J. Phys. Chem. 1993,97, 12034-12041
12034
Adhesion and Surface Energy of Mica in Air and Water H. K. Christenson Experimental Surface Physics, Research School of Physical Sciences, Australian National University, Canberra A.C.T. 0200, Australia Received: July 7, 1993; In Final Form: August 31, 1993’
The adhesion between mica surfaces in nitrogen, dry air and water has been studied with a surface force apparatus. The adhesion (pull-off force) measured between air-cleaved mica surfaces in dry air or nitrogen is large, with initial values as high as 1.2-1.6 Nm-l This corresponds to energies per surface of 130-170 mJ m-2, in agreement with measurements of the energy required to cleave mismatched and resealed mica sheets. Exposure to the atmosphere, whether laboratory air or dry nitrogen, slowly reduces the adhesion with time. Details of the experimental procedure are important for obtaining reproducibly high values of the adhesion. In particular, the use of volatile compounds (such as sym-diphenylcarbazide, used in early surface force measurements) to glue the mica to the silica disks may lead to significantly reduced adhesion. The adhesion measured between hydrogen mica surfaces (where the surface potassiums ions have been exchanged for hydrogen ions by dipping in acidic solution) is similar, but the ion-exchange procedure appears often to clean the surface of adsorbed contaminants, leading to values that are more consistently high. The magnitude of the adhesion cannot be explained by dispersion interactions alone and indicates that polar contributions are important. The contact adhesion in water is much smaller than in nonpolar liquids, due to shielding of ionic contributions. Surface energies from the adhesion measured in water and in dry nitrogen together with the equilibrium spreading pressures of water on mica calculated from recently measured adsorption isotherms of water agree with the Young equation. It is suggested that the adsorbed layer on mica surfaces consists chiefly of water and potassium carbonate, formed by reaction with carbon dioxide on initial cleavage in air. This layer affects the properties of water condensates on (potassium) mica, but no evidence for the presence of “water-soluble organics” is found. The time dependence of the adhesion, however, may be related to the slow adsorption of contaminants, possibly of organic origin.
Introduction Naturally occurring muscovite mica cleaves easily to yield molecularly smooth areas of macroscopic dimensions. This property, along with its inertness, makes it the ideal substrate for studying a variety of surface phenomena. A great many experiments with mica have been carried out to investigate the fundamental principles of friction,” vapor adsorption,’38 contact angles,g and surface forces in gases, vapors, and l i q ~ i d s . ~ O Despite -~~ the widespread use of mica, there are a number of vexing uncertainties in our knowledge of its properties. One of these is the apparent lack of agreement between the adhesion of mica surfaces2.3 in dry nitrogen as measured with the surface force apparatus (SFA)13J4 and surface energy values obtained from direct cleavage experiments.lGl9 There is also the unresolved issue of the importance of the water-soluble,adsorbed layer that is present on air-cleaved mica surfaces.lO.12 We do not know how many layers of water remain between mica surfaces in “contact” in pure water. A number of publications has dealt with direct cleavage of mica.1g22 The direct cleavage technique determines the work required to split a macroscopic crystal of mica along a plane of the layered structure. Under ideal conditions this is equal to twice the surface free energy of the mica in the medium. In practice, the required work depends greatly on the precise conditions under which the cleavage takes place. The first time a sheet is cleaved, the work involves the energy needed to overcome the ionic forces between the charges in the mica lattice. This energy is reduced by adsorption from the medium to the virgin surface. The sheets may then be brought together again, and the energy required to recleave such a “healed” sample depends on whether or not the original crystallographic orientation is preserved. It is thus difficult to define a unique surface energy Abstract published in Advance ACS Absrructs, October 15, 1993.
of mica, but in what follows I shall use the term “surface energy” to mean half the free energy of adhesion of two mica surfaces. This quantity depends on the history of the surfaces and is not necessarily equivalent to or comparable to the surface energy of mica determined by any other means. Surface adsorption is responsible for lowering the cleavage energy (=twice the surface free energy 70) of mica from the thousands of mJ m-2 measured in ultrahigh vacuum (UHV)Zou21 to only about 500 in room (The very high values of 10 000 mJ m-2 measured in UHV may be due partly to the accumulation of electrostatic charges on the insulating surface ~ ~ ~ The . ~ surface ~ ) . energy decreases with increasing relative humidity but does not appear to be affected by inert gases such as argon or nitrogen. The adsorbed water that is mainly responsible for lowering the surface energy of mica is so strongly bound that it cannot be removed by outgassing under UHV conditions or by gentle heating. Adsorbed water is a natural part of the mica surface that has been cleaved under ambient condition, but it is not the only important factor in determining the properties of air-cleaved mica surfaces. The nature of the mica surface under ambient conditions has been the subject of extensive speculation,and recently Guzonas and Hair have briefly reviewed relevant experiments.23 A number of surface spectroscopystudies indicates the presence of significant amounts of carbon on the surface of air-cleaved mica.24 Some of the carbon is undoubtedly of organic origin, although a recent study has demonstrated reactivity of the surface toward carbon dioxide.*s There are also results showing depletion of potassium from air-cleaved surfaces due to “weathering”.26 It has long been realized that water condensed on mica surfaces does not behave as pure water. Its refractive index is higher than that of bulk water, its vapor pressure lower and it behaves much like “polywater” in silica ~ a p i l l a r i e s . ~On ~ . ~evaporation ~ a pile of involatile material is left behind. It has not been possible to
0022-3654/93/2097-12034$04.00/0 0 1993 American Chemical Society
