Interaction between Surfaces of Fused Silica in Water. Evidence of

Langmuir 1998, 14, 3223-3235

3223

Interaction between Surfaces of Fused Silica in Water. Evidence of Cold Fusion and Effects of Cold Plasma Treatment V. V. Yaminsky,*,† B. W. Ninham, and R. M. Pashley‡ Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, and Chemistry, the Faculties; The Australian National University, Canberra, A. C. T. 0200, Australia Received December 15, 1997. In Final Form: March 19, 1998 Silica colloids and silica glass surfaces have often been used as “model” systems to study coagulation, rheology, contact angles, and surface forces. But the silica-water interface is highly changeable and reactive. It has stubbornly refused to conform to theoretical models of an ideal hydrophilic substrate. In this study we show why this is and demonstrate some of the diverse properties of this surface. Surfaces of fused quartz swell under water to form layers of silica gel. We report here on how this well-known effect shows up in surface force measurements. Peculiar effects occur already at normal pH. Over a period of time after the surfaces are immersed in water, identical interaction patterns occur on approach and on separation. The double-layer repulsion extends from large distances down to the contact. Interaction hysteresis develops later. Adhesion and other specific interactions, particularly at short range, develop with time. The evolution that extends for hours and days is variable in its manifestations from experiment to experiment. Precise conditions of solidification from the melt, and aspects of the thermal history of the glass transition during preparation of vitreous silica samples, can be factors in this variability. Surface degradation by formation of silica gel layers on contact with water can be enhanced by cold plasma treatment and by UV radiation. Pull-off forces increase with an increase of contact time. They also show a memory of conditions of previous contacts. Electrolytes enhance the adhesion. Complicated polycondensation equilibria, influenced by nonspecific and specific ion effects, pH, nonionic solutes, and temperature distinguish the chemistry of silicic acid. All are involved in the interaction. These curious, history-dependent, surface forces were first reported half a century ago. They were attributed by Malkina and Derjaguin to “water structure”. The effects that led later to contentious and disputed notions of hydration forces can be manifest as an “extra” repulsion or an “extra” attraction. They are here related to surface gelation. These surface force observations have distinct parallels in thixotropy and other peculiarities of “anomalous” coagulation and rheological behavior of concentrated and diluted dispersions of colloid silica in water. The effect of “cold fusion” between macroscopic surfaces of pure silica in pure water is here studied at room temperature with a new interfacial gauge force measuring technique. This spontaneous welding due to the presence of water can be hindered by stray contact shear, which interferes with observation by colloid probe and surface force techniques. The peculiar properties of the silica-water interface are discussed in connection with earlier experimental work that led to theoretical notions of polywater and non-DLVO forces.

1. Background 1.1. Introduction. For more than half a century the DLVO theory1-4 has remained as the foundation for the theory of stability of thin liquid films and hydrophobic sols. A colloid was termed hydrophobic (lyophobic) in a broad sense when its coagulation behavior matched the predictions of the theory. These problems of hydrophobic colloids are understood. But hydrophilic colloids have always caused difficulty. Many such systems are literally hydrophilic. Mica (clays) and silica (sands and glass), when clean and fresh, are indeed water wet. To ensure surface hydrophilicity as a prerequisite of surface cleanness and hydroxylation, hard * To whom all correspondence should be addressed. phone (06) 249 4693. Fax: (06) 249 0732. E-mail: [email protected]. † On leave from the Institute of Physical Chemistry, the Russian Academy of Sciences (Moscow). ‡ Chemistry, the Faculties. (1) Derjaguin, B. V. Acta Physicochim. URSS 1939, 10, 333. (2) Derjaguin, B. V.; Landau, L. D. Acta Physicochim. URSS 1941, 14, 633. (3) Verwey, E. G. W.; Overbeek, J. Th. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (4) Overbeek, J. Th. G. In Colloid Science Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1952.

oxidizing and hydrolytic treatments are applied industrially and indeed for surface force measurements. The hydrophilic notion can be made explicit in terms of contact angle or of the product of its cosine and the surface tension of water, which is the wetting tension. This entity, subject to exact thermodynamic interpretation, applies to the boundary line between the solid-liquid and the solid-vapor interfaces. It equals the difference between the two interfacial tensions. The wetting tension for a solid-liquid-fluid system as a parameter that can be measured plays the same role as does that of surface tension for a liquid-fluid interface. Over the last 20 years of extensive experimenting with surface forces many supposedly new forces have been postulated beyond those of the older DLVO theory.5 These occur in both hydrophilic and hydrophobic systems. Much of the confusion on these results can be resolved at a more general level of theory that does not demand new parameters6 and experimentally by independent thermodynamic characterization of the surfaces involved in the interaction through contact angle and adsorption (5) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York. (6) Ninham, B. W.; Yaminsky, V. V. Langmuir 1997, 13, 2097.

S0743-7463(97)01376-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/08/1998

3224 Langmuir, Vol. 14, No. 12, 1998

measurements. This is known. But, as we shall see, much more is involved. 1.2. Derjaguin-Malkina Forces. The first direct measurements of surface forces in a liquid were reported almost 50 years ago.7 This paper of Malkina and Derjaguin dealt with pull-off forces between fused quartz (vitreous silica) filaments in aqueous electrolytes. Adhesion was found to increase from zero at short contact times (seconds to minutes), with a gradual increase to high values after several hours and then days spent in contact. A detailed corpus of experimental evidence was collected. There was a vast amount of experimental work involved. Indeed, it is salutary to recall that to take a single data point took from several minutes to several days. There are many points on each curve, and the paper contains many curves: for various electrolytes, concentrations, and temperature dependence, all as a function of contact time. The rate of contact strengthening was found to increase with salt concentration and depended too on the type and valency of the ions. Such results could not be used to “confirm” the DLVO theory. The effect was attributed to a decoration called “water structure”, a concept that became fashionable after a review on water by Bernal and Fowler. This was not yet the infamous polywater. Much of the work of B. V. Derjaguin and his numerous co-workers is widely known, but virtually no reference to this particular paper was ever made even in the time of the polywater boom. Written in Russian, and apparently swept under the carpet by the authors themselves, it attracted no attention. It is now completely forgotten. The vast experimental material remains unknown even to specialists in the field. In the modern literature on the subject of direct measurement of surface forces in general, and between silica surfaces in particular, similar observations have not been made. This is in spite of the fact that the system has been reinvestigated by different techniques and many times.8-13 Interest in surface forces in liquids revived after the availability of new techniques associated with the SFA. The first measurements and most subsequent work were done with hydrophilic surfaces of mica in water.14 The expected long-range exponential repulsion of electric double layers was confirmed. A blurry “non-DLVO” adhesion pattern observed for mica in water was mentioned in this paper with a reference to the importance of hydrophilic systems. A new “secondary” hydration force due to ion adsorption was later invoked to account for discrepancies with the theory in a range of short separations.15 It was much later that silica surfaces came back into the arena. A SFA experiment with fused silica surfaces carried out by Horn et al. attracted attention. Then the attachment of a microbead of soda glass to the end of a (7) Malkina, A. D.; Derjaguin, B. V. Kolloidn. Zh. 1950, 12, 431. (8) Yaminsky, V. V.; Yusupov, R. K.; Amelina, E. A.; Pchelin, V. A.; Shchukin, E. D. Surface Tension at Solid-Liquid Boundaries. Cohesive Forces between Smooth Elastic Particles. Kolloidn. Zh. 1975, 37 (5) 918-925 (in Russian; English translation in: Colloid J. USSR 1975, 37 (5), 824-829). (9) Rabinovich, Ya. I. Kolloidn. Zh. (Colloid. J. USSR) 1977, 39, 1094. (10) Peschel, G.; Belouschek, P.; Muller, M. M.; Muller, M. R.; Konig, R. Colloid Polym. Sci. 1982, 260, 444. (11) Horn, R. G.; Smith, D. T. J. Non-Cryst. Solids 1990, 120, 72. (12) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (13) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367. (14) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (15) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531.

