Direct measurements of the force between hydrophobic surfaces in water

Advances in Colloid and Interface Science 91 Ž2001. 391᎐436

Direct measurements of the force between hydrophobic surfaces in water Hugo K. Christenson a,U , Per M. Claesson b,c a

Department of Applied Mathematics, Research School of Physical Sciences, Australian National Uni¨ ersity, Canberra ACT 0200, Australia b Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden c Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden

Abstract Direct measurements of the force between hydrophobic surfaces across aqueous solutions are reviewed. The results are presented according to the method of preparation of the hydrophobic surfaces. No single model appears to fit all published results, and an attempt is made to classify the measured interactions in three different categories. The large variation of the measured interaction, often within each class, depending on the type of hydrophobic surface is emphasized. ŽI. Stable hydrophobic surfaces show only a comparatively short-range interaction, although little quantitative data on this attraction have been published. ŽII. Many results showing very long-range attractive forces are most likely due to the presence of sub-microscopic bubbles on the hydrophobic surfaces. Such an interaction is typically measured between silica surfaces made hydrophobic by silylation. Between self-assembled thiol layers on gold surfaces very short-range attractive forces are possibly due to the presence or nucleation of bubbles. The reason for the apparent stability of these bubbles is not clear and warrants further investigation. ŽIII. Results obtained with LB films of surfactants or lipids on mica appear to give rise to a different type of force that fits neither of these two categories. This force is an exponentially decaying attraction, often of considerable range. The force turns more attractive at smaller separations, and may at short range be similar to the interaction measured between stable hydrophobic surfaces. An apparently similar, exponential attraction is also found between mica surfaces bearing surfactants adsorbed from cyclohexane, between silylated, plasma-treated mica surfaces and between

U

Corresponding author. Present address: Department of Physics and Astronomy, The University of Leeds, Leeds, LS2 9JT, UK. E-mail address: [email protected] ŽH.K. Christenson.. 0001-8686r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 1 - 8 6 8 6 Ž 0 0 . 0 0 0 3 6 - 1

392 H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436

both mica and silica surfaces with surfactants adsorbed in situ. This type of force also occurs between some surfaces of relatively low hydrophobicity as well as between one such hydrophobic surface and a hydrophilic surface. No convincing model can explain this third type of interaction for all systems in which it has been observed. This review of work to date points to the importance of the morphology and structure of the hydrophobic surface, and how it may change during the interaction of two surfaces. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrophobic attraction; Hydrophobic force; Hydrophobic surface; Force measurements

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392 1.2. Force measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 1.3. Hydrophobic surfaces for force measurements . . . . . . . . . . . . . . . . . . . . 399 1.4. Hydrophobic surfaces in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400 2. Forces measured between hydrophobic surfaces . . . . . . . . . . . . . . . . . . . . . . . 402 2.1. LB films deposited on mica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402 2.2. Surfactant films adsorbed to mica from cyclohexane . . . . . . . . . . . . . . . . 407 2.3. Polymerized LB films deposited on mica . . . . . . . . . . . . . . . . . . . . . . . 410 2.4. Surfaces prepared by in situ adsorption of surfactants . . . . . . . . . . . . . . . 412 2.5. Hydrophobic surfaces of silylated silica and mica . . . . . . . . . . . . . . . . . . 417 2.6. Bulk polymer surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422 2.7. Thiols self-assembled on gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 2.8. Measurements in non-aqueous solution . . . . . . . . . . . . . . . . . . . . . . . . 426 2.9. Forces between one hydrophobic and one hydrophilic surface . . . . . . . . . . 426 2.10. Long-range attractive forces between nucleic acid surfaces . . . . . . . . . . . . 428 3. Classification of interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 3.1. Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 3.2. Short-range, but strongly attractive force between apparently very stable surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 3.3. An attraction of variable strength and range, due to the presence of bubbles . 430 3.4. A very long-range, attractive force with exponential decay . . . . . . . . . . . . 431 3.5. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .433 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434

1. Introduction 1.1. Background The direct measurement of forces between macroscopic surfaces is a relatively recent area of surface science, but one that has evolved dramatically in the last 20

H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436 393

years. The technique has now expanded to encompass subjects such as rheological and tribological studies of thin films w1x, investigations of phase behaviour in confinement w2x as well as the adhesion and deformations of surfaces in contact w3x. Nevertheless, the study of the forces acting between surfaces across liquids remains of central importance. Double-layer and van der Waals forces across aqueous solutions have been extensively studied w4x, as have solvation or structural effects in both aqueous w4,5x and non-aqueous liquids w4,6x, interactions in surfactant systems w7x, forces across solutions of polymers w8x, proteins and other macromolecules w9x, etc. One of the most intensively studied areas for the past 15 years has been the interaction between hydrophobic surfaces across water and aqueous solutions w10x. The work has centred on understanding the origin of a strong and often very long-range attractive interaction measured between hydrophobic surfaces. The topic is an intriguing one because theory is as yet unable to account for a force of such range and magnitude. The first direct indications that the measured force between two hydrophobic surfaces is of greater magnitude than the continuum van der Waals force were obtained by Israelachvili and Pashley in 1982 w11,12x. They reached their conclusion by subtracting the expected classical interaction ᎏ a van der Waals interaction and a double-layer repulsion extrapolated from results fitted at larger distances ᎏ from the force measured between mica surfaces made hydrophobic by adsorption from solution of the cationic surfactant hexadecyltrimethylammonium bromide Žcetyltrimethylammonium bromide, CTAB.. The result was an exponentially decaying attractive force in the 0᎐10-nm regime ŽFig. 1., and they postulated that this was related to the attractive force found between small hydrophobic solute molecules in water. This ‘hydrophobic effect’ is a familiar concept in the thermodynamics of non-polar molecules in aqueous media, and it has been extensively invoked in connection with amphiphile aggregation and the conformation in solution of hydrophobic macromolecules such as proteins w13x. Despite questionable assumptions involved in making the analogy with the hydrophobic effect, and a number of experimental uncertainties, this was the impetus for a widening investigation into the forces between macroscopic hydrophobic surfaces. Two spectacular examples of an attractive force of exceptional range between hydrophobic surfaces were published in 1988, independently by Christenson and Claesson w14x and by Rabinovich and Derjaguin w15x. The measured force between mica surfaces made hydrophobic by Langmuir᎐Blodgett ŽLB. deposition of a monolayer of dimethyldioctadecylammonium bromide ŽDDOABr. on mica, and between silica surfaces silylated with dimethyldichlorosilane vapour, was shown to be an exponentially decaying attraction with a decay length of 12᎐13 nm in the range 20᎐80 nm ᎏ up to 100 times stronger than the predicted van der Waals attraction ŽFigs. 2 and 3.. The very good agreement between these separate series of measurements with differently prepared surfaces suggested that there was indeed a universal force, albeit of unknown origin, between hydrophobic surfaces. This force could not be explained by classical concepts such as DLVO theory, but could perhaps be related to some measure of the hydrophobicity of the surfaces such as the advancing contact angle of water on the surface. Both types of surface

394 H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436

Fig. 1. Attractive force law deduced from forces measured between mica surfaces immersed in CTAB solutions. Each point is the difference between a measured force and the expected DLVO interaction, i.e. a double-layer repulsion and a van der Waals attraction. For comparison, the shaded region shows the limits of the van der Waals force computed from Lifshitz theory for two mica surfaces and two hydrocarbon surfaces interacting across water. ŽFrom J.N. Israelachvili, R.M. Pashley, J. Colloid Interface Sci. 98 Ž1984. 500..

used in these studies were considerably more hydrophobic than the ones used by Israelachvili and Pashley, and the apparent absence of surfactant molecules in solution lent added credibility to the results. Remarkably, the measured range of this attractive force was close to what Blake and Kitchener had found to be the lower limit of stability of water films between an air bubble and hydrophobed silica ᎏ an early indication of long-range hydrophobic forces w16x. In the dozen years since, however, it has become increasingly clear that the situation is very much more complicated than at first realized, and at present Ž1999. the apparent state of affairs appears at first sight to be more confusing than ever. Nevertheless, we will here suggest that thanks to a number of recent experiments and ideas the picture is beginning to clear. Firstly, it now appears firmly established that certain hydrophobic surfaces do not show the very long-range attraction. Instead, there is a short-range attraction that is stronger than the van der Waals interaction, although detailed information is lacking. Secondly, in many cases it is undoubtedly the presence of microscopic bubbles that gives rise to the force. The presence and apparent stability of these bubbles have not yet been satisfactorily explained. In many instances there is no evidence of bubbles, and an exponentially decaying attraction is measured in the range 20᎐100 nm or beyond. The origin of this force is still a mystery. In what follows we will give a brief summary of facts necessary for the apprecia-

H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436 395

Fig. 2. Force measured in water between LB films of dioctadecyldimethylammonium Žbromide. deposited on mica. The different symbols denote separate series of measurements. The solid line is the van der Waals interaction expected between mica surfaces across water.

tion of the different experiments, as well as some simple thermodynamic arguments about the possible nature of the expected interaction. We will survey the literature results on experimental measurements of hydrophobic forces obtained with a variety of direct force-measuring techniques. Finally, we will attempt to classify the results and give some hints for future work, with a view to better understanding the origin and puzzling variety of force laws measured between hydrophobic surfaces across aqueous solution. The various theoretical models and

Fig. 3. A comparison between the total attractive force measured between LB films of dioctadecyldimethylammonium Žbromide. deposited on mica Ž`. and between methylated silica Ž䢇.. Note the apparent exponential decay of the attractive interaction. ŽFrom H.K. Christenson, in: M.E. Schrader, G. Loeb ŽEds.., Modern Approaches to Wettability: Theory and Applications, Plenum Press, New York, 1992..

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ideas that have been advanced to explain the experimental results do not form part of this review and will only be mentioned briefly in Section 3. This merely reflects that the authors, being experimentalists, feel that it is beyond their ability to critically review and discuss all models proposed to explain the long-range attraction between hydrophobic surfaces. We hope that this task will be considered by others. 1.2. Force measurements Several different instruments have been used for measuring forces between hydrophobic surfaces in water. These are the surface force apparatus ŽSFA. w17,18x, the MASIF Žmeasurement and analysis of surface interaction and forces. w7x, the atomic force microscope ŽAFM. w19,20x, the interfacial gauge ŽIG. w21x, a device known as the light-lever instrument for force evaluation ŽLLIFE. w22x and the method of crossed-quartz filaments w15,23x used by Rabinovich et al. Detailed descriptions of these can be found in the appropriate references, and the relative merits of each method have been the subject of much discussion. For the present purposes we will merely emphasize those features that are necessary for understanding the results to be reviewed, and these are largely common to all techniques. We will also point out what particular information may only be obtained with some of the devices. To determine the force vs. distance curve between two surfaces in the range 0᎐200 nm Žin all but a few cases no interaction is measurable at larger surface separations. it is necessary to have some means of independently both measuring and controlling the surface separation. The surface configuration is either that of two crossed cylinders, two spheres or a sphere and a flat plate. The force is detected by monitoring the deflection of a spring Žeither mechanical or piezoelectric. supporting one of the surfaces. The surfaces are displaced relative to one another with a piezoelectric device, and the separation is usually determined by suitable calibration of this displacement, except in the SFA Žsee below.. Often, there are inaccessible regions of the force curve where the gradient of the force law exceeds the spring stiffness k. This is due to mechanical instability of the spring system, or insufficient response of any force-feedback system. When this situation arises a sudden change in surface separation to the next stable region occurs, a phenomenon normally reported as a ‘jump’ and illustrated by an arrow in many figures. It may hence be difficult to determine strongly attractive interactions such as those often found between hydrophobic surfaces at small separations. In many cases the spring supporting one of the surfaces is so weak and the force resolution so low that no deflection is detectable before the surfaces jump together. The measurements in such cases yield only one point per force curve ᎏ the separation of the inward jump, or jump distance. This separation is then an indication of where the gradient of the force exceeds k, and this is often taken as a direct measure of the range of the attractive interaction. By changing the spring constant it is possible to measure the gradient of the force as a function of surface

