ABSTRACT
Smooth Zinc Sulphide (ZnS) surfaces were prepared by magnetron sputtering and the interaction forces were measured between them as a function of pH. At the isoelectric point (iep) of pH 7.1 the attractive force was well described by the van der Waals interaction calculated using Lifshitz theory for a layered system. Away from the iep, the forces were fitted using DLVO theory extended to account for surface roughness. At pH 9.8 the surfaces acquire a negative charge and an electrostatic repulsion is evident. Below the iep the surfaces acquire a positive charge leading to electrostatic repulsion. The forces in the range 3.8 < pH < 4.8 show an additional attraction on approach and much greater adhesion than at other pH values. This is attributed to the hydrophobic attraction being amplified by a small degree of charge on the surface as has previously been reported for adhesion measurements. The range of the measured forces is attributed to the long-range orientational order of water (>5 nm).
ABSTRACT
The interactions between colloidal particles and nanoparticles determine solution stability and the structures formed when the particles are unstable to flocculation. Therefore, knowledge of the interparticle interactions is important for understanding the transport, dissolution, and fate of particles in the environment. The interactions between particles are governed by the surface properties of the particles, which are altered when species adsorb to the surface. The important interactions in the environment are almost never those between the bare particles but rather those between particles that have been modified by the adsorption of natural organic materials. Citric acid is important in this regard not only because it is present in soil but also as a model of humic and fulvic acids. Here we have studied the surface forces between the model metal oxide surface hafnia in the presence of citric acid in order to understand the stability of colloidal particles and nanoparticles. We find that citric acid stabilizes the particles over a wide range of pH at low to moderate ionic strength. At high ionic strength, colloidal particles will flocculate due to a secondary minimum, resulting in aggregates that are dense and easily redispersed. In contrast, nanoparticles stabilized by citric acid remain stable at high ionic strengths and therefore exist in solution as individual particles; this will contribute to their dispersion in the environment and the uptake of nanoparticles by mammalian cells.
ABSTRACT
The surface forces and yield stress of titanium dioxide were measured in the presence of dicarboxylic acids in order to understand the molecular basis for the observed rheological response. The yield stress was measured using the static vane technique, and the surface forces were characterized using an atomic force microscope. The trans and cis isomers of butenedioic acid (fumaric and maleic acids, respectively) were chosen as the relative orientation of the carboxylic groups differs substantially. This enables us to test the hypothesis that an increase in adhesion leads to an increase in yield stress as a consequence of the dicarboxylic acids participating in highly directed bridging. Unlike fumaric acid, maleic acid caused a yield stress reduction in the titanium dioxide suspensions. Surface force measurements between approaching surfaces found that at low pH, fumaric and maleic acids did not induce any additional attraction between the titanium dioxide surfaces. However, significant differences in adhesion were observed, which can be explained in terms of the configuration of the acids at the surface. The observations are consistent with highly directed bridging in the presence of fumaric acid but not in the presence of maleic acid due to the molecular architecture of the dicarboxylic acids.
ABSTRACT
Low molecular weight benzenedicarboxylic acid has a very well-defined molecular structure because of its rigid and planar backbone. Therefore, it is hypothesized to have high potential for highly directed bridging between surfaces. However, phthalic acid cannot participate in particle bridging because the two carboxylic acid groups on the benzene ring are located adjacent to each other which prevent the molecule from bridging between two surfaces. Yield stress measurements showed that isophthalic and terephthalic acid failed to cause significant rheological changes to alumina slurries within their solubility limit. However, upon increasing the concentration beyond the solubility limit, terephthalic acid increased the yield stress by a factor of 7 and isophthalic acid by a factor of 2 when compared to the same colloidal alumina system without additive. Benzoic acid, which has low solubility at low pH, also showed an increase in yield stress by a factor of 2 even though it lacks the second carboxylic group to link neighboring surfaces. These observations suggest that highly directed bridging is unlikely to operate when these acids are present in high concentration. Instead, the dominant mechanism is most likely attraction between the negatively charged precipitates and the positively charged alumina particles and/or capillary bridging.
ABSTRACT
Adsorbed low molecular weight charged molecules are known to give rise to a range of surface forces that affect the rheological behavior of oxide dispersions. The behavior of dicarboxylic acid bolaform compounds in alumina slurry was investigated to determine the influence of the molecular structure on the nanoscale interactions between alumina surfaces and on the macroscopic properties of the slurry. The surface forces in dispersions and between a single particle and a flat surface were characterized by yield stress and atomic force microscopy (AFM) respectively. Absorbed muconic acid increased the yield stress of the alumina system, which indicates an additional attractive interaction between the particles. Adsorbed trans,trans (TT) muconic acid resulted in a much higher yield stress than cis,cis (CC) muconic acid. Force-distance data obtained via AFM displayed features indicating the presence of a capillary force attraction at low pH between the alumina surfaces when TT and CC muconic acids were adsorbed at high surface coverage. This force appeared to explain the high yield stress at low pH (pH 3.6), but the absence of a net attractive force at higher pH (pH 5) did not correlate with the yield stress results. At low pH, the muconic acids become less soluble in the confined space between the interacting surfaces resulting in the formation of an "oily" muconic acid phase located between the interacting surfaces. The nanosized "oil" phase is the source of the capillary force.
ABSTRACT
Adsorbed phosphate on smooth platelet alpha-Al2O3 particles at saturation surface coverage gives rise to strong interparticle attractive forces in dispersion. The maximum yield stress at the point of zero charge was increased by 2-fold. This was attributed to a high density of intermolecular hydrogen bonding between the adsorbed phosphate layers of the interacting particles. Adsorbed citrate at saturation surface coverage, however, reduced the maximum yield stress by 50%. It adsorbed to form a very effective steric barrier as intramolecular hydrogen bonding between -OH and the free terminal carboxylic group prevented strong interactions with other adsorbed citrate molecules residing on the second interacting particle. This steric barrier kept the interacting platelet particles further apart, thereby weakening the van der Waals attraction. The platelet alpha-Al2O3 dispersions were flocculated at all pH level. These dispersions displayed a maximum yield stress at the point of zero zeta potential at the pH approximately 8.0. They also obeyed the yield stress-DLVO force model as characterized by a linear decrease in the yield stress with the square of the zeta potential.
