Experimental measurements of solvation forces in nonpolar liquids

Forces between molecularly smooth mica surfaces immersed in tetrachloromethane, benzene, and 2,2,4-trimethylpentane have been measured. In tetrachloromethane and benzene the force law is an oscillatory function of the separation between the mica surfaces, with a periodicity equal to the mean molecular diameter and a measurable range of appoximately ten periods, or about 5 nm. Beyond this separation, the oscillations merge into a purely attractive ‘‘tail’’. In 2,2,4-trimethylpentane the force law shows only short-range oscillatory behavior, below 2 nm; at larger separations the force is everywhere attractive. The results are compared with measurements on cyclohexane, octamethylcyclotetrasiloxane and n-octane, including some repeat measurements. When the distance scale is normalized by the mean periodicity of the force curve for each liquid, the results for the four fairly rigid molecules benzene, tetrachloromethane, cyclohexane, and octamethylcyclotetrasiloxane are very similar; minor differences possibly being due to varying trace amounts of water in the liquids. In the above systems the force laws are oscillatory functions of the surface separation, with a periodicity roughly equal to the molecular diameter and a measurable range of about ten periods. The peak-to-peak amplitude shows an essentially exponential decay. By contrast, the existence of free intramolecular rotation in the two flexible molecules 2,2,4-trimethylpentane and n-octane is sufficient to reduce the range of the solvation forces to 2 nm or less. The results indicate that the amount of liquid structuring for inert liquids depends mainly on the internal rigidity of the molecules, while the effect of molecular shape, size, and temeperature is of secondary importance.