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obtain agreement with the Kelvin equation for the case of water, although cyclohexane showed good agreeme11t.~9 Transmission electron microscopy of air-cleaved mica surfaces has shown the presence of small (50.1 pm) crystallites on the s~rfaces.3~ These crystals emerge as the surface is dried and they coalesce and grow with time. N o such crystallites are found on hydrogen mica, i.e., if the surface potassium ions of the mica are exchanged for hydrogen ions by dipping in weakly acidic (pH I 4) solutions before drying. The mica used in the surface force apparatus is cleaved in laboratory air of ambient humidity and then dried with nitrogen or a drying agent such as phosphorous pentoxide. The adhesion (pull-off force) measured in the surface force apparatus, where the mica sheets are brought together in a random orientation, should be comparable to the work of cleavage measured with reoriented, rehealed sheets in dry air or nitrogen, for which values of 120 mJ m-Z16 or 150-200 mJ m-2 l9 have been obtained. (In refs 17-19 the energies quoted are per unit area of two surfaces, but whenever they are referred to here I have converted them to energies per surface.) When attempting to relate quantitatively the adhesion force obtained with the SFA to the surface energy, another problem becomes important. Because of the large radius of curvature and the rather soft glue (usually an epoxy resin) used to hold the mica to supporting silica discs in the SFA the system is very susceptible to surface deformations. The effect of surface deformations on the measured forces has been a concern since the inception of the technique." Theoretically, the pull-off force F between identical elastic bodies (sphere-on-a-flat, equivalent to the crossed-cylinder configuration used in the SFA) is related to the surface free energy per surface yo of the solid and the radius of curvature R by32-34 instead of the expected
F 47rRyo (2) between rigid solids. The second relation follows from the Derjaguin a p p r o ~ i m a t i o nviz. ,~~ F = 27rRE (3) where E is the free energy of interaction per unit area of parallel flats, and (the assumption made here) that the free energy of interaction a t contact is twice the surface energy. A number of publications has dealt with the question of the validity of eq 1 for the compositemica-glue-silica system.2.36.37 As might be expected the problem is to find independent, reliable values of y to confirm the results of the force measurements. The matter has not yet been unambiguously settled, although most authors seem to agree that eq 1 with the factor 37r is the correct one to use. By contrast, there is no question that in the presence of capillary condensates, a t high vapour pressures of condensable vapours, the pull-off force is related to the free energy of the liquid-vapor interface YI~
F = 47rRyLV
(4)
Most measurements with the surface force apparatus to date have yielded much smaller values of the surface energy than direct cleavage experiments. Early SFA measurements were affected by a number of experimental artefacts but later work has given consistent results (Horn et al.2 found values of 54-80 mJ m-2 using eq 1; Christens0113 quoted 64 f 8 mJ m-*). Nevertheless, the discrepancy between thesevalues and the results of direct cleavage, 120-200 mJ m-2,16,19 is obviously too large to be accounted for by uncertainties in the use of eqs 1 and 2. In the remainder of this paper I will use JKR theory (eq 1) to calculate surface energies from measured pull-off (adhesion) forces. If eq 2 were used instead none of the conclusions drawn from the results would be affected.
In the study of the adhesion between mica surfaces quoted above3I found that the measured adhesion in nitrogen was usually much larger with hydrogen mica. In three experiments out of four significantly larger values of the surface energy, or 141,157, and 173 mJ m-2, were obtained. Thegreater adhesion of hydrogen mica surfaces in water vapor compared to potassium mica reflected the presence of an adhesive solid-solid contribution to the total force between the surfaces with hydrogen mica. This is consistent with the attractive force found between mica surfaces in pure water,l2 where the dominating ionic species is H+. The lack of agreement between the results of force measurements and those of direct cleavage has implications that go far beyond the adhesion measurements themselves. Anything that affects the surface energy or adhesion of mica in nitrogen or dry air may influence the forces measured in other media. It is in practice impossible to avoid the adsorption of water to the virgin mica surface, but what about other adsorbing species? Reduced adhesion usually indicates some type of surface adsorption and at thevery least it is desirable to attempt to minimize its influence on the measurementsobtained with the SFA. There is an intimate connection between adhesion and friction measurements, and the nature of the mica surface is obviously important for contact angles, vapor adsorption, or whatever surface property one is studying. In this paper I will describe the results of an extensive investigation of the adhesion between mica surfaces in nitrogen or dry air as well as in saturated atmospheres of water vapor. The experiments were motivated by the very large adhesion that was sometimes measured between mica surfaces in nitrogen during a recent study of capillary condensation and solvation forces in nonpolar liq~ids.3~ In this study the contact adhesion in nonpolar liquids was deduced from measurements of the contact adhesion in vapor close to saturation. In the presence of a capillary condensate the adhesion force is composed of two terms, viz., the Laplace pressure term (eq 4) and the contribution due to the solid-solid adhesion within the condensate Fs-s,3or
F = 47rRy,, + F ,
(5) If the surfaces are brought into contact before water vapor is introduced the first separation after the condensate has formed yields a measurement where the FW term reflects the contact adhesion. This may then be compared with subsequent measurements when the surfaces are brought into contact in the presence of vapor and the surfaces separated by a few layers of molecules. The F, term is then small and negligible compared to the Laplace pressure term, which is unchanged from the first adhesion measurement. The contact adhesion may be obtained by subtraction of these subsequent adhesion values from the first and largest measurement. In this manner the contact adhesion between mica surfaces in nonpolar liquids was measured and values of up to 100 mJ m-2 were obtained, as described in ref 39. In the present study measurements were carried out with potassium and hydrogen mica, both in dry atmospheres and in water vapour close to saturation. I will show that the results of direct force measurements under optimal conditions agree well with those of direct cleavage experiments, as well as with predictions of the Young equation using experimental values of the equilibrium spreading pressure of water on mica. This is particularly important, as it suggests that solid surface energies obtained from essentially irreversible measurements such as pulloff forces are consistent with surface energy estimates from contact angles (which may be similarly subject to irreversibilities by virtue of contact angle hysteresis). I find no direct evidence of the existence on the mica surface of a "phyisisorbed layer of water-soluble organic contaminants", to which the literature often r e f e r ~ . 2 ~ -These 3 ~ , ~ results and those of a number of previous studies are, however, consistent with the presence of inorganic, ionic material, probably potassium carbonate or bicarbonate, on air-cleaved mica surfaces. This does not significantly affect the measured adhesion, and the increased
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12036 The Journal of Physical Chemistry, Vol. 97,No. 46, 1993
adhesion usually seen after ion exchange is mainly because other contaminants (possibly of organic origin) are washed away by dipping in water.
