The disjoining pressure between a mineral and soluble salt crystal in concentrated aqueous solution has been successfully measured with atomic force microscopy

Hall, Christopher

Hamilton, Andrea

Koutsos, Vasileios

English

Andrea Hamilton

Vasileios Koutsos

Christopher Hall

Crossref

Chemical Communications

Direct measurement of salt–mineral repulsion using atomic force microscopy

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryHamilton, Andrea and Koutsos, Vasileios and Hall, Christopher (2010) Direct measurement ofsalt–mineral repulsion using atomic force microscopy. Chemical Communications, 46 (29). pp.5235-5237. ISSN 1359-7345Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/Direct measurement of salt–mineral repulsion using atomicforce microscopyAndrea Hamilton,* Vasileios Koutsos and Christopher HallReceived 31st July 2009, Accepted 28th April 2010First published as an Advance Article on the web 22nd June 2010DOI: 10.1039/b915709cThe disjoining pressure between a mineral and soluble saltcrystal in concentrated aqueous solution has been successfullymeasured with atomic force microscopy.Salt crystals can grow in the pores of stone, generating stress inthe host matrix and leading to serious damage1 in buildingmaterials and works of art. Crystallization stress is also acause of natural rock weathering in terrestrial and planetarylandform alteration.2 The ability of salt crystals to causedamage depends on the existence of a ﬁlm of solution betweenthe crystal and pore wall to allow continued growth, and arepulsive interaction between the crystal and the pore tomaintain the ﬁlm.3 If the salt–mineral interaction throughthe ﬁlm of solution is attractive, the salt crystal and pore wallmake adhesive contact and no damage should be observed.The thickness of the solution ﬁlm3 is estimated to be a few nm,highlighting the importance of understanding and eventuallytailoring the nanoscale interaction between salt crystals androck forming minerals. The disjoining pressure is the forceneeded to push unit area of crystal and of pore wall into hardcontact, and therefore is the upper bound on the real crystal-lization pressure. Our aim was to measure directly for the ﬁrsttime the disjoining pressure between a rock-forming mineral(using silica as an analogue for quartz) and a soluble saltcrystal. Sodium sulfate, the most damaging2 salt, has suchcomplex hydrate chemistry4 that force–distance AFM workwas impractical. Therefore we used potassium sulfate (arcanite),chosen for its high solubility and its stability in air.Potassium sulfate crystals were grown by evaporation on afreshly cleaved mica sheet to produce a smooth surface.Crystals were attached to the AFM stub with Araldites2014 and left to cure at room temperature in a desiccator forseveral weeks. We used a Multimode/Nanoscope IIIa (Veeco)AFM with a ﬂuid cell. Room temperature was maintained at21 1C. Heating from laser irradiation raises the temperature inthe ﬂuid cell by 3–4 1C.5 Better surface stability was achievedby injecting solutions saturated at ca. 8 1C above roomtemperature with added crystal growth inhibitor. Potassiumsulfate solutions saturated at 29 1C were used (0.595 M, FisherAR grade, +99%), to which 5 ppm of Cr3+ was added to thesolution as KCr(SO4)212H2O (Sigma-Aldrich, ACS reagentgrade, +99%).Solutions were injected through syringe ﬁlters (pore size0.2 mm) to remove foreign particles. Custom-made CrAucoated AFM cantilevers (Bioforce Nanosciences, USA) weredesigned with one standard imaging tip and a particle tip(5 mm silica) of nominal spring constants 0.12 and 0.58 N m1,respectively, on the same side of the chip. The ﬂuid cell andﬁttings were initially cleaned with a 2% solution of Micro-90in an ultrasonic bath at 65 1C for at least 30 min. Cantilever,ﬂuid cell and ﬁttings were rinsed in deionized water (Millipore,18.2 MO cm resistivity), ethanol, exposed to UV/ozone for atleast 30 min and dried in a nitrogen stream. The springconstant of the particle tip was measured using the thermalnoise method (1D MFP, Asylum Research).Previous studies of potassium sulfate surfaces5 have shownthat the {0 2 1} faces always exhibit striations but these werefound to be the largest faces and it was not possible to orientthe much smaller but less striated faces in a workable way.Shown in Fig. 1 is a {0 2 1} surface imaged in K2SO4 solution(no Cr3+ added) which is obviously striated. We found wewere able to suppress the striations by using Cr3+ as a crystalgrowth inhibitor as shown in Fig. 2. The eﬀect of templatedgrowth against mica with addition of 5 ppm of Cr3+ to thesolution produces dramatic improvements in ﬂatness. Tocheck no residue of mica was present on the crystal surface,several templated arcanite crystals were etched slowly withFig. 