Yaminsky et al.

cantilever became a routine process for AFM investigations for colloid forces.12,16 We can ask what in fact these experiments showed. There was a double-layer repulsion at larger distances but no transition to a van der Waals attraction at a shorter range demanded by the DLVO theory. Long before experimental force-distance profiles in liquids became practicable, one of us undertook some studies on his own of adhesion between glass silica balls. Like everyone else subsequently, he did not observe any signs of attraction in pure water.8 These molecularly smooth macroscopic surfaces of fused silica adhered in air, in nonpolar liquids, and, after hydrophobization, almost everywhere else including water/surfactant solutions and nonaqueous solvents, but not, we repeat, for the bare hydrophilic silica surfaces in water. A thermodynamic justification for this absence of adhesion was based on considerations of surface free energy. The theory was straightforward. On the basis of rigorous thermodynamic grounds it seemingly suffered no ambiguity. The essential result here is that the specific (per unit area) free energy of adhesion between two glass balls in air is smaller than the wetting tension. It is then further smaller than the sum of the values of the surface pressure of adsorbed water vapor and the wetting tension of water. The latter gives the gross free energy effect of adhesion of water to glass. A “hydration repulsion” is a logical consequence of this consideration to account for the result. The explanation is self-consistent, but the older observations of Malkina and Derjaguin still remain a mystery. Putting aside the water structure “explanation” as prejudice, the results are too extensive and too detailed in their systematic trends to be ascribed either to extravagant imaginative powers of the respected authors or to an artifact. Furthermore, taken together and as given, they present a complex pattern of behavior distinctly parallel to a gallimaufry of events that occur with aqueous silica dispersions. 1.3. Sol-Gel Transitions. Indeed the literature on the colloid stability of silica sols provides a confusing divertissement.17 To be convinced of the parallels, one can put the literature to one side and carry out one’s own experiment by this simple recipe: Add gradually 25 g of fumed silica (the well-known aerosil of Degussa) to 100 mL of water, sonicating each time when the composition becomes too thick after a new portion of the light dry powder is stirred in. The primary particles are originally fused together due to the method of preparation by vaporphase condensation. These very loose aggregates fill the entire volume of water and are immobilized. Sonication will break them down and convert the slurry back into an opalescent fluid. A sufficiently high power may be necessary to do this. By the next day the sol has solidified into a gel (an interesting entity used in contact angle studies under the name of “solid water”, not to be taken as ice or polywater). The gelation is faster if salt is added. The gel can be redispersed by sonication, and then the process of the slow solidification is repeated. It is instructive and of interest that the effect does not occur with lyophilic dispersions of methylated aerosil (e.g., R-972, Degussa). Prepared the same way, ethanol dispersions of this hydrophobic powder remain fluid for days, even months, and show no signs of aggregation. Such ethanol dispersions can, however, be coagulated by adding water. This sits in consonance with the fact that adhesion between the hydrophobic particles increases by orders of (16) Meagher, L. J. Colloid Interface Sci. 1992, 152, 293. (17) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.

Surfaces of Fused Silica in Water

magnitude on going from pure ethanol to water. This is not an ordinary DLVO effect. It occurs by an increase of the interfacial energy that occurs by de-wetting. The critical value of the free energy of adhesion at the onset of coagulation is several mJ/m2 in this case. A detailed thermodynamic account of the effect can be given.18 The coagulation/gelation takes place immediately after ethanol is partly substituted for water. It is a typical pattern that might be expected by the von Smoluchowski theory of effective collisions, by adding the notion of coagulationpeptization equilibrium. By contrast with rapid hydrophobic coagulation observed for the methylated dispersions, gelation of concentrated dispersions of bare silica in pure water occurs over hours and days. The process is many orders of magnitude slower than the theoretical expectation based on the theory of rapid coagulation of von Smoluchowski. It remains slow even when electrolyte is added to cancel any electrostatic barriers. Salts speed the gelation, but only several times, and this does not remove the discrepancy, which is one of orders of magnitude. By considering the results of Malkina and Derjaguin, one can readily notice that it is over such times typical for gelation of concentrated silica hydrosols that the apparent free energy of adhesion increases to values of several millijoules per square meter. Interestingly, this is in turn the energy at the coagulation threshold, as is observed experimentally and is explained theoretically for dispersions of methylated silica in mixed solvents. In the latter case, however, the explanation is done by invoking shortand long-range forces between colloid particles, as measured between macroscopic surfaces, with thermodynamic considerations of the sol-gel equilibrium.18,19 The interaction here is reversible and a new level of adhesion is reached immediately each time when the concentration of water in ethanol is changed. This corresponds to immediate enhancement of coagulation when the critical level of adhesion is reached by adding water. In contrast to this, for dispersions of bare silica in pure water, the role of a coagulant is played by the contact time. A more detailed consideration confirms the conclusion that adhesion develops in time as the particles stay in contact. While concentrated dispersions gradually solidify, more dilute silica hydrosols do not coagulate even on extended time scales, either in pure water or in sodium chloride solutions. This is in contrast to coagulation of methylated silica by water, which proceeds rapidly and in both concentrated and dilute dispersions. There are other striking parallels that occur between colloid properties of silica dispersions and the crossed filament experiments of Malkina and Derjaguin. Electrolytes speed the contact strengthening. The ultimate contact strength of 40 dyn/cm (mN/m) is reached after more than a day in water and after several hours in salt solutions. These are characteristic times of the sol-gel transitions that are enhanced by electrolytes in a similar way. These considerations show that prolonged contact of bare silica particles in pure water, forced together by the effect of their high volume fraction, is an essential condition for the gel to form. This is how it is observed in this system: the longer the particles remain in contact, the stronger is the adhesive bonding that develops. (18) Shchukin, E. D.; Yaminsky, V. V. Colloids Surf. 1988, 32 (19), 33. (19) Yaminsky, V. V.; Yaminskaya, K. B.; Shchukin, E. D. Adhesion of Particles and Formation of Coagulation Structures. Kolloidn. Zh. 1987, 49, 967 (in Russian; English translation in: Colloid J. USSR 1987, 49, 843).

Langmuir, Vol. 14, No. 12, 1998 3225

The phenomenon is in contrast to the ordinary type of coagulation observed by addition of water to ethanol dispersions of methylated silica. These, and also when diluted, flocculate upon addition of water. At higher concentrations of particles the same aggregation effect takes the form of a rapid solidification of the entire volume. The composition thickens immediately after water is added. No changes occur thereafter with time. Kinetic effects visible on experimental time scales arise only near the coagulation threshold. These are explained here by consideration of a reversible sol-gel transition, through dynamic equilibrium between coagulation and peptization by Brownian effects of collisions and bond instability. There are no time-dependent forces between surfaces involved here. 2. Results 2.1. Preliminary Considerations. An ordinary SFA/ AFM experiment typically occurs on much shorter time scales than those associated with gelation of aqueous silica dispersions or the similar times involved in the crossed filament experiments of Malkina and Derjaguin. This might explain why other authors have not observed this adhesion. However, one of the authors (V.V.Y.) once tried a long-duration experiment in a crossed filament mode and could not detect any adhesion between silica glass surfaces in water even after 12 h of contact. This might seem to disprove the old report but of course not the gelation phenomenon observed with colloid silica. But then, how could the latter be explained? This is not a matter of choice of a particular silica sample. We have chosen the Degussa product for our demonstration experiment for the mere reason that the pure colloid silica is synthesized here at high temperatures by condensation from the vapor phase. This fumed silica then could be deemed to be more relevant in its surface properties to model surfaces obtained by flame polishing. This, however, is not at all critical. Different brands of concentrated dispersions of colloid silica including those obtained by hydrolytic polycondensation show similar effects of slow aggregation. Milky spots of adhered particles gradually develop on the walls of glass flasks in which silica hydrosols are stored. These are then almost impossible to clean away. The glass stopper becomes glued so strongly that after some time the flask cannot be opened without breaking. 2.2. New Observations on Forces between Silica Surfaces in Water. By experimenting more recently with the interfacial gauge,20,21 we came across some further confusing circumstances. This instrument allows one to do rapid and accurate force-displacement measurements between freshly molten glass balls or in fact any other objects. A distinguishing feature of the setup is the high mechanical spring constant (500 N/m) of the solid-state sensorsa piezoelectric bimorph. It is stiffer by many orders of magnitude than the torsion balance8 or a quartz filament18 of those older adhesion studies or an AFM cantilever. It is hundreds of times stiffer than the spring of an SFA. The axial mode of magnetic loading used in the gauge further diminishes any stray effects of contact shearing. Through investigation with this instrument we obtained a representative compilation of data on interaction of silica glasses in water. Initially they were not collected systematically. This was the starting point for other attempted experiments with added electrolytes, surfactants, (20) Yaminsky, V. V.; Ninham, B. W.; Stewart, A. M. Langmuir 1996, 12, 836.

