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Surface Forces between Spherical ZnS Particles in Aqueous Electrolyte Gary Toikka, Robert A. Hayes, and John Ralston* Ian Wark Research Institute, University of South Australia, The Levels, South Australia 5095, Australia Received December 12, 1995X
An atomic force microscope has been used to measure the forces of interaction between ZnS spheres in aqueous electrolyte. The ZnS surface, characterized by scanning electron microscopy, tapping mode atomic force microscopy, and gas adsorption, was found to be physically heterogeneous. The measured interaction behavior is not well described by conventional theory where interacting surfaces are assumed to be physically ideal. In particular the van der Waals attraction could not be measured at pHiep and elevated ionic strength, where the electrical repulsion between surfaces is minimal. In addition no short range repulsion that could be attributed to hydration was observed. If the range of a surface force is small compared to the roughness of the surface, then it may have little effect on particle interaction. In contrast only forces of long range compared to roughness, electrical and hydrophobic in this case, are directly measurable. The hydrophobic attraction which becomes evident at low pH was found to be dependent upon electrolyte concentration.
Introduction The production of metallic zinc commences with the separation of its naturally occurring form, zinc sulfide (sphalerite), from other metal sulfides and oxides. The basis of this separation is the process of mineral flotation, which relies on the selective hydrophobicity of the constituent minerals. While collectors are added to facilitate this separation on an industrial scale, it is generally accepted1-4 that under carefully controlled conditions sulfide minerals may be floated without collector as a sulfur-rich, hydrophobic surface is produced by the manipulation of redox potential. In an entirely different context, high-purity synthetic zinc sulfide forms the basis of the “phosphors” applied to the inside of television screens, generally from an aqueous suspension. In both cases the interaction behavior of ZnS particles, both with like and unlike surfaces, in an aqueous medium is of considerable interest. The DLVO5,6 theory quantitatively predicts the interaction between macroscopic surfaces in a liquid. For like bodies, the symmetric case, the interaction depends upon the balance of electrical repulsion and van der Waals attraction. The validity of the theory has been proven by direct measurements using the surface forces apparatus (SFA), based upon the interaction of crossed mica cylinders.7 Deviations from the theory have been observed, but these occur at small separations, where the intervening medium may not be regarded as a continuum, or are due to the action of non-DLVO forces, such as hydration forces.8 Since it can be cleaved atomically smooth, mica is an ideal test substrate for this verification. Little progress9-12 has * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Gaudin, A. M.; Miaw, H. L.; Spedden, H. R. Proceedings of the 2nd International Congress on Surface Activity III; Butterworths: London, 1957; pp 202-219. (2) Fuerstenau, M. C.; Sabacky, B. J. Int. J. Miner. Process. 1981, 8, 79. (3) Hayes, R. A.; Price, D. M.; Ralston, J.; Smith, R. W. Miner. Process. Extr. Metall. Rev. 1987, 2, 203. (4) Hayes, R. A.; Ralston, J. Int. J. Miner. Process. 1988, 23, 55. (5) Derjaguin, B. V.; Landau, L. Acta Physicochim. 1941, 14, 633. (6) Verwey, E. J. W.; Overbeek, J. Th.G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (7) Israelachvili, J. Chemtracts: Anal. Phys. Chem. 1989, 1, 1. (8) Grabbe, A.; Horn, R. G. J. Colloid Interface Sci. 1993, 157, 375.
