Study of two-component colloidal particles at air/water interfaces using Langmuir–Blodgett techniques

Interfacial Viscoelasticity of Self-Assembled Hydrophobic/Hydrophilic Particles at an Air/Water Interface

Hydrophobic/hydrophilic mixtures of latex particles at an air/water interface self-assemble, creating space filling, interconnected aggregates as the relative surface fractions of the dissimilar particles approach 0.5, which is reflected both in qualitative observation and fractal dimension of the microstructure. It is hypothesized that this change in microstructure occurs due to an asymmetry in the electrostatic interaction between similar and dissimilar particles caused by polarization of hydrophilic particles by hydrophobic particles. The changes in both microstructure and interparticle interactions significantly impact the interfacial viscoelasticity. As greater shape complexity is observed, interfacial complex moduli can increase by as much as 3 orders of magnitude and interfaces become more elastic.

Phase Inversion of Colored Pickering Emulsions Stabilized by Organic Pigment Particle Mixtures

Pickering emulsions stabilized by a mixture of colored organic pigment particles of different hydrophobicity have been investigated at equal oil/water ratio and a fixed overall particle concentration by several complementary methods. Transitional phase inversion of emulsions from water-in-oil to oil-in-water can be effected by increasing the fraction of hydrophilic pigment orange in mixtures with either hydrophobic pigment yellow, red, indigo, or blue. In two mixtures, we find that a distinct change in the color of emulsions occurs at phase inversion. Although the fraction of pigment orange required depends on the particular hydrophobic pigment selected, phase inversion occurs at a similar surface energy of the particle mixture, which lies between that of the hydrophilic pigment and those of the hydrophobic pigments. We show that both pigment types are present at the oil-water interface simultaneously, giving rise to emulsions that are extremely stable to coalescence. The average drop size for both emulsion types increases toward phase inversion, in line with an increase in the extent of sedimentation/creaming at long times.

Bending modulus of bidisperse particle rafts: Local and collective contributions

The bending modulus of air-water interfaces covered by a monolayer of bidisperse particles is probed experimentally under quasistatic conditions via the compression of the monolayer, and under dynamical conditions studying capillary-wave propagation. Simple averaging of the modulus obtained solely with small or large particles fails to describe our data. Indeed, as observed in other configurations for monodisperse systems, bidisperse rafts have both a granular and an elastic character: chain forces and collective effects must be taken into account to fully understand our results.

Dilational surface elasticity of spread monolayers of polystyrene microparticles

The dependence of the dilational surface elasticity on the surface pressure of the spread monolayers of polystyrene microparticles is studied at the water-air interface. The surface rheological measurements together with the data from optical methods allow the division of the whole range of surface pressures into three zones characterized by different monolayer structures. The extremely high surface elasticity (∼500 mN m(-1)) at surface pressures close to 30 mN m(-1) is similar to the results for the adsorption layer of the complexes formed between silica particles and surfactant molecules and is probably caused by strong hydrophobic attraction between the particles. At the same time, some other characteristic features of the viscoelasticity of the monolayers of polysterene microparticles differ strongly from the properties of previously studied systems.

Surface pressure isotherm for a monolayer of charged colloidal particles at a water/nonpolar-fluid interface: experiment and theoretical model

Monolayers from electrically charged micrometer-sized silica particles, spread on the air/water interface, are investigated. Because of the electrostatic repulsion, the distances between the particles are considerably greater than their diameters, i.e., we are dealing with nondensely packed interfacial layers. The electrostatic repulsion between the particles occurs through the air phase. Surface pressure vs area isotherms were measured by Langmuir trough, and the monolayers' morphology was monitored by microscope. The mean area per particle is determined by Delaunay triangulation and Voronoi diagrams. In terms of mean area, the surface pressure for monolayers from polydisperse and monodisperse particles obeys the same law. The experiments show that Π ∝ L(-3) at large L, where Π is the surface pressure and L is the mean interparticle distance. A theoretical cell model is developed, which predicts not only the aforementioned asymptotic law but also the whole Π(L) dependence. The model presumes a periodic distribution of the surface charge density, which induces a corresponding electric field in the air phase. Then, the Maxwell pressure tensor of the electric field in the air phase is calculated and integrated according to the Bakker's formula to determine the surface pressure. Thus, all collective effects from the electrostatic interparticle interactions are taken into account as well as the effects from the particle finite size. By evaporation of water, the particle monolayers are deposited on a solid substrate placed on the bottom of the trough. The electrostatic interparticle repulsion is strong enough to withstand the attractive lateral capillary immersion forces that are operative during the drying of the monolayer on the substrate. The obtained experimental results and the developed theoretical model can be useful for prediction and control of the properties of nondensely packed interfacial monolayers from charged particles that find applications for producing micropatterned surfaces.

Surface dilational moduli of latex-particle monolayers spread at air-water interface

Latex particles prepared by radical dispersion polymerization of styrene and diacetone acrylamide (DAAM) in the presence of potassium persulfate as an initiator were spread as particle monolayers at the air-water interface. The surface pressure isotherms of the latex-particle monolayers are almost reversible during the compression-expansion cycles. The surface dilational moduli of the latex-particle monolayers at a fixed surface pressure of 20 mN/m and a fixed frequency of 10 mHz are almost independent of the strain. Moreover, at a fixed strain of 10% and fixed surface pressures of 10 or 15 mN/m the surface dilational moduli of the latex particle monolayers were measured as a function of frequency. The Lissajous orbits of the latex-particle monolayers exhibit positive hysteresis loops for all surface pressure ranges examined. The crossover between the magnitude of the surface elastic modulus and the magnitude of the surface viscous modulus occurs between the frequencies of 10 and 12 mHz and beyond the frequency of 12 mHz the former is larger than the latter. Such crossover indicates that the strain response of the latex-particle monolayers behave changes from liquid-like viscoelastic behavior to solid-like behavior with increasing frequency.