Stabilization of colloidal suspensions: competing effects of nanoparticle halos and depletion mechanism

Competitive effects of salt and surfactant on the structure of nanoparticles in a binary system of nanoparticle and protein

Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) experiments have been carried out to study the competitive effects of NaCl and sodium dodecyl sulfate (SDS) surfactant on the evolution of the structure and interactions in a silica nanoparticle-Bovine serum albumin (BSA) protein system. The unique advantage of contrast-matching SANS has been utilized to particularly probe the structure of nanoparticles in the multi-component system. Silica nanoparticles and BSA protein both being anionic remain largely individual in the solution without significant adsorption. The non-adsorbing nature of protein is known to cause depletion attraction between nanoparticles at higher protein concentrations. The nanoparticles undergo immediate aggregation in the nanoparticle-BSA system on the addition of a small amount of salt [referred as the critical salt concentration (CSC)], much less than that required to induce aggregation in a pure nanoparticle dispersion. The salt ions screen the electrostatic repulsion between the nanoparticles, whereby the BSA-induced depletion attraction dominates the system and contributes to the nanoparticle aggregation of a mass fractal kind of morphology. Further, the addition of SDS in this system interestingly suppresses nanoparticle aggregation for salt concentrations lower than the CSC. The presence of SDS gives rise to additional electrostatic repulsion in the system by binding with the BSA protein via electrostatic and hydrophobic interactions. For salt concentrations higher than the CSC, the formation of clusters of nanoparticles is inevitable even in the presence of protein-surfactant complexes, but the mass fractal kind of branched aggregates transform to surface fractals. This has been attributed to the BSA-SDS complex induced depletion attraction along with salt-driven screening of electrostatic repulsion. Thus, the interplay of depletion and electrostatic and hydrophobic interactions has been utilized to tune the structures formed in a multicomponent silica nanoparticle-BSA-SDS/NaCl system.

Total internal reflection microscopy: a powerful tool for exploring interactions and dynamics near interfaces

The occurrence of many micro/macrophenomena is closely related to interactions and dynamics near interfaces. Hence, developing powerful tools for characterizing near-interface interactions and dynamics has attached great importance among researchers. In this review, we introduce a noninvasive and ultrasensitive technique called total internal reflection microscopy (TIRM). The principles of TIRM are introduced first, demonstrating the characteristics of this technique. Then, typical measurements with TIRM and the recent development of the technique are reviewed in detail. At the end of the review, we highlight the great progress of TIRM during the past several decades and show its potential to be more influential in measuring interactions and dynamics near interfaces in various research fields.

Structural correlations in highly asymmetric binary charged colloidal mixtures

We explore structural correlations of strongly asymmetric mixtures of binary charged colloids within the primitive model of electrolytes considering large charge and size ratios of 10 and higher. Using computer simulations with explicit microions, we obtain the partial pair correlation functions between the like-charged colloidal macroions. Interestingly the big-small correlation peak amplitude is smaller than that of the big-big and small-small macroion correlation peaks, which is unfamiliar for additive repulsive interactions. Extracting optimal effective microion-averaged pair interactions between the macroions, we find that on top of non-additive Yukawa-like repulsions an additional shifted Gaussian attractive potential between the small macroions is needed to accurately reproduce their correct pair correlations. For small Coulomb couplings, the behavior is reproduced in a coarse-grained theory with microion-averaged effective interactions between the macroions. However, the accuracy of the theory deteriorates with increasing Coulomb coupling. We emphasize the relevance of entropic interactions exerted by the microions on the macroions. Our results are experimentally verifiable in binary mixtures of micron-sized colloids and like-charge nanoparticles.

