Direct measurements of forces between surfaces in liquids at the molecular level

Abstract

A knowledge of the forces between surfaces or particles in liquids, especially their magnitude and range, is essential for gaining insight into the fundamental intermolecular interactions occurring in both simple and complex multicomponent systems and for understanding many everyday phenomena at the molecular level. Examples of such phenomena include the swelling of soils; the properties of colloidal and polymeric systems (such as paints); the strengths of materials such as ceramics; the action of adhesives, detergents, and lubricants; biological organization and the interactions of biological membranes; and many technological and industrial processes. During the last 20 years tremendous advances have been made in measuring some of these forces, due mainly to the development of sophisticated force-measuring techniques, some of which can now directly measure the forces between molecularly smooth surfaces in vapors or liquids, with a sensitivity of 10 nN (1 jxg on the earth's surface) and with a distance resolution (between the surfaces) of better than 1 A-i.e., a resolution that is smaller than the size of the intervening liquid molecules. Here I shall briefly describe one of these techniques, reviewing some of the more interesting results that have been obtained by it (as well as by other methods), and then end by speculating on future possibilities. The Surface Forces Apparatus. The surface forces apparatus was developed some 10 years ago (1, 2), a result of years of experience by Tabor, Winterton, Israelachvili, and others who had developed earlier devices for measuring surface forces in air or vacuum. A recently improved version of this apparatus (shown in Fig. 1) allows for both attractive and repulsive forces to be measured over a range of six orders of magnitude. Though mica surfaces are the primary surfaces used in these measurements, it is possible to deposit or coat these surfaces with polymer layers, metal films, surfactant or lipid monolayers and bilayers, etc., so as to alter the nature and chemistry of the interacting surfaces while keeping them smooth by virtue of the molecularly smooth mica substrate surface underneath. Applications of the Surface Forces Apparatus. The surface forces apparatus has been used to identify and quantify the fundamental long-range interactions between surfaces in various liquids and liquid mixtures, polymer and surfactant solutions, etc. (3). These fundamental interactions include: (i) attractive van der Waals forces; (ii) repulsive or attractive electrostatic double-layer forces, which arise when surfaces carry a net charge; (iii) solvation or hydration forces, which arise from the structuring or ordering of liquid (solvent) molecules at surfaces (these can be attractive or repulsive); and (iv) entropic (steric or undulation) forces,