AFM studies of interaction forces between surfaces in alcohol-water solutions

The interaction and adhesive forces between a mica plate and SiO2-surfaces in water-alcohol (methanol, ethanol and n-propanol) mixtures and their correlation with the microstructure of the layer adsorbed on the interface are investigated on the molecular level using the atomic force microscope (AFM). It is found that the characteristics of solutions of low alcohol concentration are not too different from those of aqueous solutions, but very different features on the interaction and adhesive forces are found when the alcohol weight fraction w(alc) is greater than 0.9, as summarized below. (1) The long-range interaction force is successfully expressed by the DLVO theory at w(alc)<0.7. (2) The apparent surface potential and ionic concentration decrease with w(alc) at high w(alc) which can be explained by the ion association mechanism proposed by Fuoss. (3) The short-range interaction force increases gradually with the alcohol concentration but a step-like force curve appears at w(alc)>0.9. This step-like force will be attributable to the vertical adsorption of alcohol molecules. (4) The adhesive force decreases with the alcohol concentration at w(alc) <0.7, which can be explained by the variation of medium property. (5) There exists a large attractive force at w(alc) similar to 0.9 whose magnitude increases with the molecular weight of alcohol. A possible mechanism for these attractive forces is proposed. (C) 1998 Elsevier Science B.V. All rights reserved.