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The technique and equipment used have been described extensively e l ~ e w h e r e . ~ JThe ~ J ~force between crossed cylinders of mica (radius of curvature R ) in dry air, nitrogen, and water vapor near saturation (relative humidity p / p l 1 0.95) was measured with a simplified surface force apparatus39using only a translation stage and a piezoelectriccylinder tomove the surfaces. As previously, the surface separation was measured with multiplebeam interfer~metry?~ which also permits the measurement of refractive indices of the medium between the surfaces and the study of surface deformations, contact diameters, etc. The force was measured by determining the deflection of a double-cantilever spring with approximate normalized stiffness k / R = 5 X lo4and a "rigid" support of k / R 3 X lo6 N m-2, The mica was from Mica Supplies Ltd., U.K., and a few sheets were cleaved and cut from a 12-year old batch obtained from Brown Mica Co., Sydney. The water was processed through a Millipore U H Q unit and the nitrogen was evaporated from a liquid nitrogen tank. The mica sheets were glued to the cylindrically polished silica disks with an epoxy resin (Epon 1004 from Shell Chem. Co.) or sym-diphenylcarbazide (Aldrich Chemical Co.), mounted in the apparatus and dried by passing dry nitrogen through for 1 h and/or placing a small container of P2Os in the chamber. All work involving the mica (Le., cleaving and gluing) was carried out in a clean-air cabinet with horizontal laminar flow. The total exposure of the mica to the laboratory atmosphere during both cleavage and gluing was of the order of 15 min or less. Water vapor was introduced by injecting a few milliters of water processed through a Millipore UHQ unit onto the bottom of the chamber. In some experiments the mica was ion exchanged by dipping the silica disks to which the mica sheets had been glued in a dilute ( ~ 1 0 - 4M) solution of hydrochloric acid (prepared with U H Q water) for 1 min, withdrawing, and immediately blowing off the excess solution with a jet of dry nitrogen. The surfaces were then dried with nitrogen and/or P2Os as for the experiments with unmodified (potassium) mica. The measurements were carried out in a temperature-controlled room at 22 f 0.1 OC, usually with the apparatus suspended from springs to minimize the influence of external vibrations. Most measurements were made by recording the interference fringes on video tapes using an intensified video camera (DAGE MTI, Model 65) a t the exit slit of the spectrometer. Fringe shifts, etc., were then measured from the tapes with a Colorado video micrometer, Model 305. The largest source of error in the measurements is the determination of the radius of curvature (typically f5%) and the spring constant (f2% for the double-cantilever spring, f 10% for the rigid support). Errors in the distance determination are f0.2 nm for small changes in the surface separation, such as those measured in Figure 6. The error in the distance when calculating the adhesion force from large jumps, which applies to the data in Figures 1-4, is less than 2%. Special conditions apply to the data in Figure 5-see Results. The errors in the spring constant and radius do not affect the spread of values obtained at one and the same contact position in an experiment but do influence comparisons between experiments and comparisons with other values of forces and energies. The limits of measured values indicated in what follows show the standard deviation of sets of measurements and do not include errors in the radius and the spring constant. Results
The results of a typical experiment are shown in Figures 1 and 2. The adhesion force was monitored over a period of 2 weeks
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Figure 1. Measured pull-off or adhesion force (normalized by mean radius of curvature of the undeformed surfaces) between mica surfaces as a function of time after gluing the mica surfaces and mounting them in the apparatus. The epoxy resin 1004 was used to glue the mica to supporting silica disks and the chamber was dried with P2O5 and dry nitrogen. Open squares show measurementsafter the surfaces had been left in contact for 15-20 min (normal time in contact I 5 min). Open diamonds show measurements after an excess load of 3 4 N m-l had been applied to the surfaces.
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The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 12037
MICA in Air and Water
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Figure 3. Measured pull-off or adhesion force (normalized by mean
radius of curvature of the undeformed surfaces) between mica surfaccs in dry nitrogen or dry air in a series of 33 experiments between May 1, 1992 (=day 1) and April 30,1993. Each point is the average of at least three values obtained during the first few hours after the start of the experiment. The right-hand scale gives the surface energy per surface yo calculated using eq 1. Filled squares and circles are measurements with potassium mica glued with epoxy 1004, unfilled squares are measurements with hydrogen mica and epoxy 1004glue. The filled circles are from experimentswhere no nitrogen was used to purge the apparatus after gluing the mica. Filled diamonds are data taken with potassium mica glued with sym-diphenylcarbazide;the open diamond is from an experiment with hydrogen mica and sym-diphenylcarbazideglue.
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Figure 4. Measured adhesion between mica surfaces as a function of time after gluing showing initial results obtained with epoxy resin 1004 as glue (filled circles)and results after exposure of these surfacesto water vapor and subsequentdryingat a diffcrcnt contactposition (filled squares). After ion exchange the adhesion has increased to the values indicated by the unfilled squares. Diamonds show results from an experiment with sym-diphenylcarbazideas glue, before (filled) and after (unfilled) ion exchange.
