Surface Forces Measured in Liquids

The attractive forces between smooth mica surfaces immersed in liquids have been much measured over the past 30 years by Israelachvili and his colleagues. Israelachvili joined the staff at the Australian National University in Canberra during 1973 and started to do the unthinkable by carrying out experiments in the Mathematics Department. He began to work with Adams in modifying the surface force apparatus previously built in Cambridge. The idea was to fill it full of water and other liquids. A schematic of the equipment is shown in Fig. 6.10. 

The early experiments showed that the attraction between the mica was much reduced by the presence of water. Results displayed in Fig. 6.11 show that adhesion dropped by an order of magnitude compared to previous measurements in air. The benefit of this smaller force was that measurements could be made at smaller gaps. 

The main difference between air and liquid is that other forces start to intrude, especially double-layer forces at long range and solvation forces at ranges less than a nanometer. The double-layer forces can be seen quite easily in the surface force apparatus. At pH around 6 in potassium nitrate (KNOj) solutions of various concentrations, the force was a repulsion which increased exponentially as the gap closed. For higher concentrations of salt, the slope increased as expected from DLVO theory (see Chapter 10). Results are shown in Fig. 6.12 for several concentrations ofKNOj, ranging from 10 M to 10 M. 

The results were found to fit the DLVO theory over a wide range of separations above 10 nm, when the Hamaker constant was taken as 2.2 X 10 . At closer approach the repulsion increased faster. There was no clear-cut evidence of the primary minimum of attraction which should be 

Fi iim fi.l2. Doiitiln-layttr ropulsion [orces medsuruci in KNO,i solutions,  

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