Heißler, Stefan

Laye, Fabrice

Melzak, Kathryn A.

English

KITopen

Nanoscale-Specific Reaction in a Precursor Film: Mixing SodiumCarbonate, Calcium Chloride, and an Organic Thiol to ProduceCrystals of Calcium sulfateKathryn A. Melzak,* Fabrice Laye, and Stefan HeißlerABSTRACT: The ultrathin precursor film surrounding droplets ofliquid on a solid surface is used here as a confined reaction medium inorder to drive a reaction that would not occur in bulk fluid. Sodiumcarbonate and calcium chloride mixed together in the presence of theorganic thiol dithiothreitol (DTT) produced crystals of gypsum, orcalcium sulfate, instead of the otherwise expected calcium carbonate.The possible sources of sulfate in the system are contaminants in theDTT or the oxidation product of the DTT sulfhydryl. The amount ofgypsum produced implies that contaminants do not account for thetotal sulfate present in the system, suggesting that the DTT could beoxidized. The reaction quotient may be skewed in favor of this unexpected reaction by a combination of efficient removal of sulfateby precipitation and the concentration of DTT at the leading edge of the precursor film through the coffee ring effect during a briefdrying step.INTRODUCTIONThe behavior of liquids in thin films differs greatly from theirbehavior in bulk. As films of liquid get thinner, the effectiveviscosity goes up, increasing by as much as a factor of 106 atnm scale distances from hydrophilic surfaces.1 The behavior ofsolute molecules in such films can also vary: the diffusion ofhydrated sodium in thin films may be up to 40 times slowerthan diffusion in bulk solution.2 One application of theseeffects has been the use of picoliter droplets to slow down thecrystallization of CaCO3, permitting the observation ofintermediate forms in the crystallization process.3 In thework presented here, nanoscale reaction conditions result in aproduct that would not be obtained in the bulk solution. Thesmall scale crystals of the reaction product are formed inpatterns, a result that also has some potential relevance tobiomineralization, in which well defined crystalline arrays areformed in confined surroundings.Drops of liquid applied to solid substrates can take differentforms: they can remain as cap shaped drops, they can spreadoutward to generate flat pancake shapes, or they can exist in anintermediate regime in which a droplet retains a spherical capshape but is surrounded by a thin film of liquid that spreadsoutward to cover the surrounding surface.4 The thickness ofthis precursor film is predicted to be on the order of a fewangstroms,4 sufficient to allow the passage of small solutemolecules or ions. It is therefore possible to modify a surfacefar beyond the macroscopically visible boundaries of an applieddrop of solution, as illustrated in Figure 1.The original intent of the experiments described here was touse the precursor film surrounding drops of DTT on gold as aconfined volume for the formation of calcium carbonatecrystals, inspired by experiments described elsewhere in whichthe early stages of calcium carbonate crystal formation wereobserved in picoliter droplets on thiol layers on goldsubstrates.3 The unexpected result, however, was that gypsumcrystals were formed instead of calcium carbonate.Figure 1. Breath pattern illustrating the extent of the spread of aprecursor film around the base of a drop of 50 mM DTT. The dropon the left is 50 mM DTT; the drop on the right is water, applied tothe surface of the gold at the same time. The scale bar is 1 mm. Thebrightness and contrast have been adjusted.EXPERIMENTAL SECTIONFor these experiments, a small drop (0.2−0.5 μL) of a freshlyprepared so lu t ion o f 50 mM di th io thre i to l (DTT;CH2SHCHOHCHOHCH2SH), 5 mM CaCl2, and 1 mM Na2CO3was applied to a sputter coated or else plasma cleaned gold surfacewithin 2−5 min of the surface being exposed to air; the film wasallowed to spread outward for 5−10 min at room temperature in achamber saturated with water vapor; the chamber lid was then openedfor 5−10 s to allow the precursor film to dry, after which the lid wasreplaced, allowing the outward spread of the precursor film tocontinue. The samples were allowed to sit overnight and thenanalyzed after being dried in air. Experiments were also carried outwith no carbonate.RESULTS AND DISCUSSIONCrystals were found to form in a ring outside the boundaries ofthe spot of the solution. The crystals were identified as gypsumon the basis of the crystal morphology (Figure 2) andidentification was confirmed by Raman spectroscopy (Figure3). Raman bands sensitive to the orientation of polarized lightalso supported the identification of gypsum (see SupportingInformation Figure S1). The crystals formed only when theprecursor films were allowed to dry briefly, an observationsupported by the production of multiple lines of crystals inassociation with multiple drying steps (Figure 4). Controlexperiments with no carbonate produced only small crystals,which were confirmed as gypsum by Raman bands at 1009.4cm−1 (strong), 494.94 (w/med), and 416.04 (w/med).We can clearly identify gypsum, which indicates that there issufficient sulfate