3226 Langmuir, Vol. 14, No. 12, 1998

and so forth. The results spanned the interaction of pure silica and commercial silica glasses at different times after melting and after immersion in pure water. A remarkable observation was that in many cases, but not in all instances, a quite strong adhesion between the surfaces occurred in water. Any attempt to identify precisely the conditions under which this phenomenon can be observed seems doomed to failure. This feature underlines a basic point which follows from our observations: It undermines the foundations by introducing a new uncertainty into colloid science. The effect is a persistent feature but occurs at random. It escapes definition through the circumstance of events under which it occurs. The fundamental principles that govern the phenomenon are buried in an apparently inevitable chaos. The sporadic character of these observations seems unavoidable. Details of events that can be triggered by following what are presumed to be exactly equivalent procedures are variable. In each particular case the evolution is difficult to predict. However, a statistically stable pattern emerges by consideration of repeated experiments. This feature of the results, together with the experiments of Malkina and Derjaguin, explains much of the silica sol mystery and other related observations. Such results are difficult to present along the conventional clean lines of a scientific communication. Nevertheless the evidence is compelling and the issue is important. It cannot just be ignored. We remark that this is not the first publication since that of ref 7 that claims to detect history-dependent adhesion forces between silica surfaces in water and in salt solutions. In a previous report21 on interaction between Pyrex balls in aqueous CTAB we have already noted that the initial state of interaction in pure water is not as simple as theory might expect. We extended these observations with various brands of freshly molten silica glasses in water vapor. This led us on inevitably to the vexed “polywater” issue.22 A similar observation on adhesion, with memory effects for fused silica in aqueous electrolyte, has been reported with SFA.23 We do not consider here other reports which relate to adhesion of silica surfaces in solutions of cationic surfactants or divalent cations. These may well be relevant to our thesis but would lead us a step away from the main subject. 2.3. Materials and Methods. For surface force measurements we used different silica substrates, basically those of our recent polywater study in vapor.22 Pasteur pipets represented the class of low-melting soda glasses. Pyrex was taken as a characteristic chemical glass with a high melting point. More systematic experiments were carried out with pure amorphous silica in its vitreous state (fused quartz). The procedure by which a surface is created can be traced back to pioneers of surface force studies in the first half of this century.24,25 The end of a glass rod, or a tube, typically 2 mm in diameter, is introduced into a flame, propane-oxygen for pure quartz or propane-air for lower melting point glasses. After a liquid drop forms, the sample is taken quickly out of the flame. The drop solidifies in air. The sample cools and is installed in the interfacial gauge.20,21 Whether the installation procedure (the time of tightening of a screw to clamp the sample) takes place inside a laminar flow cabinet or outside is not (21) Yaminsky, V.; Jones, C.; Yaminsky, F.; Ninham, B. Langmuir 1996, 12, 3531. (22) Yaminsky, V. V. Langmuir 1997, 13, 2. (23) Chapel, J.-P. J. Colloid Interface Sci. 1994, 162, 517. (24) Tomlinson, G. Philos. Mag. 1928, 6, 695. (25) Bradley, R. S. Philos. Mag. 1932, 13, 853.

Yaminsky et al.

critical for the results. Only flame-polished parts of the samples which have not contacted any surfaces are then immersed in water that fills an all-glass cell (a small beaker). The water was purified by a reverse osmosis-distillation-Millipore SFA routine. To avoid other concerns as to this mode of purification, we tried water treated in different ways. In particular, the Millipore product was distilled again in an all-glass apparatus under charcoal. The charcoal was washed in advance by boiling with large amounts of Millipore and was decanted. In some experiments other common chemicals were used: sodium hydroxide pellets of analytical grade, out of which alkaline solutions in Millipore were prepared, or sodium chloride or acetate for saline, filtered through a 0.2-µm porous Teflon membrane or not filtered. The facility of gauge measurement allows plenty of different tests to be done. We mention only some, so as not to draw attention from the main points through information overload. A cold argon plasma treatment was used in some tests, as recommended in ref 12. This requires the samples to be placed into the reactor for a period of time before being installed in the gauge. An alternative way of surface treatment with an ultraviolet lamp could be done in situ. In this case the lamp was positioned at a distance of about 1 cm from the samples already installed in the gauge. The treatment for 1 min was done in water vapor to facilitate formation of ozone; larger treatment times were tested as well. Curious effects follow after this treatment. The results are discussed in the next section. 2.4. Experiments. For freshly molten surfaces of pure quartz, pull-off forces in ambient air (around 50% humidity) are up to 80-90 mN/m in the scaled F/(2πR) representation. By this “Derjaguin approximation”, forces are converted into free energy per unit area provided the radii are large, contact deformations are small, and the interaction is reversible.26 In this context we do not consider the 25% difference between the factors, which makes the distinction between the two limiting (Derjaguin and JKR) approximations for dry adhesion. Washing with water brings the adhesion slightly down to a value of about 70 mN/m. The fact that it almost coincides with the surface tension of water is not surprising. This is the level to which the surface energy can be reduced by adsorption of water vapor. 2πRγ is altogether the limiting expression for the attractive force created by concave menisci of small annuli formed by capillary condensation of wetting liquids from their saturated vapors in a narrow slit around the contact position.26 With exposure for hours and days to air at ambient humidity, the adhesion reduces further. Typically it does not reduce below a value of about 50 mN/m. While the adhesion has a tendency to decrease with time, the jump-in distance does not change. It stays at a value about 7 nm. This is the value expected from the Hamaker formula for the given spring stiffness and radii of the spheres (about 2 mm). The estimate assumes a Hamaker constant of 10-19 J, with an allowance for the amplitude of the vibrational noise (typically 1 nm for the experiments in air). Unlike the case for mica samples in SFA, which have modulated surface geometry, contact radii acting for the glass balls coincide with their measured macroscopic radii. As a result, pull-off forces may not vary as much by changing contact positions. An increase of humidity toward 100% increases the jump-in distance considerably, (26) Derjaguin, B. V. Kolloid-Z. 1934, 69, 155.


