Preparation, type and stability of emulsions stabilised by solid particles
The preparation, type and stability of emulsions stabilised solely by solid silica particles has been investigated in detail using a combination of conductivity, stability, light diffraction, optical microscopy and contact angle measurements. The wettability of the particles in situ at the oil-water interface plays a major role in such systems. The stability of aqueous colloidal dispersions in the presence of various electrolytes and over a range of pH is shown to be important in determining the stability of toluene-in-water emulsions formed from them. Partial flocculation of the silica dispersions enhances the emulsion stability whereas strong flocculation results in unstable emulsions. Inversion of emulsion type occurs via two mechanisms. Catastrophic inversion of water-in-oil emulsions stabilised by hydrophobic silica particles occurs upon increasing the volume fraction of water in the system. Transitional inversion of emulsions at fixed volume fraction of water has been achieved by varying the mass fraction of hydrophilic (or hydrophobic) particles in systems containing particle mixtures. Neither inversion mechanism exhibits hysteresis, in contrast to surfactant-stabilised systems. The energy of attachment of a particle to the oil-water interface is shown to be dependent on the contact angle and the interfacial tension. The contact angle was varied by changing the hydrophobicity of the silica particles. Particles of intermediate hydrophobicity were most effective at stabilising emulsions which is thought to be due to the contact angle at the oil-water interface being close to 90°. The interfacial tension was affected by changing the nature of the oil phase or by'replacing water with various polar liquids. Undecanol-water emulsions stabilised by silica particles of intermediate hydrophobicity break down via gel formation followed by coalescence. The wettability of the silica surface was varied in situ by varying the pH of the aqueous phase. Increasing the pH causes dissociation of silanol groups, which increases the affinity of the particles for the aqueous phase. The type and stability of emulsions and the contact angle of an aqueous drop under toluene on hydrophobically modified glass slides were measured as a function of pH. The results are modelled using simple theory. The wettability of hydrophobic pigment surfaces is dependent on the type and concentration of added surfactant. The findings are discussed in terms of surfactant adsorption at the relevant interfaces.