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separation, a method that is usually less accurate and more time-consuming than the normal force measuring procedure. The radius of curvature of the surfaces varies across a wide range in the different types of measurements ᎏ from approximately 2 cm in the case of the SFA to effectively 0.1 ␮m or less in some AFM work. Force vs. distance data are presented by normalizing the measured force by the mean radius of curvature, e.g. R s Ž R1 R 2 .1r2 for crossed cylinders at right angles. Through use of the Derjaguin approximation w24x the normalized force FrR may be related to the free energy of interaction per unit area between parallel, flat surfaces Gf , Fcc r2 ␲ R s Fsfr2 ␲ R s Fss ␲ R s Gf

Ž1.

where the subscript cc stands for crossed cylinders, sf for a sphere against a flat and ss for two spheres. The results should thus be independent of the force-measuring technique used and direct comparison between measurements with different instruments should be valid, except in cases where surface deformations affect the measured results. In most of the results discussed here surface deformation effects may be ignored except for measurements of the pull-off force or adhesion force. Nevertheless, there are indications that some results with surfaces of small radii of curvature show much larger hydrophobic forces than those obtained with larger surfaces, even after normalization w25x. In particular, forces due to bridging bubbles may not scale with the radius of the interacting surfaces w26x. In the SFA multiple-beam interferometry w27,28x is employed to measure the surface separation, and this optical technique allows the simultaneous determination of the refractive index of the medium between the surfaces. The surface separation is always calculated relative to an initial measurement with the surfaces in contact in dry nitrogen, and an absolute zero of distance is obtained in these experiments. By contrast, in all other techniques contact is assigned to the point at which the spring deflection becomes linear with increasing load, the so-called constant compliance regime. Consequently, a number of features of the measurements are only accessible by use of the SFA. This includes direct observation of both phase changes in the interlayer between the surfaces Ži.e. cavitation ᎏ the formation of a vapour or gas bubble between the surfaces ᎏ see Section 1.4., and surface deformations occurring during the measurements. Other techniques cannot readily distinguish between surface deformations and strongly repulsive forces ᎏ in both cases an increase in the applied load and consequent surface flattening will lead to a shift in the apparent surface separation. It should be noted that surface flattening always invalidates the Derjaguin approximation and that it in all cases, except in SFA measurements, results in an error in the surface separation. In many of the force-measuring devices used the surfaces are always brought into contact once before force vs. separation data are collected. A measurement on first approach is consequently not possible, or at least very difficult. This applies in particular to experiments with the AFM, and this is not always discussed in the published material. At 2 cm, the radius of curvature of the surfaces in SFA measurements is larger

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than in any of the other experiments. Under the influence of the large adhesion of hydrophobic surfaces in water this should cause greater surface deformations than when any other technique is used. Indeed, the observed effects are substantial, with contact diameters of the flattened contact zone reaching 25᎐50 ␮m. None of the other techniques, however, gives a direct measure of the extent of surface flattening. Theoretical arguments w29x suggest that surface flattening should be large enough to affect the measured pull-off force in all experiments except perhaps in AFM measurements with hydrophobed silicon nitride tips or micrometre-size silica spheres. Theoretically w30,31x, the adhesion force between ideally rigid, crossed cylindrical surfaces, or a sphere against a flat, is related to the surface or interfacial energy ␥ by FrR s 4 ␲ ␥

Ž2.

whereas in the presence of elastic surface deformations this is expected to change to FrR s 3␲ ␥

Ž3.

In case of two interacting spheres, the right hand side of Eqs. Ž2. and Ž3. should be divided by a factor of two. In the SFA the surfaces are often observed to come apart with a finite contact diameter, as predicted by theory for strongly adhesive forces. Given the typical spread in measured values, and the uncertainty in the precise value of the energy of a given hydrophobic surface᎐water interface, it is in most cases not possible to experimentally distinguish between the two limiting cases of rigid and deformable surfaces w34x. In addition, the adhesion measured between hydrophobic surfaces in air has been found to be highly dependent on the particular surfactant or lipid monolayer used to coat the mica substrate. The differences have been interpreted in terms of chain interdigitation and energy losses in the monolayers during the separation process w33x. The mode of separation of the surfaces has been found to greatly affect measured adhesion values between some surfaces in air w32x. In particular, use of a single-cantilever or leaf spring, such as is employed in most devices except the SFA, leads to an underestimation of the true adhesion force. The extent to which these factors might be important in aqueous solutions is not clear. A further complication is the occurrence of cavitation. The presence of a bridging vapour or air bubble between the surfaces is expected to change the pull-off force between deformable surfaces from that given by Eq. Ž3. to a value more in accordance with Eq. Ž2. Žsee further Section 1.4.. This has been shown both experimentally w32x and theoretically w35x for the case of bridging liquid condensates in vapour, where the surfaces clearly regain their undeformed shape before coming apart. In the SFA measurements the bubble is often observed to have formed only after separation from contact, which causes further uncertainty in the interpretation of the adhesion values. In the detailed review of the results we will quote only the normalized, measured adhesion forces. In view of the above uncertainties it may not be prudent to place to much emphasis on the exact values quoted. We merely note that empirically it

H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436 399

has been found that the adhesion ranges up to approximately 500 mNrm. Using Eq. Ž3. or Eq. Ž2. this becomes equivalent to surface energies of 40᎐53 mJrm2 , which is reasonable for a smooth, hydrophobic surface in water. 1.3. Hydrophobic surfaces for force measurements The fundamental problem in attempting to measure directly the force between hydrophobic surfaces is the preparation of a suitable surface. A number of methods has been used, most of which are based on the modification of either silica or mica. Each of these has its advantages and its drawbacks, and as we shall see there is no single method that is obviously superior to all others. Mica is atomically smooth over macroscopic Žcm2 . areas, and fused silica or glass has a surface roughness of at most a few Angstroms, and any modification method relies on maintaining this smoothness as far as possible. As both mica and silica are negatively charged in dilute aqueous solutions of electrolyte at normal pH, modification by cationic surfactants is a straightforward means of producing a hydrophobic surface. Most early experimental studies of hydrophobic forces were carried out with such surfaces, either by allowing the cationic amphiphile to adsorb from dilute solution, or by depositing spread monolayers by the LB method. In either case the measured forces have been found to vary considerably with the type of surfactant or lipid used, and in particular the charge of the resultant surface varies widely. It is clear that details of the structure and stability of the hydrophobic surfaces thus produced are of crucial importance for the nature of the interaction. The second method of choice for making hydrophobic surfaces is based on chemical modification of silica or mica surfaces, usually with silanes. Alkylsilanes may react with free hydroxyl groups on the silica surface under suitable conditions to form covalent siloxane Ž ᎐Si᎐O᎐ . bonds w36x. Mica essentially lacks surface hydroxyl groups, but these may be introduced by treating the mica with water-vapour plasma, and the ‘activated’ mica will then react with silanes in a similar manner to silica w37,38x. Experimental results with silylated surfaces have also shown great variability depending on exact preparation conditions, and further underscored the critical connection between details of surface chemistry, surface morphology and the measured forces w39᎐42x. LB deposition and silylation have been combined in a technique involving the deposition of insoluble monolayers of silanes on mica w43x or silica w44x. The monolayers are polymerized at the air᎐water interface and then deposited and allowed to react with the silica or activated mica surface. Further techniques relying on the use of a mica or glass substrate are plasma polymerization from the vapour phase w45x and spin coating of hydrophobic polymer films onto mica. In both cases smoothness and the strength of bonding of the hydrophobic layer to the underlying substrate are cause for concern. Hydrophobic surfaces not based on modified silica or mica surfaces include bulk polymers. With such materials smoothness, possible porosity, and reactivity need to be considered. Gold surfaces made hydrophobic by self-assembly of long-chain

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alkyl thiols w46x are a very recent example of hydrophobic surfaces used in direct force measurements. The strength of the gold-sulfur bond seems to ensure that such surfaces are less prone to degradation than the other hydrophobic surfaces employed in direct force measurements. 1.4. Hydrophobic surfaces in water A hydrophobic surface in water has a large surface energy. The interfacial energy between a surface consisting of mainly ᎐CH 2 groups and water is approximately 50 mJrm2 w47x, which is considerably more than expected from simple considerations of van der Waals forces. Arguments based on the calculated Hamaker constant A of hydrocarbon across water would predict a value of an order of magnitude less. The use of a somewhat arbitrary ‘cut-off’ distance D 0 s 0.165 nm in the relationship for the van der Waals energy Gvdw w4x Gvdw s Ar12 ␲ D 2

Ž4.

where A is the Hamaker constant, would give a free energy at contact per surface of 3 mJrm2 instead of 50 mJrm2 . The very fact that this interfacial energy is larger than expected proves in itself that a ‘hydrophobic’ attraction must exist. If the energy at contact, or the interfacial energy, is larger than expected it is reasonable to conclude that the force at finite distances must also be larger than predicted by continuum theory. No prediction about the range of the interaction can be made, of course. The surface energy of a hydrophobic surface in air or water vapour is lower than in water. This statement is equivalent to saying that the contact angle of water on the surface ␪ is larger than 90⬚. This is expressed by the classical Young equation, ␥ SV s ␥ SL q ␥ LV cos␪

Ž5.

where ␥ SV is the energy of the hydrophobic surface-vapour interface, ␥ SL is the energy of the hydrophobic surface-liquid interface and ␥ LV is the surface energy of the liquid. It follows that it may be energetically favourable to replace water between two hydrophobic surfaces at small separations with vapour w48x. The necessary condition is that the area of the resulting water᎐vapour interface is small by comparison, as the creation of this interface increases the total free energy. In SFA experiments the configuration of the interacting surfaces is such that this criterion is often fulfilled already at rather large surface separations ᎏ up to 100 nm or even 1 ␮m, depending on the exact value of the contact angle. That fact that cavitation or capillary evaporation is thermodynamically favoured does not mean that it will occur. Estimates of the energy barrier involved show that it is an extremely unlikely process for separations larger than typical molecular dimensions w48x. Indeed, cavitation phenomena have been observed only at or close to contact with the SFA w14,49᎐51x, where the refractive index measurements allow the unambiguous detection of vapour between the surfaces ŽFig. 4.. In other cases

H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436 401

Fig. 4. The appearance of the multiple-beam interferometry fringes Žleft. and the actual surface conditions Žright. for two interacting mica surfaces coated with LB films of dioctadecyldimethylammonium Žbromide.. Ža. Surfaces at a separation of 100 nm in water. Žb. Surfaces in contact with flattening due to the strong adhesion between the hydrophobic surfaces. Žc. Surfaces after separation from contact, with a bridging vapour cavity. Žd. Surfaces with a narrowing vapour bridge after increasing the separation. ŽFrom H.K. Christenson, P.M. Claesson, Science 239 Ž1988. 390..

cavitation or the presence of vapour can often be inferred w15,23x, although there is sometimes considerable uncertainty in the correct interpretation of the experimentally determined force curves. It follows that the force between hydrophobic surfaces is, strictly speaking, often measured under non-equilibrium conditions. A long-range hydrophobic force may be measured on approach, but on separation from contact there is an equilibrium capillary force due to a bridging vapour bubble. The Laplace equation places limits on the possible size and shape of vapour

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cavities between two hydrophobic surfaces. The pressure change ⌬ P across a vapour᎐liquid interface of principal radii of curvature r 1 and r 2 is given by ⌬ P s ␥ LV Ž 1rr1 q 1rr 2 .

Ž6.

