Nanoscale forces of interaction between glass in aqueous and non-aqueous media: A theoretical and empirical study

The interfacial forces that control colloid stability in aqueous and non-aqueous systems are functions of the polar and apolar properties of the medium. This study presents a complete empirical and theoretical description of the forces and energies between a glass microsphere and a glass surface in aqueous and non-aqueous n-decane solutions that were amended systematically to enhance various colloidal interactions. Force data were measured using colloid probe atomic force microscopy (AFM) and both classical DLVO and extended-DLVO (XDLVO) theories were used to calculate the free energy of each system. The successes and failures of each theory in predicting force data were evaluated and the results indicate the importance of minor amounts of impurities, especially in the non-aqueous solutions, which impart measurable electrostatic charges on the colloidal particles. Data were also collected in a complex non-aqueous solution (commercial motor oil) and the recorded forces were significantly different compared to data collected in n-decane; long-range repulsive forces were measured by AFM. These results document the significance of aqueous impurities in non-aqueous systems which will likely dominate interactions between colloids in natural systems. Our results also indicate that DLVO and XDLVO theory predict colloidal interactions in aqueous systems. However these theories are unable to predict colloid stability in natural and chemically complex non-aqueous media.