up to 10 nm. This 50% increase leaves the adhesion unchanged at a level of 70 mN/m. This is explained by adsorbed water films. These observations are similar to those reported for mica with SFA.27 We have already shown that the interaction pattern in saturated water vapor can be different if a soda glass is taken instead of pure silica. The surfaces, if not washed with water after melting, display a very long-range attraction in water vapor. The soda leaches out of the glass. It then promotes formation of thick aqueous films and large contact condensates. Through this process of leaching, silicic acids have to be released. The polysilicic condensate can further participate in the bridging phenomenon. On immersion in water, be this silica spheres of fused quartz or soda glass balls, one may end up with a pattern reported by many authors. This is a case of low-charge surfaces with no adhesion (Figure 1). The repulsion at not too short distances can be modeled by fixed-charge or constant-potential fits (about 50 mV typically). Interaction at shorter range as we observe it here is a less persistent feature. It shows greater variability between experiments. Experimental curves may appear to mimic theoretical constant-potential (roughly exponential down to the contact) or, more often, constantcharge plots (a steeper repulsion occurs at shorter range). In some cases this repulsion is even stronger and steeper at short separations than that permitted by the constantcharge limit. This is not an unexpected result. Such observations have lead to notions of hydration forces,15 silica hairs,13 and so forth. What we observe here and has also been missed is that the interaction at short range can change with time. This has not been stated clearly by previous authors. Initially the interaction can be repulsive at all distances and the pattern, whatever it issconstant charge, constant potential, extra, and so forthsappears almost identical on approach and on separation. We note in this connection that the simplest candidate for the source of hysteresis which later appears could be simply viscosity. However, this does not explain the situation. Why in some instances is hysteresis is absent, and when it occurs, why does it change with time? The story cannot be simply a matter of the viscosity of water. For the given set of loading rates (in the range between 10 and 100 nm/s in these experiments), known sphere radii, and the spring stiffness, the viscous drag forces which follow from solution for the corresponding kinetic equation of state are insignificant. As we just noticed, in many experiments the surfaces immediately after immersion were nonadhesive. However, after some time the situation changes. One hour after immersion (or much earlier or much later or even not at all) the force-deflection curves on approach and on separation are no longer identical. The interaction on approach does not change. The change shows up during unloading and separation of the surfaces. When the pressing force applied to the surfaces decreases, a step outward of the hard-wall contact occurs. The surfaces leave the hard wall not by a gradual increase of separation with decreasing external force but by jumping out suddenly when the load is reduced to a critical value. While still pushed together by the load, they jump out of contact and end up at a distance apart in the repulsive double-layer regime. The step can be up to several nanometers wide. Its heightsthe critical separation loadsdecreases further in succeeding cycles. The kinetics of this evolution is (27) Christenson, H. K. J. Phys. Chem. 1993, 97, 12034.

Langmuir, Vol. 14, No. 12, 1998 3227

Figure 1. Surfaces of molten quartz with a long history in air (3 days after melting) interacting in millipore water with a small amount of electrolyte added (10-4 M NaAc). The approach (thicker dots) and the separation records are taken 1 h after the immersion. The nonlinear Poisson-Boltzmann formula gives a reasonable constant charge fit (large diamonds) down to a distance of 1 nm. From here and down to contact the repulsion can still be considered exponential, but with a different exponent. Its decay length is an order of magnitude shorter than the fitted Debye length (the latter is 25 nm in this experiment). There is a small (a few angstroms) hysteresis at short range. Variability of these “non-DLVO” forces, which circumstantially can be reversible or irreversible or repulsive or attractive, is a very typical feature of our observations. In this particular plot the interaction can be approximated by a hard wall when the load exceeds 4 mN/m. Not to confuse further the analysis by invoking van der Waals forces, a zero Hamaker constant was assumed throughout the DLVO fits. To show more detail, the results are plotted on different scales. The scaled force is in millinewtons per meter.

variable, as we shall see. Among other factors it changes from experiment to experiment. A similar step out occurs symptomatically when conditions are changed, for example by adding sodium hydroxide to increase the pH. In some situations, such as in the example in Figure 2, the surfaces step apart not from the hard-wall regime but at a later stage, after some gradual separation by a finite distance already has occurred. There is something dubious about this interaction hysteresis that tends to suggest that it is rather of chemical than of physical nature. Interestingly, a similar step out of contact can be observed when freshly molten soda glass surfaces interact in saturated water vapor. Here this event usually takes place several minutes after the beginning of the exposure to the vapor. It is typically after such a step that the subsequent interaction becomes astonishingly long range. A cooperative structural transformation of the surface layers with leaching of soda and release of silicic acid has

3228 Langmuir, Vol. 14, No. 12, 1998

Figure 2. History of the surfaces: UV ozonized after melting, spent hours in Millipore water; the record was taken 1 h after sodium hydroxide was added to a pH of about 11. The PoissonBoltzmann constant charge (diamonds) fits the approach (ticker dots) down to distances less than 1 nm. The surface potential is high (150 mV), as might be expected for silica at alkaline pH when all silanols happen to dissociate. The Debye length (about 15 nm) accords with the ionic strength (stuffed pellets give pH slightly less than the value based on stoichiometry). In a range of short distances from 0 to 1.5 nm no hysteresis occurs. However, on the way out a discontinuity shows up: when the distance increases to 1.5 nm, the surfaces make a jump further apart to a distance of 3 nm. The subsequent exponential longrange repulsion is stronger than that seen on the approach. Similar steps out can be observed in experiments in pure water, and jumps can occur straight from the hard wall. When the jump occurs in the negative range of loads, as shown in Figure 3, the event concurs with an ordinary concept of pull-off force. The adhesion depends on time under water, time in contact, plasma treatment, and other factors. In this experiment the step-out will disappear 1 h later. It will be substituted by a monotonic repulsion in the way shown here on approach. F/(2πR) is in millinewtons per meter.

been suggested as a possible explanation.22 The shearing resistance might be here affected, and through this, a stray motion under load can also be changed. This possibility cannot be entirely prevented or ruled out in the current setup. However such an effect cannot explain the subsequent evolution of interaction patterns that follows these events. For interaction in the vapor a sudden increase of the range of attraction occurs after such a step. The range increases in succeeding cycles, but no more steps occur on unload. When the interaction takes place in liquid water, the stepout observed at the onset of the event shows up in each cycle as runs into contact continue. It becomes smaller in height from cycle to cycle. The interaction pattern can be even more involved. What first appears often is a sort of staircase. Up to several

Yaminsky et al.

Figure 3. This record in water was taken after the surfaces stayed immersed for more than a day. A long Debye length accompanies the interaction on approach (shown here in the thin dots) in the pure water. The apparent surface charge (fitted value of 100 mV) is larger in this case than what one observes during the first hours after immersion. Otherwise the fit is reasonable, and the force on approach is repulsive. It remains so almost down to the contact. But a slow jump into the hard wall from a distance of less than 1 nm cannot be ruled out for this particular record. The hard wall extends in this case into the range of negative loads down to a value of about -6 mN/m. This force makes the surfaces jump apart. A record from this experiment taken during the first day of the immersion is shown in Figure 1. At that stage the surfaces were not adhesive. F/(2πR) is in millinewtons per meter.

sequential steps can be observed before the surfaces are finally liberated into the double layer. The events are initiated in the range of positive (compressive) loads. The step-out height moves down. At some stage it transforms in depth. In other words, the jumpout eventually occurs in the range of negative (disruptive) external loads. The surfaces now separate when the force of interaction is net attractive! The strength of this attraction continues to increase. If runs into contact continue in ramps for which frequency, amplitude, and effective contact duration remain unchanged, the adhesion stabilizes after a period of time. At this stage pull-off forces become quite large (Figure 3). Values up to several millinewtons per meter are typical (values up to 40 mN/m were reported by Malkina and Derjaguin for very long times of contact). Associated with this type of adhesion there is often some increase of the separation between the surfaces by up to a few nanometers before the jump-out occurs. In other cases as in Figure 3 a fast jump can proceed directly out of the hard-wall contact. It would seem to be difficult to quantify the effect any better at this stage as long as the possibility of stray shearing is not totally eliminated.