S0743-7463(95)01534-4 CCC: $12.00
been made on the extension of the theory to the interaction of nonideal surfaces, specifically those that are rough, due to mathematical intractability. This is unfortunate as essentially all solids of practical interest are rough, most grossly so on a surface forces scale. In spite of this complication it is still common to describe the behavior of aqueous dispersions in the context of DLVO theory, for example, the addition (removal) of salt is observed to cause coagulation (dispersion). This observation demonstrates qualitative, rather than quantitative, agreement with theory as one would expect some decrease in the forces due to roughness. Indeed the behavior should be controlled by either long range forces or those prevailing in the immediate neighborhood of asperity contacts. This issue has both practical and fundamental implications. At a practical level the fact that interaction forces are smaller in magnitude than those predicted by theory impacts upon strategies for modification of dispersion behavior. The more fundamental implication relates to the now widespread use of the atomic force microscope (AFM) for force measurements.13-15 In principle the surface forces associated with any particle (1 < R < 10 µm) that can be attached to an AFM cantilever can be measured, the technique being less restrictive with respect to solid geometry and chemistry than the SFA. However this broadening of application is inevitably accompanied by sacrificing physical ideality as either one or both of the component surfaces studied with the AFM “colloid probe” technique are not as smooth as mica. Despite this complication measured forces are routinely quantitatively “fitted” to DLVO theory, and for this agreement to be soundly based one can only conclude that the roughness must be insignificant when compared to the range of surface forces. The validity of this approach can now be tested as the development of scanning probe microscopes, (9) Czarnecki, J.; Itschenskij, V. J. Colloid Interface Sci. 1984, 98, 590. (10) Czarnecki, J.; Dabros, T. J. Colloid Interface Sci. 1980, 78, 25. (11) Miklavic, S. J. Philos. Trans. R. Soc. London, A 1994, 348, 209. (12) Fogden, A.; Mitchell, D. J.; Ninham, B. W. Langmuir 1990, 6, 159. (13) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (14) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Phys. Chem. 1995, 99, 2114. (15) Ott, M. L.; Mizes, H. A. Colloids Surf., A 1994, 87, 245.
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such as the AFM, for imaging, also allow the assessment of roughness, albeit in a different mode of operation. The interaction of ZnS spheres, similar to those used in the work reported here, with mica in aqueous electrolyte has been described.16 An attempt was made to verify the theoretical prediction of charge reversal, and therefore attraction, between surfaces with potentials of the same sign but differing magnitude. Instead the measured interaction at pH 5.6 in 10-3 M KCl was found to be monotonically repulsive at all separations. At higher electrolyte concentrations (10-1 M), where the electrical interaction is effectively screened, the forces appeared to be well modeled by the van der Waals attraction alone. The zeta potential, rheological behavior, and surface forces affecting the behavior of ZnS spheres in 10-3 M electrolyte have also recently been reported.17 The zeta potential was found to be positive at low pH with an isoelectric point (iep) occurring in the neighborhood of pH 7, which corresponded to the maximum in the yield stress obtained from rheological measurements. Surface force measurements in sphere-sphere geometry showed the expected monotonic repulsion at pH 10, but an attraction at pH 4. This attraction, which correlated well with rheological measurements where the yield stress exhibited a timedependent increase at low pH, could not be explained by van der Waals attraction and was ascribed to hydrophobicity. The decay length of the hydrophobic attraction was 7 nm (fitted to a single exponential), which raised some doubts about the magnitude of the respective forces because of its similarity to the Debye length for the electrical repulsion. The aim of the work reported here was to examine the interaction of ZnS spheres in more detail as a function of pH and electrolyte concentration in the context of a thorough physical characterization. This work has particular relevance to the interaction of particles that are physically heterogeneous.
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Figure 1. SEM (Camscan 44FE) micrograph showing asperities on the surface of ZnS spheres.
Materials and Methods High-purity water was produced from a reverse osmosis fed Elga UHQ PS system. In the final purification stage the water was filtered at 0.05 µm prior to discharge via a dispenser that was subject to continuous UV irradiation. The resistivity of this water was 18 MΩ cm, and the surface tension was 72.8 mN/m at 20 °C. All reagents were of analytical reagent grade and were used as received. Electrolyte solutions were made up to the desired concentration from KCl. Hydrochloric acid and sodium hydroxide were used for pH adjustment. ZnS spheres were precipitated according to the procedure described by Williams et al.,18 with hydrochloric acid being used for acidification. Solutions of ZnO and thioacetamide were mixed at room temperature in a 250 cm3 volumetric flask, filled to the neck to minimize cooling, and placed in a water bath at 80 °C for 1 h. The precipitate was then separated from the supernatant on a 0.45 µm cellulose acetate filter, washed thoroughly with dilute ammonium hydroxide then water, vacuum dried, and stored in a desiccator. On the scale of the particle radius (1-4 µm) both light microscopy and SEM16,17 confirm the spherical and smooth nature of the particles. However at higher resolution the scanning electron micrographs show clear evidence of asperities (Figure 1). The topography of the particles was measured in air by AFM in tapping mode (Figure 2). The physical characteristics were further assessed by the measurement of a N2 adsorption/desorption isotherm from which a BET specific surface area and pore size distribution were obtained. A Nanoscope III (Digital Instruments) equipped with an E-head was used in force mode for the acquisition of raw data (cantilever (16) Atkins, D. T.; Pashley, R. M. Langmuir 1993, 9, 2232. (17) Muster, T. H.; Toikka, G.; Hayes, R.; Prestidge, C. A.; Ralston, J. Colloids Surf., A 1996, 106, 203. (18) Williams, R.; Yocom, P. N.; Stofko, F. S. J. Colloid Interface Sci. 1985, 106, 388.