The pressure in the bulk aqueous phase is 1 atm, and the pressure in a vapour bubble can be no less than 0. It follows that the only permitted bubbles are those that either have two very small radii of curvature of nearly equal magnitude but opposite sign, or are larger than a minimum size, i.e. the negative curvature is not too great. Depending on the contact angle at the three-phase line Žhydrophobic surface᎐vapour᎐water. only certain bubble sizes are permitted. Observations with the SFA have been found to be in semi-quantitative agreement with these conditions, which account for why bridging bubbles that form after separation from contact disappear when the surfaces are brought into renewed contact. With the surfaces in contact only discrete bubbles around the perimeter of the contact area form, and a single annular bubble must exceed a minimum size w14x.

2. Forces measured between hydrophobic surfaces 2.1. LB films deposited on mica Hydrophobic surfaces produced by deposition of the double-chain cationic surfactant dimethyldioctadecylammonium ŽDDOA. bromide on mica were used in the first study to show the existence of a truly long-range interaction, i.e. the detectable range of the hydrophobic attraction was considerably larger than that of any possible van der Waals force w52x. The surfaces were weakly charged, as shown by the presence of a small double-layer repulsion, but the advancing contact angle of water Ž94⬚., the measured thickness of the deposited layers Ž2 nm., and surface coverage measurements with X-ray photoelectron spectroscopy ŽXPS. suggested a well-packed monolayer surface w53x. The low value of the receding angle Ž50᎐70⬚., however, hinted at heterogeneity of the deposited layers. The hydrophobic attraction inferred from these SFA experiments Žby subtracting the double-layer repulsion obtained by fitting at larger separations. could be expressed as the sum of two exponentially decaying attractive terms, FrR s C1exp Ž yDr␭ 1 . q C2 exp Ž yDr␭ 2 .

Ž7.

The decay lengths were ␭ 1 s 1.2 nm and ␭ 2 s 5.5 nm, and the pre-exponential factors were C1 s y0.36 Nrm and C2 s y6.6 mNrm. ŽIn subsequent accounts of results the parameters of Eq. Ž7. will often be used as a quantitative estimate of the range of the measured interaction.. The extrapolated value at D s 0 Žcontact., or F0rR s y0.37 Nrm agreed within the estimated error with the measured adhesion force Žy0.43 Nrm., and this was consistent with the measured adhesion force in air and the advancing contact angle of water using the Young equation ŽEq. Ž5... Addition of KBr to 10y2 M modified the interaction only slightly ᎏ there was

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no measurable double-layer repulsion and the parameters for the hydrophobic attraction changed to ␭ 1 s 1.2 nm, ␭ 2 s 4.5 nm, C1 s y0.36 Nrm and C2 s y6.3 mNrm. A large number of measurements of the force between hydrophobic surfaces coated with LB films of DDOA has been carried out since these first investigations w14,50,54᎐58x. Some of these results were mentioned in Section 1.1 and are shown in Figs. 2 and 3. A certain amount of variability in the apparent surface charge and the contact angle of water on the surfaces depending on deposition conditions has been found. For example, five separate experiments with apparently uncharged surfaces showed decay lengths at long range of 12᎐14 nm Ž ␭ 2 in Eq. Ž7.. and pre-exponential factors w C2 in Eq. Ž7.x in the range y2.5 to y 3.5 mNrm w59x. The short-range force was not accurately determined in most of these experiments, but approximate values of ␭ 1 s 3 nm and C1 s y0.10 Nrm were obtained in one case w54x. Hato w58x, in a later study, obtained ␭ 1 s 2 nm and C1 s y0.38 Nrm, and Žapprox.. ␭ 2 s 15᎐20 nm and C2 s y1 mNrm with surfaces of ␪a s 96⬚. With one of the surfaces in the SFA mounted on a stiff spring the jump apart from contact was small enough that cavitation could be observed between the DDOA-coated surfaces ŽFig. 4.. It is possible that a vapour or air bubble is always created during separation of the surfaces, but if a weak spring such as that commonly used for force measurements is employed, the jump apart is very large. The bubble will then break and disappear before the surfaces can be observed after they have come to rest again. If the surfaces were brought into renewed contact after separation with a rigid spring, the vapour bubble was found to disappear again, as expected Žsee Section 1.4.. The variation in the measured adhesion between the DDOA surfaces may be related to differences in cavitation behaviour. No correlation with the strength of the long-range attraction was found, even though the adhesion varied from 200 to 500 mNrm. The force between hydrophobic surfaces prepared by deposition of several other double-chain cationic surfactants on mica was reported by Hato w58x ŽFig. 5.. Interestingly, no correlation with the hydrophobicity as determined by ␪a of water was found. Indeed, the most long-range attraction, with ␭ 2 s 30 nm and C2 s y3.5 mNrm Ž ␭ 1 s 1 nm; C1 s y0.17 Nrm. was reported for surfaces showing a ␪a of only 82⬚ Žan equimolar mixture of arachidic acid and ␻-hydroxyeicosyloctadecyldimethylammonium bromide.. In all cases the attraction became much stronger, with a considerably shorter Žexponential. decay length at small separations. Mixed monolayers of eicosylamine and arachidic acid deposited on mica Ž ␪a s 113⬚. gave a long-range attraction of roughly the same range w60x Žno detailed data were presented. as that found between DDOA-coated surfaces. If the hydrophobicity of the mixed monolayer was reduced by incorporating the bifunctional acid docosandioic acid Žthus exposing carboxylic acid groups to the solution. the attraction decreased and an electrostatic repulsion became evident. These monolayers were not stable on repeated approach-separation cycles, presumably because of the lack of electrostatic binding to the mica surface for these essentially neutral lipids. An attractive interaction of exceptional range was found by Kurihara et al. using polymerized LB films deposited on mica w61,62x. A double-chain quaternary am-

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Fig. 5. Forces measured between various LB films deposited on mica. Circles: dimethyldioctadecylammonium bromide Ž ␪a s 96⬚, ␪ r s 66⬚.; triangles: ␻-Žhydroxyhexadecyl.dimethyloctadecylammonium bromide and arachidic acid 1:1 Ž ␪a s 91⬚, ␪r s 55⬚.; diamonds and squares: ␻-Žhydroxyeicosyl.dimethyloctadecylammonium bromide and arachidic acid 1:1 Ž ␪a s 82⬚, ␪r s 69⬚.; crosses: ␻-Žhydroxyeicosyl.dimethyloctadecyl-ammonium bromide and arachidic acid 1:3 Ž ␪a s 95⬚, ␪ r s 81⬚.. The arrows indicate positions of inward jumps. ŽFrom M. Hato, J. Phys. Chem. 100 Ž1996. 18530..

monium surfactant Žname not given. was photopolymerized in aqueous dispersion and dissolved in an organic solvent ŽC 6 H 6rCH 2 Cl 2rEtOH in the ratio 8:1:1. and spread as an insoluble monolayer on water. This was deposited on mica in the downstroke mode at a surface pressure of 35 mNrm. The resulting force curves were attractive out to surface separations of 250 nm, with ␭ 2 s 62 nm and C2 s y1.7 mNrm, decreasing to ␭ 2 s 42 nm and C2 s y0.25 mNrm at a concentration of 10y2 M NaBr. The force became more strongly attractive at separations below approximately 50 nm, although this was not accurately determined. Hydrophobic forces between LB films of fluorinated cationic surfactants on mica have also been the subject of detailed study w54,62᎐64x. The overall picture was very similar to that found with DDOA. With monolayers of N-Ž ␣-Žtrimethylammonio.acetyl.-0,0⬘-bis-Ž1 H,1 H,2 H,2 H-perfluorodecyl.-L-glutamate chloride Ž2CF8. deposited at 20 mNrm on mica neutral surfaces were obtained and the measured force was attractive out to separations of 90 nm ŽFig. 6., with ␭ 2 s 16 nm and C2 s y2.2 mNrm and a more steeply increasing attraction at separations below 20 nm Ž ␭ 1 f 2 nm and C1 f y0.3 Nrm.. Cavitation was observed, but unlike the case with DDOA numerous discrete vapour bridges formed around the flattened contact zone as the surfaces came into contact. These surfaces were very hydrophobic but the contact angle hysteresis was very large with ␪a s 113⬚ and ␪r s 50᎐60⬚. Evidence for the presence of heterogeneities in these monolayers was discussed in a subsequent report w65x.

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Fig. 6. Force measured between LB films of the double-chain fluorocarbon surfactant N-Ž ␣-Žtrimethylammonio.acetyl.-0,0⬘-bis-Ž1 H,1 H,2 H,2 H-perfluorodecyl.-L-glutamate chloride Ž2CF8. deposited on mica, plotted on a semi-logarithmic scale. The different symbols are measurements with springs of different stiffnesses. Note the very good fit to an exponential function for D G 20 nm.

In a study of the adsorption of the non-ionic surfactant dimethyldodecylphosphine oxide to LB films of DDOA on mica it was found that the long-range attraction was only slightly reduced at low concentrations of the amphiphile w56x. Up to surfactant concentrations of 10y5 M the contact angle remained above 90⬚ and the surfaces jumped together from large separations. At 10y5 M the amount of surfactant between the surfaces at contact corresponded to considerably less than a monolayer ᎏ the total shift in the contact was only 0.6 nm per surface. The adhesion was reduced by than an order of magnitude, decreasing from 430 mNrm in water to 40 mNrm at 10y5 M. At higher concentrations the adsorbed layer increased to monolayer thickness and the long-range attraction disappeared. Adsorption of a charged surfactant, dodecylammonium chloride, to hydrophobic LB films leads to the development of a repulsive double-layer force and the disappearance of the long-range attraction already at a surfactant concentration of 5 = 10y6 M w57x. A further increase in surfactant concentration leads to an increased double-layer force, but an attraction larger than the expected van der Waals force persists at distances below 5᎐15 nm. The adhesion force decreases gradually as the concentration is increased, but more slowly than when the uncharged DDPO was added. No experimentally significant change Ži.e.- 0.5 nm. in the contact separation was found at these low concentrations, indicating that the dodecylammonium surfactants either were incorporated in the DDOA monolayer or desorbed as the monolayers were brought into contact. The effect of added electrolyte on the long-range attraction measured between hydrophobic surfaces consisting of deposited LB films has been found to be

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Fig. 7. Forces measured between monolayers of a double-chain fluorocarbon surfactant Ž2CF8 ᎏ as in Fig. 6. deposited on mica in solutions of tetrapentylammonium bromide. Crosses: 5 = 10y5 ; triangles: 10y4 M; circles: 10y3 M, squares: 10y2 M. The solid lines are fits to a hydrophobic force of the form FrR s CexpŽyDr␭ . and a repulsive double-layer interaction. With increasing electrolyte concentration the factor C increases in magnitude and ␭ decreases. ŽFrom H.K. Christenson, in: M.E. Schrader, G. Loeb ŽEds.., Modern Approaches to Wettability: Theory and Applications, Plenum Press, New York, 1992..

dramatic ŽFig. 7.. The appearance of a double-layer force indicates that the charge on the surfaces increases with increasing electrolyte concentration. Concomitantly, the attractive interaction decreases, but fits to Eq. Ž7. often do not yield unambiguous results. Equally reasonable fits may in some cases be obtained by postulating an unchanged decay length ␭ 2 and a decreasing pre-exponential factor C2 , or by allowing ␭ 2 to decrease and C2 to increase w59x. The effect on the short-range interaction Ž ␭ 1 and C1 . is also uncertain, but it is often much less pronounced for dilute concentrations of electrolyte and short times of exposure to the salt solution. In many cases the results at high concentrations of salt show unambiguously that the LB films are destroyed, with multilayer formation occurring w67x. In view of what is now known about the surface morphology of these films Žsee below. it is clear that the surfaces become increasingly heterogeneous with the addition of electrolyte. The various attempts at fitting the results do consequently not yield any fundamental information on the effect of electrolyte on the interaction between hydrophobic surfaces. Nevertheless, the fact that a very long-range attraction has often been found to persist in relatively high concentrations of electrolyte is noteworthy. Thus, even in 0.1 M MgSO4 Žequivalent to 10y2 M dissociated electrolyte. an attraction with ␭ 2 s 13᎐14 nm was present between DDOA-coated surfaces w55x. Similarly, a weak attraction was measurable for 20 F D F 40 nm in 10y2 M monovalent salt with both DDOA and 2CF8 surfaces ᎏ beyond the range of the double-layer repulsion w59x. Spread monolayer films of DDOA on the air᎐water interface show large-scale heterogeneities related to the formation of crystalline domains at surface pressures of 15 mNrm or above w67,68x. To some extent this domain structure disappears on

