Proc. Nati. Acad. Sci. USAVol. 84, pp. 4722-4724, July 1987Symposium Paper

This paper was presented at a symposium "Interfaces and Thin Films," organized by John Armstrong, Dean E.Eastman, and George M. Whitesides, held March 23 and 24, 1987, at the National Academy of Sciences,Washington, D.C.

Direct measurements of forces between surfaces in liquids at themolecular levelJACOB ISRAELACHVILIDepartment of Chemical and Nuclear Engineering, and Materials Engineering Department, University of California, Santa Barbara, CA 93106

A knowledge of the forces between surfaces or particles inliquids, especially their magnitude and range, is essential forgaining insight into the fundamental intermolecular interac-tions occurring in both simple and complex multicomponentsystems and for understanding many everyday phenomena atthe molecular level. Examples of such phenomena includethe swelling of soils; the properties of colloidal and polymericsystems (such as paints); the strengths of materials such asceramics; the action of adhesives, detergents, and lubricants;biological organization and the interactions of biologicalmembranes; and many technological and industrial pro-cesses.During the last 20 years tremendous advances have been

made in measuring some of these forces, due mainly to thedevelopment of sophisticated force-measuring techniques,some of which can now directly measure the forces betweenmolecularly smooth surfaces in vapors or liquids, with asensitivity of 10 nN (1 14g on the earth's surface) and with adistance resolution (between the surfaces) of better than 1A-i.e., a resolution that is smaller than the size of theintervening liquid molecules. Here I shall briefly describe oneof these techniques, reviewing some of the more interestingresults that have been obtained by it (as well as by othermethods), and then end by speculating on future possibilities.The Surface Forces Apparatus. The surface forces appara-

tus was developed some 10 years ago (1, 2), a result of yearsof experience by Tabor, Winterton, Israelachvili, and otherswho had developed earlier devices for measuring surfaceforces in air or vacuum. A recently improved version of thisapparatus (shown in Fig. 1) allows for both attractive andrepulsive forces to be measured over a range of six orders ofmagnitude. Though mica surfaces are the primary surfacesused in these measurements, it is possible to deposit or coatthese surfaces with polymer layers, metal films, surfactant orlipid monolayers and bilayers, etc., so as to alter the natureand chemistry of the interacting surfaces while keeping themsmooth by virtue of the molecularly smooth mica substratesurface underneath.

Applications of the Surface Forces Apparatus. The surfaceforces apparatus has been used to identify and quantify thefundamental long-range interactions between surfaces invarious liquids and liquid mixtures, polymer and surfactantsolutions, etc. (3). These fundamental interactions include: (i)attractive van der Waals forces; (ii) repulsive or attractiveelectrostatic "double-layer" forces, which arise when sur-faces carry a net charge; (iii) solvation or hydration forces,which arise from the structuring or ordering of liquid (sol-vent) molecules at surfaces (these can be attractive orrepulsive); and (iv) entropic (steric or undulation) forces,

which arise from the interactions of protruding mobile sur-face groups such as polymers, surfactant head-groups or fluid(viz. mobile or "undulating") interfaces; these, too, can beattractive or repulsive. Other types of interactions have alsobeen studied by using this apparatus, such as short-rangeadhesion forces and surface deformations arising duringadhesive contacts (4), and-more recently-dynamic inter-actions, including the viscosity of liquids in very thin films (5,6). In what follows we shall briefly review some of the moreimportant of these interactions in turn.van der Waals and Electrostatic "Double-Layer" Forces. A

series of experiments on the forces between mica andsurfactant/lipid bilayer surfaces in aqueous electrolyte solu-tions containing both mnonovalenlt 1:1 electrolytes (such asNaCl) and divalent 2:1 electrolytes (such as CaCl2) show thatthe force laws are excellently described by a superposition oflong-range attractive van der Waals forces (based on theLifshitz theory) and repulsive double-layer forces (based onthe Poisson-Boltzmann equation) (7-10). This constitutes adirect verification of the celebrated Derjaguin-Landau-Ver-wey-Overbeek (DLVO) theory (3) which has been the basisfor analyzing colloidal and biocolloidal interactions since the1940s. However, in the case of high surface charge in thepresence of divalent counterions (as occurs for certainnegatively charged surfactant or lipid bilayer surfaces withCa2l in the bathing medium) there appears to be an enhancedattraction (11) (in excess of the van der Waals attraction) atsurface separations below about 30 A. This effect may be dueto ion correlations, which have been predicted (12) to arise injust such situations, and may underlie the well-known ten-dency of colloidal particles and biological membranes toaggregate or fuse in the presence of divalent cations such ascalcium.

Solvation Forces. Experiments on a variety of differentsystems suggest that there are two types of solvation forces(13, 14). The first is quite general and arises quite simply fromthe finite size or "geometry" of molecules. Thus whereas theDLVO theory treats the liquid solvent medium as astructureless continuum, a more realistic treatment that takesthe finite-sized solvent molecules specifically into accountpredicts that the force law between any two surfaces,particles, or solute molecules should be oscillatory at sepa-rations extending over a few molecular diameters (3). Thisdecaying oscillatory force has a periodicity equal to thediameter of the liquid molecules and reflects their tendencyto pack into discrete, but diffuse, layers at surfaces-aphenomenon that is entirely analogous to the "radial distri-bution function" and "potential of mean force" charactersistic of molecular interactions in pure liquids.The second type of solvation force is more difficult to

understand. This is characterized by a monotonically decay-ing (usually exponential) force law, and it appears to bespecifically associated with the interactions of water (hence

4722

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Proc. Natl. Acad. Sci. USA 84 (1987) 4723

different force-measuringsprings

upper rod

0 cm 5I I.

T

...,.......,.

.- main suppor.......................