Figure 2. Tapping mode AFM image of a ZnS sphere obtained in air. A cantilever with a high aspect ratio silicon tip was used. deflection versus scanner position). The methodology regarding the setting up of the AFM for force measurements in fluids, sphere-sphere alignment, and deconvolution of the raw data to yield normalized force (F/R) as a function of separation (D) is described in detail elsewhere.17 The AFM cantilevers were calibrated by the resonant frequency technique.19 To calibrate the z-piezo scanner either a compact disk stamper was imaged or the method recently proposed by Jaschke and Butt20 was used. In the latter case the diode laser was reflected off a mirror and the cantilever simultaneously, during scanning, into the split photodiode. The signal obtained had a wavelength which corresponded to a whole number of half-wavelengths of the incident radiation from which the scanner could be routinely checked and/or recalibrated. These simple procedures allowed both the calibration of individual cantilevers and the calibration of the z-scanner on a routine basis. Calibration was further confirmed by the slope of the log(F/R) versus D data which at extended separation in all cases corresponded to the Debye length (1/κ) calculated from the analytical concentration of electrolyte. Sphere radii (R1, R2) were determined by light microscopy and combined according to R ) (R1R2/R1 + R2) for the calculation of normalized force. Cantilever spring constants were 0.027 ( 0.002 N/m with sphere radii in the range 3-4 µm. The ZnS spheres (19) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403. (20) Jaschke, M.; Butt, H.-J. Rev. Sci. Instrum. 1995, 66, 1258.
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Figure 3. Nitrogen sorption (Coulter Omnisorp) isotherm for ZnS. The sample was outgassed at 110 °C for 16 h to a pressure of less than 10-5 Torr prior to analysis. Open (closed) symbols correspond to adsorption (desorption) data. were attached to the end of an AFM cantilever and to a silicon wafer with a resin (Epikote 1004, Shell) and cleaned by rinsing in ethanol prior to plasma irradiation (Harrick PDC-329) for 20 s prior to assembly within the fluid cell. All AFM work and associated preparation was carried out in a Class 100 clean room and, where possible, a laminar flow cabinet within this room.
Results and Discussion Physical Characterization of ZnS Spheres. SEM (Figure 1) indicates that the asperities on the surface of the spheres were approximately 50 × 50 nm2 in area. SEM does not provide an unequivocal representation of topography due to the carbon coating of the sample surface, and one would expect the asperities to be smoothed out by this pretreatment. An AFM was therefore used in tapping mode (in air) to focus on the surface topography directly at high resolution. The polycrystalline nature18 of the ZnS surface prepared by this method of precipitation was confirmed. It has been proposed18 that the particles consist of a single crystal core, probably of cubic ZnS, surrounded by a mantle of very small crystallites of cubic ZnS having a minimum diameter of 12 nm. AFM images suggest that the sizes of these crystallites in our preparation were about 50 nm. The roughness of the surface resulting from its polycrystalline nature was typically 40 nm over an area of 1 × 1 µm2. This value can be regarded as an upper bound for the roughness as it corresponded to the difference in height between the highest and lowest points on the surface. The root mean square roughness obtained from the height-mode image (175 × 175 nm2) corresponding to Figure 2 was 3.5 nm. This latter value is probably more relevant given the area of interaction in the current work. The BET specific surface area obtained from nitrogen adsorption data (Figure 3) was 56 m2/g, which compares well with the value of 66 m2/g obtained by Matijevic and co-workers21 in their pioneering work on the preparation of ZnS spheres. As the geometric surface area of the spheres is only 1 m2/g, porosity, rather than simply roughness, is implicated. Indeed the uptake of nitrogen in the relative pressure range 0.6-0.9 (Figure (21) Wilhelmy, D. M.; Matijevic, E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 563.