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deposition, possibly due to the increase in packing density caused by the electrostatic attraction between the negatively charged mica substrate and the cationic surfactant. The amphiphiles in the deposited layers may be well-ordered on the molecular scale, as has been found for DDOA self-assembled on mica from aqueous solution w66x, but for SFA measurements the large-scale structure is crucial. The deposited layers show a large number of pinhole type defects, typically 30 nm in diameter and making up approximately 10% of the area ŽFig. 8.. From the height profiles it is likely that these defects are bare, or nearly bare, mica patches. The size of these bare patches increases on passage over the surface of the three-phase line at the air᎐water interface, and on exposure to electrolyte, albeit more slowly w67,68x. While such studies have not been undertaken with the other LB films used in the force measurements, the general observations of large contact angle hysteresis and sensitivity to salt and passage through the air᎐water interface lead to the conclusion that these surfaces are not good model hydrophobic surfaces. The pinhole defects explain the slight variation in surface charge from experiment to experiment, and even between different contact positions of the surfaces in the same experiment. With increasing salt concentrations the surface becomes more and more mica-like, with a resultant increase in surface charge and double-layer repulsion, as observed. The results thus reflect an increasingly heterogeneous surface, not the effect of electrolyte on the interaction between hydrophobic LB films. Surfaces produced by deposition of LB films on mica are far from ideal hydrophobic surfaces for fundamental studies. The existence of a very long-range attractive interaction between these surfaces is not, however, open to doubt. While its change or disappearance with electrolyte concentration is comparatively easy to rationalize, its presence and ultimate origin are not understood. 2.2. Surfactant films adsorbed to mica from cyclohexane Hydrophobic surfaces consisting of double-chain quaternary ammonium surfactants were produced by adsorption from cyclohexane by Tsao et al. w69᎐71x The surfactant was dissolved in warm Žf 50⬚. cyclohexane and allowed to adsorb to mica. After washing with cyclohexane and drying the surfaces were treated with warm water Žf 60⬚. prior to measurements at room temperature. Direct immersion of similarly prepared surfaces into room-temperature water results in hydrophilic surfaces w72x. Results of SFA measurements with amphiphiles of varying chain length from C 16 to C 22 showed force curves that were very similar to those obtained by LB deposition, i.e. a long-range attraction with two separate exponential decay constants ŽFig. 9.. These results were obtained by measuring the jump separations Žthe gradient of the force. rather than the force itself. Direct comparisons with the results described in the previous section are made possible because the exponential form is retained on integration. Using dimethyldioctadecylammonium acetate Žthe same cation as in the LB film studies above. the parameters of the fit were

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Fig. 9. Derivative of the forces measured in water at 25⬚C between monolayers of dialkyldimethylammonium salts adsorbed to mica from cyclohexane solution. DHDA, dihexadecyl Žacetate .; DODA, dioctadecyl Žacetate .; DEDA, dieicosyl Žbromide.. The dotted line is the predicted van der Waals force. ŽFrom D.F. Evans, H. Wennerstrom, ¨ The Colloidal Domain, 2nd ed., Wiley, New York, 1999..

␭ 1 s 2.1 nm and ␭ 2 s 25 nm, and the pre-exponential factors were C1 s y0.34 Nrm and C2 s y1.2 mNrm. The attraction was found to be sensitive to the alkyl chain length, and it decreased with increasing temperature, but the effect was larger for the shorter-chain homologues ᎏ thus the C 20 compound showed only a minor decrease on going from 25⬚ to 50⬚C. Arguments have been presented as to whether this could be interpreted as a genuine temperature effect on the interaction, or whether it merely indicated a change in the structure and thereby hydrophobicity of the surface with temperature w73x. Qualitatively similar results were obtained when the surfactants were adsorbed Žfrom cyclohexane. to a silicon nitride tip and the force between this and a surfactant-coated mica surface was measured using an AFM. The effects of electrolyte on the attractive forces were similar to what was found with the LB surfaces. A successive reduction in the magnitude of the force with increasing salt concentration up to 10y3 M divalent electrolyte ŽMgSO4 . was found. The apparent decay length of the long-range attraction decreased, while the pre-exponential factor increased, as was concluded in one analysis of data with LB surfaces w54x. As Fig. 8. AFM images of LB films of dioctadecyldimethylammonium Žbromide. deposited on mica. The image size is 5 = 5 ␮m. The top image shows a freshly deposited surface, the middle image is after 1 h exposure to 10y4 M KBr solution, and the bottom after 1 h in 10y2 M KBr. ŽFrom L.G.T. Eriksson, P.M. Claesson, S. Ohnishi, M. Hato, Thin Solid Films 300 Ž1997. 240..

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with the LB films, stability problems at higher electrolyte concentrations were encountered. Contact angle data or large-scale AFM scans were not reported in the original publications, although detailed scans showed characteristics of crystalline monolayers w69x. AFM scans of similarly prepared surfaces subsequently showed that that the surfaces are patchy, with large, micrometre-size Žcrystalline?. domains and intervening areas of bare or only very sparsely covered mica w72x. Cavitation was not reported in the original work, but repeat measurements have shown that bridging bubbles may form on separation from contact, and in some cases apparently even on approach, at small Žnanometre. separations, before contact is reached w72x. The force measured between these surfaces consisting of double-chain surfactants adsorbed to mica from cyclohexane thus shows great similarity to that measured between LB films deposited on mica. There is a long-range, exponentially decaying attraction and a much stronger, short-range attraction. In both cases the surfaces show large-scale heterogeneities and are apparently unstable in electrolyte solution. 2.3. Polymerized LB films deposited on mica In an attempt to prepare a more stable and better-characterized hydrophobic surface by LB deposition on mica Wood and Sharma used a combination of polymerization at the air᎐water interface and chemical grafting to the substrate w43,50,51x. Octadecyltriethoxysilane was spread on a nitric acid subphase at pH 2 and allowed to stand for 30 min to encourage polymerization before compression to a surface pressure of 20 mNrm. The mica substrate was treated with water vapour plasma to ‘activate’ the surface by introduction of hydroxyl groups that can react with the ethoxylated silane headgroups Žsee further, below, Section 2.4.. The compressed monolayer was deposited on the plasma-treated mica, dried in air and heat-treated in a vacuum oven at 100⬚C at 100 mtorr for 2 h. The contact angles of water on this surface were found to be ␪a s 112⬚ and ␪ r s 93⬚ ᎏ a much smaller contact angle hysteresis than has been found with any of the conventionally deposited LB films. Moreover, the contact angle was unchanged in 0.1 M KNO 3 ᎏ ␪a s 110⬚ and ␪r s 94⬚. AFM scans of the surfaces showed the existence of discrete, crystalline domains of diameter 12.5 " 1.2 ␮m, with intervening areas having a reduced surface density of molecules. The mean thickness across the domains was 2.1 nm, with intervening low-density areas having a thickness of 0.9 nm. These values were inferred from comparisons with SFA measurements of the mean layer thickness. The force curve measured between these surfaces across water showed no evidence of any long-range attraction, i.e. < FrR < - 10 ␮Nrm for D ) 17 " 2.5 nm, at which point a jump into contact occurred ŽFig. 10.. The actual force was not measurable within the resolution of the SFA employed, but it could be concluded that a strong attraction suddenly appeared once the surfaces were sufficiently close. The adhesion between the surfaces in water was 480 mNrm, in air a value of 279 mNrm was measured. The formation of vapour cavities around the contact

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Fig. 10. Forces measured in water ŽpH s 5.6. between mica surfaces coated with polymerized monolayers of octadecyltrimethylethoxysilane Žfilled symbols, from separate series of measurements.. The open symbols are the results of force measurements between mica surfaces coated with monolayers of dioctadecyldimethylammonium bromide. The solid lines show the van der Waals forces expected between mica surfaces Ž1. and hydrocarbon surfaces Ž2. across water. ŽFrom J. Wood, R. Sharma, J. Adhesion Sci. Technol. 8 Ž1995. 1075..

zone, similar to those found with the Žunpolymerized. fluorocarbon LB films, occurred after contact was reached. No change in the measured interaction on addition of KNO 3 to 0.1 M was found. The measured jump separations, in both water and 0.1 M electrolyte, suggested a force that is stronger than the continuum van der Waals force, but of a similar measurable range, i.e.- 20 nm. It was concluded that no long-range interaction is found between stable hydrophobic surfaces, and that the Žshort-range. attraction is insensitive to electrolyte. Support for these observations has been obtained by using the same technique to deposit fluorinated layers on mica and measure forces across water w74x. Heptadecafluoro-1,1,2,2-tetrahydrododecyltriethoxysilane was spread on a pH 2 nitric acid subphase, allowed to polymerize and deposited on plasma-activated mica at 15 mNrm with subsequent heat treatment. No long-range interaction Ži.e. N FrR N- 50 ␮Nrm for D ) 12 nm. was measured in water and the surfaces jumped into contact from D f 12 nm, although the surfaces were strongly hydrophobic Ž ␪a f 110᎐120⬚. and cavitation occurred on separation from contact. Repeat measurements with identical surfaces to those used by Wood and Sharma has essentially confirmed their findings w75x.

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2.4. Surfaces prepared by in situ adsorption of surfactants The first indications of a hydrophobic attraction were obtained by measurements with mica surfaces immersed in solutions of CTAB w11,12x, as described in Section 1 ᎏ see Fig. 1. Since then there have been many published accounts of hydrophobic forces measured between adsorbed layers of CTAB with mica substrates w11,12,76x and, more recently, with silica substrates w25,77᎐79x. The first measurements were carried out under conditions that could later not be reproduced ᎏ in particular ␪a of water was only approximately 64⬚, as opposed to the 94⬚ found in most later investigations. The amount of adsorption for a given bulk concentration of CTAB was less and the contact adhesion was smaller ᎏ only approximately 130 mNrm compared with 250᎐300 in other investigations. Note also that the hydrophobic attraction deduced from these measurements was calculated by subtracting both a double-layer repulsion and a van der Waals force with the empirical Hamaker constant for mica interacting across water from the measured results. Most subsequent investigators have considered the total attractive force, i.e. the measured force minus a double-layer repulsion, when discussing the results in terms of hydrophobic forces. Despite the large number of studies with mica, there are no measurements sufficiently accurate to permit very detailed conclusions to be drawn about the hydrophobic force in this case. What is clear is that with increasing CTAB concentration the charge on the mica surface first decreases, and at some concentration close to 3 = 10y6 M the surfaces becomes neutral, after which there is a build-up of charge again. Close to the point-of-zero charge Žpzc. there appears a long-range attraction, which is much larger than any possible van der Waals force. On silica the trends with increasing solution concentration of CTAB are similar, but the isoelectric point occurs at a higher bulk concentration Žf 2 = 10y5 M. because of the lower charge of the silica surface compared with mica. A study of the force between glass surfaces in CTAB w77x demonstrated the presence of a stronger than van der Waals attraction at CTAB concentrations of approximately 5 = 10y5 M using a modified SFA instrument ᎏ an early version of the MASIF. Later Rutland and Parker extended the study to high pH w78x, and Fig. 11 shows results of the measured force at various CTAB concentrations close to the isoelectric point at pH s 10. This behaviour is typical for charged surfactants adsorbing to oppositely charged surfaces. Kekicheff and Spalla published a detailed study of the forces acting between a ´ glass sphere Ž R s 50᎐100 ␮m. and a glass plate of identical material immersed in CTAB solution. The measurements were carried out with a commercial AFM. An exponentially attractive force was found close to the pzc Ž2.2 = 10y5 M. with long-range results ␭ 2 f 30 nm and C2 f y0.15 mNrm. At smaller separations the force became more attractive, but no detailed analysis was presented. The range of the attraction decreased markedly with added salt and was shown to be a function of the ionic strength in the range 0᎐2.3 = 10y3 M KBr, with the decay length of the attraction being simply ␬y1 r2 . The pre-factor of the attraction increased with increasing concentration of added KBr and was shown to be proportional to the

