Thermodynamic properties of the S/L interfacial layer: Stabilization of the colloidal system in binary liquids
In the knowledge of the adsorption and wetting properties of solid particles the influence of particle-liquid and interparticle interaction has been examined on the stability and structure formation of suspensions obtained for hydrophobic Sio, colloids in the mixtures benzene+n-heptane and methanol+benzene. The Hamaker constants of the adsorption layer of composition range. The Hamaker constant have been determined by optical dispersion measurements in the entire mixture composition range. The Hamaker constant of the adsorption layer of composition different from that in the bulk have been calculated at several mixture compositions on the basis of the adsorption results. Having the excess isotherms available enabled us to determine the adsorption layer thickness as a function of the bulk composition. For interparticle attractive potentials, model calculations have been effected on the basis of the Vincent model. In the case of hydrophobic particles dispersed in benzene+n-heptane and methanol+benzene mixtures it was established that the change of the attractive potential was in accordance with the interactions obtained from rheological measurements. INTRODUCTION Rheological, adsorption and immersional wetting of solid particles formed by highly-dispersed hydrophobic silica have been discussed separately in a number of publications. Our previous work (ref. 1) pointed out a close relation between rheological parameters characteristic to the structure formation of a suspension and the composition of the adsorption layer. It was found, that in a certain composition range, i.e. in the environment of the adsorption azeotropic point the free energy minimum of the adsorption layer results in optimal wetting (lyosorption) and the corresponding dipsersion stability. The results of optical density measurements of mixture series pointed out the importance of dispersion interactions in binary liquids. There must be an important relation between the polarizability of the dispersed particles and the dispersion medium and accordingly, between the optical properties and the attractive potential. In the present study we examined the thickness of the adsorbed layer (lyosphere) on the surface of particles and the layer thickness as a function of the mixture composition based on the adsorption excess isotherms. For the characterization of the interaction on the solidliquid interface the free energy of wetting was calculated according 901