Investigation of the concentration- and temperature-dependent motion of colloidal nanoparticles

Although the motion of a single nanoparticle suspended in a fluid can be easily modeled, things get complicated for non-infinitely diluted systems. Coincidentally, these are the systems of interest in relevant fields such as, nanomedicine, microfluidics and miniaturized energy storage devices. Hence, a better understanding of the dynamics of colloidal nanoparticles is utterly needed. Herein, the motion of colloidal suspension of plasmonic nanoparticles (i.e., gold nanoshells) is investigated via laser speckle imaging. The method relies on the analysis of the speckle pattern generated by colloidal suspensions forced to flow at specific velocities. Temperature-dependent measurements corroborated that the dynamics of non-infinitely diluted nanoparticle suspensions are better described through a diffusive model rather than by the equipartition theorem. Under the tested experimental conditions, an average diffusion velocity between 0.37 and 1.57 mm s-1 was found. Most importantly, these values were largely dependent on the nanoparticle concentration. These results are in agreement with previous reports and indicate the existence of long-range interactions between nanoparticles.

pH-Induced reorientation of cytochrome c on silica nanoparticles

The orientation of cytochrome c molecules at the surface of silica nanoparticles was studied in a wide pH range by combining small-angle neutron scattering, adsorption measurements, and molecular dynamics simulations. The results indicate a reorientation of the ellipsoidal protein from head-on to side-on as the pH is increased. This is attributed to changes in the surface charge distribution of both the protein and the nanoparticles.

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.

Effective electrostatic interactions in colloid-nanoparticle mixtures

Interparticle interactions and bulk properties of colloidal suspensions can be substantially modified by the addition of nanoparticles. Extreme asymmetries in size and charge between colloidal particles and nanoparticles present severe computational challenges to molecular-scale modeling of such complex systems. We present a statistical mechanical theory of effective electrostatic interactions that can greatly ease large-scale modeling of charged colloid-nanoparticle mixtures. By applying a sequential coarse-graining procedure, we show that a multicomponent mixture of charged colloids, nanoparticles, counterions, and coions can be mapped first onto a binary mixture of colloids and nanoparticles and then onto a one-component model of colloids alone. In a linear-response approximation, the one-component model is governed by a single effective pair potential and a one-body volume energy, whose parameters depend nontrivially on nanoparticle size, charge, and concentration. To test the theory, we perform molecular dynamics simulations of the two-component and one-component models and compute structural properties. For moderate electrostatic couplings, colloid-colloid radial distribution functions and static structure factors agree closely between the two models, validating the sequential coarse-graining approach. Nanoparticles of sufficient charge and concentration enhance screening of electrostatic interactions, weakening correlations between charged colloids and destabilizing suspensions, consistent with experiments.

Investigation on the use of graphene oxide as novel surfactant to stabilize weakly charged graphene nanoplatelets

This paper presents a unique synergistic behavior between a graphene oxide (GO) and graphene nanoplatelet (GnP) composite in an aqueous medium. The results showed that GO stabilized GnP colloid near its isoelectric point and prevented rapid agglomeration and sedimentation. It was considered that a rarely encountered charge-dependent electrostatic interaction between the highly charged GO and weakly charged GnP particles kept GnP suspended at its rapid coagulation and phase separation pH. Sedimentation and transmission electron microscope (TEM) micrograph images revealed the evidence of highly stable colloidal mixtures while zeta potential measurement provided semi-quantitative explanation on the mechanism of stabilization. GnP suspension was confirmed via UV-vis spectral data while contact angle measurement elucidated the close resemblance to an aqueous solution indicating the ability of GO to mediate the flocculation prone GnP colloids. About a tenfold increase in viscosity was recorded at a low shear rate in comparison to an individual GO solution due to a strong interaction manifested between participating colloids. An optimum level of mixing ratio between the two constituents was also obtained. These new findings related to an interaction between charge-based graphitic carbon materials would open new avenues for further exploration on the enhancement of both GO and GnP functionalities particularly in mechanical and electrical domains.

Tuning the particle-surface interactions in aqueous solutions by soft microgel particles

