Electrostatic interaction in macro-ionic solutions and gels

Colloidal Clusters and Networks Formed by Oppositely Charged Nanoparticles with Varying Stiffnesses

Colloidal clusters and gels are ubiquitous in science and technology. Particle softness has a strong effect on interparticle interactions; however, our understanding of the role of this factor in the formation of colloidal clusters and gels is only beginning to evolve. Here, we report the results of experimental and simulation studies of the impact of particle softness on the assembly of clusters and networks from mixtures of oppositely charged polymer nanoparticles (NPs). Experiments were performed below or above the polymer glass transition temperature, at which the interaction potential and adhesive forces between the NPs were significantly varied. Hard NPs assembled in fractal clusters that subsequently organized in a kinetically arrested colloidal gel, while soft NPs formed dense precipitating aggregates, due to the NP deformation and the decreased interparticle distance. Importantly, interactions of hard and soft NPs led to the formation of discrete precipitating NP aggregates at a relatively low volume fraction of soft NPs. A phenomenological model was developed for interactions of oppositely charged NPs with varying softnesses. The experimental results were in agreement with molecular dynamics simulations based on the model. This work provides insight on interparticle interactions before, during, and after the formation of hard-hard, hard-soft, and soft-soft contacts and has impact for numerous applications of reversible colloidal gels, including their use as inks for additive manufacturing.

Ion fluctuations and intermembrane interactions in the aqueous dispersions of a dialkylchain cationic surfactant studied using dielectric relaxation spectroscopy and small- and wide-angle X-ray scattering

A dialkylchain cationic surfactant forms the so-called α-gel in water showing virtually no fluidity, which is transformed into a highly fluidic dispersion upon addition of a small amount of salt. This intriguing phenomenon is utilized in household industries. However, the underlying mechanisms remain unclear. Here, we use dielectric relaxation spectroscopy (DRS) and simultaneous small- and wide-angle X-ray scattering (SWAXS) to shed light on this issue. We find that an excess amount of CaCl₂ induces an α-gel-to-multi-lamellar vesicle (MLV) transition accompanied by a marked increase of the reservoir volume fraction. This resembles an unbound lamellar-to-bound lamellar transition that cannot be explained without invoking a weak long-ranged electrostatic attraction. The DRS data provide evidence that the counterions fluctuate both vertically and laterally at the interface, whose relaxation amplitudes sharply depend on a percolating state of an aqueous phase. The strikingly small bulk-water amplitude is likely to reflect depolarizing electric fields induced by the MLV architecture, along with genuine hydration effects. The modified Caillé approach to the SAXS intensities reveals sensitive salt-concentration dependent membrane-membrane interactions. The least undulating membranes are formed at a salt concentration of ca. 10 mmol L^-1. Above 25 mmol L^-1, where small surface separation (<2.5 nm) is attained, far more undulating membranes than those predicted by the Helfrich interaction are produced. This suggests that the hydration forces, generally believed to induce strong short-range repulsion, do not suppress the membrane undulation fluctuations.

Smectite clay--inorganic nanoparticle mixed suspensions: phase behaviour and rheology

Smectite clay minerals and their suspensions have long been of both great scientific and applications interest and continue to display a remarkable range of new and interesting behaviour. Recently there has been an increasing interest in the properties of mixed suspensions of such clays with nanoparticles of different size, shape and charge. This review aims to summarize the current status of research in this area focusing on phase behaviour and rheological properties. We will emphasize the rich range of data that has emerged for these systems and the challenges they present for future investigations. The review starts with a brief overview of the behaviour and current understanding of pure smectite clays and their suspensions. We then cover the work on smectite clay-inorganic nanoparticle mixed suspensions according to the shape and charge of the nanoparticles - spheres, rods and plates either positively or negatively charged. We conclude with a summary of the overarching trends that emerge from these studies and indicate where gaps in our understanding need further research for better understanding the underlying chemistry and physics.

Influence of the surface charge on the homogeneity of colloidal crystals

Five groups of suspensions composed of polystyrene particles, having similar size but different effective surface charge, were adopted to investigate the effects of surface charge and volume fraction on the homogeneity of colloidal crystals through checking the difference between D(exp) and D(uni) by reflection spectroscopy method (D(exp), D(uni) are the experimental and the expected value of the average nearest neighbor interparticle distance by assuming a uniform structure, respectively). We found volume fractions (ranging from 0.006 to 0.02) and structure types basically have no influence on the values of D(exp)/D(uni). Moreover, for crystals formed by lowly charged particles, D(exp)/D(uni) is approximately equal to 1, implying the crystals are homogeneous. With the increase of effective surface charge, D(exp) gradually deviates from D(uni) and the formed crystals become inhomogeneous. Our experimental observations are in accordance with the previous simulation results. Additionally, we also found D(exp)/D(uni) initially drops quickly with increasing effective surface charge and then it tends to an asymptotic value (~0.85), it is supposedly due to the saturation of effective charge. Our relevant computer simulations confirmed that the study scheme that using D(exp)/D(uni) as an indicator to assess the homogeneity of crystal structure is tenable and the simulation results are consistent with experiments.