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Figure 4. Raw data for the interaction between ZnS spheres at pH 7 in 2 × 10-3 M KCl.
3) provides clear evidence of mesoporosity.22 A pore size distribution otained by the BJH method23 from the desorption branch of the nitrogen isotherm indicated that the maximum pore diameter was 10 nm. This value correlates well with the width of cracks between adjacent crystallites evident in the tapping mode AFM image. Only the pore entrances can be probed by the technique due to the radius of curvature of the AFM tip (nominally 5-10 nm, however tip radii much larger than nominal values have been reported24), which effectively prevents it from entering pores of this size. van der Waals Forces. The interaction (Figure 4) between ZnS spheres at pH 7, close to the iep,17 and an electrolyte concentration of 2 × 10-3 M was monotonically repulsive on approach, and the separation curve was effectively coincident. The absence of a jump to contact, and particularly the absence of adhesion, make it clear that attractive forces do not dominate the interaction behavior at any separation. One would expect the van der Waals attraction to be evident at small separations. This issue is explored further in Figure 5 where the interaction data have been transformed to normalized force-separation (D ) 0 corresponding to minimum separation). Increasing the electrolyte concentration to 2 × 10-2 M reduces the Debye length to 2.1 nm and effectively neutralizes the electrical repulsion between the surfaces. However even in this case no attraction that could provide evidence of a van der Waals attraction was observed. The expected van der Waals attraction (Figure 5) is significant in both range and magnitude in the separation range studied. For silica surfaces no attraction is observed at elevated ionic strength close to the iep as the hydration of the surface provides an additional short range repulsion.8 However for ZnS surfaces it is clear from Figure 5 that the surface is not hydrated due to the absence of any short range repulsion. It was concluded that the van der Waals force was (22) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66, 1739. (23) Barrett, E. P.; Joyner, L. G.; Halenda, P. H. J. Am. Chem Soc. 1951, 73, 373. (24) Drummond, C. J.; Senden, T. J. Colloids Surf., A 1994, 87, 217.
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Figure 5. Normalized force versus separation for ZnS spheres at at pH 7 with varying electrolyte concentration. The nonretarded van der Waals interaction, calculated with a Hamaker constant17 of 2 × 10-20 J, is included for comparison.
decreased to below the sensitivity of the AFM (0.01 mN/ m) because of the effective separation between the surfaces at asperity contact and also because the area fractions associated with asperity contacts were very small. In the interaction between a ZnS sphere and mica in 10-1 M KCl16 an attraction, ascribed to van der Waals forces alone, was measured. By bringing a ZnS sphere up against a mica surface, one would expect the separation at asperity contact to be approximately halved. Under these circumstances it is not surprising that the van der Waals force is not decreased to the degree observed in the present work. However one might have expected at least some reduction of the van der Waals attraction due to the roughness of a single ZnS sphere. The Effect of pH. The interaction between ZnS spheres in 2 × 10-4 M KCl was investigated from pH 10 down to pH 4 (Figures 6 and 7). Above the iep (Figure 6) the interaction was monotonically repulsive and was fitted simply by an electrical term, using both constant charge and constant potential boundary conditions, the fitted potential increasing with pH. No van der Waals term was used in the light of the results described earlier. Comparison between fitted and experimental data indicated that the surfaces were interacting closer to constant charge conditions as the pH was increased. This is perhaps not surprising given the iep for this system indicates that the surface is significantly oxidized and Zn2+ hydrolysis products affect the surface chemistry. This finding has been supported by surface analysis17 where the surface stoichiometry is best represented as (ZnS)x(Zn(OH)2)1-x with x ) 0.25. Below the iep (Figure 7) there was clear evidence of an attraction as separation was decreased as reported elsewhere.17 In the current work however the decade reduction in electrolyte concentration extended the range of the electrical repulsion to well beyond that of the attractive component. As a result any uncertainty in the magnitude of the respective components to the total measured interaction evident in previous work17 was overcome. The experimental data were fitted by combining a single exponential (attractive component) with a repulsive electrical component. The fitted potential decreased from 32 mV at pH 6 to 23 mV at pH 5. At pH
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Figure 6. Normalized force versus separation for ZnS spheres at mildly alkaline pH in 2 × 10-4 M KCl. The symbols represent experimental data and the solid lines to fits to the PoissonBoltzmann equation calculated under constant charge (upper line) and constant potential (lower line) conditions using the program of Alexis Grabbe´.8 The Debye length used, 21.6 nm, corresponded to the analytical concentration of electrolyte. The fitted potentials were -38 mV at pH 10 and -32 mV at pH 8.
