Computer simulation of the microstructure of a nanoparticle monolayer formed under interfacial compression

Use of Silica Nanoparticle Langmuir Films to Determine the Effect of Surface Roughness on the Change in the Forces between Two Silica Surfaces by a Liquid Flow

We aimed to determine how surface roughness changes the effect of a liquid flow on the forces between two charged surfaces. This is because many applications require liquid to flow through charged confined areas and because all surfaces show a degree of roughness. We prepared films of different roughness by making mixed Langmuir films of silica nanoparticles (NPs) of two different diameters at air-100 mM NaCl aqueous interfaces and then by transferring and sintering these films to silicon wafers. Atomic force microscope (AFM) imaging showed that the film roughness decreased (1) with an NP diameter decrease and (2) an increased ratio of small NPs in a mixed film of small and larger NPs. This decrease was explained by a decreased NP aggregation in the film, due to the increased Brownian velocity that accompanies an NP diameter decrease. Force-separation curves were next measured with an AFM between a microsized silica particle (probe) and a smooth substrate (silicon wafer) or the rough NP films in 1 mM NaCl. In the absence of a liquid flow, the repulsive forces decreased with an increased substrate roughness. This reduction was explained by an increased difference between the real zero separation distance and apparent zero separation distance (the distance between the first point of mechanical contact between the probe and substrate) with an increased surface roughness. In the presence of a liquid flow, the repulsive forces decreased in the case of a smooth substrate. However, the repulsive forces were reduced less by a liquid flow for rough substrates. This result was explained by the difference in the effect of liquid flow on the diffuse layer for the smooth and rough surfaces. Surface roughness is postulated to modify the liquid flow trajectory near the surfaces and to cause ion concentration gradients near the surface. These factors are proposed to lessen the change in the diffuse layer brought about by a liquid flow. This would then reduce the change in the forces.

Use of Mixed Langmuir Films of Nanoparticles to Form Metal Oxide Materials with the Optimal Surface Charge

We aimed to prepare metal oxide materials with the optimal surface charge by preparing mixed films of non-modified metal oxide nanoparticles (NPs) with dissimilar isoelectric points (iep). The purpose of preparing such surfaces was to expand the use of metal oxide materials in environments where the solution pH cannot be changed. Langmuir films of SiO₂ (iep: pH 2-3) and TiO₂ (iep: pH 5-6.6) NPs were first prepared at air-100 mM NaCl aqueous interfaces. This subphase allowed the formation of stable films of the NPs without the need to modify the NPs with surface-active chemicals, whose presence may detrimentally change the properties of the films. The Langmuir films were then transferred and sintered to silicon wafers and their physical properties were characterized using atomic force microscopy (AFM). The AFM images showed that the films were composed of NP aggregates. The average size of the aggregates decreased, and the uniformity of the aggregate sizes and their inter-aggregate spacing increased with the addition of SiO₂ NPs to the film of TiO₂ NPs. These changes were explained by an increased electrostatic and steric repulsion between the aggregates formed at the air-100 mM NaCl interface due to the adsorption of negatively charged SiO₂ NPs to the slightly positively charged TiO₂ aggregates. The force-distance curves measured between a SiO₂ probe and the sintered films of SiO₂ and/or TiO₂ NPs in a 1.0 mM NaCl solution adjusted to pH 4 showed that the magnitude of the repulsive force decreased with an increased number of TiO₂ NPs in the film. This force change indicated that the surface charge changed when different types of NPs were mixed. These results indicate that mixing different NP types in a Langmuir film at an air-aqueous interface can help change the physical properties of the transferred film.

Collapse of Particle-Laden Interfaces under Compression: Buckling vs Particle Expulsion

Colloidal particles can bind to fluid interfaces with a capillary energy that is thousands of times the thermal energy. This phenomenon offers an effective route to emulsion and foam stabilization where the stability is influenced by the phase behavior of the particle-laden interface under deformation. Despite the vast interest in particle-laden interfaces, the key factors that determine the collapse of such an interface under compression have remained relatively unexplored. In this study, we illustrate the significance of the particle surface wettability and presence of electrolyte in the subphase on interparticle interactions at the interface and the resulting collapse mode. Various collapse mechanisms including buckling, particle expulsion, and multilayer formation are reported and interpreted in terms of particle-particle and particle-interface interactions.

Structure and rheology of colloidal particle gels: insight from computer simulation

A particle gel is a network of aggregated colloidal particles with soft solid-like mechanical properties. Its structural and rheological properties, and the kinetics of its formation, are dependent on the sizes and shapes of the constituent particles, the volume fraction of the particles, and the nature of the interactions between the particles before, during and after gelation. Particle gels may be permanent or transient depending on whether the colloidal forces between the aggregating particles lead to irreversible bonding or weak reversible interactions. With short-range reversible interactions, network formation is typically associated with phase separation or kinetic arrest due to particle crowding. Much existing knowledge has been derived from computer simulations of idealized model systems containing spherical particles interacting with well-defined pair potentials. The status of current progress is reviewed here by summarizing the underlying methodology and key findings from a range of simulation approaches: Monte Carlo, molecular dynamics, Brownian dynamics, Stokesian dynamics, dissipative particle dynamics, multiparticle collision dynamics, and fluid particle dynamics. Then it is described how the technique of Brownian dynamics simulation, in particular, has provided detailed insight into how different kinds of bonding and weak reversible interactions can affect the aggregate fractal structure, the percolation behaviour, and the small-deformation rheological properties of network-forming colloidal systems. A significant ongoing development has been the establishment and testing of efficient algorithms that are able to capture the subtle dynamic structuring effects that arise from effects of interparticle hydrodynamic interactions. This has led to an appreciation recently of the potentially important role of these particle-particle hydrodynamic effects in controlling the evolving morphology of simulated colloidal aggregates and in defining the location of the sol-gel phase boundary.

