Aqueous Foam Stabilized By Short-chain Amphiphiles/Particles And Surfactants/Particles

Foams are dispersions in which gas is dispersed in liquid. Foams are thermodynamically unstable systems due to their high gas-liquid interfacial area. The system has the tendency to decrease the interfacial area. It is important to stress that pure liquids cannot form foam unless a surface active material is present. In considering surface active foaming materials in an aqueous environment, three types materials are included, surfactants, polymers and solid particles.Small solid particles can be irreversibly adsorbed at fluid-fluid interfaces, enabling the stabilization of emulsions and foams. Recent reviews have covered different aspects of foam stabilization by solid particles. The enormous stability of particle-stabilized foams results from the interplay among the ability of the particles to form dense coherent particle shells around the bubbles, to stabilize the liquid films separating the bubbles and to form a three-dimensional network in the bulk aqueous phase. Particles form a dense layer on bubble surfaces preventing coalescence and slowing down or halting disproportionation. Evidence also exists that additional stabilization occurs via particle network formation between adsorbed and non-adsorbed particles, which reduces, in this case, drainage of water between the bubbles.In most cases, spherical silica and latex particles modified through prior chemical surface treatments are used as foam stabilizers. Aqueous suspensions of certain solid particles with inherent hydrophobicity are able to make extremely stable foams in the absence of any surfactant. The optimum particle hydrophobicity has been achieved by appropriate chemical synthesis or surface modification or after dispersing the particles in the aqueous phase by adjusting the pH and electrolyte concentration. The present dissertation focuses on the foams prepared by short-chain amphiphiles/particles and surfactants/particle. Some important results for such systems are obtained. These studies are of significance in the basic theory and practical application. The present dissertation includes three topics.1. Aqueous foams stabilized by hexylamine-modified Laponite particlesStable foams have been generated in low concentration aqueous dispersions of hexylamine-modified Laponite particles. The particles of primary size 30 nm, are modified with hexylamine molecules to render them partially hydrophobic or moderate flocculants. Laponite particles have an initially low negative potential (close to -64 mV). The zeta potential first increases with increasing hexylamine concentration and then remains at about-30 mV, indicating saturation of hexylamine on the particle surface. The spectral similarity of modified particles and neat hexylamine also supports the idea that hexylamine is present on the particles. Lyophobization occurs as a result of the relatively strong interaction between the anchoring group and the particle surface, thus leaving the amphiphiles' hydrophobic tails in contact with the aqueous solution.The foamability, drainage behavior and microstructure of wet foams were studied in terms of their dependence on the content of Laponite particles and the concentration of hexylamine. Laser-induced confocal microscopy observations confirm that stable bubbles appear to be surrounded by a thin layer of hexylamine-modified Laponite particles, which is crucial to the stability of foams. The interfacial rheology of the same systems has also been investigated by measuring the dilational viscoelasticity as a function of hexylamine concentrations. The adsorption of particles at the air/water interface has the effect of increasing dilational surface elasticity, indicating that the gel-like layer at the interface inhibits foam drainage and bubble coalescence.2. Aqueous foam stabilized by plate-like particles in the presence of sodium butyrateThe addition of salt promotes the adsorption of layered double hydroxide (LDH) particles onto the air-water interface, but stable foams cannot be prepared from LDH dispersions at all the concentrations of NaCl or sodium acetate. We generated stable foams using positively charged plate-like LDH particles in the presence of sodium butyrate.The effects of adding sodium butyrate to LDH on the particle zeta potential, adsorption behavior and the adsorption of modified particles at the air-water interface were studied. The adsorption isotherms are characteristic of the Langmuir type, which is typically used to describe the adsorption behavior of species that adsorb strongly on the surface at small concentrations and reach a saturated condition at higher concentrations. At low equilibrium concentrations of sodium butyrate, the molecules lie flatly on the particle surface. As the sodium butyrate concentration increased, due to a strong electrostatic attractive interaction, the molecules quickly cover the surfaces of the particles until complete monolayer coverage is reached. Upon adding sodium butyrate, the potential gradually decreases in magnitude, butyrate anions electrostatically adsorb on the LDH surfaces mainly as a monolayer with their hydrocarbon chains exposed to solution. This configuration can screen the electrostatic repulsion between particles, and thus phase separation occurs. At a fixed LDH particle concentration, adding a trace amount of sodium butyrate maximizes flocculation of the aqueous particle dispersion.The LDH particles alone are poor foam stabilizer at any dispersion concentration. The positively charged LDH particles are very hydrophilic, and can hardly stay on the air-water interfaces due to low attachment energy. With low concentrations of sodium butyrate added to the LDH particles, the particles are not sufficiently hydrophobic to adsorb to the air-water interface and are therefore not able to stabilize air bubbles. With increasing sodium butyrate concentration, particles become sufficiently hydrophobic to adsorb at the surface of freshly incorporated air bubbles. Foams stable to coalescence and disproportionation are formed, although they release water through drainage with time. The bubbles are stable when drying at 80℃with little change in size. Laser-induced fluorescent confocal micrographs and scanning electron microscopy observations clearly confirm the adsorption of LDH particles on the foam surfaces, and the bubbles are armored by an interfacial particle multilayer.3. Foams stabilized by Laponite nanoparticles and alkylammonium bromides with different alkyl chain lengthsWe investigated the behavior of foams stabilized by Laponite nanoparticles combined with alkylammonium bromides with different alkyl chain lengths. A four-region model based on electrostatic and hydrophobic interactions adequately explains the adsorption of the cationic surfactants on the negatively charged Laponite particles. The results indicate that chain length has a minimal influence on surfactant adsorption via cation exchange, but a longer alkyl chain length can induce a stronger hydrophobic interaction among the adsorbed alkylammonium molecules and hence a higher surfactant adsorption.Foam stability is closely related to the adsorption conformation of the surfactants on Laponite. In regionⅠ, surfactant ions adsorb individually with their headgroups down. The particles are still hydrophilic so no stable foams can be produced. In regionⅡ, a sharp increase of adsorption is observed with increasing surfactant concentration. Particles become coated with a surfactant monolayer causing them to aggregate as their hydrophobicity increases. The foam stability increases with increasing surfactant concentration. Foams are most stable when the aqueous dispersions are most sedimented, corresponding to their lowest charge and maximum hydrophobicity. In regionⅢ, a bilayer surfactant forms on the particle surfaces, rendering them hydrophilic again. Foam stability decreases in this range. In regionⅣ, adsorption remains constant in this region. The adsorbed layer possesses the structure of a bilayer. Foam stability is almost the same as that of pure surfactant.Increasing the chain length of the monomer by four methylene units, from C12 to C16, lowers the concentration at which characteristic features of the isotherm occur by approximately an order of magnitude. Therefore, for longer chained surfactants stable foams can be obtained at lower concentrations.