4 there was no measurable electrical interaction between the surfaces. Below the iep the experimental data indicated that the ZnS spheres were interacting closer to constant potential conditions, a trend which was more obvious as the pH was reduced. At pH 4, where only an attraction was observed, the decay length of the attractive component was 5 nm and similar to the value (7 nm) reported previously.17 An identical attraction, when coupled with the repulsive electrical components at pH 5 and 6, was found to give an acceptable fit to the experimental data at all separations. It is difficult to quantify any pH dependence of this attractive component in the presence of the electrical term given the large separation between the constant charge and potential curves at separations where this attractive component, presumed to be hydrophobic in nature, is operative. The magnitude of the hydrophobic component, as measured by the pre-exponential factor (0.128 mN/m) is significantly less than that reported previously17 (0.28 mN/m), when the fitted potential at pH 4 was assumed to be significant on the basis of measured zeta-potential data (Figure 8). The cause of the hydrophobicity at acidic pH is the dissolution of zinc hydroxy species and the consequent exposure of the zinc sulfide surface, which becomes sulfur rich.17 The Effect of Electrolyte Concentration on the Hydrophobic Interaction. The interaction at pH 4, where no electrical repulsion was evident, was investigated further (Figure 9). In 2 × 10-3 M KCl the attraction was effectively coincident with that measured at the lower concentration. However as the electrolyte concentration was increased further, to 2 × 10-2 M and then to 2 × 10-1 M, the magnitude and range of the attraction decreased monotonically. While there have been previous reports that the hydrophobic interaction is electrical in nature,25,26 and therefore dependent upon ionic strength as found here (25) Tsao, Y.-H.; Evans, D. F. Langmuir 1993, 9, 779. (26) Christenson, H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. J. Phys. Chem. 1989, 93, 1472.
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Figure 9. Normalized force versus separation for ZnS spheres at pH 4 as the electrolyte concentration (KCl) is varied. The symbols represent experimental data and the solid lines a single exponential fit. At 2 × 10-4 and 2 × 10-3 M the data were effectively coincident and the best fit to the data corresponded to a pre-exponential factor of -0.127 ( 0.004 and a decay length of 5.1 ( 0.2 nm. At 2 × 10-2 M the values were -0.070 ( 0.003 mN/m and 3.2 ( 0.2 nm, while at 2 × 10-1 M the values were -0.050 ( 0.006 mN/m and 2.0 ( 0.3 nm. Figure 7. Normalized force versus separation for ZnS spheres at mildly acidic pH in 2 × 10-4 M KCl. The symbols represent experimental data and the solid lines a theoretical fit. The fitting equation combined a single exponential (A exp[-D/B]) with the solution to the Poisson-Boltzmann equation calculated under constant charge (upper line) and constant potential (lower line) conditions, using the program of Alexis Grabbe´.8 The Debye length used, 21.6 nm, corresponded to the analytical concentration of electrolyte. The fitted potentials were 32 mV at pH 6, 23 mV at pH 5, and 0 mV at pH 4. The attractive component, with A ) -0.128 ( 0.004 mN/m and B ) 5.0 ( 0.2 nm was fitted at pH 4 and incorporated in the fitted curves at pH 5 and pH 6 without modification.