present in the system to form the observedcrystals. A rough calculation of the amount of sulfate requiredto produce the observed volume of crystals was done with apreparation that produced a relatively well defined line (FigureS3); the calculations were based on the assumption that therewas sufficient gypsum to form a continuous track two crystalswide and two crystals high with a diameter of 3 mm, forcrystals 0.3 μm wide, which would correspond to sulfate beingpresent as 0.25% of the initial DTT in the drop applied to thegold surface. This is a plausible level of impurities but it wouldonly be sufficient to produce the observed crystals if all thesulfate left the bulk of the drop and traveled at the advancingfront of the precursor film. This seems unlikely and is also notconsistent with the observation that repeated drying steps canproduce multiple rings of crystals. Additionally, Ramanmapping of the dried remnants of the applied drop did notreveal any calcium sulfate peaks; although this would beconsistent with the suggestion that all the hypotheticallypresent sulfate leaves the drop and concentrates at the front ofthe precursor film, it would also, more plausibly, suggest thatthe amount of sulfate present as a contaminant is too low toexplain our results.The solubility product of calcium carbonate is 104× lowerthan that of calcium sulfate;7 for reactions in which carbonatewas added to the solution, the preferential formation ofgypsum implies that the sulfate is present at much higherconcentrations than the carbonate. One possibility is that thecarbonate does not travel through the precursor film; if wefollow the model in which the source of the sulfate is acontaminant of the DTT that appears, based on requiredamounts for the observed crystals, to travel at the front of theprecursor film, we would then have to assume a very efficientseparation of the two ions. Solubilities will vary with pH, andpH can be affected close to surfaces, but the formation ofcalcium carbonate crystals that has been observed in picoliterdroplets on gold surfaces3 implies that the calcium carbonatehas not been made soluble for some reason when in closeproximity to a modified gold surface.Figure 2. SEM image of a single crystal formed from a solution of 50mM DTT, 5 mM CaCl2, and 1 mM Na2CO3, on a sputter coated goldsurface prepared with a Bal Tec sputter coater. The image wasacquired after sputter coating the sample with a 5 nm layer of gold.Scale bar indicates 200 nm; the crystal is approximately 3.5 × 0.6 μm.Figure 3. Raman spectra of crystal (red line, top, axis at left)compared to reference spectrum for gypsum (blue line, at bottom,axis at right);5 crystals were prepared on a gold coated slide withsolution conditions as for Figure 2, but with a 0.5 μL drop; theprecursor film was allowed to spread outward for 10 min, followed bya 15 s drying step, after which the drop was left to dry overnight. Insetshows image of the measured sample, scale bar 20 μm. Imagesacquired after the measurement did not show evidence of sampledestruction. The weak band at 670 cm−1 is a polarization dependentfeature (see Figure S1) present in some gypsum reference spectra6 butnot in others.5Figure 4. Three lines of crystals produced by three successive dryingsteps; scale bar on large image is 0.5 mm; scale bar on insets is 10 μm.The insets a−c show details of the corresponding circled regions onthe large image. See also Supporting Information Figure S2 forcomparison to an SEM image.The other possible source of the sulfate comes from the thiolgroups of the DTT, which could potentially be oxidized;oxidation of organic thiols to sulfates has been notedpreviously in bacteria, occurring via complex pathways.8 Thiolscan interact with gold to form self assembled monolayers; it isthis interaction that probably drives the spread of the precursorfilm through reactive wetting of the surface.9 If we are toexplain an unusual reaction of thiols near a gold surface, wewould have to have unusual conditions. In the present case,this includes the thickness of the precursor film. As mentionedabove, the behavior of highly confined liquids differs from thatin bulk. The structural relaxation time for water in thin filmsbetween molecularly smooth surfaces also differs from the bulkvalue: a value of t ≈ 0.01 s has been measured for 0.6 nm thickfilms between mica sheets, as measured with a surface forceapparatus, in contrast to values on the order of ps measured inthe bulk liquid.10 In addition, reactions with ambient oxygencould be affected because there will not be a requirement forthe oxygen to diffuse a substantial distance away or evenpossibly any distance away from the air−water interface. Thecrystallization of calcium carbonate may be slow in confinedconditions,3 but it seems unlikely that kinetic control can slowdown the calcium carbonate formation to negligible levelswhile permitting the formation of calcium sulfate.The gypsum crystals described here are formed only whenthere is a brief drying step. Drying could provide nucleationsites to initiate crystal growth, but lack of nucleation sites doesnot appear to be the factor limiting the crystal growth