Modeling the structure formation of particulate langmuir films: the effect of polydispersity

Two-dimensional molecular dynamics computer simulation has been developed to model the compression of Langmuir films composed of spherical nanoparticles with arbitrary size distribution. We demonstrate that the usual assumption in the determination of interparticle potentials from the surface pressure vs area isotherms (i.e., monodisperse particles in perfect hexagonal order) leads to a systematic overestimation of the characteristic length of the interaction. On the basis of the results of the simulation, we propose a correction method to improve the traditional way of determining the interparticle potentials. We use the corrected particle-particle interactions to explore the correlation between the broadness of the size distribution and several structural parameters (decay length of pair-correlation function, global orientational order parameter, mean, and standard deviation of number of neighbors). Due to the uniaxial compression and the stiffness of the particulate layer, the surface pressure is not a scalar field. We investigate the effect of polydispersity on the anisotropy and the fluctuation of the surface pressure tensor in Langmuir films during uniaxial compression.

Compression of Langmuir films composed of fine particles: collapse mechanism and wettability

The collapse mechanism of microparticulate Langmuir films was studied experimentally in the present work. Using a Wilhelmy film balance, surface pressure vs area isotherms were determined, and the particle removal during the compression was examined by video-microscope and by naked eye. Upon compressing partially wettable 75 microm diameter surface modified glass beads at liquid (water or aqueous surfactant solution)-air (or n-octane) interfaces, different collapse mechanisms were visualized depending on the wettability of the particles. At low contact angles (below 40 degrees ) irreversible particle removal was observed as a consequence of a particulate line-by-line collapse mechanism. At higher contact angles a buckling-type collapse mechanism was revealed without particle removal from the liquid interface. In the case of irreversible particle removal we assessed the contact angles from the nondissipative part of the isotherm. These values were found to be in reasonable agreement with those determined directly on the beads.

Colloid science of mixed ingredients

Some recent advances in the colloid science of heterogeneous systems containing food biopolymer ingredients are reviewed. Understanding the instability processes controlling the shelf-life and rheology of food colloids requires a detailed knowledge of the factors affecting the nature of the interactions in emulsions and gels containing mixtures of protein + protein, protein + surfactant and protein + polysaccharide. Against this background, theoretical modelling and computer simulation are useful tools for predicting effects of system composition on stability mechanisms. Confocal microscopy combined with image analysis is providing new experimental insight into the microstructural origins of changes in macroscopic properties during processing and storage.

Importance of physical vs. chemical interactions in surface shear rheology

The stability of adsorbed protein layers against deformation has in literature been attributed to the formation of a continuous gel-like network. This hypothesis is mostly based on measurements of the increase of the surface shear elasticity with time. For several proteins this increase has been attributed to the formation of intermolecular disulfide bridges between adsorbed proteins. However, according to an alternative model the shear elasticity results from the low mobility of the densely packed proteins. To contribute to this discussion, the actual role of disulfide bridges in interfacial layers is studied. Ovalbumin was thiolated with S-acetylmercaptosuccinic anhydride (S-AMSA), followed by removal of the acetylblock on the sulphur atom, resulting in respectively blocked (SX) and deblocked (SH) ovalbumin variants. This allows comparison of proteins with identical amino acid sequence and similar globular packing and charge distribution, but different chemical reactivity. The presence and reactivity of the introduced, deblocked sulfhydryl groups were confirmed using the sulfhydryl-disulfide exchange index (SEI). Despite the reactivity of the introduced sulfhydryl groups measured in solution, no increase in the surface shear elasticity could be detected with increasing reactivity. This indicates that physical rather than chemical interactions determine the surface shear behaviour. Further experiments were performed in bulk solution to study the conditions needed to induce covalent aggregate formation. From these studies it was found that mere concentration of proteins (to 200 mg/mL, equivalent to a surface concentration of around 2 mg/m(2)) is not sufficient to induce significant aggregation to form a continuous network. In view of these results, it was concluded that the adsorbed layer should not be considered a gelled network of aggregated material (in analogy with three-dimensional gels formed from heating protein solutions). Rather, it would appear that the adsorbed proteins form a highly packed system of proteins with net-repulsive interactions.

Brownian dynamics simulation of adsorbed layers of interacting particles subjected to large extensional deformation

We present Brownian dynamics simulations of the compression and expansion of monolayers adsorbed at a planar interface. The surface-active species are modelled as monodisperse spherical particles that can form particle-particle elastic bonds. The objective is to model the large compression and expansion of viscoelastic protein films investigated in Langmuir trough experiments. We determine the stress-strain response of the system and the associated microstructural changes induced by the large deformation of the interface as a function of particle adsorption energy, and bond breakability and stiffness. We also study the effect of the velocity of compression and the type of compression (uniaxial or homogeneous) on the mechanism of collapse of the adsorbed films. Furthermore, we present simulations on complex mixed systems containing both bond-forming particles (modelling protein) and nonbond-forming particles (modelling surfactant). We find that the preferential desorption of one type of particle or the other, upon compression, is sensitive to the extent of bond breakability of the bond-forming species.