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Fig. 11. Forces measured between glass surfaces across solutions of varying concentration of CTAB at pH s 10. The arrows on the points show the positions of measured inward jumps. The solid lines are fits to DLVO theory, with the arrows showing the expected inward jumps. The arrows on the fits show the expected jump separations. ŽFrom M.W. Rutland, J.L. Parker, Langmuir 10 Ž1994. 1110..

ionic strength of the solution ŽFig. 12.. The measured force was in accordance with theoretical calculations of the force between two surfaces bearing adsorbed ion pairs w80x. Craig et al. subsequently used silica spheres and silica plates in another forcemeasuring device ᎏ the LLIFE ᎐ and obtained somewhat similar results w25x. The maximum range of the attraction was once again found at approximately 2.2 = 10y5 M CTAB, but the magnitude was considerably larger, for reasons that were not clear. The authors fitted the results to a sum of two exponential functions with decay lengths of 6 nm and 20 nm. These measurements were thus in apparent contradiction to the results of Kekicheff and Spalla, although the effect of added ´ electrolyte was not investigated. Other systems of adsorbing single chain or gemini surfactants that have been studied in the context of hydrophobic forces are dodecylammonium chloride w81᎐84x, octylammonium chloride w81x and the cationic gemini surfactant 1,2-bisŽ ndodecyldimethylammonium. ethane dibromide w85x, and the polymerizable surfactant cetyl p-vinylbenzyldimethylammonium chloride adsorbing to mica w86x, as well as cetylpyridinium chloride w25,87x and octadecyltrimethylammonium chloride w88x adsorbing to silica. With the surfactants adsorbing to mica the expected increase in adsorption, consequent charge neutralization and ultimately charge reversal was

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Fig. 12. Force measured between a glass sphere and a flat glass surface immersed in aqueous solutions of hexadecyltrimethylammonium bromide for various concentrations of added salt. The solid line is the van der Waals force for two silica surfaces across water calculated from Lifshitz theory. ŽFrom P. Kekicheff, O. Spalla, Phys. Rev. Lett. 75 Ž1995. 1851.. ´

observed. In the vicinity of the pzc the attraction was of maximum range, although any difference between the measured force and a van der Waals force showed up mainly in the adhesion values. Cetylpyridinium chloride ŽCPC. adsorbed to silica gave very long-range attractive forces. This series of measurements was carried out with a constant background concentration of electrolyte y0.1 M NaCl. Once again a maximum range of the attraction was found for a certain concentration of surfactant, 5 = 10y6 M in this case ŽFig. 13.. These results thus showed that a strong, long-range interaction could persist in the presence of high salt concentrations, as had been found previously with some LB surfaces. Further studies by the same authors focussed on the effects of dissolved gas, neutron irradiation of the solutions and variation of the approach velocity of the two surfaces during the measurements. The results showed relatively minor variations in the results with these parameters, but in all cases a long-range interaction was seen to persist. Ishida et al. have recently published a comparative study of the forces measured between silica surfaces made hydrophobic by adsorption from solution and surfaces rendered hydrophobic by silylation Žsee below, Section 2.5. w88x. In both cases C 18 -alkyl chain compounds were used. The forces between silica surfaces were measured in octadecyltrimethylammonium chloride, and the authors concluded that the exponential attraction increased in range and magnitude ᎏ with a maximum decay length of 15 nm ᎏ with increasing contact angle of water

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Fig. 13. Force measured between silica surfaces in cetylpyridinium chloride ŽCPC. solutions of different concentrations, all with a background of 0.1 M NaCl. The letters denote the points of the inward jumps for CPC concentrations of 1.0 = 10y5 M Ža., 3.0 = 10y6 M Žb., 8.0 = 10y6 M Žb., 5.0 = 10y6 M Žd.. The calculated retarded van der Waals force for silica surfaces across water is indicated by ^. ŽFrom V.S.J. Craig, B.W. Ninham, R.M. Pashley, Langmuir 14 Ž1998. 3326..

Žmeasured by the sessile drop method after withdrawing and drying the surfaces . in the range 55᎐82⬚. The relevant solution concentrations were not given. Fig. 14 shows the decay length of the attractive force and the pre-exponential factor as a function of the contact angle of water measured on the surfaces. A few studies using surfaces prepared by adsorption of double-chain cationic surfactants to mica have been performed. In one of the earliest studies Pashley et al. used dihexadecyldimethylammonium acetate adsorbed at a bulk concentration of 2.2 = 10y5 M w49x. ␪a on these surfaces was 95⬚ and the measured layer thickness on mica 1.5 " 0.1 nm. The measured force was comparatively short-range and could be fitted to a single exponential for 5 - D - 15 nm with ␭ 1 s 1.4 nm and C1 s y0.35 Nrm, and this extrapolated to give good agreement with the measured adhesion. Cavitation was sometimes observed after separation of the surfaces from contact, as was later found with deposited LB films ŽSection 2.1.. The adsorption had in this case clearly not reached equilibrium as with time, additional material adsorbed and the hydrophobic attraction vanished, to be replaced by a double-layer repulsion.

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Fig. 14. Dependence of the parameters of the exponential fitting equation FrR s yCexpŽyhrh 0 . on the contact angle of the surfaces for surfaces silylated with octadecyltrichlorosilane Žmodification-type. and silica surfaces with octadecyltrimethylammonium bromide adsorbed from solution Žadsorption type.. The lines show the limits of the expected capillary attraction due to a bridging bubble between the surfaces ᎏ see reference for details. Note the gradual increase in strength of the force with contact angle for the adsorption-type surfaces, and the more sudden increase at ␪ s 90⬚ for the modification-type surfaces. ŽFrom N. Ishida, N. Kinoshita, M. Miyahara, K. Higashitani, J. Colloid Interface Sci. 216 Ž1999. 387..

The fluorinated cationic surfactant used in the measurements with deposited LB films was also used to prepare hydrophobic surfaces by in situ adsorption w54,64x. Here too, the adsorption process was complex, and time-dependent effects appeared to play a significant role. The solution was prepared by sonication of the surfactant, as the monomeric solubility appeared to be very low. Depending on the adsorption time and how far apart the surfaces were kept in the SFA Žthereby presumably affecting the rate of adsorption by limiting the diffusion of the surfactant aggregates. different results were obtained. For example, at a bulk concentration of 10y4 M and with the surfaces far apart a 1.9 nm thick monolayer on each surface formed within 2 h. At this stage the force was net attractive and similar to that found between the equivalent LB films

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deposited on mica. It showed the same functional form with a long-range exponential decay and a stronger, shorter-range attraction below 15 nm. With time, the attraction was replaced by a repulsion, but the surfaces ultimately still came into monolayer contact. Cavitation occurred on separation of the surfaces from contact. The above observations, both with dihexadecylammonium bromide and the fluorocarbon amphiphile, indicate that similar surfaces to those produced by LB-deposition may form during adsorption from solution. The maximum attractive force is of similar but slightly smaller range and magnitude, and is usually achieved only transiently during the slow adsorption from these solutions that consist of aggregates, often present under non-equilibrium conditions. The surface morphology of surfactant films adsorbed to flat, solid surfaces has been studied with the AFM, and diverse surface structures such as parallel cylinders and hemi-micelles, often arranged in large-scale domains, have been found w89᎐91x. These surfaces are thus often far from homogeneous over areas involved in the interaction of two surfaces during force measurements. In particular, the classical picture of flat monolayer and bilayer structures on the solid substrate does not appear to be the rule. We note that it is very difficult to image monolayer-coated surfaces Ži.e. at solution concentrations corresponding to those used in most measurements of hydrophobic forces. due to the strong tip-substrate attraction. However, this was possible for the gemini surfactant 1,2-bisŽ n-dodecyldimethylammonium. ethane dibromide w85x, and it was shown that the monolayer formed was rather heterogeneous and with time was transformed into a bilayer structure. 2.5. Hydrophobic surfaces of silylated silica and mica A large number of experiments on measuring forces between silylated Žsilanated. surfaces has been carried out w15,26,37,38,88,92᎐99x. As with the previously discussed hydrophobic surfaces, considerable variability in the results with only minor changes in the preparation procedure has often been found. The general basis of the technique is the reaction of surface hydroxyl groups on silica with silane molecules, either in the gas-phase or in a suitable organic solvent. Such silylated silica surfaces are much used in surface science whenever a hydrophobic substrate is required w100x. The simplest modification procedure is the one employed by Rabinovich and Derjaguin w15x who used dichlorodimethylsilane vapour to produce hydrophobic surfaces of fused quartz with quoted contact angles of ␪a s 100⬚ and ␪ r s 80⬚. The results of these studies were briefly mentioned in the introduction ŽFig. 3.. The results could be fit with two separate exponential decay lengths, or ␭ 1 s 3 nm, ␭ 2 s 12.2 " 1 nm, C1 s y0.3 Nrm and C2 s y2.5 mNrm. A second set of parameters for the long-range attraction was deduced from measurements of the jump separation by varying the response characteristics of the feedback circuit. These were ␭ 2 s 13.5 " 0.7 nm and C2 s y2.4 mNrm. The remarkable agreement between these results and those with the LB films on mica was mentioned in Section 1 ᎏ see Fig. 3. Cavitation appeared to occur on separation from contact, as had been discussed in an earlier study with similarly prepared surfaces w23x.