This attraction has some features in common with, but cannot be reduced entirely to, the trivial effects of water viscosity. For the given set of conditions (the spring constant, sphere radii, and loading rates), experimental forces, obviously kinetic in nature, are nevertheless orders of magnitude larger than any figure for “hydrodynamic” attraction that might follow from well-known formulas for viscous drag. The latter could not be experimentally resolved by these experiments (in particular, load rates are smaller while the spring that we use here is hundreds of times stiffer than that used in ref 28, results of which are described by the same equation of motion). At the same time the attraction that we observe experimentally is not merely of static origin. The loading speed and the time under load both influence the adhesion. Generally, the longer the contact time, the larger is the pull-off force. This is essentially what Derjaguin and Malkina observed. The way this adhesion depends on dynamic factors shows different trends from that which follows from theoretical considerations on Poiseulle flow. For example, if the speed is reduced by increasing the period, through which effect the contact time is increased accordingly, then pull-off forces become larger. Smaller attraction at lower speeds should rather be a general expectation for “hydrodynamic” attraction originating from Newtonian viscosity. The issue is somewhat more involved than this because the equation of motion based on the formula for viscous drag diverges in the limit of small distances, as is discussed in ref 28. But the considerations above do essentially apply. To show this experimentally the speed can be reduced, with the contact time kept constant. In this way the compression at each instant of time during the contact part of the period also decreases. Both effects, the smaller force and the smaller speed, favor a reduction of hydrodynamic adhesion. Experimental pull-off forces are here independent of the speed. This shows that the simple hydrodynamic argument does not even hold qualitatively. Pull-off forces can be larger for smaller loads if contact times are longer! This adhesion shows other peculiar features. What we observe in the present case is that after the surfaces were separated for a longer period of time, or moved to a new contact position, the adhesion in the first approachseparation cycle is typically quite small. It can even be shifted back into the repulsive regime or not show up at all. But then it builds up again: pull-off forces becomes larger from cycle to cycle and stabilize at a constant level, as already occurred before. If the loading speed does not change but the contact is maintained for a longer period of time, the stationary level of adhesion, as we have already observed, becomes higher. However the way this adhesion responds to conditions of contact loading varies from experiment to experiment. Within each particular observation one can end up with a nice kinetic plot of for example adhesion versus time of contact (Figure 4). Qualitative trends are repeated, but outgoing parameters can differ from experiment to experiment and change with time within one and the same experiment. In some cases observed pull-off forces depended only slightly on contact duration. In some other instances the force of adhesion was found to be almost proportional to the time which the surfaces spent in contact before they (28) Steblin, V. N.; Shchukin, E. D.; Yaminsky, V. V.; Yaminsky. Hydrodynamic Surface Interactions in an Electrolyte Solution. New method of Investigating Surface Forces Using Capacitance Ultradynamometer. Kolloidn. Zh. 1991, 53, 684 (in Russian; English translation in: Colloid J. USSR 1991, 53, 577).

Langmuir, Vol. 14, No. 12, 1998 3229

Figure 4. This figure gives an example of how pull-off forces depend on contact duration. These surfaces were immersed 3 days after melting and then stayed several days more under water. Qualitatively similar trends occur for adhesion that may arise shortly after melting, and immersion though forces of adhesion are typically smaller in absolute magnitude and their dependence on times of contact is less pronounced. The time spent in contact was varied by an offset while the period (about 1 min) and the amplitude of the ramp did not change. The rate of the load corresponds to a 10 nm/s speed of displacement of the spring freed when the surfaces are out of contact. F/(2πR) is in millinewtons per meter.

separate, as illustrated in Figure 4. Also here, if the contact time is changed, the new stationary value of adhesion is usually not reached immediately but only after several cycles. This retardation effect displays a memory of previous contacts, an effect quite striking in observation. We also consider experiments done in a mode for which contact time is held constant but the rate of the external magnetic loading is increased. The pressing force at each moment of time thereby becomes larger and rises to a larger maximum value. But the increase of the adhesion is usually less pronounced than that when contact time is increased. As was already remarked by Malkina and Derjaguin, this dependence on the time in contact rather than on the load in contact is a distinguishing feature of this adhesion. One more peculiarity: this attraction in most cases does not show up on approach. Here the interaction remains repulsive at all distances down to the point at which the surfaces effectively come into hard-wall contact. This point shows up as a distinct event in the plots. In accord with what we have already said, the force on entering the hard-wall contact can be larger than the preexponential factor of the double-layer exponent. This theoretical limit for the maximum value of the repulsive force follows by extrapolation of the Poisson-Boltzmann result to the effective zero of distance. If one makes an allowance for a van der Waals attraction, then the net repulsion could be expected to be smaller still. In some cases, however, the exponential DLVO form extends down to the last few angstroms away from hardwall contact. The double layers operate all the way down to the contact, and the only complaint against the DLVO approach is that van der Waals attraction is “switched off”. However, in some of the experiments which showed this constant-potential type of interaction we indeed observe a jump into contact from a finite distance of up to 2-3 nm. Such a jump is generally indicative of attractive forces that are devised theoretically to operate at this short range. The jump, however, proceeds at much slower rates than what might be expected by ordinary DLVO considerations

3230 Langmuir, Vol. 14, No. 12, 1998

of van der Waals forces. Over the entire length of the jump the surfaces move toward each other with a speed which does not exceed a value of about 100 nm/s. For comparison, the jump into contact that occurs for freshly molten silica surfaces in air is several orders of magnitude faster. In this case, however, the Hamaker constant is large while air viscosity is low. But also for water theoretical estimates show that the jump has to be much faster than experimentally observed. To confirm this we simulated a shorter van der Waals jump by measurement with freshly molten silica spheres in di-iodomethane. According to a smaller Hamaker constant for interaction across a condensed medium, the jump-in distance is here 2-3 nm. This jump-in as it occurs in methylene iodide is similar to what we have observed in the above-mentioned experiments in water: in the size of the jump but not in the speed with which it proceeds. The speed here is indeed many times higher than that in water even though the viscosities of the two liquids are not much different. This shows that the slow jump-in in pure water whenever it occurs cannot be simply attributed to the action of van der Waals forces. The jump not only is too slow but also occurs from a too large distance to be explained this way. Indeed, one can expect the Hamaker constant for silica in water to be an order of magnitude smaller than that in di-iodomethane. The theoretical jump-in distance then would be several times smaller than the values observed in some experiments. 2.5. Question of Artifacts. The simplest explanation for the whole set of the results is to invoke the word artifact. This could result from contamination of water or from a surface contaminant. More detailed considerations that we extend here rule out such a convenient hypothesis. The variability of the effect seems not to be correlated to the “quality” of the water. To justify the contaminant hypothesis, we need to assume that the amount of a postulated contaminant in water has to be variable. However, one and the same batch of water can lead to results that differ from experiment to experiment. On the other hand, results can be similar by use of different batches of Millipore water or with water specially distilled over charcoal. With reference to the latter observation, we are compelled to assume that such a mysterious contaminant cannot be removed by adsorption. Nonetheless it influences the interaction. Water is easily saturated with carbon dioxide from air. But any influence on the interaction other than through a change of the pH is difficult to envisage here. If nevertheless this environmental gas makes such a dramatic effect on forces between the surfaces, the question which one then has to face is: why has this problem has not been noticed by other researchers? Of course, the “contaminant” could be silicic acid itself. It leaches from the walls of glass containers involved in the purification procedures and in fact out of the silica samples themselves. This is a more serious problem. It relates to the question of the thermodynamic status of surface forces in general. The issue is rarely addressed in the literature. Comprehensive treatises tend to ignore this fundamental principle. It deserves serious attention as follows: All substances are soluble in water. All formally have finite vapor pressures. These are basic postulates of molecular kinetic theory. However small the absolute value of the analytical concentration might be, only its relative value with respect to the solubility is here important. This ratio is the scaling parameter in a con-

Yaminsky et al.