measuring chamber was dried with PZOSbut no nitrogen was used-these are indicated in Figure 3 by the filled circles. Open points show the adhesion between hydrogen mica surfaces. The mica sheets used in the experiments shown in Figure 3 were of varying thickness in the range 1.7-8.6 pm, and orierited randomly successive measurements in the early stages of an experiment, with respect to the crystallographic axes of the two opposing when the adhesion is large, appears to be related to details of the sheets (the birefringence varied between zero and the maximum unloading of the surfaces. The contact diameter of the surfaces often does not decrease smoothly to the theoretically e x p e ~ t e d ~ q ~ ~possible value of 0.0048 for sheets with the crystallographic axes in close to perfect alignment). The measured adhesion showed value of 0.63a0, where a. is the contact diameter under zero load, no correlation with the birefringence, but any very sharp maximum predicted by JKR theory. Instead, the diameter shrinks in little centered around perfect alignment, such as that found for mica jumps, and usually the surfaces come apart at values of the surfaces in water,40would most probably have been missed. There diameter a > 0.63 ao. The measured contact adhesion is slightly was no correlation with the thickness of the mica sheets or with smaller if separation occurs a t larger contact diameters. These the experimentally determined effective Young's modulus of the phenomena are currently under closer study-they do not affect layered glue-mica system. (Values of the elastic constant E in the overall behavior discussed here and the variation they cause the range (0.7-9) X 1OloJ m-3 were calculated from the contact between successive measurements of the adhesion is less than the diameter under zero load and surface energies calculated from absolute error in the values due to errors in the radius of curvature the measured pull-off forces according to JKR theory.2J2 The and the spring constant (see Materials and Methods). average of 38 measurements from 16 different experiments using Over a period of six days the adhesion drops to about one-third epoxy 1004 was (4.6 1.8) X 1010 J m-3.) The adhesion between of its initial value (Figure 2). The adhesion may at this stage thesheets from theold batch of mica (see Materials and Methods) vary somewhat a t different contact positions (compare filled was in the same range as between the new mica surfaces-1252 squares and filled triangles after about 50 h). If the surfaces are taken out after a week and ion exchanged as described in the f 36 mN m-l. The adhesion between the hydrogen mica sheets Materials and Methods section, the adhesion goes up significantly was invariably in the high range of values (1200-1600 mN/m). (open squares). The decrease in adhesion with the hydrogen The average of all 33 experiments was 1150 260 m N m-l. mica then continues, as before with the potassium mica. Figure 4 shows examples of the increase in adhesion caused Throughout this time there is no experimentally significant change by the ion-exchange procedure. The initial adhesion measured in the contact separation of the surfaces. This means that any in an experiment using epoxy 1004is indicated by the filled circles. change in thickness due to, for example, adsorption is less than If the surfaces are then exposed to water vapor (p/pl 2 0.95) and 0.1 nmpersurface. It isnotpossible, however, tomakeanaccurate dried again 24 h later the adhesion a t a new contact position is comparison of the contact separation before and after ion shown by the filled squares. This is a general observation-after exchange. One cannot ensure that the surfaces are remounted exposure to water vapour and subsequent drying the adhesion in exactly the same position relative to each other and the decreases markedly. By contrast, exposure to cyclohexane vapor apparatus after the ion exchange. followed by flushing with nitrogen has no discernible effect on The adhesion measured between mica surfaces in dry nitrogen the slow rate of decrease in the adhesion with time (not shown). or dry laboratory air in a series of 33 experiments carried out After ion exchange and drying the adhesion increased to the during approximately 1 year is shown in Figure 3 (day 1 is May values indicated by the unfilled squares. The diamonds are from 1,1992). Each point is the average of a t least three measurements an experiment with sym-diphenylcarbazide. Filled symbols were taken during the first 1-3 h after mounting the mica. As can be measured before ion exchange, and open symbols after ion seen, there is considerable scatter with values in the range 700exchange and subsequent drying with nitrogen. 1600 mN/m. Squares and circles show the results from In water vapor close to saturation capillary condensation of experiments where epoxy 1004 was used to glue the mica sheets water around the surfaces in contact dominates the total adhesion. to the supporting silica disks, diamonds show experiments where In the case of potassium mica it is difficult to obtain reproducible the glue was sym-diphenylcarbazide. In three experiments the
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12038 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 400
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Figure 5. Adhesion force between mica surfaces in water as a function of the adhesionmeasured in dry nitrogen. The adhesionforce was deduced by subtracting theoretical values of the adhesion due to the Laplace pressure in the capillary condensatefrom the measured adhesion at p / p l 2 0.95 (see eq 5 ) . Because of the subtraction the relative error is large (see text). Filled squares are initial measurements with hydrogen mica (made by introducingwater vapor with the mica surfaces in contactsee text) and unfilled squares are the results of subsequent measurements when the surfacesare brought into contact in the presence of water vapor. Filled and unfilled circles are the corresponding measurements with potassium mica surfaces.
Figure 6. Force measured between hydrogen mica surfacesinthe presence of a capillary-condensed bridge of water at p / p , 2 0.997 showing the presenceof twoadhesiveminima. The forcedoesnotincludetheattractive Laplace pressure contribution to the adhesion. The inner force barrier has been set at D = 0.0 nm, this is about 0.3 nm inside contact in nitrogen after the ion-exchange procedure. Triangles (filled and unfilled) are the results of outward jumps from adhesive minima, squares and circles are the results of force runs measured on approach (filled) and separation (unfilled). The height of the force barrier between the two minima could not be accurately determined due to effects related to the compression of the mica sheets (see text), but it is of the order of several N m-*.