Figure 8. Comparison between zeta potential data17 measured in 10-3 M electrolyte and fitted potentials for ZnS.
for the interaction of ZnS spheres, the majority of reports have found little or no dependence upon electrolyte concentration.27 The decrease in the magnitude of the
attraction as the electrolyte concentration was increased provided additional confirmation that the attraction was not due to van der Waals forces. The Decrease of Surface Forces Due to Roughness. Clearly the magnitude of surface forces will be affected by surface roughness. The range of a surface force, in the context of the prevailing roughness, will determine whether it will effectively play a role in a given interaction. Obviously forces of short range will be the most affected, with the absence of a measurable van der Waals force in the present work a good example. One may also expect longer range forces, such as the electrical and hydrophobic forces measured here, to be decreased. To quantify this more subtle effect, interaction measurements between ZnS surfaces that are significantly smoother are required. This is not possible at the present time. The fitted potential is normally equated with the surface potential, ψo, and the zeta potential, ζ, represents the potential at the plane of shear, some small but finite distance away from the surface. As a result ζ < ψo for ideal (smooth) surfaces. In the work reported here the fitted potential was found to be of similar magnitude to the zeta potential. Measurements of hydrophobic forces have recently been compared.28 While the decay length of the hydrophobic force measured between ZnS spheres is well within the very broad range found by others, the magnitude of the force, as a result of the pre-exponential term, is significantly lower. This is presumably a consequence of surface roughness. Comparison of Fitted Potential with the Zeta Potential. The fitted potentials have been compared with zeta potential determinations (Figure 8). The trend with pH is very similar for pH 10 down to pH 6. However, below pH 6 the fitted potential and zeta potential diverge. The cause of this divergence is not known. It is possibly (27) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (28) Meagher, L.; Craig, V. S. J. Langmuir 1994, 10, 2736.
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due to kinetic factors, in that the two measurements were carried out at significantly different volume fractions,17 the force measurement being carried out closer to infinite dilution. The implication, then, would be that at low pH the zeta potential would continue to decrease with time until it coincided with the fitted potential. Comparison of Direct Force Measurements with Rheology. For ZnS particles in aqueous electrolyte a maximum in yield stress at the iep has been observed17 and attributed to van der Waals attraction. The inability of the AFM to sense this attraction given its sensitivity limit was initially surprising. However in direct force measurements the interaction occurs between “tethered” solids which have little freedom in the plane of interaction whereas in a typical dispersion the interacting particles are essentially free. For surfaces that are rough, one would expect that free solids are able to approach each other more closely than tethered solids and therefore are more likely to respond to forces of shorter range. In the present case rheological studies appear to be more diagnostic of dispersion behavior than direct force measurements. Conclusions The ZnS spheres studied here can be thought of as intermediate in “ideality” between surfaces normally used in direct force measurements and those typically encountered in large-scale particle processing. While it is the behavior and control of the latter that typically motivate investigation, inevitably this is done by extrapolation of fundamental studies on ideal, well-characterized, systems. The results presented here raise obvious questions as to the validity of this approach given that prevailing theory, and fundamental measurements, do not address the
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important issue of surface roughness. In practical terms one needs to consider the range of surface forces in the context of surface topography, specifically the “effective” separation at asperity contact. It is indeed fortunate that surface topography can now be accessed at high resolution by scanning probe microscopy. Clearly, particle interaction will be dominated by long range forces, of which the electrical force is a good example. It is also possible for interactions to be affected by short-range effects of significant magnitude, for example a chemical, as opposed to physical, interaction or bond. The practical implication of this roughness-induced reduction of surface forces is that the balance between dispersion/coagulation, which is often so critical in particle processing, will be very delicate. This may explain why the behavior of a particle dispersion in practice can be influenced by either reagent addition or the use of mechanical means. In the case of ZnS, particle interaction in electrolyte can be described by a combination of electrical and hydrophobic forces depending on pH. At low pH the hydrophobic attraction was found to be dependent on electrolyte concentration implying that the nature of this force, which is the subject of much debate, is possibly linked to the influence of salt on water structure.29 Acknowledgment. We thank Andrew Robinson for measurement and analysis of nitrogen isotherms, Len Green for scanning electron microscopy, and Clive Prestidge for some interesting discussions. LA951534U (29) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Nature 1993, 364, 317.