sideration of capillary condensation phenomena that relate to phase separation. For silica with its sophisticated condensation-depolymerization equilibria, the situation can be more complicated still. The equilibrium solubility could be difficult to define, even in principle. The problem cannot be simply solved by collecting water in a plastic bottle, or with allgold distillation hardware. The latter piece of purification equipment that dominated early studies on sol stability at the turn of the last century is out of fashion nowadays. Clearly, the problem extends far beyond this particular experimental report. It refers to any surface force measurement in principle and in general. For general discussion of this issue, we refer the reader to a separate review.29 At this stage we add one more relevant observation. If the surfaces prove to be adhesive in one batch of water, they usually continue to adhere after being transferred into another lot, and the same is true with nonadhesive silica surfaces. Of course, it is possible to enhance adhesion artificially by creating a supersaturation by for example dissolution of sodium silicate. We later shall consider more evidence that will show us that the effects are not precisely due to background silicic acid. However, the mere notions of “background” and “intrinsic” cannot be consistently discriminated in this context as a matter of principle. A lot of the confusion could be explained if one assumes that it is the surfaces themselves, not the water, which are contaminated. In a consideration of possible surface contaminants other than the silica of the samples themselves (this paradox is the essence of our treatment) we note that at the moment when it is taken out of the flame the liquid silica drop shows an intense white glow. Its temperature at this stage exceeds 1000 °C. The droplet continues to radiate for a few seconds before it dims and solidifies in air. One can nevertheless argue that there might be a risk of a carbon deposit not burnt out. A common route to try to skirt around this sort of a problem is to use a low-temperature plasma or some other oxidizing treatment devised to eliminate any organic or carbon contaminant. We consider the outcome of such procedures in some detail later. Whatever hidden obstacles there might be, by use of the current setup, the risk of surface contamination is reduced as compared to that for other studies. In some of our experiments the time interval between the melting of glass and the immersion in Millipore or freshly distilled water was no greater than several minutes. This time span however does not introduce any differences in the results. The same phenomena can be observed when the samples were exposed to air for hours or days before the immersion. Any contacts other than those of juvenile silica of the flame-polished surfaces with the water in an all-glass beaker are avoided with this setup. The beaker can be cleaned thoroughly by hot chromic acid and multiple rinsing, by soaking with sodium hydroxide, or by annealing, or it can be substituted by a Teflon cell. It can be filled with water without interrupting runs into contact, so that measurements can be started immediately after the gap between the surfaces instantaneously fills with water. Any stray diffusion which can bring a contaminant into the interaction zone if not originally present in the water should go via a longer way and take a longer time. Such a diffusion could then explain long-term effects observed in these experiments if, of course, the source of a diffusing material could be allocated. However, just to (29) Yaminsky, V. V.; Ninham, B. W. In preparation.


confuse the matter, some experiments showed build-up of adhesion straight after immersion. We also observed by chance that sometimes adhesion did not occur even after days under water. 2.6. Plasma- and UV-Treated Surfaces. To add more confidence to the issue of surface cleanness, we tried two common techniques currently employed for surface cleaning. These are widely in use with AFM studies of colloid forces. The techniques are cold plasma and UV radiation. Both have an effect, and the effects of the two are similar. In one experiment which did not display any adhesion under water over a period of time the surfaces were taken out of water and dried. An ordinary pattern with a jump-in from a distance of 6-7 nm and a pull-off force of 70 mN/m in air was restored. This presumably showed that the surfaces remained smooth and hydrophilic. The UV procedure was then applied for 1 min. The subsequent measurement in air was a revelation: the adhesion went up by almost twice, up to values over 100 mN/m! Also the jump-in distance was increased more than two times, up to 15 nm. Two comments can be made in connection with these two results. An increased surface energy for cleaned and hydroxylated silica surfaces is an explanation that cannot be dismissed. However, an increased jump-in distance is a result which makes us reconsider that “activation” hypothesis with considerable caution. This is because the Hamaker constant should not have changed so dramatically. Only minor changes to the long-range van der Waals attraction might be expected by surface hydroxylation or by the burning out of carboorganic contaminants that affect only the surface monolayers. In any case Hamaker constants needed to account for such a long jump-in would be unreasonably high, by an order of magnitude. Subsequent measurements with these surfaces under water were appealing. When the surfaces were immersed, strong contacts of about 10 mN/m were immediately displayed. This adhesion is even larger than the ultimate values reached under water with samples without cold plasma treatment (recall that to make the samples we flame silica, which is a hot plasma treatment of the juvenile surfaces in statu nascendi). An “above constant potential” repulsion seen on approaching the contact was well pronounced. No jump-in occurred on hitting the “hard wall”. This pattern was preserved during the several hours the samples spent under water. A slight tendency for the adhesion to decrease with time was observed. The surfaces were then taken out of water, and the wetting films were allowed to evaporate for about half an hour. Then measurements in air were repeated. This time results proved to be different from those seen after melting, or after the first immersion in water before the UV treatment. It was also different from that observed after the UV treatment prior to the subsequent immersion. The surfaces now experienced a much longer range attraction in air with variable and irregular patterns on approach. A complex jump-in pattern appeared. The surfaces accelerated and slowed several times before being stopped at contact. Pull-off forces were more scattered and had smaller magnitudes than those observed in air after the UV treatment before the immersion. In one of the experiments with UV-treated silica surfaces, a solution of sodium hydroxide was added to water to a pH of 11. A manyfold increase of the doublelayer repulsion was observed. This is an expected result (Figure 2). It is conventionally attributed to the high charge attained by silica at alkaline pH. The surfaces

Langmuir, Vol. 14, No. 12, 1998 3231

displayed signs of adhesion in this case also. There is seemingly nothing that could alarm us about the results of this measurement as compared to those obtained at moderate values of pH. In the latter case no notice of a dissolution is ever taken. The surfaces then were taken out of the alkaline solution and placed back into water. The interaction changed back to a common pattern with less developed double layers. One again can be assured that no irreversible changes occurred. Finally the surfaces were taken out of water and dried. Subsequent measurements in air pointed to some fault in this “final conclusion”. The interaction in air in this case appeared to be particularly long ranged. It showed features in common with those of experiments for freshly molten soda glasses in water vapor.22 The polywater pattern persisted for several days in ambient air of moderate humidity. A similar pattern in air occurred for surfaces with no plasma treatment, after being stored under water for a week. Plasma treatment of silica surfaces at still higher powers for longer periods of time damages the surfaces even more severely. They display a further longer range “non-DLVO” repulsion but also no adhesion under water. The surfaces are still adhesive in air, but the long-range attraction is irregular and pull-off forces are scattered. A degradation similar to that caused by hard oxidizing treatments apparently proceeds at much slower rates simply by contact with water. An example of typical results which can be obtained with such conditioned surfaces when measurement is done in water is shown in Figure 5. The adhesion which develops after immersion diminishes later. It can disappear after several days spent under water. At the same time the “extra” repulsion increases both in magnitude and in range. Its relevance to surface degradation by hydrolytic decomposition of silica is further evidenced by the less regular shape of the curves, effective damping of vibration and other peculiar features that occur here. 3. Discussion 3.1. Self-Decompositions of Silica Surfaces in Water. This hysteretic behavior manifests mechanical and chemical irreversibility. It clearly points to nonNewtonian rheology of transient jelly films. Fused silica is a nonequilibrium entity, and by contact with a water surface gelation occurs. The gel layers further undergo gradual evolution under water. The interaction pattern that arises when two surfaces are in contact and the gel layers overlap is a result of a complicated interplay of elements comprising the high viscosity, elasticity, and plasticity of these layers. These rheological properties are interlinked with ionexchange equilibria, as these occur in the DLVO notion for double layers and other forces for diffuse and gel layers of polymers. These are living polymers in our case which can react with each other over any of their monomeric units. The surface layers of polysilicic acids can undergo a capillary type condensation to form linkages on contact, with chemical polycondensation cross-links superimposed on this physical effect. These are subject to change with time. The viscoelastoplasticity of the diffuse layers of the self-reacting weak polyelctrolyte mediated by ionic forces and chemical and physical transformations is reflected in interaction patterns that change accordingly. The gel layers show up in air as well. When taken out of water and dried, the surfaces experience a super longrange attraction. Capillary bridging is now enhanced by the surface tension effect. The “polywater” condensate

