Foams Stabilized By Laponite/Surfactants And HMHEC/Surfactants

Foams are dispersions in which gas is dispersed in liquid. Gas is the dispersed phase and liquid is the dispersant. 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 particles. In practical industries, such as oil exploitation, mineral flotation and food processes, particle and surfactant or polymer coexist. So it is important to study the foams stabilized by the mixtures of particle and surfactant or polymer and surfactant. The present dissertation focuses on the foams prepared by particle/surfactants or polymer/surfactants. Some important results for such systems are obtained. These studies are of significance in the basic theory and practical application.Synthetic Laponite is a layered hydrous magnesium lithium silicate consisting of two tetrahedral silica sheets sandwiching a central octahedral magnesia sheet. Laponite is composed of rigid disk-shaped crystals with a negative charge on the surface. Synthetic Laponite is often used as a model of study because of the high purity and narrow size distribution. So the synthetic Laponite was chosen and the foams stabilized by Laponite and surfactants (CTAB, C12E4) were studied. Hydrophobically modified polymers have been a target of extensive studies because of their special salt-resistance and thickening effect. HMHEC was chosen and foams stabilized by HMHEC and various surfactants (SDS, CTAB and C12E4) were investigated.The present dissertation includes three topics.1. Foams stabilized by Laponite and CTABThe adsorption of CTAB (cetyltrimethylammonium bromide) on the Laponite surface was studied first, including the adsorption isotherm, TGA/DSC, the zeta potential and contact angle of particles. Then the foams were investigated. The zeta potential of the particles decreases initially with increasing CTAB initial concentration, and changes to a positive value around 1.0 cec of CTAB. Then the zeta potential of the particles increases and attains a plateau with increasing CTAB concentration. In the presence of CTAB, the contact angle first increases with increasing CTAB concentration up to a maximum value at 1.7 cec. Further increases of CTAB concentration lead to a decrease of the contact angle. Here, the hydrophobicity is relative and the contact angles are still less than 90°. The TGA/DSC results have confirmed that a phase transition of the double layer occurs at 1.7 cec, which also confirms the result of particles hydrophobicity. At a fixed particle concentration, the foam stability increases gradually with increasing CTAB concentration and reaches a maximum at 1.7 cec. Alter that, the foam stability decreases and then remains constant. When the concentration of CTAB is lower than 1.0 cec, the adsorption results from the electrostatic interaction between particle surface and head groups of the ionic surfactant. After that, the adsorption process is no longer driven by electrostatic interaction and that hydrophobic interactions probably are involved. Patches of two-dimensional aggregates on the particle surface seem to be formed. The Laponite particles become more hydrophobic. When the concentration of CTAB reaches about 1.7 cec, the hydrophobicity and flocculation of particles both attain the maximum values, and the foam stability also obtains the maximum. After this point, a second surfactant layer with head groups orients toward the solution, that is, the reverse hemimicelles form gradually. So, the hydrophobicity of particles begins to decrease and the foam stability declines. It can be seen that the foam stability is related with the adsorption feature of surfactant on particles. The most stable foams are obtained by the particles with the maximum hydrophobicity, not with the zero charge. The hydrophobicity of the particle is crucial to obtain stable foams compared with the electrical property of particle for CTAB/Laponite system. At intermediate CTAB concentrations, adsorption of modified particles on the bubble surface is confirmed by the dried foams experiments and the laser-induced fluorescent confocal microscopy. An aggregated structure is formed by the armored bubbles and particles in the bulk phase. When the particles become hydrophilic again, the foams are only stabilized by CTAB molecules. It is indirectly confirmed in three ways. First, the drainage time of foams prepared by these dispersions is close to that prepared by pure CTAB solutions (above cmc). Second, there are no residual particles left in the dried foam phase. Third, foam stability is independent of the particle concentrations.2. Foams Stabilized with Laponite and Nonionic SurfactantsThe aqueous foams prepared by mixtures of Laponite and nonionic surfactant (C₁₂E₄, tetraethylene glycol monododecyl ether) were studied in detail. The adsorption of C₁₂E₄ on Laponite particles is different from the other surfactant with longer hydrophilic group. C₁₂E₄ molecules are adsorbed on the Laponite in the form of multilayer or aggregations. The