Interfacial Interactions Between Clay Particles And Surfactants And The Emulsions Stabilized By Clay Particles/Surfactants

The emulsion is a system consisting of dispersed droplets of at least one immiscible liquid in another immiscible liquid. Emulsions are inherently unstable and must be stabilized against coalescence with suitable emulsifying agents such as surfactants or particles. Emulsions have wide applications in different fields such as chemical engineering, material science, biology, environmental science, food industry, detergency, and enhanced oil recovery. Emulsions stabilized by surfactants have been known for a long time, and a thorough understanding of those stabilized by particles has also been attained recently. Up to now, many particles have been used as emulsifiers. However, many raw particles in practice with extreme hydrophilicity or hydrophobicity cannot produce stable emulsions. Thus different methods have been applied to modify the particle wettability, for instance (1) in situ modification via the adsorption of amphiphilic molecules,(2) surface coating/grafting via chemical treatment. Comparatively, the in situ modification by adding amphiphilic molecules is much simpler and inexpensive. In fact, many emulsions applied in industries contain both surfactants and particles. Hence it is important to investigate the interaction between surfactants and particles and the stabilization mechanism of the emulsions containing particles and surfactants. In recent years, investigations about system containing surfactants and particles have been carried out, and synergistic and competitive effects in stabilizing emulsions have been reported, but further systematic studies are necessary for a fully understanding about the emulsions stabilized by mixtures of particles and surfactants.In this context, we studied the interactions between clay particles and nonionic and cation surfactants at water-oil interface, and the emulsions containing these emulsifiers. First, the emulsions stabilized by clay particles and lipophilic surfactant (Span80) were investigated, and the effects of particle concentration and ionic strength on the emulsions were evaluated. Second, oil-in-water (o/w) emulsions were investigated, and the interactions between clay particles and cation surfactants at water-oil interface were studied.The present dissertation includes the following three topics.1. The effect of clay concentration on the emulsions stabilized by lipophilic surfactantIn our previous work, the type of emulsions stabilized by a mixture of Laponite and Span80is always o/w when the particle concentration is1.0wt%and the concentration of Span80ranges from0to100mM. Here we report that an abnormal inversion of emulsions containing fixed concentration of surfactant from o/w to w/o was induced by increasing the concentration of hydrophilic clay particles. When the concentration of particle is low, emulsions are mainly stabilized mainly by particles. Surprisingly, the emulsion inverted to water-in-oil (w/o) when the concentration of the hydrophilic clay particles was increased. Here, we increased the concentrations of the hydrophilic clay particles, and the emulsions should retain o/w according to the theories that hydrophilic particles prefer to stabilize o/w emulsion. In order to find out why the emulsions inverted to w/o at high clay concentrations, a systemically study was carried out. Based on the results of laser-induced confocal microscopy and rheological measurements, we proposed that the second inversion of the emulsions is induced by the particle networks formed at high particle concentrations. When gels were formed at high clay concentrations, the gelled aqueous phase easily became fragments during emulsification which make it likely to be the inner phase. The gel structures also inhibit the transition of particles from aqueous phase to the interface. Finally, rheological measurements about the emulsions were carried out. For the o/w emulsions, the viscosity increases with the initial particle concentration, whereas the viscosity of the w/o emulsions is similar to the emulsion stabilized by Span80alone, and changes little with the variation of the particles concentration because the particles are mainly in the inner phase. 2. The effect of salt concentration on the emulsions stabilized by Laponite particles and Span80The effects of salt on emulsions containing sorbitan monooleate (Span80) and Laponite particles were investigated, and a double phase inversion of emulsions induced by salt concentration was observed. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. To confirm this hypothesis, laser-induced confocal scanning microscopy and cryo-TEM were used to observe the emulsion droplets, and the results confirmed the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.3. Effect of cetyltrimethylammonium bromide addition on the emulsions stabilized by montmorilloniteThe wettability of montmorillonite could be in situ modified by cationic surfactant cetyltrimethylammonium bromide (CTAB). The type and stability of emulsions prepared from montmorillonite with different concentrations of cationic surfactant were investigated, and a double phase inversion of emulsions was observed. The adsorption of CTAB on montmorillonite particles was studied by surface tension and zeta potential measurements, and the variation of the wettability of particles with the concentration of CTAB was characterized by the contact angle measurements. The adsorption of particles at the surface of emulsion droplets was observed by laser-induced confocal scanning microscopy. At low surfactant concentrations, the adsorption of CTAB on montmorillonite increased the hydrophobicity of the particles, and the stability of o/w emulsions was enhanced. With the increase of the CTAB concentration, montmorillonite particles changed from hydrophilic to hydrophobic, and w/o emulsions were obtained. However, at higher surfactant concentrations, the emulsions inverted to o/w again because montmorillonite particles were reconverted into hydrophilic due to the formation of CTAB bilayer on the surface of montmorillonite.