NaCl equivalents, the values of S(0) obtained from FDPM matched those predicted by the Percus-Yevick model, and decreased with volume fraction, consistent with prediction by the Carnahan-Starling equation.

Interaction potentials, structural ordering and effective charges in dispersions of charged colloidal particles

As colloidal dispersions of charged particles exhibit a wide variety of commercial, technological and scientific applications, a considerable theoretical effort has been devoted to finding an effective interaction potential from primitive models. The forces derived from this potential should justify the spatial ordering experimentally observed under certain conditions. This paper reviews the advances in these theoretical studies as well as some experiments (based on the mentioned order) that try to corroborate them. Special attention has been paid to the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential. Nowadays, many of these theoretical investigations suggest that it could be applied if some of its parameters are renormalized. Nevertheless, to achieve a renormalization procedure in a strict way (from a primitive model) is a difficult task as a result of the size and charge asymmetries between small ions and macroions. Thus, several procedures for computing renormalized charges in a simple way have been developed. However, the notion of effective charge has also been widely used (as a adjustable parameter) in order to justify results found for several kinds of colloids (like solid particle dispersions or micellar systems) by means of quite different experimental techniques. Renormalization (as well as ion condensation) approaches, experiments and the controversial relationship between theoretical and phenomenological effective charges are also reviewed in this work.

Probing Electrostatic Forces in Colloidal Suspensions through Turbidity Data

This paper deals with the use of turbidimetry to obtain information on forces between negatively charged polymeric particles at very low ionic strength. The specific turbidity of deionized colloidal suspensions is measured and compared with the values obtained for suspensions of noninteracting particles. From these data, the integrated structure factors are experimentally obtained for several latex samples. These functions are fitted using the repulsive part of the DLVO potential, the Ornstein-Zernike equation, and the hyper-netted chain closure, which enables us to interpret the results in terms of an effective charge. The reliability of these values is discussed. In addition, a comparison between turbidimetry and light scattering data is presented. Copyright 1999 Academic Press.

Interactions between Silica Colloids with Magnetite Cores: Diffusion, Sedimentation and Light Scattering

The magnetic and Van der Waals attraction between uncharged silica spheres with a magnetic core has been studied using sedimentation and static and dynamic light scattering techniques. Specifically the effect of the interactions on the concentration dependence of the sedimentation velocity, diffusion, and the apparent radius of gyration was investigated. It was found experimentally that the concentration dependence is decreased significantly as a result of the Van der Waals and magnetic attractions and even may change sign in comparison to the hard-sphere case. Calculations of the (linear) concentration dependence for weak interactions predict this decrease and also indicate that for the silica-magnetite particles the second virial coefficient passes a maximum with increasing silica layer thickness. Copyright 1999 Academic Press.