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The mica surface lacks reactive hydroxyl groups and covalent bonding of silanes is only possible after such groups have been introduced by some suitable means. Parker et al. showed how this can be achieved by subjecting the freshly cleaved mica surface to a water-vapour plasma w37,38x. Water vapour at approximately 10y4 torr is subjected to a high-voltage radio-frequency discharge, which causes ionization and the production of hydroxyl and other radicals in the gas phase. Through a combination of sputtering and surface reaction the mica is altered and made slightly rougher, and hydroxyl groups introduced. These then react in the same way as the surface silanols of silica. Parker et al. used a monofunctional fluorosilane, viz. tridecafluoro-1,1,2,2-tetrahydrooctyl-1-dimethylchlorosilane ŽFSCl1 . to render plasma-treated mica hydrophobic by vapour-phase reaction, and then study the interaction of such surfaces in aqueous solution w37x. The surfaces had ␪a s 93᎐96⬚ and ␪ r s 70᎐75⬚ and they were charged in aqueous solution. An additional attraction Ž ␭ 2 s 12 nm and C2 s y0.8 mNrm. was inferred by subtracting the double-layer force fitted at long-range. The adhesion at contact was large Ž400᎐500 mNrm., and cavitation was sometimes observed after separation from contact. As with the LB films, the magnitude of the attraction was reduced by the addition of electrolyte ŽKBr to 10y4 M.. Apart from the studies using the polymerized silane LB films described in Section 2.3, subsequent measurements with silylated surfaces have focussed on the use of silica as the substrate. All experiments have been carried out with instruments other than the SFA, and are thus all subject to the uncertainties common to these techniques Žsee Section 1.2.. The extent of surface deformations, the exact location of D s 0 and the occurrence of cavitation could not be directly monitored. Rabinovich and Yoon used 10᎐30 ␮m silica beads glued with an epoxy resin to an AFM cantilever and measured the interaction between such a bead and a silica plate w93,94x. The beads and plates were allowed to react with trimethylchlorosilane ŽTMS. in the gas phase or with octadecyltrichlorosilane ŽOTS. in cyclohexane solution. The hydrophobicity of the surfaces, as determined by measurements of ␪a , was varied by changing the immersion time in cyclohexane. In this manner ␪a could be varied from 88⬚ to 116⬚, while ␪ r changed from 62⬚ to 89⬚, although the angles measured on the beads and on the plates were somewhat different. AFM scans of the hydrophobed plates showed the presence of distinct clusters of OTS molecules on the silica substrate. The measured forces were attractive and their range depended on the contact angle, varying from approximately 30 nm for the TMS surface Ž ␪a s 88⬚. to over 100 nm for the most hydrophobic OTS surface Ž ␪a s 115⬚. ŽFig. 15.. The authors presented fits of the force curves to exponential functions and to power laws Žnot shown.. It was also demonstrated that the attractive interaction became considerably stronger Žby about a factor of two. when the water was saturated with argon w94x. In a subsequent study forces were measured between a very hydrophobic glass sphere of ␪a s 109⬚ and silylated glass plates of varying contact angles in the range

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Fig. 15. Force measured between a silica plate and a silica sphere coated with trimethylchlorosilane ŽTMCS. and octadecyltrichlorosilane ŽODTCS. for various measured contact angles of water on the surfaces. ŽFrom Ya.I. Rabinovich, R.-H. Yoon, Langmuir 10 Ž1994. 1903..

75᎐109⬚ w95x. The force was found to increase smoothly in both range and magnitude with increasing contact angle, i.e. in a fit to Eq. Ž7. both C2 and ␭ 2 increased. The behaviour was quantified by fitting the results to a power law and it was then noted that the force constants for these asymmetric systems could be predicted with a combining rule similar to that applicable to Hamaker constants w4x. Silica hydrophobed with a fluorosilane was studied by Claesson and co-workers, using the MASIF. In an initial study they used a vapour-phase reaction and the same silane as previously employed in the modification of mica discussed above ŽFSCl1 . w92x. The contact angles measured on these surfaces were ␪a s 98⬚ and ␪ r s 80⬚. The surfaces were slightly charged in water at pH 5.6, but a pure hydrophobic attraction with ␭ 2 s 5.6 nm and C2 s y3.2 mNrm was found at pH 2. The magnitude and decay of the hydrophobic attraction varied with electrolyte concentration, and time-dependent effects related to hydrolysis of the surface siloxane bonds were observed. The results were thus similar to those found with many LB films deposited on mica. A further investigation using both the same monofunctional silane ŽFSCl1 . as well as the similar Žtridecafluoro-1,1,2,2-tetrahydrooctyl.dimethyldichlorosilane ŽFSCl 2 . and Žtridecafluoro-1,1,2,2-tetrahydrooctyl.trichlorosilane ŽFSCl 3 . showed the remarkable effect on the results of varying details of the preparation procedure w26x. In contrast to the initial study, FSCl1 was allowed to react with the silica surface at an elevated temperature Ž120᎐150⬚C., and the surface thus produced was apparently uncharged in water and considerably more hydrophobic than the one produced by reaction at room temperature. FSCl 2 and FSCl 3 can form polymeric species by cross linking, and the heat treatment was here applied after reaction at room temperature. All three surfaces had ␪a s 110⬚ and ␪ r s 90⬚, and AFM scans were indicative of smooth, non-crystalline surfaces with a maximum height variation of 0.2 nm over 400 nm lateral extension. The force measured between these surfaces in water was strongly attractive from

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Fig. 16. Force measured between surfaces silylated with Žtridecafluoro-1,1,2,2-tetrahydrooctyl.dimethyldichlorosilane. The inset shows the difference between forces measured on approach and on separation. Note the distinct steps on approach at D f 120 nm. ŽFrom J.L. Parker, P.M. Claesson, P. Attard, J. Phys. Chem. 98 Ž1994. 8468..

separations of 60᎐250 nm. The exact range varied considerably between experiments and also depended on the particular surface used, with FSCl1 in general showing the longest range of the attraction. The force curves measured on approach showed small steps, which were absent on separation ŽFig. 16.. Addition of salt up to 5 M ŽNaCl. had no major effect on the interaction. Raising the temperature from 22⬚ to 41⬚C caused an increase in the range of the attraction, and on subsequent cooling the range only returned to its original value very slowly. A detailed analysis of the force curves showed that these were consistent with the presence of a number of bridging bubbles, which explained the relative insensitivity to electrolyte and the greater temperature sensitivity. Bubbles at the surfaces would be expected to grow with increasing temperature as the solubility of air decreases. Carambassis et al. later used the FSCl 2 surfaces in an AFM study, and found evidence of the presence of bubbles on the interacting surfaces w96x. A small repulsion, presumably due to a hydrodynamic drainage repulsion and a double-layer force between a bubble and a silica surface Žor two bubbles. was usually measured before a stepwise increase in the attraction occurred as bubble coalescence took place. Occasionally, in the absence of bubbles only a hydrodynamic repulsion and a double-layer repulsion Žbut no attraction . between the bare, silylated surfaces were measured. Experiments were carried out at electrolyte concentrations ŽNaCl and NaClO4 . in the range 10y4 ᎐10y1 M, and with the differences attributed to changes in the double-layer interaction. As part of a study referred to earlier ŽSection 2.4. Ishida et al. carried out

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experiments with silylated surfaces using an AFM w88x. The surfaces Žsilica beads and plates. were treated with OTS in toluene, and the hydrophobicity controlled by varying the reaction time. Contact angles from 83⬚ to 115⬚ were studied, and AFM scans showed the presence of clusters on the surface as found by Rabinovich and Yoon on similarly silylated glass beads. The authors found attractive forces for contact angles above 90⬚ and the presence of clear steps in the force curves led them to conclude that bubbles were responsible for the attraction. This was supported by studying the hysteresis during repeated approach and separation cycles, and the force as a function of approach speed. A comparison between the dependence of the measured force and the contact angle with that expected due to the presence of a bridging bubble provided further evidence for this interpretation. Fig. 14 shows the dependence of the strength of the measured interaction on the contact angle of water on the silylated surfaces, together with similar data for adsorbed surfactants described above ŽSection 2.4.. Yaminsky and co-workers w97᎐99x have used the interfacial gauge to measure forces between fused Pyrex-glass surfaces hydrophobed by reaction with various silanes, including dimethyldichlorosilane and octyldimethylchlorosilane. The results have highlighted the importance of the precise silylation conditions and in particular the influence of reactions occurring between fused glass and ambient moisture, i.e. the presence of polywater-like layers on the surfaces. Some of the results can be explained by the presence of bubbles, and an extremely good fit to the capillary equation has been obtained ŽFig. 17.. Recent results obtained with Pyrex surfaces made hydrophobic by LB deposition of polymerized heptadecafluoro-1,1,2,2-tetrahydrododecyltriethoxy-silane have confirmed the absence of a long-range attraction between such apparently more stable surfaces w99x. The strong attraction found below 15 nm was in good agreement with the force measured in this regime in an earlier study of fluorocarbon LB films on mica w54x. In general, the large variability in the results of these force measurements with silylated surfaces indicates a number of largely unresolved problems with the preparation method. AFM scans show that the silylated surfaces may be far from homogeneous w88,93x. There is often evidence of substantial amounts of excess, polymerized material that is not incorporated into an ordered monolayer. Spectroscopic techniques that depend on average surface coverage may not identify the presence of such undesirable aggregates w42x. Some results, particularly the earlier ones, with silylated surfaces were similar to those obtained with LB films, and a long-range, exponentially decaying attraction was measured. In contrast, many of the more recent experiments with slightly more hydrophobic silylated surfaces show the undoubted presence of bubbles on the interacting surfaces. The measured force curves appear to largely reflect the interaction and coalescence of these bubbles, rather than the true interaction of the hydrophobic surfaces themselves. The presence and apparent stability of these very small bubbles is an unresolved issue. One would expect small bubbles of sub-micrometre radius to dissolve very rapidly due to the large excess pressure in

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Fig. 17. Force measured in water between glass surfaces silylated with octyldimethylchlorosilane ŽI.. The dots show the force due to a constant-volume bridging bubble of volume 10y1 3 cm3 . ŽFrom V.V. Yaminsky, Colloids Surf. A: Physicochem. Eng. Asp. 159 Ž1999. 181..

these convex structures. It is noteworthy that no evidence of the presence of bubbles on LB films deposited on mica prior to contact has been found. 2.6. Bulk polymer surfaces Plasma polymerization of monomers onto mica substrates from vapour was used to prepare surfaces for force measurements by Proust et al. w101x. With hexadimethylsiloxane, a 4.5 " 0.4-nm thick film of polydimethylsiloxane with ␪a s ␪r s 95 " 0.5⬚ was produced. On first approach of the surfaces in a SFA they jumped together from a separation of 12 nm, somewhat larger than expected from van der Waals forces alone. There was no long-range attractive force. On the first separation from contact the surfaces were damaged, presumably due to delamination of the polymer film from the substrate and no further measurements could be carried out. Much thicker Žf 50 nm. polydimethylsiloxane films on mica were used in another SFA study by Parker et al. w102x. No long-range attraction was found in water, but the initially very hydrophobic surface Ž ␪a s 109⬚, ␪ r s 98⬚. became

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increasingly hydrophilic, and a weak surface charge present initially increased with time. It was further noted that a steric force appeared, indicating that the surfaces were gradually broken up during the measurements. These studies suggested for the first time that no long-range force was present between some hydrophobic surfaces, but the limited stability of the surfaces made accurate measurements impossible, and no firm conclusions were drawn. Forces between polystyrene surfaces were reported by Karaman et al. w103x. The experiments were carried out with a 5-␮m polystyrene latex particle glued to an AFM cantilever and a fused polystyrene block. Attractive forces pulled the surfaces into contact from a separation of 30 nm, as opposed to the F 5 nm expected from the action of van der Waals forces. No further details of the attractive forces were presented. The polystyrene block was reported to have a rms roughness of 0.4 nm, no data were given for the particles. Precise contact angle measurements were not reported, although it was stated that ␪ was larger than 90⬚. In contrast to the above results, experiments with two fused polystyrene surfaces ŽR f 1 mm. using the MASIF yielded no long-range attraction ŽFig. 18. w104x. Contact angles measured on spin-coated polystyrene films were ␪a s 89 " 3⬚, ␪ r s 83 " 3⬚, and AFM scans of the fused surfaces used in the force measurements gave a rms roughness of 0.4 nm with a maximal topographical height difference of 3.0 nm. The force measured in water showed a weak repulsion at long range, attributed to double-layer forces, and a jump into contact at 7 nm. A careful analysis of the effect of surface deformations on the apparent zero of separation Žsee Section 1.2. led to the conclusion that no hydrophobic attraction was found between these surfaces. Polypropylene surfaces were used in an AFM study of forces in dilute NaCl solution by Meagher and Craig w105x. A flat surface was prepared by heat compression of a polypropylene bead between two mica sheets, and a roughly spherical surface was made by fusing a polypropylene bead of R f 100 ␮m onto the AFM cantilever. The surfaces were quite rough, with a rms value of 1.3 nm and a maximum range Žpeak-to-trough. of 7.6 nm. Measured contact angles of water or NaCl solutions Žof ‘different concentrations’. were ␪a s 103᎐111⬚ and ␪ r s 90⬚. The forces showed the presence of an electrostatic double-layer force and an attraction that caused a jump into contact at approximately 20 nm. On increase of the NaCl concentration from 10y4 to 10y2 M the double-layer force decreased in range, as expected, but the attraction did not change significantly. Measurements with a stiff cantilever Ž200 times stiffer than the normal AFM cantilever. showed the existence of an attraction stronger than the van der Waals force that was independent of electrolyte concentration between 10y4 and 1 M NaCl. The effect of outgassing the solution was investigated in this study. The inward jump distance in 1.4 = 10 ᎐ 4 M NaCl was found to decrease on outgassing the solution, although there was a substantial spread in the measured values and some overlap in the data before Ž21.0 " 5.2 nm. and after Ž15.2 " 4.2 nm. treatment. No quantitative information on the levels of dissolved air before or after outgassing was given.

