Effects of long-chain surfactants, short-chain alcohols and hydrolizable cations on the hydrophobic and hydration forces

The DLVO theory states that the interaction between two lyophobic particles in aqueous media can be predicted by the sum of two surface forces, i.e., the electrical double-layer and van der Waals forces. This theory, which was developed 50 years ago, served as a backbone for colloid chemistry. However, various experiments conducted in recent years showed that it is applicable only to those particles whose advancing water contact angles (θa) are in the range of 15-60. For example, direct surface force measurements conducted between silica substrates, whose θa values are less than 15, exhibited the existence of repulsive hydration forces at relatively short separation distances. On the other hand, substrates, for which θa is greater than 60, exhibit long-range attractive hydrophobic forces not considered in the DLVO theory . These extraneous attractive forces play important roles in many industrial applications. It is, therefore, the objective of the present study to measure the hydrophobic and hydration forces under different conditions. The measurements were conducted using both the Surface Forces Apparatus (SFA) and the Atomic Force Microscope (AFM). Mica and Silica were used as substrates, and the effects of dioctylammonium-hydrochloride (DOAHCl), octanol, methanol, ethanol, trifluoroethanol (TFE), pyridine, CaCl2, MgCl2 and sodium oleate were studied.