results. As noted earlier, the condensate leaves behind a residue on drying, so the reversibility of the results cannot be checked. With hydrogen mica the condensates can be dried and the reversibility at the same contact position checked, although there is usually a substantial decrease in the adhesion. Also, the ionexchange procedure does entail an increased risk of particulate contamination during subsequent stages of the experiment. The adhesion between mica surfaces in water as deduced from the vapor-phase measurements using the procedure outlined in the Introduction (see eq 5 et seq.) is shown in Figure 5 as a function of the adhesion in nitrogen for both hydrogen (squares) and potassium (circles) mica. Filled symbols are first measurements on separation of surfaces initially brought into contact in nitrogen and open symbols are subsequent measurements. The measured values show that the total adhesion for hydrogen mica is significantly higher than expected from only the Laplace pressure of the water condensates (not shown). The solid-solid contribution FH is hence calculated by subtracting a theoretical value of the Laplace pressure adhesion using the bulk surface tension of water (eq 5), for both hydrogen and potassium mica. Because of the subtraction procedure the error in the values is rather large. The significant conclusion from Figure 5 is that the adhesion between hydrogen mica surfaces (1 43 f 15 mN m-l) is larger and more reproducible than between potassium mica surfaces (60 f 27 mN m-1). The adhesion measured with the surfaces in contact before introduction of vapour was slightly higher than after subsequent contacts, although the difference was not experimentally significant in the case of hydrogen mica. With potassium mica the difference was greater but rendered uncertain because of the very large spread in the measured values. The contact measured in the condensates was approximately the same, to within the error of -0.2 nm, as that measured in dry nitrogen, for both hydrogen and potassium mica. If the measurements are carried out with a very rigid spring ( k / R 3 X 106 N m-2) the surfaces can be moved in and out of contact without any instability due to the Laplace pressure force of the condensate, provided it is big en0ugh3~(the effective radius of curvature of the interface has to exceed about 150 nm, which implies a relative vapor pressure p / p s of 0.996, according to the Kelvin equation29). This permits a "force curve" in the liquid condensate to be measured, as described for nonpolar liquids
in ref 39. Such a force curve obtained in hydrogen mica is shown in Figure 6. The error in the force is rather large because the spring constant of the rigid support cannot be accurately measured. Also, thermal drifts and the error in the distance determination have a large effect when small changes in the surface separation are involved in calculating the forces. Nevertheless, there are clearly two minima with an average spacing of 0.28 f 0.15 nm (the result of a second experiment was 0.26 f 0.28 nm). The forces measured a t the two minima were -95 f 21 and -34 f 5 mN m-1(-47 f 8 and-27 f 2 m N m-I in the second experiment). The height of the maximum between the two minima could not be accurately determined as the surface separation did not appear to change simultaneously across the entire contact zone. What seemed to be a gradual change occurred under applied forces in the range 1-5 N m-1 (not shown). By dividing the applied load with the area of the flattened contact zone (readily measured from the interference fringes), one may obtain a rough idea of the magnitude of the average pressure necessary to expel this layer of water molecules. The result is about lo7 N m-2, or 100 atm. The pressure at the midpoint is expected to be higher than this. Measurements of the absolute surface separations under such large loads are rendered difficult by compression (and/or a possible change in the refractive index under pressure) of the mica itself, and this compression is not uniform across the contact region. This shows up as a bulge of the initially flat contact zone, as monitored by the interference fringes. The apparent difference in separation between the edges and the middle of the contact zone is several tenths of nanometers a t these forces. From estimates of the compressibility of mica1 of about 2.X 1011 N m-2 the compression Ax of 4.4 pm of mica (the thickness of two sheets in the experiment shown in Figure 6)under a pressure of 100 atm is found to be Ax = lo7 X 4.4 X 106/2 X 1011 = 2.2 X 10-10 or about 0.2 nm. This is of the order of the spacing between the minima and makes correct interpretation of the measured separations difficult a t these applied loads. With potassium mica the short-range force measured in large condensates (of radius >200 nm) did not show the same force curve at small separations. There appeared to be a repulsive force with a range of about 1 nm, with one or more adhesive minima at separations of about 1 nm. This is consistent with the
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The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 12039
TABLE I: Adhesion and Surface Energy of Mica in Dry Atmospheres mica no. of meas. study FIR (mN m-’) ref YO (mJ m-2) ? K+ mica cleavage 120 16 K+ mica 9 contact angles 120 3 H+ mica 1480 f 150 force meas 157 f 16 3 5“ K+ mica cleavage 18 70-1400 24“ K+ mica cleavage 19 150-2000 2?’ H+ mica cleavage 19 110-2400 all 246 K+ mica force meas 123 23 1163 f 220 this work 12 highestb force meas K+ mica 1338 f 91 142 f 10 this work K+ mica 6 highestb force meas 1403 f 87 149 f 9 this work 4 H+ mica force meas 1482 f 117 157 f 12 this work K+/H+ mica force meas ads 150 f 6 this work a Estimated from figures. * Measurements with epoxy glue only. Note that the surface energies as given in refs 18 and 19 are for two surfaces. expected presence of an oscillatory hydration force between mica surfaces in water when the surface concentration of potassium ions is high.41.42 In view of the previous problems encountered with water condensates on potassium mica no attempt was made to study the details of the force law.
Discussion Adhesion in Nitrogen and Dry Air. The adhesion between mica surfaces in dry air or nitrogen is variable, as seen by Figure 3, although the results of any one experiment are reproducible (Figure 1) over short periods of time. The lack of any correlation with time in contact or load (for moderate times and loads) shows that adhesion results may be obtained with the SFA to a high degree of precision. The cause of variability between different experiments and changes with time must be varying degrees of adsorption from the atmosphere. It is reasonable to assume that the highest measured values are the “most correctn ones, since the surface energy can only decrease through adsorption. A number of different averages are displayed in Table I, which also shows comparisons with direct cleavage results. The range of surface energy values obtained by the different averaging procedures-123 f 23,142 f 10, and 149 f 9 mJ m-L-agrees well with the direct cleavage results of 120’6 and 150-200 mJ/ m-2.19 The reason for the higher adhesion values compared to those of earlier work is probably due to a combination of factors. There has been no obviously significant change in the experimental procedure other than the installation of a new air-conditioning system in the laboratory. It is possible that a slightly improved cleanliness of the atmosphere has at least contributed to the difference. More importantly, the importance of time-dependent effects was not realized earlier, a t least not by myself. The present emphasis on minimising exposure to the laboratory atmosphere during both cleaving and gluing was not made. Many of the results included in the 64 f 8 mJ m-2 figure from ref 3 were in fact obtained after leaving the surfaces for about 1 day in the apparatus after gluing the mica. In the experiment shown in Figure 1 this caused a 25% reduction in the adhesion. The lowest values obtained in the current study correspond to surface energies of the order of about 75 mJ m-2, which are in the high range of the earlier results.2.3 Unfortunately, few other laboratories around the world have published values of the adhesion measured in nitrogen so there is little else with which to compare. The adhesion found with hydrogen mica is the same in this study as was found earlier-F/R = 1482 f 117 mN m-l or yo = 157 f 12 mJ m-2 compared to FIR = 1480 f 150 mN m-1 or yo = 157 f 16 mJ m-2 (excluding one obviously anomalously low value quoted in ref 3). This suggests that the reason for the larger adhesion measured with hydrogen mica is simply that the ion-exchange procedure “washesn the mica surface and that the ionic species present on the surface is of minor importance. After washing the pull-off force decreases slowly with time in the same manner as potassium mica, as shown in Figure 2. Recent direct cleavage experiments have also yielded a surface energy measured