different self-assembly structures of the surfactant in solutions may lead to the different adsorption conformations on the particles. For Laponite/C₁₂E₄ dispersions, a synergistic effect on foam stability occurs and becomes more obvious with increasing C₁₂E₄ concentration. The bubble sizes in the Laponite/C₁₂E₄ foams are smaller than those in the C₁₂E₄ foams. Three different locations such as bubble surface, plateau border and lamella between bubbles were clearly distinguished in the fluorescence micrographs. Particles located on the lamella between bubbles build up a three-dimensional network structure in the coherent phase, and the bubbles are trapped in the array of particles. This kind of steric network structure also promotes the foam stability. The quantity of particles attached to the bubble surface increases as the C₁₂E₄ concentration increased. The increased attachment of particles would lead to a higher interfacial coverage of the bubbles and promote the foam stability. The decrease of zeta potential of the particles would favor their adsorption on the bubble surface.For comparision, the behavior of foams prepared by silica/C₁₂E₄ and Laponite/C₁₂E₂₃ aqueous dispersions were explored. It is found that synergism is not displayed in the foam system containing Laponite and C₁₂E₂₃. The adsorption of C₁₂E₂₃ on Laponite particles cannot impart them proper hydrophobicity because of the long hydrophilic groups. The modified Laponite particles cannot be adsorbed on the bubble surface. In addition, the viscosity of Laponite/C₁₂E₂₃ dispersions is very low, so no synergistic effect on foam stability is found. For silica/C₁₂E₄ systems, the synergistic effect on foam stability occurs at high C₁₂E₄ concentrations. Foam stability does not come from the adsorption of particles on bubble surface but the increased viscosity of the dispersions. So the synergistic effect is much weaker than that of the Laponite/C₁₂E₄ system.3. Foams Stabilized by Hydrophobically Modified Hydroxyethyl Cellulose (HMHEC) and Different SurfactantsThe properties of foams stabilized by hydrophobically modified hydroxyethyl cellulose (HMHEC) with different surfactants, CTAB, SDS (sodium dodecylsulfate) and C₁₂E₄ are investigated. Different results were found. The properties of mixed aqueous solutions were studied first, and then the foams prepared by these systems were studied. The properties of HMHEC/SDS and HMHEC/CTAB solutions are similar. The viscosity of these solutions obtains a maximum value at a certain ratio of surfactant and HMHEC. Above the ratio, the hydrophobic chains of polymer are solubilized by the surfactant micelles and the viscosity decreases. For both HMHEC/SDS and HMHEC/CTAB systems, the foam stability first increases with the increase of surfactant concentration and reaches a maximum, and then decreases. The maximum viscosity corresponds with the maximum foam stability. In addition, the foam stability of mixed solutions is higher than that of each component. But differences also exist between HMHEC/SDS and HMHEC/CTAB systems. First, at high SDS concentrations, the stability of HMHEC/SDS foams is similar with that of pure SDS solution (above cmc) and independent of the polymer concentration. The bubble sizes become bigger. It can be deduced that the foams are mainly stabilized by SDS molecules. But for HMHEC/CTAB systems, at high CTAB concentrations the foam stability is higher than that of CTAB system alone (above cmc) and dependent on the polymer concentration. That is to say, the foam is stabilized by the complex of HMHEC and CTAB. Second, the stability of HMHEC/SDS foams is higher than that of the HMHEC/CTAB foams. The difference could be due to the different charged groups of SDS and CTAB that interact in different ways with the -OH groups of the cellulosic backbone.Indirect methods were often used to confirm the adsorption of polymer and surfactant on the air-water interface in before studies, such as surface tension. Here, the adsorption of the complex formed by HMHEC/surfactants on the bubble surface was directly observed by laser-induced confocal microscopy. The bubbles are trapped in the array of three-dimensional network.For HMHEC/C12E4 solutions, the viscosity shows a continual increase with C12E4 concentration, which is different from the above two systems. The scattered lamellar phase of surfactant will enhance the association of the HM-polymers and the network will not collapse. Foam stability also shows an increase with C₁₂E₄ concentration. The bubble size decreases with increasing surfactant concentration and is smaller than that of the sole C₁₂E₄ or HMHEC bubbles.For the three different HMHEC/surfactants solutions, the foamability is similar and affected by both the surface tension and viscosity of the solutions. At lower surfactant concentrations, the foamability is mainly controlled by the surface tension. At higher surfactant concentrations, the foamability is mainly controlled by the viscosity of solutions.