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Fig. 18. Force measured between fused polystyrene surfaces in water. The squares and circles are results with two different samples. ŽFrom T. Ederth, Ph.D. Thesis, Royal Institute of Technology, Stockholm, Sweden, 1999..

2.7. Thiols self-assembled on gold Self-assembled thiol surfaces are very hydrophobic, and they have been used in a few recent studies of hydrophobic forces with and AFM and the MASIF w106᎐109x. In the first study hexadecanethiol was adsorbed onto 9 cold-coated glass spheres Žradius f 10 ␮m. and the forces measured in water and NaCl solution w106x. No long-range attraction was measured, but the inward jump was larger than expected from van der Waals forces. This jump-in separation showed only minor and experimentally insignificant variation with electrolyte concentration up to 1.5 M. The measured adhesion values indicated that the surfaces were slightly rough. A second, more comprehensive series of investigations has been carried out with the MASIF w107᎐109x. Borosilicate glass was flame polished and coated by electron-beam evaporation with a 1-nm-titanium layer, followed by 10-nm gold. The surfaces were then immersed in 1 mM ethanol solutions of the relevant thiols ŽC 16 .. The gold surfaces were slightly rough, with peak-to-trough values of approximately 1.5 nm and a rms roughness of 0.15᎐0.20 nm over a 1 = 1 ␮m area. Contact-angle measurements after reaction with 16-thiohexadecane yielded ␪a s 107 " 1⬚ and ␪r s 92 " 1⬚, and these values did not change on prolonged immersion in 1 M NaCl. Forces measured between these hydrophobic surfaces showed very abrupt steps at varying separations in the range 30᎐50 nm. At larger separations no experimen-

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Fig. 19. Force measured between thiol layers adsorbed to gold surfaces in water. The surfaces are mixed layers of thiohexadecane and 16-thiohexadecanol. The solid lines Ž ␪ s 98⬚. are for surfaces consisting of 80% thiohexadecane, the dashed lines Ž ␪ s 88⬚. are for surfaces consisting of 65% thiohexadecane. Note the presence of steps in the force curves for the 98⬚ surfaces. ŽFrom T. Ederth, Ph.D. Thesis, Royal Institute of Technology, Stockholm, Sweden, 1999..

tally significant force could be measured. For comparison, measurements between surfaces prepared with 16-thiohexadecanol Ž ␪a s 29 " 2⬚ and ␪r s 15 " 1⬚. showed no attraction beyond that expected from van der Waals forces for these largely hydrophilic surfaces. The measured adhesion was large, with values in the range 350᎐500 mNrm recorded Žreportedly near the limit of the maximum adhesion force that can be measured with the MASIF.. The presence of a connecting vapour cavity could sometimes be inferred from the behaviour of the surfaces after separation, and the authors concluded that the steps in the force curves on approach were due to the formation of, or the bridging of the two surfaces by, vapour bubbles. In a subsequent study the wettability of the surfaces was controlled by changing the ratios of thiohexadecane and 16-thiohexadecanol. In this manner surfaces with contact angles ␪a of 88⬚, 94⬚ and 98⬚ were prepared and the interactions in water measured ŽFig. 19.. The discontinuities, or steps, in the force curve on approach were found for the 98⬚ surface, only occasionally for the 94⬚ one, and never for the least hydrophobic surface. The results were found to be consistent with bridging vapour bubbles for contact angles above 90⬚, and the formation and presence of

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these bubbles was discussed with reference to surface heterogeneities, in particular grain boundaries of the underlying gold film. Interestingly, when hexadecanol was adsorbed onto the bare gold surface Ž ␪a s 93⬚, ␪ r s 63⬚. a very long-range exponentially decaying attraction was observed Ž C2 s 0.3, ␭ 2 s 30 nm. w109x. Just as for the LB films on mica, no steps were present in the force curve, and an amplification of the attraction was observed at distances below 15 nm. 2.8. Measurements in non-aqueous solution The possible presence of a long-range attraction between surfaces in nonaqueous liquids has been an obvious point of interest. Such ‘solvophobic’ systems would involve polar liquids having high contact angles on non-polar surfaces. Parker and Claesson used the double-chain fluorocarbon surfactant 2CF8 previously employed in water ŽSection 2.1. to measure the interaction across ethylene glycol w110x. The surfactant was allowed to adsorb to mica from a 2 = 10y5 M solution in ethylene glycol, and forces then measured in the presence of increasing amounts of water in solution. In pure ethylene glycol the force was close to what is expected for a van der Waals interaction, but on addition of water the range and magnitude of the attraction increased substantially. At 51% water the attraction was of the same order as in water between LB films of DDOA. The contact angle of ethylene glycol on similarly prepared surfaces was found to be ␪a s 96⬚ and ␪r s 80. Tsao et al. w71x used double-chain cationic surfactants adsorbed to mica from cyclohexane to measure forces in pure ethylene glycol. Using an AFM with silicon nitride cantilevers and mica surfaces they found an attractive interaction of similar magnitude to that measured in water for surfactants with C 20 and C 22 chains, but a slightly weaker attraction for a C 18 surfactant ŽFig. 20.. Ederth used gold surfaces modified with 16-thiohexadecane and studied the wetting properties in ethanol᎐water mixtures w109x. The forces were measured at various solvent compositions and it was found that an attraction starting with a sudden step, was present when ␪a was larger than 90⬚. This was interpreted in terms of bridging bubbles. The range of the attraction decreased with decreasing contact angle, and for ␪a - 90⬚ no steps were observed in the force curve and the measured attraction was consistent with a van der Waals force. Thus, for thiol surfaces, it appears that essentially the same result is obtained whether the contact angle is varied by changing the chemical composition of the surface or by changing the solvent composition. 2.9. Forces between one hydrophobic and one hydrophilic surface A few investigations have been concerned with the interactions in asymmetric systems, viz. one hydrophobic and one hydrophilic surface. The interpretation of these results is complicated by the fact that the double-layer force between

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Fig. 20. Forces Žnot normalized. measured between an AFM Si 3 N4 tip and a mica surface, both bearing monolayers of dialkyldimethylammonium salts adsorbed from cyclohexane. The top graph shows two sets of data obtained with dioctadecyldimethylammonium monolayers in water. ŽThe solid line is derived from SFA measurements with similarly prepared surfaces, see Ref. for details.. The bottom graph shows forces measured in ethylene glycol with dioctadecyldimethylammonium Žtriangles., dieicosyldimethylammonium Žsquares. and didoeicosyldimethylammonium Žcircles. monolayers. Note the similar range and magnitude of the interactions measured in water and in ethylene glycol. ŽFrom Y. Tsao, D.F. Evans, H. Wennerstrom, ¨ Science 262 Ž1993. 547..

surfaces of unequal charge may also be attractive and at low electrolyte concentrations is of similar range to that of many examples of hydrophobic forces. In the case of one bare mica surface and one mica surface coated with an LB film of DDOA, the persistence of a strongly attractive force up to 0.1 M KBr was taken as an indication of the presence of a non-DLVO attraction w111x. The range of this force was no more than 20 nm, however, although in pure water the attraction was measurable at 80 nm or more. There was some uncertainty as to whether to attribute the long-range attraction in Žsalt-free . water to a double-layer attraction or to some type of ‘hydrophobic’ force. One mica surface and one hydrophobic surface prepared by adsorption of surfactant from cyclohexane to mica also yielded long-range attractive forces w70x. These appeared to be stronger than in the symmetrical system of two identically prepared hydrophobic surfaces, but their functional form and range were very similar. Electrolyte was found to have the same effect as that found with two hydrophobic surfaces, i.e. the strength of the attraction was reduced for the

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moderate Ž10y3 M. concentrations investigated. The results did suggest strongly that the interaction in this asymmetric system was related to that found in the symmetrical case. Koehler et al. have published results showing the presence of long-range attractive forces between a hydrophobic surface of polysulfone spin-coated on mica Ž ␪a s 92⬚, ␪ r s 74⬚. and a hydrophilic surface of lysozyme adsorbed to mica w112x. The measurements were carried out in 10y2 M salt ŽKOHrHNO3 . and the range of the attraction was up to 200 nm. No long-range attractive force was measured between one bare mica surface and one polysulfone-coated surface. No detailed analysis of the attractive force was published, and measurements between two spin-coated surfaces were not presented. There have been some measurements showing good agreement with DLVO theory in asymmetric systems, i.e. an apparent absence of any additional, long-range attraction. These include one untreated glass surface and one silylated one w92x, as well as silica and polypropylene Ž ␪a s 105⬚, ␪ r s 90⬚. w113x. It should be noted that in both these latter cases the attractive forces between two of the hydrophobic surfaces were not of very long range. 2.10. Long-range attracti¨ e forces between nucleic acid surfaces Two independent groups have published intriguing observations of the presence of long-range attractive interactions between surfaces consisting of nucleotides w114᎐116x. These were prepared by LB deposition of lipids with nucleic acid base headgroups. The deposition was either: ŽI. as second layers on LB films of dimyristoylphosphatidylethanolamine on mica w114x and LB films of DDOA on mica w115x; or ŽII. directly on mica using nucleosides end-grafted to DDOA w116x. The measured force was attractive over separations of up to 100᎐150 nm. In one of the studies it was demonstrated that the attraction between the bare hydrophobic surfaces consisting of LB films of dimyristoylphosphatidylethanolamine was weaker than between the nucleoside-coated surfaces. The contact angles of water on the surfaces consisting of nucleotide-terminated cationic surfactants were found to be 70᎐75⬚. Contact angles were not reported for the other surfaces. At first sight it thus appears likely that the measured interaction is of the same type as that measured between LB films of surfactants on mica.