On attractive interaction of a colloid pair of like charge at infinite dilution

Numerical data on the potential of mean force W(r) at infinite dilution of a highly charged colloid pair embedded in a 1:1 electrolyte are reported. The authors obtain attractive minima (W<0) at short interparticle distance in these potential functions in hypernetted chain (HNC) approximation, as salt concentration is increased. These minima, however, disappear in all system sets studied when a self-consistent Zerah-Hansen (ZH) closure is used. The authors infer that the attractive minima obtained in a HNC closure are spurious and result from the neglect of bridge diagrams in HNC approximation. An expression of bridge function, which the ZH closure in effect incorporates in W(r) to remove attractive minima, is derived in terms of modification of correlation functions. Features of repulsive pair potentials obtained using the ZH closure, their dependence on particle charge and salt concentration, and their agreement with those of the Derajguin-Landau-Verwey-Overbeek theory are investigated.

Attractive interactions in dispersions of identical charged colloidal particles: a Monte Carlo simulation

Using the Debye-Hückel pair potential, the collective interactions between identical charged particles were examined via Monte Carlo simulations. The results have shown that when the number of charges per particle and the particle volume fraction were sufficiently large, the pair long-range electrostatic repulsion generated an effective attractive interaction between identical charged particles because of many-body effects. Disordered liquid-like structures, ordered crystal-like structures, ordered structures dispersed in disordered ones, and disordered structures dispersed in ordered ones have been found. The structures are dependent on the volume fraction and charge of the particles, as well as on the electrolyte concentration.

Elementary swelling and gelling theories

It has been experimentally illustrated that it is possible to form an n-phase thermodynamic system where n> or =2 and has a value which is limited by the properties of the system. For example, to attain limited chemical reversibility of a mineral matrix such as vermiculite using specific butyl ammonium vermiculite, it is necessary to reform chemical bonds using an additive and a particular ion. In other words, to obtain a similar thin film using exfoliated vermiculite as that obtained with natural vermiculite, hydrophobic or binding electrostatic bonds must be formed or reformed. It has also been observed that complementary structural compounds can be formed from other silicates, such as glycerine/sodium silicate. These compounds can also encase the vermiculite matrices. (Film Formation) is proportional to (A/Temp). (Pressure).(Ionic Content). Hence F=L(A/T)PE, where F=film formation, L=functional constant for the matrix compound, T=temperature, A=temperature constant, P=pressure, E=electrostatic force and P=force/area. Thus, F=LTE2 for unit area. SigmaF=pfL. TE2/SigmaPefl=F1, where P=the probability for film formation and SigmaPef(L) is the partition function for the film formation. This concept can be extended to a response geometric algorithm. This means that for every mineral matrix there is an algorithm to describe its formation or its response to an applied field. Thus deltasec2(E)tan(E1) is the response for (F1). V---, V1...Vn, F1...Fn1, ..., Fp, ...Fnp.

Charge and salt-driven reentrant order-disorder and gas-solid transitions in charged colloids

Monte Carlo simulations have been performed for aqueous charged colloidal suspensions as a function of effective charge density (sigma) on the particles and salt concentration C(s). We vary the effective charge density in our simulations over a range where a reentrant solid-liquid transition in suspensions of silica and polymer latex particles has been reported by Yamanaka et al. (Phys. Rev. Lett. 80 (1998) 5806). We show that at low ionic strengths a homogeneous liquid-like ordered suspension undergoes crystallization upon increasing sigma. Further increase in sigma resulted once again in a disordered state, which is in agreement with experimental observations. In addition to this reentrant order-disorder transition, we observe an inhomogeneous-to-homogeneous transition in our simulations when salt is added to the disordered inhomogeneous state. This inhomogeneous-to-homogeneous disordered transition is analogous to the solid-gas transition of atomic systems and has not yet been observed in charged colloids. The reported experimental observations on charged colloidal suspensions are discussed in the light of present simulation results.