3232 Langmuir, Vol. 14, No. 12, 1998

Figure 5. After several days in water the strength of the adhesion decreases. So does the gradient of the force. As a result of the latter effect the jump-out instability diminishes and vanishes. Double-layer repulsion is substituted by an equally weak and long-ranged attraction seen on approach (the upper curves). This curious effect is more pronounced at lower speeds. The force changes to repulsion below distances of about 5 nm. From here the approach and the separation curves begin to merge but the hysteresis vanishes only from a shorter distance. F/(2πR) is in millinewtons per meter.

forms an order of magnitude stronger linkages between the surfaces than those that occur under water. The surface tension of the aqua gel of polysilicic acids that now contributes to the attraction is indeed close to the surface tension of water. These linkages stretch by elongation to very large distances. We have already reported on similar patterns induced by poly(dimethylsiloxane) coatings.30 Mechanical resistance of these deformable adhering shells shows up on entering contact and on separation. Even these effects now extend to quite large distances (Figure 6). One can see here how the surfaces begin to slow during the jump-in from distances of more than 20 nm before the contact. Jump-out occurs from even larger separations. When the bridging material dries out more and solidifies, which can be enhanced here through hardening by polycondensation of silicic acids back into silica, the attraction becomes hindered. At the beginning of the drying the hindrance takes the form of enhanced viscoelasticity. This reversible effect is substituted at later stages of solidification by irreversible types of mechanical behavior of plastic flow and brittle fracture. Static resistance to stress characteristic of the solid state is displayed by the dehydrated material of the former gel that now forms the bridge. (30) Yaminsky, V. V. Colloid Surf. 1997, 129-130, 415.

Yaminsky et al.

Figure 6. Gradual surface gelation induces adhesion between silica surfaces in water. This observation, illustrated with preceding figures, parallels the peculiar properties of concentrated silica dispersions. The phenomena are well-known but poorly understood. Surface gelation can be enhanced by plasma treatment, UV radiation, and soaking in alkaline solution. A very strong and very long-range attraction then occurs between the gel layers in air (thinner dots are the approach; F/(2πR) is in millinewtons per meter as usual; data for the inward and the outward runs are shown on three different scales for greater detail). Such patterns observed for soda glasses were identified with “polywater”.22 The result shows that even pure silica is not totally immune to this surface degradation. In fact fused quartz was the substrate which originally lead to the discovery that caused so much trouble.

By this progress of hardening and drying, irregular force-distance profiles emerge. Reproducible adhesion is now substituted for scattered fracture forces. We already have reported22 that if the drying occurred while the surfaces stayed in contact, they become strongly glued together. Very high loads are then needed to break the joint. After it breaks, subsequent contacts occur by irregular fracture surfaces. Pull-off forces are small and poorly reproducible. An exact picture of transformations that silica surfaces undergo in water and on drying can be far more involved. The chemistry of silica covers a wide spectrum of dynamic coexistence of numerous species polymerized in one, two, and three dimensions, from linear polymers to the crosslinked colloidal condensates that silicic acid is able to form, with stoichiometric silicon dioxide at one extreme and oligomers and silica acid monomer on the other. The variety of forms that can emerge is no less diverse than that of carboorganic compounds and polymers. In the latter case, however, monomer reactivity normally goes one way with buildup of linear chains. Stable covalent bonds rearrange only under the special reaction conditions of organic synthesis. The presence of special functional groups along the polymer chains might be needed to allow for cross-linking. For silicates polycondensation equilibria


are not so much limited. In the presence of water, sophisticated polyacid material can evolve with enthusiasm. Siloxane bond rearrangement is not hindered as for covalent bonds in organic molecules. The bonds form and break spontaneously with low activation. Polyacids form, swell, gelate, decompose, condense back into silica, and undergo other transformations. This complex surface material rearranges chemically, physically, and mechanically. The kinetic processes behind these decompositionrecondensation equilibria and the structural properties of the gel are influenced by electrolytes, pH, and other factors. It is this notion which lies behind various techniques of preparation of silica sols and gels, molecular sieves, hydrolytic and drying mechanisms of silicate glues and hardening of concrete, biomineralization, and so forth. Particularities of transformations that occur with fused silica surfaces in water can further depend on the structural peculiarities of the material of the samples and the manner in which it forms after flaming. We recall in this respect that internal stresses that occur by solidification from the melt are especially strong in glasses. Magnitudes depend on the thermal history. Glasses overstressed by fast cooling act as strong explosives. This shows that the stored energy can indeed be as high as that released in chemical transformations. Such a stress accumulated in the surface layers of the samples during their preparation is then released through depolymerization and gelation. The process triggered by water depends in its thermodynamics and the kinetics on the initial state of the surface layers. It can be very different depending on the precise temperature regime during the sample preparation. We present indirect evidence that the surface hydrolysis typically begins more like a cooperative phenomenon. However, the subsequent slow transformation extends to hours and days and occurs more gradually. Phenomena qualitatively similar to those that we here describe with reference to pure silica can be observed and are occurrences even more pronounced for commercial soda glasses. The hygroscopic silicate reacts with water eagerly and gelates in a few minutes after being exposed to water vapor.22 A more detailed comparison between different brands of silica glasses could be currently based only on very large statistics. To minimize the element of contingency, we might need first to consider how to maintain standard rates of melting and cooling and make other conditions during the glass transition identical for a given material. It might be equally important to characterize the exact chemical compositions of these glasses. (But even this would present difficulty as the surface composition is different from the bulk composition. The surface even more than the bulk of the glass is almost certainly not only structurally but also compositionally heterogeneous). Routine precautions for surface force studies continue to apply, but they are not sufficient to avoid the problem. 3.2. Experimental Techniques and Cold Welding Effects. Several questions are posed by this study. If the cold welding effect is so ubiquitous and occurs for pure silica and various glasses in saline/alkaline solutions and even in pure water, as we confirm by this study, why then are relevant observations so rare in modern literature on direct measurement of colloid forces? One can invoke a psychological explanation: experimental results that do not match our expectations and present dogma are not noticed or are blamed on artifacts. There is however a counter argument that turns the artifact problem on its head, so that instead of being

Langmuir, Vol. 14, No. 12, 1998 3233

dismissed as contamination it leads toward unsuspected insights via instrumentation. That idea is suggested by our old-fashioned measurements of adhesion between glass balls.8 Even though the balls were prepared in essentially the same way as for the gauge studies that we report here, these measurements failed to show any adhesion of clean glass surfaces, either in water or in electrolyte solutions. This is a puzzle, particularly surprising because an enhancement of adhesion by for example cationic surfactants can be detected by either of the two techniques. Both give the same results,18,31 in accordance with AFM, SFA, and so forth. Similar measurements were carried out with hydrophobic balls in a variety of liquids and solutions. Here again results obtained with different techniques agree with each other.8,18,20 Static and viscous drag forces can both be measured, and alternative techniques do not contradict each other. The main difference between the conditions of the two modes of experiments which we consider here is the stiffness of the measuring springs. The quartz filaments used in our older measurements had 10 billion times lower stiffnesses than that of the bimorph. The other and the only other essential difference lies in the mode by which force control is achieved. In the gauge setup, magnetoelectric load is directed straight into the contact with no stray tangential component. This correct way of loading is not precisely the case with other setups. In a thin filament setup that we used, one of the balls slides over the other over a long path to the point from which the jump-out occurs. The length of the jump is typically chosen to be of the order of 1 mm to allow for microscopic measurement of the force. This is done by a choice for the stiffness of the filament to match up with the magnitude of the adhesion intended to be measured. The length of the sliding path that has to be covered to reach the separation point of the outward jump depends further on the length of the filament and other details of the geometry. This travel path can be as large as a fraction of a millimeter (hundreds of microns is many contact diameters). This shearing drag during the unload could have a minor effect on measured values of static adhesion, the latter being due to ordinary van der Waals forces and/or other attractive surface energy contributions with no frictional component. This condition readily occurs in theory where surfaces are uniform planes. There is by the principle of virtuality no lateral resistance in such a case. The yield shear stress to mutual tangential displacement of the surfaces, which in other terms provides the notion of dry static friction, equals zero. Tangential effects do not interfere with normal adhesion. The latter can be approximated by a simple van der Waals form or by zero range attraction in the form of surface energy in equations of contact mechanics. Whatever these latter could be (e.g. DMT or JKR), mathematically uniform surfaces on mutual contact always glide and slide smoothly until they reach the critical point at which the normal component of the applied force rises to the critical level that makes them jump out of the contact. In the theory, which by the way is deterministic, the jump occurs from one and the same point and the pull-off force has a unique value. But this is so only as long as the surfaces do not change with time and on their travel path do not meet obstacles such as asperities and other surface irregularities that can in turn be created and (31) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706.