3. Classification of interactions 3.1. Outline The results reviewed above suggest strongly that there is no single mechanism that can account for the diversity of behaviour observed with differently prepared hydrophobic surfaces. In an attempt to classify the results according to the type of interaction measured, they have been divided into three categories, namely:

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1. A short-range, but strongly attractive force between apparently very stable surfaces. 2. An attraction of variable strength and range, due to the presence of bubbles. 3. A very long-range, attractive force with exponential decay. Most likely, the last type has to be further subdivided, especially when one considers the effect of electrolyte on the interaction. It also appears that one sometimes has forces of both types I and III operating in the same system. Likewise, an interaction of type II obviously pre-empts the occurrence of type I Žor possibly type III. in that system. This classification does not in itself explain the origin of the force in each case, it merely sets out to rationalize experimental observations. There are many instances where the classification is open to doubt, or where different features of the results give contradictory indications. Nevertheless, it is hoped that possible avenues for continued research, both experimental and theoretical, will emerge from this scheme. 3.2. Short-range, but strongly attracti¨ e force between apparently ¨ ery stable surfaces The strongly attractive forces of comparatively short range that have been measured between certain hydrophobic surfaces appear to constitute a distinct category. Such forces have been found with polymerized LB films that have been chemically grafted to plasma-treated mica surfaces w43,74,75x or to silica w99x, with plasma-polymerized films on mica w101,102x Žalthough the films readily delaminate from the mica during experiments., with some bulk polymer surfaces w104x and thiol-modified gold surfaces w106x. These surfaces are very hydrophobic in that the contact angle of water is well over 90⬚, with comparatively little hysteresis Žthe receding contact angle also usually exceeds 90⬚.. None of the published papers has given an accurate measurement of the force between these surfaces. We know only that it is stronger than the van der Waals force but of a similar range, i.e. measurable out to approximately 20 nm. This is consistent with expectations based on the larger-than-predicted Žfrom Lifshitz theory. interfacial tension between water and a hydrophobic surface. We may speculate that this is indeed the true ‘attraction between macroscopic hydrophobic surfaces’. As expected, electrolyte appears to have only a minor influence on the measured interaction w43,106x. No conclusive information on the effects of temperature or pressure is available. Cavitation has been shown to occur on separation of the surfaces in the case of the polymerized and grafted LB films w43x, as expected in view of the high values of both the advancing and receding contact angles of water. There are a few other systems that may fit into this category. Surfaces of fused polystyrene, although very hydrophobic, give only an attraction of short range w104x. Certain silylated surfaces w88x and some of the thiol-modified gold surfaces w107᎐109x show only short-range attractive forces when the contact angle of water is below 90⬚. Note that in the absence of cavitation, theory does not give reason to

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expect a drastic change in the measured interaction for a contact angle of 90⬚. The nature of the force would, of course, change from being an equilibrium force below 90⬚ to being an interaction across a metastable liquid phase at larger contact angles. The short-range interaction Žtypically for D - 20 nm. measured in addition to the exponentially decaying long-range attraction in many systems of deposited LB films and adsorbed surfactant layers Žtype III. also fits into this category w54,58x. Here, too, there are insufficient data to give an accurate idea of the precise functional form of this force, although recent work by Ohnishi et al. shows that it is in agreement with the force between stable, polymerized surfaces w99x. Moreover, this short-range interaction is likely to be very sensitive to details of the surface morphology and chemistry. Factors such as roughness, the precise nature of chemical groups involved Žsuch as ᎐CH 2 ᎐ or ᎐CH 3 . and non-equilibrium, orientational effects may play a significant role. The variability of these factors between different systems and even from one similarly prepared surface to another may contribute to the large spread in the adhesion measured between most hydrophobic surfaces. Stable hydrophobic surfaces thus show a short-range attraction, stronger than the van der Waals force, across aqueous solution, and this force is relatively insensitive to added electrolyte. This is in agreement with the type of interaction that has been predicted by a number of theoretical calculations and simulations w117᎐120x. Experimentally, it is not clear whether or not the observations in this regime are consistent with a postulated drying transition Žcavitation. between two hydrophobic surfaces at small separations w121᎐124x 3.3. An attraction of ¨ ariable strength and range, due to the presence of bubbles Experimentally, a number of systems appears to show a sharp onset of strongly attractive forces at finite separations ᎏ ranging from 25 to 250 nm w26,88,96,107᎐109x. In many cases the force curves show clearly discernible steps on approach. The surfaces showing this type of behaviour are commonly silylated silica surfaces with high contact angles of water ᎏ usually well in excess of 90⬚. The strength of the attraction often depends strongly on the contact angle, provided it exceeds 90⬚. Less hydrophobic silylated surfaces appear instead to show a long-range, exponential attraction of type III w37,38,88,92x. Nevertheless, there are results with very hydrophobic surfaces where the long-range attraction lacks obvious steps and appears to decay exponentially w93,94x. Thiol-modified gold surfaces appear to give rise to similar, stepwise increasing attractive forces, although of shorter range than most silylated silica surfaces w107᎐109x. These observations are probably due to the presence of bubbles on one or both surfaces at large separations Žwell before the surfaces are interacting .. The measured force curves have in some cases been fitted to the interaction expected in the presence of one or more bubbles w26,88,96,98x, and visual observations have lent further credence to the explanation. A transition from steps to no discernible steps

H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436 431

for contact angles of approximately 90⬚ has been identified in at least two cases Žwith silylated silica surfaces w88x and with self-assembled thiols on gold w108,109x.. When the steps disappear a strong, short-range attraction is measured instead. It is likely that this is a transition from type II to type I. While the measured attraction of this type is readily accounted for by the bubbles, their presence on the surfaces remains an unsolved problem. Microscopic bubbles of convex curvature are expected to have extremely short life times w125x and for these bubbles to exist on the surfaces there must hence be some stabilizing factor. This may be surface-active material, perhaps originating from the silylation procedure itself, and large amounts of excess, partly polymerized material are known to occur on these surfaces w41,42x. Alternatively, sub-microscopic cracks in the surfaces may provide sites where bubbles may be lodge. It seems unlikely that bubbles would be present on smooth and perfect hydrophobic surfaces w126x. It is noteworthy that this type of bubble-induced attraction has not been observed with LB films on mica, or with silylated, plasma-activated mica. Finally, it should be noted that the results are explicable in terms of pre-existing bubbles, and there has been no unambiguous report of any attractive force at finite surface separations caused by bubble nucleation itself. Many theoretical models and suggestions, on the other hand, have dealt explicitly with such surface-induced phase-separation or cavitation w121᎐124x. If such a transition is to be found among the reviewed experiments, the very sharp onset of the attraction between thiolmodified gold surfaces w107᎐109x would perhaps be the best candidate. 3.4. A ¨ ery long-range, attracti¨ e force with exponential decay This category includes the most puzzling results, and the origin of the interaction still appears to defy explanation in all but Žpossibly. a few special cases. A long-range, exponentially decaying attraction has been measured in a variety of systems, particularly with LB films on mica, but also with some silylated surfaces Žboth silica and mica. and with many surfaces of surfactant layers adsorbed from cyclohexane solution or in situ, from aqueous solution. For the case of LB films on mica little or no apparent dependence of the magnitude or decay length of the measured force on the contact angle of water on the substrate has been found. Indeed, very long-range forces have been measured between surfaces on which the contact angle of water is less than 90⬚, e.g. the results with nucleic acids discussed in Section 2.10 w114᎐116x, and some mixed LB films w58x. The decay length has been found to vary from approximately 5 up to approximately 50 nm, and the force Žwhen measured over a large enough range. always appears to turn more attractive at small separations Žtypically below 20 nm.. Differently prepared surfaces show a large spread in the measured decay lengths and magnitudes, but for identically prepared surfaces the results are reproducible. The measured adhesion shows large variability, and cavitation may occur on separation from contact, and in some cases as the surfaces come into contact. The above essentially also applies to those surfaces prepared by adsorption to mica of

432 H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436

double-chain surfactants from aqueous solution w63,64x and from cyclohexane w69᎐72x, as well as a few silylated surfaces, particularly in some of the earlier reports w37,38,92x. Electrolyte acts to reduce the extra attraction, but precise analysis is complicated due to changes in surface charge and the known instability of the LB films and the other surfaces in salt solution. Significantly, however, the long-range, exponentially decaying attraction has been found to persist to high salt concentrations in several w52,55,59,62x ᎏ but not all ᎏ systems. The decay length of the attractive force sometimes remains considerably larger than the Debye screening length ␬y1 at high electrolyte content. In the case of hydrophobic surfaces formed by equilibrium adsorption of charged surfactants to oppositely charged surfaces the long-range Žexponentially decaying where measured. attraction is apparently found only in the vicinity of the isoelectric point w25,77,78,87x. An attempt has been made to qualitatively rationalize the attractive force between such surfaces as a separation-induced shift in the adsorption equilibrium caused by electrostatic interactions between the surface and the polar part of the surfactant as well as between surfactant headgroups, and favourable hydrophobic interactions between surfactant tails w127,128x. While this may fit some published results, it cannot easily account for the presence of a long-range attraction at high salt concentrations, as the necessary electrostatic interaction between the surfaces would be screened. Note also that similar ideas were advanced earlier in an attempt to explain specifically the interactions between surfaces in CTAB solution w129,130x. In the type III systems formed by adsorption from solution the interaction often turns more attractive at small separations, although there are little precise data. Cavitation has only rarely been observed, and possibly only with double-chain surfactants, which also often show the largest adhesion. Addition of electrolyte will affect the adsorption equilibrium and consequently the attraction, and this complicates the interpretation. In one system an electrostatic mechanism for the attraction has been strongly indicated. The results of Kekicheff and Spalla w79x, where the force shows a ␬y1 r2 ´ dependence on electrolyte concentration, has been attributed to correlations between adsorbed ion pairs on the overall neutral surfaces. Note that several theoretical models involving some type of correlation between charged or transiently charged entities on net neutral surfaces have been advanced to explain experimental results with hydrophobic surfaces w71,131᎐134x. The large number of cases where the range and decay of the attraction greatly exceed ␬y1 r2 suggests that any such model cannot have general applicability. In fact, the apparent success of an electrostatic explanation in this case and its failure in many others strongly supports the view that no single mechanism can be the origin of the long-range interaction in all cases that we have classified as type III. Type III may also encompass some of the long-range attractive forces measured between one hydrophobic and one hydrophilic surface. At least three different studies have found an exponentially decaying, long-range attraction in asymmetric systems w70,111,112x.

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Surfaces showing interactions of type III seem to be largely those consisting of mobile hydrophobic groups w128x. The hydrophobic molecules that make up these surfaces undergo adsorptionrdesorption during interaction, or are laterally mobile on the surfaces Žas in LB films, and possibly some silylated surfaces if the bonding to the substrate is incomplete.. This applies also to the asymmetric systems showing long-range attractive forces w70,111,112x, and the long-range, exponentially decaying attraction recently measured between gold surfaces coated with hexadecanol w109x. Many of the LB films and films self-assembled from non-aqueous liquids are clearly very inhomogeneous over intermediate length scales Žmicrometres. and are destroyed by electrolyte. This, and the rather sizable contact angle hysteresis exhibited by these surfaces, suggest that these surfaces can undergo diverse physical and chemical transformations during interaction. It is hence not surprizing that a universal mechanism behind the long-range attractive force measured between these surfaces has not been found. However, a recent Monte Carlo simulation which considers the interaction of charged surfaces that are neutralized by amphiphilic molecules does perhaps point in the right direction w135x. A very long-range interaction is predicted when highly charged aggregates are formed by the amphiphiles at the surfaces. 3.5. Future work On the assumption that there is some merit to the above classification we may give some suggestions for questions to be addressed in future work. It is essential to aim for a proper understanding of surface structure ᎏ before, during and after the interaction in each experiment. The stability and smoothness of the interacting surfaces is clearly a central issue in clarifying the mechanism of the interaction. Such information has up until now rarely been accorded the importance it deserves. There are hence very few surfaces that have been fully characterized by AFM, both at short Žmolecular size. and longer length scales. To our knowledge no comparative scans before and after a measurement have been published. In view of the large perturbations that phenomena such as mechanical deformation and cavitation may cause between two interacting hydrophobic surfaces this would seem to be a prudent check on any experimental system. We believe that such studies are the key to explaining the types of forces classified under type III. The reasons behind the presence of bubbles at macroscopic hydrophobic surfaces used in the type II experiments need to be clarified. Conflicting and inconclusive reports on the effect of dissolved gas on some of the published results should be rationalized. Lastly, and most importantly ᎏ more accurate data on the interaction between stable hydrophobic surfaces Žtype I. are needed!

Acknowledgements V.S.J. Craig, M. Hato and V.V. Yaminsky are thanked for helpful suggestions

434 H.K. Christenson, P.M. Claesson r Ad¨ ances in Colloid and Interface Science 91 (2001) 391᎐436

and comments on the manuscript, and A. Luzar for useful correspondence. K. Abe, T. Ederth, M. Hato, K. Higashitani, P. Kekicheff, S. Ohnishi, O. Spalla and V.V. ´ Yaminsky are thanked for kindly supplying material for some of the figures. H.K.C. is indebted to F.C. Meldrum for encouragement and support during the writing of this work.

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