Due to the softness and deformability, interaction between colloidal surfaces induced by soft particles varies in a more complex way than for solid particles and thus has attracted much attention in recent years. In the present study, we use total internal reflection microscopy (TIRM) to directly measure the interaction between polystyrene (PS) microparticles and a flat glass surface in a poly(N-isopropylacrylamide) (PNIPAM) microgel dispersion with concentration varying from dilute (0.1 wt %) to highly concentrated regime (7.5 wt %). Our result shows that the PS particle-surface interactions mediated by the soft microgels are greatly affected by the particle concentration, the configuration of those microgels adsorbed on the surfaces, and the structure and packing of microgels in bulk solution. With increasing the microgel concentration (Cmicrogel), the interaction between the PS particle and surface turned from bridging attraction to steric repulsion, and then depletion attraction, which were mainly governed by the adsorption amount and configuration of microgels on the two surfaces. By further increasing Cmicrogel to condensed situation, structural force with oscillated energy wells was detected. The variation of interactions induced by the soft microgels was further confirmed by optical imaging. Crystallization of the PS microparticles appeared at moderate Cmicrogel; however, crystallization was hindered at higher Cmicrogel where the microgels are highly packed in the bulk solution. Furthermore, using TIRM, microgel packing with local energy well (0.1-1.0 kBT) at the highly condensed state (7.5 wt %) was resolved from the interaction profiles. Therefore, the shear force and modulus generated by such microgel packing can be determined as ∼0.2 pN and tens of mPa, respectively, which are much weaker than data given by conventional active methods.

Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation

This review summarizes the current understanding of adsorption of polyelectrolytes to oppositely charged solid substrates, the resulting interaction forces between such substrates, and consequences for colloidal particle aggregation. The following conclusions can be reached based on experimental findings. Polyelectrolytes adsorb to oppositely charged solid substrates irreversibly up to saturation, whereby loose and thin monolayers are formed. The adsorbed polyelectrolytes normally carry a substantial amount of charge, which leads to a charge reversal. Frequently, the adsorbed films are laterally heterogeneous. With increasing salt levels, the adsorbed mass increases leading to thicker and more homogeneous films. Interaction forces between surfaces coated with saturated polyelectrolyte layers are governed at low salt levels by repulsive electric double layer interactions, and particle suspensions are stable under these conditions. At appropriately high salt levels, the forces become attractive, principally due to van der Waals interactions, but eventually also through other forces, and suspensions become unstable. This situation can be rationalized with the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Due to the irreversible nature of the adsorption process, stable unsaturated layers form in colloidal particle suspensions at lower polyelectrolyte doses. An unsaturated polyelectrolyte layer can neutralize the overall particle surface charge. Away from the charge reversal point, electric double layer forces are dominant and particle suspensions are stable. As the charge reversal point is approached, attractive van der Waals forces become important, and particle suspensions become unstable. This behaviour is again in line with the DLVO theory, which may even apply quantitatively, provided the polyelectrolyte films are sufficiently laterally homogeneous. For heterogeneous films, additional attractive patch-charge interactions may become important. Depletion interactions may also lead to attractive forces and suspension destabilization, but such interactions become important only at high polyelectrolyte concentrations.

Direct measurements of particle–surface interactions in aqueous solutions with total internal reflection microscopy

This feature article reviews the experimental studies of the interactions between designed colloidal surfaces in the presence or absence of macromolecules/nanoparticles including depletion attraction, steric repulsion, bridging flocculation, and specific interactions by using Total Internal Reflection Microscopy.

Large counterions boost the solubility and renormalized charge of suspended nanoparticles

Colloidal particles are ubiquitous in biology and in everyday products such as milk, cosmetics, lubricants, paints, or drugs. The stability and aggregation of colloidal suspensions are of paramount importance in nature and in diverse nanotechnological applications, including the fabrication of photonic materials and scaffolds for biological assemblies, gene therapy, diagnostics, targeted drug delivery, and molecular labeling. Electrolyte solutions have been extensively used to stabilize and direct the assembly of colloidal particles. In electrolytes, the effective electrostatic interactions among the suspended colloids can be changed over various length scales by tuning the ionic concentration. However, a major limitation is gelation or flocculation at high salt concentrations. This is explained by classical theories, which show that the electrostatic repulsion among charged colloids is significantly reduced at high electrolyte concentrations. As a result, these screened colloidal particles are expected to aggregate due to short-range attractive interactions or dispersion forces as the salt concentration increases. We discuss here a robust, tunable mechanism for colloidal stability by which large counterions prevent highly charged nanoparticles from aggregating in salt solutions with concentrations up to 1 M. Large counterions are shown to generate a thicker ionic cloud in the proximity of each charged colloid, which strengthens short-range repulsions among colloidal particles and also increases the corresponding renormalized colloidal charge perceived at larger separation distances. These effects thus provide a reliable stabilization mechanism in a broad range of biological and synthetic colloidal suspensions.