A...................'..........

..........h lc ls rn

..-.............::::: ::::::::........................

:::::::::::::...........

canileersring

white light

FIG. 1. Recently modified version of the surface forces apparatus for measuring the forces between two curved molecularly smooth surfaces inliquids, showing some ofthe alternative (interchangeable) force-measuring springs suitable for different types ofexperiments. The separation betweenthe surfaces is measured (to better than 1 A) by use of an optical technique using multiple beam interference fringes. The distance between the twosurfaces is controlled by use of a three-stage mechanism: (i) coarse control (micrometer-driven upper rod), (it) medium control (differential springdriven by lower rod), and (ifT) fine control (to better than 1 A via the voltage-driven piezoelectric crystal tube supporting the upper mica surface). Thestiffness of the force-measuring spring can be varied by a factor of 1000 by shifting the position of the dove-tailed clamp with the adjusting rod.

"hydration" forces), and perhaps a small number of otherassociated liquids. Between hydrophilic surfaces (such assurfactant or silica surfaces) the hydration force is repulsiveand results in such phenomena as micelle and bilayer stabil-ity, and clay swelling. Fig. 2 shows an example of amonotonically repulsive hydration force law (with the oscil-latory component superimposed at smaller separations). Incontrast to the solvation force between hydrophilic surfaces,that between two hydrophobic surfaces is strongly attractive,but still exponential and with a decay length similar to therepulsive interaction (3, 16, 17). Such forces have been foundto extend over long range (at least as far as the van der Waalsinteraction, while being much stronger) and are responsiblefor many aggregation processes, the immiscibility of waterand oil, detergency, the spontaneous self-assembly of bio-logical membranes, and the folding of proteins.

Steric and Undulation Forces. Surfaces with long-chainedpolymers attached to them interact with each other notdirectly but via the flexible and highly mobile polymersegments that protrude from them like seaweeds. This typeof interaction is normally repulsive and can be of very longrange, so that it usually dominates over the other types ofinteractions discussed so far. Indeed, polymer additives havelong been used to "sterically stabilize" colloidal disper-sions-i.e., prevent the suspended particles from aggregating(in paints and inks, for example). Another type of interactionthat falls into this category occurs when surfaces containshort (but still mobile) surface hydrophilic groups such assurfactant or lipid head-groups in micelles and bilayers inaqueous solutions. Any oscillatory component that may be

intrinsic to the solvation force is now smeared out by thethermal fluctuations of these head-groups, leaving only an

1.0Distance (nm)

FIG. 2. Short-range hydration forces between two mica surfaceswith low and high densities (% coverage) of adsorbed hydrated K+.The 2.5-A periodicity in the oscillatory force corresponds to thesqueezing out of successive layers ofwater molecules (modified fromref. 15). Note that for hydrophobic surfaces the force law would besimilar but the monotonic (smoothly varying dashed) part ofthe forcelaw would now be attractive-i.e., below the pressure = 0 axis (1 atm= 101 kPa).

Symposium Paper: Israelachvili

I

4724 Symposium Paper: Israelachvili

E 0. -l ludchains0.

E

0)

0Svan der Waats attraction

hydration-0.. repulsion

-0.1~~~~~~~.

0 l 2 3 4 5Interbilayer separation, nm

FIG. 3. Interaction energy between two adsorbed lecithin bilay-ers in water measured with the surface forces apparatus, showing theeffect of interfacial mobility (fluidity) in enhancing the monotonicallyrepulsive hydration repulsion between the bilayers (modified fromref. 19).

effectively monotonic (again, usually exponentially decay-ing) repulsive hydration force law between the surfaces, asinvestigated in detail by Rand, Parsegian, and coworkers(18). For such surfaces, their mutual repulsion increases withthe mobility of the head-groups (Fig. 3), the undulations ofthe fluid-like interfaces, and the decreasing bending modulusof the bilayers. Such short-range interactions are difficult toseparate into their pure solvation (hydration) and stericcomponents, which act together to give the final interactionpotential. These short-range steric-like interactions betweenamphiphilic structures such as bilayers are now popularlyreferred to as "undulation forces" after Helfrich and others(20, 21), who investigated their theoretical origin and sug-gested their importance.Measurements of Other Interfacial and Thin Film Interac-

tions. Over the years the versatility and scope of the surfaceforces apparatus have been extended with the design of newattachments so that it has also been possible to measurevarious interfacial phenomena (other than forces) at themolecular level-e.g., the optical properties of thin films,adsorption from solution, the elastic deformations of adher-ing surfaces, capillary condensation and capillary forces, thefusion of lipid bilayers, and the viscosities of liquids nearsurfaces (5, 6), where it was found that the bulk viscosity ofsimple liquids such as water and hydrocarbons was already

manifest in the first layer of molecules adjacent to the(molecularly smooth) surface of mica.Future Prospects. The recent and successful attempt to

measure a dynamic property (i.e., the viscosity) of liquids inthin films suggests that other types of dynamic (as opposedto static or equilibrium) interactions could be measured, suchas the viscosity, relaxation processes, and other time-depen-dent phenomena in complex (e.g., polymeric) fluid films. Inaddition, various tribological phenomena such as friction andlubrication could be studied at the microscopic level. Thedevelopment of new surfaces and surface coatings, and newliquids and mixtures, offers an almost unlimited potential forresearch at both the fundamental and applied level. In partic-ular, the as yet untried possibility of looking at the specificinteractions of biological macromolecules such as proteins,antibodies, antigens, biomembranes, etc., should provide someunique insights that are unavailable by any other techniques.This possibility should quickly become a reality once itbecomesfeasible to attach these macromolecules onto the surfaces ofmica in some quantitative and ordered way.

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Proc. Natl. Acad. Sci. USA 84 (1987)