with hydrogen mica that was similar or only slightly larger than that found with potassium mica.19 The on average lower adhesion measured when sym-diphenylcarbazide was used to glue the surfaces ( F I R = 802 f 105 m N m-l or yo = 85 f 11 m J m-2) is consistent with a small amount of adsorption of the comparatively more volatile compound as the mica is placed on the molten glue on the hot plate. The polymeric epoxy glue would be far less likely to transfer through the vapor phase or by surface migration from the back side of the mica. If the glued mica is subsequently ion exchanged, the surface is “cleaned” by immersion in water and the adhesion is as large as with the epoxy glue (FIR = 1607 f 11 m N m-1 or yo = 171 f 1 mJ m-2 obtained in one experiment-see Figure 4) * The slow decrease in the adhesion with time must be due to surface adsorption. The exact nature of this process is not clear since it appears to be remarkably insensitive to details of the experimental procedure. It is not affected by flushing with nitrogen, and short of carrying out both the cleaving and gluing under UHVconditions there is no obvious way around the problem. The important thing is to be aware of it and to control ambient conditions toan extent where reproducibleresults can beobtained. Minimizing the exposure to air or any atmosphere is essential and the use of even slightly volatile glues must be avoided. It is likely that volatile components of varnishes, paints, and cleaning agents as well as secretions of human origin have the potential to exacerbate problems related to adsorption from the atmosphere. Such compounds may explain a t least part of the variability from experiment to experiment. The magnitude of the adhesion in nitrogen or air is considerably larger than expected if van der Waals forces alone were responsible for the adhesion. The van der Waals energy of interaction E between flats is given by
E = 27, = -A/ 121rD’
(6)
where A is the Hamaker constant with the value of (1-1.35) X 10-19 J.10J1J5 Using a “cutoff‘ distance of 0.2 nm, which gives reasonable values of the surface energy for nonpolar substances, one obtains yo = 33-45 mJ m-2, or less than about one-third of the measured values. The difference must be due to short-range polar contributions to the surface energy, which cannot be ignored for mica. It appears that the agreement between previous adhesion valuesand the estimatedvan der Waals contribution to thesurface energy was largely fortuitous. Surface adsorption was just sufficient to lower the surface energy to values consistent with the Hamaker constant for mica (without any adsorbed layers). Lawn and co-workers have in a series of recent papers argued that the adhesion of misoriented mica surfaces (as measured by direct cleavage) is largely due to a combination of van der Waals forces (to which they ascribe a similar magnitude to what was calculated above) and contributions from macroscopic domains of charge on the ~ u r f a c e s . l ~They - ~ ~ argue that misorientation of a few degrees (such as would be the case in all the SFA experiments discussed here) is more than sufficient to completely cancel any ionic bonding contributions to the surface energy. The
12040 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993
macroscopic domains they envisage have dimensions of -1 mm for their mica sheets of size =10 mm.19 It is difficult to reconcile the present SFA results with domains of such size, they would be considerably larger than the contact area of the opposing surfaces (-50 X 50pm). The present results, together with those of ref 39, show very clearly that the adhesion between mica surfaces in air or in nonpolar liquids is much larger than can be explained by van der Waalsforces. Furthermore, theadditional contribution cannot bestrongly angle-dependent over the entire range of relative orientations. The present results do not, however, rule out the existence of an angle-dependent contribution of the kind measured between mica surfaces in water.@ The increased adhesion was there found only at angles 12O from perfect alignment (maximum birefringence) and none of the pairs of mica sheets in this study was that well aligned. Any angle-dependent contribution to the adhesion in dry air or nitrogen would have to be additional to what has been measured here. McGuiggan and Israelachvilia did not, however, find any correlation with the twist angle between the surfaces in humid air of 33% rvp. Adhesion in Water. Numerous force measurements have established that mica surfaces immersed in water come into adhesive contact.l2~~~ It has been widely assumed that the force between the surfaces is a monotonic van der Waals attraction down to molecular contact, particularly as measured values of the attraction in electrolyte solution ( 2 ~ 1 0 - M) ~ at larger separations ( D 1 4 nm) show good agreement with Lifshitz theory.12 The values of the adhesion measured in water or dilute electrolyte solutionsl2.~~41 are in the range FIR = 25-100 m N m-1 or a solid-liquid interfacial free energy ysl = 3-1 1 mJ m-2, with probably the most accurate results for “distilled water” (pH not given) reported in ref 40, equivalent to 41 f 6 m N m-1 (for the “baseline” adhesion away from perfect crystallographic alignment). The spread in values between different studies may partly reflect a pH dependence due to varying surface concentrations of hydrogen ions but also undoubtedly environmental effects due to differing amounts of contaminants that adsorb to the mica surfaces, either in air before immersion or from the aqueous solution. The nature of the ions adsorbed to the mica surface determines the short-range force between two mica surfaces. In pure water and dilute electrolyte solutions the original interlayer potassium ions from the mica lattice are exchanged for hydrogen ions. At higher concentrations of electrolyte the van der Waals-like attraction between the mica surfaces may be replaced by an oscillatory hydration force, as is the case when potassium ions adsorb to the surface.42 The adhesion between mica surfaces in water is hence a function of the type and surface concentration of ions, but the important point in the context of the present work is that it is comparatively small. Similarly, recent direct cleavage measurements (of reoriented and rehealed sheets) in water gave surface energies that were close to zero.19 The adhesion measured between mica surfaces in water vapor is dominated by the Laplace pressure of the capillary condensate. The short-range forces inside the condensate give only a comparatively small contribution to the a d h e ~ i o n .Because ~ the surface potassium ions cannot be exchanged in the same manner as when the surfaces are immersed in bulk water and because of the tendency of ionic material to accumulate in thecondensates,32* the nature of the short-range interaction is difficult to rationalize. Only if the mica is ion-exchanged first, so that the surface is covered with hydrogen ions, can we be reasonably confident of obtaining quantitatively meaningful data. In this case, as Figure 5 shows, consistent values of the order of 150 mN m-l (corresponding to a surface energy of 16 mJ m-2) are found. These values are higher than any published adhesion values for water, but it is not clear to what extent the exact values are significant given the total magnitude of the adhesion. Also, in view of the results presented in Figure 6 these adhesion values may be a collection of data from two separate minima.