1 30  30 mm scan of a templated arcanite surface in 0.595 Msolution with no inhibitor. A section through the image is shown (left).Peak-to-trough max and RMS roughness are 115 nm and 20.9 nmrespectively.Fig. 2 10  10 mm scan of a templated arcanite surface in 0.595 Msolution with 5 ppm Cr3+. A section through the image is shown (left).Peak-to-trough max and RMS roughness are 9.18 nm and 0.31 nmrespectively.School of Engineering and Centre for Materials Science &Engineering, The University of Edinburgh, The King’s Buildings,Edinburgh, UK. E-mail: andrea.hamilton@ed.ac.uk;Fax: +44 (0)131-650-6554; Tel: +44 (0)131-650-5693This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 5235–5237 | 5235COMMUNICATION www.rsc.org/chemcomm | ChemCommDownloaded by University of Strathclyde on 12 February 2013Published on 22 June 2010 on http://pubs.rsc.org | doi:10.1039/B915709CView Article Online / Journal Homepage / Table of Contents for this issuewater under an optical microscope, and others imaged andmapped by SEM with EDX facility. Neither of the twocrystals used for the data presented here contained anydetectable trace of mica.After imaging the surface, 4 mL of solution was ﬂushedthrough the cell and 155 force curves were taken using thesilica colloid cantilever at a piezo speed of 0.1 Hz. The raw tipdeﬂection vs. piezo movement data were converted to force–separation curves using the measured spring constant anddeﬂection sensitivity measured from the top part of theapproach curve. The point of zero distance was determinedas described by Butt et al.6 by extrapolating between thecontact region of the curve and the non-contact region atlarge separations. The resulting force–separation curves wereaveraged to produce a single curve which was smoothed usinga 5-point FFT routine. The Derjaguin approximation is usedto obtain the corresponding interaction free energy per unitarea between two ﬂat plates and the resulting curve diﬀeren-tiated to achieve the disjoining pressure vs. separation (Fig. 3).It is clear that no obvious minima, primary or secondary,are observed and that the interaction is essentially repulsive atall separations. Modeling the system using the algorithm ofChan7 which solves numerically the non-linear Poisson–Boltzmann equation for asymmetrical electrolytes but modiﬁedto include dissimilar surfaces and van der Waals interactionspredicts an attraction beginning at 0.5–1.5 nm (see Fig. 3).Modiﬁcation for dissimilar surfaces was achieved by integratingtwice for kL (inverse Debye length multiplied by surfaceseparation L), with the lower limit of integration being thepotential minimum in both cases, but the upper limits set atthe surface potential for the two dissimilar surfaces. The totalkL value is the sum of the two kL values which together equalthe dimensionless distance between the plates. van der Waalsforces are calculated for a sphere and a plane and converted topressure via the Derjaguin approximation and diﬀerentiatingthe equation for interaction free energy per unit area.8 TheHamaker constant used is calculated for arcanite/silica–potassiumsulfate solution using the Lifshitz approximation; retardationeﬀects are not considered. Modeling was carried out forconstant potential and constant charge, as shown in Fig. 3,which are generally considered the standard lower and upperboundaries, respectively, in DLVO theory. What is clear fromthe ﬁt is that DLVO does not adequately describe either thelength scale or the nature of the interaction at short distancesbut that constant charge (CC) is a slightly better ﬁt thanconstant potential (CP). Bostrom et al.9 state that atconcentrations above 0.1 M DLVO theory is ﬂawed anddescribe a method for taking the image and dispersion forcesbetween ions into account. The result is a stronger attractionthan that predicted by DLVO (constant potential) which doesnot describe the results presented here. An approximation tothe shape of the curve, up to gap length of ca. 0.5 nm, can beobtained with the constant charge boundary condition atsurface potentials of 22 mV and 19 mV for silica andarcanite (Fig. 3). Reducing the input surface potential to muchlower values in both the CC and CP models produces aprimary minimum with little or no repulsion visible at anyseparation. This is not surprising since at high salt concentrationsthe charge and potential generally tend towards zero and theelectrostatic part of DLVO is screened out leaving the vdWcontribution only. In salt crystallization damage in rocks,crystal growth must occur in concentrated