3234 Langmuir, Vol. 14, No. 12, 1998

influenced by this drag. These could initiate the contact break prematurely. It is in fact this perfect smoothness of molten glass surfaces, uniform over macroscopic areas down to atomic dimensions and effectively lubricated by the presence of the liquid, adsorbed layers, or silane coatings, which makes such measurements certain and reproducible. Here results are indeed similar to those obtained with nonshearing experiments. However, the occurrence of contact shear may obstruct the kinetic formation of bonding linkages between silica surfaces in water. For any such bonds to form via condensation of gelated silicic acids, prolonged contact times seem and prove to be essential. In as far as such shear occurs, bond connections do not occur or are permanently destroyed by this continued stray contact motion. Through this phenomenon, no adhesion shows up on separation in the normal direction. To test further that idea, we changed the mode of external loading in the experiments carried out with the interfacial gauge to achieve a situation more in accord with other techniques. Instead of applying the force from the magnet connected to the sample on the bimorph along the axis normal to the contact plane, we moved the other surface with a piezomotor to which this sample is attached. This simulates the mode by which pull-off forces are measured macroscopically with a thin quartz filament or with AFM. This precisely is the mode of SFA experiments. What we observed is that pull-off forces measured between silica balls in water by use of this piezodrive are systematically lower than those measured with magnetic loading, by about a factor of 2. This is easily verified by switching between the two modes. For the load control mode ideally no shearing occurs when the surfaces are in the hard-wall regime. For the displacement control mode, even though the controlled translation occurs normal to the plane of contact, shear is inevitable, unless special nonshearing springs are used.32 This mode basically operates by dragging of one of the surfaces along the other. For the stiff bimorph this shearing displacement is smaller by orders of magnitude than that for flexible filaments in macroscopic measurements of adhesion. But it still can be substantial on the scale of molecular lengths and even on the scale of contact dimensions. Whichever mechanism of polycondensation and chain entanglement of the reactive swelling material is involved in the adhesion, this continuous shearing displacement hinders the binding that tends to glue the two surfaces together. Shear jumps can be occasionally observed on unload. These were detected through a stray bimorph motion readily noticeable in this case. This stick-slip frictional motion shows that cross-linking bonds that create a resistance to shear indeed form between the surfaces on contact. When by the effect of this shear the linkage breaks, a translational jump to the new equilibrium position occurs. At the point of separation in the normal direction the number of the bonds that have to break is smaller in such a case than that for an adhesion measurement carried out in nonshearing mode. At this stage we can only speculate that ordinary polycondensation equilibria for polysilicic species responsible for the entire spectrum of transformations of silica polymers and colloids are behind the molecular mechanism for these adhesional and frictional phenomena. As for filaments in macroscopic adhesion studies, stiffnesses of the AFM cantilevers are many orders of magnitude smaller than that of the bimorph. But these (32) Christenson, H. K. J. Colloid Interface Sci. 1988, 121, 170.

Yaminsky et al.

are short, and the stray effect of lateral translation is not that large. By this small length, however, angular displacement becomes important. If shear is prevented by friction, the sphere begins to roll. We do not give here sketches or numerical estimates. The elementary geometry is simple and straightforward. It provides the theory of the mode by which the optical arm of an AFM lever operates. With the displacement control mode this angular motion can be avoided and substituted for pure shear by use of double cantilever springs. Apart from these considerations, AFM measurements are ordinarily carried out with commercial glass microbeads. These are stored after manufacture in powder form in a bottle. The possibility of shearing damaging (“scratching”), aging during storage, and gluing procedures used to attach the probe adds to the uncertainty to a high degree. The classical experiment of Malkina and Derjaguin still poses questions. These wait to be answered. And this may demand new investigations. Their experiment was done in the macroscopic crossed filament mode. However, one change in their design as compared with the classical setup of Tomlinson is worth noting. The springing filament of Malkina and Derjaguin was U-shaped. Through this geometry it is possible in principle to eliminate shearing. Given this engineering solution, the principle has another important advantage over AFM/SFA techniques. Because of a very low stiffness of the macroscopic filament, any mechanical effects of thermal expansion that arise in the entire macroscopic setup due to ambient temperature instability and convection are not transmitted into the contact. This allows long-term kinetic measurements to be done. With stiffer (such as the bimorph) and shorter (AFM) springs, contact displacements become comparable to the magnitude of thermal elongation of the entire body of the macroscopic apparatus. Small variations of temperature become catastrophic. Long-term measurements then become problematic, even in principle. 3.3. Conclusions. The complex pattern of events that we have reported shows the silica-water interface is a far more involved system than modern theories of surface forces are used to dealing with. It is not an equilibrium entity. Surface gelation occurssto a greater or lesser degreesdepending on the manner of preparation and history. The gel undergoes gradual evolution under water. Silica in itself is of course a hard body, but the hydrophilic interface tends to be soft. Interaction patterns that arise when two surfaces are in contact and the gel layers overlap result of a complicated interplay of influences comprising high viscosity, elasticity and plasticity of these reactive layers that can show up also in surface segregation and capillary phase separation effects that there develop in the realm of solid-state physics and chemistry of reactive solids. Once this is recognized, much of the confusion in the literature and disputes on interpretation fall into place. The philosophic challenges posed to theories of colloid science by this situation are another story. Thus, for example, a fit of an experimental force measurement to a theory that supposes a sharp interface might invoke contributions from a double layer, with whatever decorations, and an attractive van der Waals force. Deviations between theory and experiment are then assigned to “hydration forces”. But clearly with a diffuse interface the van der Waals component in such a low-Hamakerconstant system is immeasurably small. We indeed notice that a Poisson-Boltzmann fit works much better here with no Hamaker constant postulated. Further the apparent double layer itself is modulated and drastically changed by the existence and extent of the charged silicic


acid polymeric coatings that form the real diffuse interface and of associated steric forces that inevitably arise in a range of shorter distances. So deduced, the “hydration force” is an artifact of incorrect application of physical theory to experimental measurements that on the level of chemistry makes the notion of polywater. The effects that occur there are diverse in manifestation and can vary from situation to situation. The initial state of the solid and the presence of ionic and organic solutes are equally important, and many contradictory results on surface ionization by changing pH and dehydration by ethanol that lead to an annihilation of the hydration force are explained. Any solute effects on this complicated background of forces that develop between silica surfaces in water have to be studied with great caution. The same effect that leads to an “extra” repulsion on approach can then show up as an “extra attraction” on separation. The silica welding effect occurs by capillary

Langmuir, Vol. 14, No. 12, 1998 3235

self-condensation of the reactive covalent body catalyzed by presence of water. These are all not uncommon notions of physical theories of nucleation and phase transition and solid-state chemistry. The Derjaguin-Malkina forces that arise there lead to formation of much stronger joints than just those that might be expected by action of forces of van der Waals. This effect is difficult to observe by routine SFA/AFM techniques. It gives rise to non-DLVO thixotropic patterns of coagulation and rheological behavior of colloidal silica hydrosols, with far-leading implications for hydrophilic systems in general. Acknowledgment. New acquisition and data-conditioning systems by C. Jones and F. Yaminsky incorporated in the interfacial gauge were first used in this study. The computer program for Poisson-Boltzmann fits was provided by A. M. Stewart. LA9713762

Interaction between Surfaces of Fused Silica in Water. Evidence of

Recommend Documents