asindicated by multiple lines of precipitated material that formwithout associated formation of crystals and seen in Figure S4of the Supporting Information. Drying also concentratessurface active solutes at the outer rim of the drying spot bythe coffee ring effect;11 this remains as a possible factor drivingthe formation of the gypsum crystals.CONCLUSIONIn summary, we see gypsum crystals produced underconditions where we would not expect to have much sulfatepresent. One possible source is a contaminant from the DTT,which would require the sulfate to travel at the front of theprecursor film at very efficient separations from the carbonateions; the other possible source is the oxidation product ofDTT, which would require some unexpected oxidations in air.The procedures described here provide a way to form smallscale crystals arranged in concentric lines, with reactions thatwould not occur in the bulk solution. The conditions requiredin these experiments to produce the precursor film, which is tosay high concentrations of thiol on gold, would limit potentialapplications but precursor films have been described for othersystems,12 and so other systems could possibly be employed aswell.AUTHOR INFORMATIONCorresponding AuthorKathryn A. Melzak − Institut für Funktionelle Grenzflac̈hen,Gebaüde 330, Karlsruher Institut für Technologie, EggensteinLeopoldshafen 76344, Germany; orcid.org/0000 00023140 9672; Email: kathryn.melzak@kit.eduAuthorsFabrice Laye − Institut für Funktionelle Grenzflac̈hen, Gebaüde330, Karlsruher Institut für Technologie, EggensteinLeopoldshafen 76344, GermanyStefan Heißler − Institut für Funktionelle Grenzflac̈hen,Gebaüde 330, Karlsruher Institut für Technologie, EggensteinLeopoldshafen 76344, GermanyAuthor ContributionsAuthor K.A.M. prepared samples, obtained Raman spectra(Renishaw), and wrote the paper; F.L. acquired SEM andoptical microscopy images; S.H. acquired and interpretedRaman spectra. All authors have given approval to the finalversion of the manuscript.FundingGerman Research Foundation DFGNotesThe authors declare no competing financial interest.ACKNOWLEDGMENTSThe authors thank Prof. Robin Turner at the Michael SmithLaboratories, University of British Columbia, for use of theRenishaw Raman microscope. Author K.A.M. acknowledgessupport from the German Research Foundation DFG (ME4648/2 1).ABBREVIATIONSDTT dithiothreitolREFERENCES(1) Goertz, M. P.; Houston, J. E.; Zhu, X. Y. Hydrophilicity and theviscosity of interfacial water. Langmuir 2007, 23, 5491−5497.(2) Anzalone, A.; Boles, J.; Greene, G.; Young, K.; Israelachvili, J.;Alcantar, N. Confined fluids and their role in pressure solution. Chem.Geol. 2006, 230, 220−231.(3) Stephens, C. J.; Kim, Y. Y; Evans, S. D.; Meldrum, F. C.;Christenson, H. K. Early stages of crystallization of calcium carbonaterevealed in picoliter droplets. J. Am. Chem. Soc. 2011, 133, 5210−5213.(4) Brochard Wyart, F.; di Meglio, J. M.; Queŕe,́ D.; de Gennes, P.G. Spreading of non volatile liquids in a continuum picture. Langmuir1991, 7, 335−338.(5) http://rruff.info/Gypsum/R040029 from the RRUFF database.(6) http://rruff.info/Gypsum/R060509 from the RRUFF database.(7) CRC Handbook, 61st ed. (Eds: Weast, R.C.; Astle, M.J.), CRCPress: Boca Raton, 1980; p B−242.(8) Koch, T.; Dahl, C. A novel bacterial sulfur oxidation pathwayprovides a new link between the cycles of organic and inorganic sulfurcompounds. ISME J. 2018, 12, 2479−2491.(9) Campbell, C. J.; Fialkowski, M.; Bishop, K. J. M.; Grzybowski, B.A. Mechanism of reactive wetting and direct visual determination ofthe kinetics of self assembled monolayer formation. Langmuir 2009,25, 9−12.(10) Masciovecchio, C.; Bencivenga, F.; Gessini, A. Water dynamicsat the nanoscale. Condens. Matter Phys. 2008, 11, 47−56.(11) Li, Y. F.; Sheng, Y. J.; Tsao, H. K. Evaporation stains:suppressing the coffee ring effect by contact angle hysteresis.Langmuir 2013, 29, 7802−7811.(12) Leger, L.; Erman, M.; Guinet Picard, A. M.; Ausserre, D.;Strazielle, C. Precursor film profiles of spreading liquid drops. Phys.Rev. Lett. 1988, 60, 2390−2393.      Repository KITopen  Dies ist ein Postprint/begutachtetes Manuskript.  Empfohlene Zitierung:  Melzak, K. A.; Laye, F.; Heißler, S. Nanoscale-Specific Reaction in a Precursor Film: Mixing Sodium Carbonate, Calcium Chloride, and an Organic Thiol to Produce Crystals of Calcium sulfate 2020. Langmuir, 36 doi: 10.554/IR/1000128738    Zitierung der Originalveröffentlichung:  Melzak, K. A.; Laye, F.; Heißler, S. Nanoscale-Specific Reaction in a Precursor Film: Mixing Sodium Carbonate, Calcium Chloride, and an Organic Thiol to Produce Crystals of Calcium sulfate 2020. Langmuir, 36 (35), 10490–10493.  doi:10.1021/acs.langmuir.0c01653              Lizenzinformationen: KITopen-Lizenz 

Nanoscale-Specific Reaction in a Precursor Film: Mixing Sodium Carbonate, Calcium Chloride, and an Organic Thiol to Produce Crystals of Calcium sulfate

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Nanoscale-Specific Reaction in a Precursor Film: Mixing Sodium Carbonate, Calcium Chloride, and an Organic Thiol to Produce Crystals of Calcium sulfate

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