supersaturatedsolution with little or no electrostatic component. Thissuggests the repulsive regime observed here applies to othersalt–mineral systems for which crystallization damage isobserved.Very little literature exists on surface interactions inconcentrated (>0.1 M) solution and even less on dissimilarsurfaces. Long range hydration repulsion, extending out to7 nm, between mica sheets at high NaCl concentration(60–600 mM) was observed by Alcantar et al.,10 who remarkedthat hydrated Na+ could push apart mica sheets forcedtogether under 10–50 MPa of pressure by up to 3 nm. Theyattribute this long range interaction to hydrated Na+ bindingto negative sites on the mica surface. Work on silica11,12 showsthat less hydrated ions such as K+ and Cs+ bind more easilyto the silica surface and that at high concentrations (B1 M)K+ ions condensed on the surface can produce a surfacecharge inversion. In the case of charge inversion, they describean extra short-range repulsion that takes place at separationso2 nm (0.5 M CsCl) after a pronounced vdW attraction anddescribe it as a short range electrical double layer repulsion.Ion and surface speciﬁc eﬀects and long range non-DLVOforces are also evident in the force–distance experiments ofVeeramasuneni et al.13 using a silica colloid tip and NaCl andKCl crystals in saturated solution. With NaCl they ﬁnd astrong attraction extending toB17 nm and with KCl a purelyrepulsive interaction toB4 nm. Although they do not calculatedisjoining pressure for either system their results suggest KClhas the potential to be damaging to siliceous stones whereNaCl is not. Ion and crystal face speciﬁc interactions aredescribed by Finot et al.,14 who measured forces betweensilica and gypsum and pairs of gypsum crystals in solutionsan order of magnitude less concentrated than the solutionsused here.Our results show a long range repulsion, consistent withthat of Alcantar et al.,10 and no strong attraction. A very slightsecondary minimum (not shown) is found at ca. 13 nm with aforce value of 32 pN followed by a substantial repulsion andno observable primary minimum. z-Potential measurementswere made (Malvern, Zetasizer Nanoseries) on particles ofarcanite (not more than 10 mm in diameter) and silica (AstonChemicals UK, 3 mm diameter, 50 A˚ pore size) in 0.595 MK2SO4 with 5 ppm Cr3+ which suggested a near zero butslightly negative z-potential for both particles. As the diﬀuseFig. 3 Disjoining pressure calculated from smoothed average of 155force curves (black circles). DLVO CP ﬁt (dashed line) and CC ﬁt(dotted line).5236 | Chem. Commun., 2010, 46, 5235–5237 This journal is c The Royal Society of Chemistry 2010Downloaded by University of Strathclyde on 12 February 2013Published on 22 June 2010 on http://pubs.rsc.org | doi:10.1039/B915709CView Article Onlinedouble layer shrinks, the z-potential tends towards zero butthe standard deviation, which is 3–4 mV means that theabsolute value is a rough guide. The high solubility of arcanitecrystals makes it impossible to measure the z-potential indilute solution, which would be more consistent with standardpractice. The eﬀect of adding 5 ppm Cr3+ to maintainarcanite surface stability was investigated through z-potentialmeasurements on silica spheres in 0.6 and 6 mM K2SO4solutions with and without 5 ppm Cr3+. Results were25.6 mV (0.6 mM, no Cr3+), 26.5 (0.6 mM, 5 ppmCr3+), 7.0 mV (6 mM, no Cr3+) and 7.3 mV (6 mM,5 ppm Cr3+) which are consistent with the fact that the Debyelength is not aﬀected by the addition of 5 ppm Cr3+.When the experimental curve in Fig. 3 is ﬁtted to a secondorder exponential, we obtain values of 2.3–2.7 MPa for thedisjoining pressure at 0.165–0.2 nm.10 The decay length of therepulsion is 2.9 nm, an order of magnitude higher than theDebye length but fairly consistent with the non-DLVO repulsionobserved by Alcantar et al.,10 who report a decay length of1 nm. Beyond the repulsion, hard contact is reached. Althougha primary minimum is not observed, an adhesion force ismeasured on retraction. Calculating15 the pull-oﬀ force frommeasured surface roughness and the estimated van der Waalsforce gives a value about 25 times greater than the measuredpull-oﬀ force. This suggests that either the surface roughness ismuch higher in the area sampled by the colloid tip or that evenat hard contact, repulsive non-DLVO forces eﬀectively reducethe tip–sample adhesion.An ultimate aim of studying the disjoining pressure is to ﬁnda method of tailoring the interaction at small separations topromote an attractive interaction and reduce damage from saltcrystallization. Zohar et al.16 ﬁnd charge inversion betweensilica surfaces in the presence of Z = 3 ions which dramaticallyalters their force curves from a long range repulsion to a muchshorter range attraction. To see if this can occur in silica/arcanite, saturated potassium sulfate solutions were preparedat 29 1C with 5, 50 and 500 ppm of crystal growth inhibitor.An equilibration time of 15 min was allowed before forcecurves were captured. At least 4 mL of each new solution wasﬂushed through the cell prior to data collection. 