Christenson It is uncertain why the force measured in the water condensates between hydrogen mica surfaces shows two minima. This was observed in two separate experiments, and several of the experiments with water vapor and the double-cantilever spring suggested the presence of a force barrier close to contact as the initial adhesion (when the surfaces were brought together before vapor was introduced) was slightly larger than on subsequent contacts. The difference was not, however, large enough to be experimentally significant. Layering of water between mica surfaces has previously not been detected when the surface is covered by hydrogen ions (as is the case in bulk water at low or moderate pH values). It is difficult to predict what the p H of the water in the condensates is because of the large surface areato-volume ratio and the lack of a reservoir. The ion-exchange behavior of the mica surface in contact with capillary condensates might be quite different from bulk. Clearly, more experiments are needed to resolve the matter. The important conclusion for the present study is that in agreement with previous work, the adhesion between mica surfaces in water, irrespective of the surface ions involved, is small. Using eq 6, with the experimental value of A = 2.2 X 10-20 512 to estimate the van der Waals contribution to the surface energy of mica in water, gives ysl = 7 mJ m-2, of the same order as the 10 and 5 mJ m-2 obtained for the inner minima in the two experiments with the rigid spring (one shown in Figure 6). In water, polar contributions to the adhesion are screened and the surface energy is dominated by dispersion energy effects. The surface energy of mica in water ysl may be related to the surface energy of mica in air/nitrogen yo with the Young equation
(7) and the use of the equilibrium spreading pressure re Ysv
YO - *e
(8)
giving YO = + 71”COS 6 + r e (9) Here 0, the contact angle of water on mica is small, = 5 O , so that cos 6 o 1, ylvrthe surface tension of water, is 72 mJ m-2. The spreading pressure of water on mica may be calculated from the adsorption isotherms of water on mica in ref 8, using Gibbs’ adsorption equation
- d r / d p = I’
(10)
in the form
withps the vapor pressure a t saturation, where the surface energy is ysv.This gives *,(water) = 69 and 67 (results from two separate isotherms, the first one shown in Figure 4 of ref 8). With re= 68 f 1 and ysl = 10 f 5 (complete range of the measured values) eq 9 then predicts yo = 150 f 6 mJ m-2, which is within error of the experimentalvalues, whatever average of these is calculated (see Table I). There is one additional estimate of the surface energy of mica in the literature, which is also in reasonable agreement with these results and those of direct cleavage. Schultz et al. obtained a value of 120 mJ m-2.9 using the contact angles of water droplets on mica immersed in a range of nonpolar and polar liquids to calculate dispersive and polar parts of the surface energy. Adsorbed Layers on Mica Surfaces. A number of inexplicable features of the force measurements has been attributed to the adsorbed layer that forms on mica surfaces on cleavage in laboratory air. It has, for example, been suggested” that it accounts for the hysteresis in contact diameter found on loading and unloading cycles in nitrogen2 and that the lack of an angle dependence of the adhesion in humid air is due to the presence of this 1ayer.a The thickness of this adsorbed layer after 1-h
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 12041
MICA in Air and Water
of the contact diameter under load it should be possible to detect exposure has been measured to be 0.35 f 0.1 nm/surfaceI0J2 and a time dependence of the behavior. Alternatively, if the potassium its refractive index has been reported to be very high, 1.8 0.1 .I2 carbonate layer is responsible, the effects should vanish or a t The adsorbed layer has often been referred to as being “organic” least change radically on ion exchange. In any case, it no longer or “carbonaceous”, because the presence of organic carbon has seems appropriate to simply dismiss all inexplicable features of been detected by surface spectroscopic techniques such as force measurements between mica surfaces by referring to some SSIMS.24 alleged properties of adsorbed layers. The results presented in refs 10 and 12 were obtained 15-20 years ago and the experimental procedure has changed somewhat Acknowledgment. I am indebted to V. V. Yaminsky for many since then. In the present study there was no indication of the helpful discussions and for patiently explaining many features of contact separation changing with time, as would happen if an surface thermodynamics to me. D. Beaglehole is gratefully adsorbed layer were building up, as observed in ref 12. The acknowledged for allowing me to use his unpublished water vapor thickness and composition of the adsorbed layer on mica will adsorption isotherms. I thank T. Sawkins and A. Hyde for undoubtedly vary somewhat depending on the laboratory atmotechnical assistance. sphere and the experimental procedure used, but it seems obvious that it consists mainly of a monolayer of water (which accounts References and Notes nicely for the 0.35-nm change in thickness). There is undoubtedly some carbon present on the surface, both organic, perhaps partly (1) Gaines, G. L.; Tabor, D. Nature (London) 1956, 178, 1305. of human origin and from carbon dioxide in the a t m o ~ p h e r e . ~ ~ (2) Horn, R. G.; Israelachvili, J. N.; Pribac, F. J . Colloid Interface Sci. 1987, 115, 480. The organic carbon may well be chiefly in particulate form, (3) Christenson, H. K. J . Colloid Interface Sci. 1988, 121, 170. scattered over the mica surface. Surface spectroscopic methods (4) Bailey, A. I.; Courtney-Pratt, J. S.Proc. R . SOC.London, A 1955, would not differentiate this from uniformly adsorbed compounds. 227, 500. (5) Briscoe, B. J.; Evans, D. C. B. Proc. R . Soc. London, A 1982,380, The fact is, however, that none of the known properties of the 389. adsorbed layer suggests the presence of organic carbon. (6) Homola, A. M.; Israelachvili, J. N.; Gee, M.L.; McGuiggan, P. M. Instead, I suggest that all the known properties of the mica J . Tribol. 1989, 111, 675. (7) Bangham, D. H.; Mosallam, S. Proc. R . Soc. London, A 1938,165, surface in air and liquids are consistent with the presence on the 552. air-cleaved surface of a monolayer of water and small amounts (8) Beaglehole, D.; Christenson, H. K. J. Phys. Chem. 1992, 96, 3395. of an inorganic salt, probably potassium carbonate or hydrogen (9) Schultz, J.; Tsutsumi, K.; Donnet, J.-B.J . Colloid Interface Sci. 1977, 59, 277. carbonate. This compound is formed by some reaction involving (10) Tabor, D.; Winterton, R. H. S. Proc. R . Soc. London, A 1969,312, carbon dioxide, water and potassium ions.25,30It accounts for all 435. quantifiable properties of the adsorbed layer and capillary(1 1) Israelachvili, J. N.; Tabor, D. Proc. R . SOC.London, A 1972, 331, condensed water on mica, as has been previously d i s ~ u s s e d . * ~ ~ ~91.~ (12) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans. 1 The high refractive index of small condensed water bridges,28 1978, 74, 975. and of the adsorbed layer itself,12 is indicative of inorganic salt; (13) Christenson, H. K. J. Dispersion Sci. Technol. 1988, 9, 171. of organics only polycyclic aromatics could possibly have such (14) Parker, J. L.;Christenson, H. K.;Ninham, B. W. Reu.Sci. Instrum. 1989, 60, 3135. high refractive indexes, and they are not water soluble. The (15) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; water-soluble inorganic salt accounts for the decreased vapor Academic Press: New York, 1991. pressure and positive deviations from the Kelvin equation of (16) Bailey, A. I.; Kay, M. K. Proc. R. SOC.London 1967, A301, 47. (17) Wan. K.-T.: Aimard. N.: Lathabai. S.: Horn. R. G.: Lawn. B. R. J . capillary-condensed water, and the lack of a significant effect on Muter: Res. 1990, 5, 172. the surface tension of water (as measured by the pull-off force). (18) Wan, K.-T.; Lawn, B. R. Acta Mer. 1990, 38, 2073. Drying of the mica produces crystals of K2C03or KHC03 by (19) Wan, K.-T.; Smith, D. T.; Lawn, B. R. J. Am. Ceram. Soc. 1992, 75, 667. surface diffusion.30 If no potassium ions are present no salt can (20) Obreimoff, J. W. Proc. R . SOC.London 1930, A127, 290. crystallize, as with hydrogen mica. The presence of a water(21) Bryant, P. J. Trans. 9th National Vacuum Symposium; Macmillan: soluble layer on air-cleaved mica does not significantly affect the New York, 1962; p 311. (22) Metsik, M. S. J . Adhesion 1972, 3, 307. adsorption or capillary properties of nonpolar liquids, to which (23) Guzonas, D. A,; Hair, M. L. Lmgmuir 1991, 7, 2346. the results of several investigators te~tify.2~93~ The lack of an (24) Dowsett, M. G.; King, R. M.; Parker, E. H. C. J. Vac. Sci. Technol. effect on capillary-condensed organic liquids is good evidence for 1977 14 711 .__. Bhattacharyya, K. G. Lungmuir 1989, 5, 1155. the absence of any significant amounts of low molecular weight Baun, W. L.; Solomon, J. S. US.Air Force Materials Laboratory organic compounds. In bulk solutions the influence of the 1980, AFML-TR-79-4203. adsorbed layer is, of course, negligible. The shift of the contact (27) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (28) Christenson, H. K. J . Colloid Interface Sci. 1985, 104, 234. to negative distances when water is introduced is most likely due (29) Fisher, L. R.; Israelachvili, J. N. J . Colloid Interface Sci. 1981,80, to removal of the potassium ions (and/or potassium carbonate) 528. by exchange and possibly loss of one monolayer of water (as (30) Christenson, H. K.; Israelachvili, J. N. J . Colloid InterfaceSci. 1987, illustrated in Figure 6 of ref 40). In nonpolar liquids the potassium 117, 576. (31) Hughes, B. D.; White, L. R. J . Chem.Soc.,Faraday1 1980,76,963. ions and potassium carbonate remain on the surfaces, as does all (32) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R . SOC.London the adsorbed water and there is no change in contact separation. 1971, A324, 301. Since an important constituent of the adsorbed layer, the (33) Muller,V. M.;Yushchenko,V.S.;Derjaguin,B.V.J.ColloidInterface Sci. 1983, 92, 92. potassium carbonate, is produced by a chemical reaction the layer (34) Maugis, D. J. Colloid Interface Sci. 1992, 150, 243. should not be referred to as “physisorbed”. (35) Derjaguin, B. V. Kolloid. Zh. 1934, 69, 155. Only the variation in the adhesion from experiment to (36) Christenson, H. K.; Claesson, P. M. J . Colloid Interface Sci. 1990, 139, 589. experiment and especially its decrease with time may point to the (37) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. J . Phys. Chem. 1991, involvement of organic adsorbents. Significantly, the time 95, 10737. dependence is similar for potassium mica and hydrogen mica. It (38) Fogden, A.; White, L. R. J. Colloid Interface Sci. 1990, 138, 414. (39) Christenson, H. K.; Yaminsky, V. V. Lungmuir, 1993, 9, 2448. is clear that this adsorption process is very slow and quite different (40) McGuiggan, P. M.; Israelachvili, J. N. J . Mater. Res. 1990,5,2232. from the rapid adsorption of water that occurs immediately on (41) Pashley, R. M. J . Colloid Interface Sci. 1981, 80, 153. exposure to the atmosphere. If such “organic” contaminants (42) Pashley, R. M.; Israelachvili, J. N. J . Colloid Interface Sci. 1984, 101, 511. influence properties such as the angle dependence or hysteresis
*
_.