155, 47 and 50force curves were analyzed for the 5, 50 and 500 ppm solutionsrespectively.Fig. 4 shows a typical set of force–separation curves withadditions of 5, 50 and 500 ppm inhibitor Cr3+. It is clear thatat 500 ppm, an attraction appears on approach at ca. 9 nm butthat it is still followed by repulsion. The Debye length changesvery little on addition of 500 ppm Cr3+ but z-potentialmeasurements suggest that surface charge inversion by 500 ppmof Cr3+ is more eﬀective than predicted charge inversion fromvery high concentrations of monovalent cations,16 1.19 M K+in this case. z-Potential values for silica spheres in 0.6 mMK2SO4 with 5, 50 and 500 ppm added are 26.5, 24.2 and19.2 mV respectively. For 6 mM K2SO4 with 5, 50 and500 ppm Cr3+ added the z-potential results are 7.3,4.7 and1.4 mV respectively. At 500 ppm Cr3+ 6 out of the 11samples analyzed had positive values under +3 mV but againa standard deviation of 3–4 mV per measurement. While theabsolute value of samples at close to 0 mV cannot be taken tooliterally, the trend towards charge inversion is clear. Analysisof arcanite crystals in 0.595 M K2SO4 solutions with the sameset of Cr3+ additions also suggests the surface could bepositively charged in 500 ppm Cr3+. Ion correlation eﬀects(vdW forces between polarisable counterions) are responsiblefor the attraction at ca. 9 nm and the repulsion at shorterrange suggests Cr3+ is not tightly bound to the surface. Theuse of Cr3+ shows that it is possible to tailor salt–mineralinteractions, for example through charge inversion. This opensa possible important new approach to prevent salt crystal-lization damage in building materials.Results from a total of 455 measurements on two crystals atCr3+ concentrations of 5, 50 and 500 ppm were analysed toarrive at a disjoining pressure for silica–arcanite in potassiumsulfate solution of 2.4 to 4 MPa. This is at the lower end of therange (ca. 2–9 MPa) of tensile strength for most sedimentaryrocks17 and may explain damage through slow crack propagationin cyclical crystallization tests. Charge inversion by Cr3+produces an attraction at long range between silica andarcanite with repulsion at short range. This is the ﬁrstmeasurement of disjoining pressure and charge inversion ina salt–mineral system and proof that a repulsive interactioncan exist between minerals and salt crystals in salt solution.Notes and references1 N. Tsui, R. J. Flatt and G. W. Scherer, J. Cult. Heritage, 2003, 4, 109.2 A. S. Goudie and H. A. Viles, Salt Weathering Hazards, Wiley,Chichester, 2007.3 G. W. Scherer, Cem. Concr. Res., 1999, 29, 1347.4 A. Hamilton and C. Hall, J. Anal. At. Spectrom., 2008, 23, 840.5 A. Mauri and M. J. Moret, J. Cryst. Growth, 2000, 208, 599.6 H.-J. Butt, B. Cappella and M. Kappl, Surf. Sci. Rep., 2005, 59, 1.7 D. Y. C. Chan, J. Colloid Interface Sci., 2002, 245, 307.8 J. N. Israelachvili, Intermolecular and Surface Forces, AcademicPress, London, 1991.9 M. Bostrom, D. R. M. Williams and B. W. Ninham, Phys. Rev.Lett., 2001, 87, 168103.10 N. Alcantar, J. N. Israelachvili and J. Boles, Geochim. Cosmochim.Acta, 2003, 67, 1289.11 M. Dishon, O. Zohar and U. Sivan, Langmuir, 2009, 25, 2831.12 G. V. Franks, J. Colloid Interface Sci., 2002, 249, 44.13 S. Veeramasuneni, Y. Hu, M. R. Yalamanchili and J. D. Miller,J. Colloid Interface Sci., 1997, 188, 473.14 E. Finot, E. Lesniewska, J.-C. Mutin and J.-P. Goudonnet,J. Chem. Phys., 1999, 111, 6590.15 E. R. Beach, G. W. Tormoen, J. Drelich and R. Han, J. ColloidInterface Sci., 2002, 247, 84.16 O. Zohar, I. Leizerson and U. Sivan, Phys. Rev. Lett., 2006, 96,177802.17 R. J. Flatt, J. Cryst. Growth, 2002, 242, 435–454.Fig. 4 Force–separation curves for the three solutions of 0.595 Mpotassium sulfate containing 5 ppm (dotted line), 50 ppm (dot-dashline) and 500 ppm (dashed line) of Cr3+.This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 5235–5237 | 5237Downloaded by University of Strathclyde on 12 February 2013Published on 22 June 2010 on http://pubs.rsc.org | doi:10.1039/B915709CView Article Online

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Direct measurement of salt–mineral repulsion using atomic force microscopy

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