The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

Abstract As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg2+ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg2+ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.

Abstract: As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms.Inthis study,we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg 2+ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals.U sing a"Crystal Hotel" microfluidic device to providewell-defined, nanoliter volumes,weobserved that calcite crystals form via an amorphous precursor phase.S urprisingly,t he first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1-0.5 mmf or Mg 2+ and 1-2 mmf or PSS.T he crystals then continue to growt od evelop morphologies characteristic of these additives.T hese results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by ac lassical mechanism.
Soluble additives are widely used to generate crystals with defined morphologies,s izes,a nd structures.H owever,o wing to the difficulty of studying dynamic phenomena on the nanoscale,s urprisingly little is known about the effects of additives on the initial growth of crystals.M onitoring the development of crystals by isolating them from solution at different reaction times is challenging as crystals initially change rapidly in size and can undergo dissolution and regrowth. Thebest insight into the growth of small crystallites has probably come from the study of metal [1] and metal oxide nanoparticles, [2] which terminate at small particle sizes and have well-defined morphologies.I nsitu analytical methods are particularly promising as demonstrated in ar ecent study of the surfactant-directed growth of Pd nanocubes by liquidphase transmission electron microscopy (LP-TEM). [3] We herein made use of afascinating phenomenon, namely that crystallization processes are retarded in small volumes, [4] to investigate the effects of the contrasting soluble additives Mg 2+ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite single crystals formed from an amorphous calcium carbonate (ACC) precursor phase.W ith its rich polymorphism, CaCO 3 provides am uch-studied model system that is also biologically,e nvironmentally,a nd industrially important. Te chniques such as time-resolved wide/ small-angle X-ray scattering [5] and light scattering [6] have been used to study the precipitation pathway of CaCO 3 in bulk solution while cryo-TEM has provided snapshots of its development. [7] LP-TEM has also been used to follow the precipitation of CaCO 3 in additive-free solutions, [8] and in the presence of PSS. [9] However,l ittle is known about the initial morphological development of calcite crystals in the presence of additives.T his study addresses this challenge by using a" Crystal Hotel" microfluidic device [10] as as imple and versatile means of investigating the morphological development of crystals.Each device provides an array of independent "rooms" with volumes of just 19 nL ( Figure 1) that not only slow down the rate of crystal growth, but also avoid the problems associated with impurities,i ncomplete mixing,a nd convection that are often found in bulk solution. Some adsorption of PSS on poly(dimethylsiloxane) (PDMS) can be expected, but this has an egligible effect at the additive concentrations used. Studying CaCO 3 precipitation within the Crystal Hotel provides strong evidence that the crystallization of ACCinsolution begins at the surface of the ACCparticles, and that ACCc an be transformed directly into calcite,i n contrast to previous suggestions. [8] Surprisingly,t he morphological changes characteristic of Mg 2+ and PSS are not observed until the crystals reach sizes of at least 100 nm, which we rationalize with as imple model based on classical crystal growth and additive binding to kink sites.
Crystal Hotel devices were created from PDMS using common lithographic methods and bonded to ag lass slide. Based on adesign by Sun and co-workers, [11] each device has asequence of 48 cylindrical rooms with adiameter of 400 mm and ah eight of 150 mm, which are linked by microchannels ( Figure 1). Ther ooms were filled with CaCl 2 solution ([Ca 2+ ] = 1.25-5 mm)c ontaining either [Mg 2+ ] = 0-5 mm or [PSS] = 0-500 mgmL À1 (M w = 70 000), and then CaCO 3 precipitation was induced by introducing (NH 4 ) 2 CO 3 vapor at 50 mLmin À1 . [12] Thee stimated range of supersaturations is given in the Supporting Information, Figure S1. Thereactions were allowed to proceed for 5-30 min, and were then terminated by circulating ethanol through the device (Figure 1c). Thep articles deposited on the glass slide in the Crystal Hotel were then characterized by scanning electron microscopy (SEM) and Raman microscopy ( Figure S2) after removal of the PDMS layer.
ACCw as the first phase formed and subsequently transformed into calcite at different times depending on the nature of any additives present. Thet ransformation into calcite occurred within 5min in the absence of additives,a s compared with 20 [13,14] which has been attributed to an inhibition of the nucleation and growth of the new crystalline phase rather than ad irect effect on the ACC. [13] As crystallization processes are retarded within small volumes,i tw as possible to use the Crystal Hotel to observe the transformation mechanism. A large proportion of ACChemispheres formed in the presence of [Mg 2+ ] = 1.25 mm at [Ca 2+ ] = 2.5 mm exhibited small rhombohedral particles,morphologically consistent with calcite,on their surfaces (Figures 2a,b). This provides strong evidence that the transformation of ACCi nto calcite is initiated adjacent to the ACC/solution interface,a nd that the calcite crystallites then grow at the expense of the amorphous material. ACCparticles imaged by SEM prior to the onset of crystallization showed no evidence of surface calcite crystal- Figure 1. The Crystal Hotel microfluidic device. a) Photographof aPDMS device bonded to ag lass slide and filled with asolution of red dye. b) Chip design of aC rystal Hotel with 48 "rooms". c) Crystallization in asingle room (i). An aqueouss olution (light blue) is introducedt hrough inlet 2t ofill the channel and rooms (ii). Subsequently,a ir (white) is introduced through inlet 1topush the solution out of the channel and isolate the solution contained in each room (iii). (NH 4 ) 2 CO 3 vapor (green) is then pumped through inlet 1( iv), and CaCO 3 precipitation is initiated by diffusion of CO 2 and NH 3 gas into the solution (v). Once crystals have formed, ethanol (yellow) is pumped through the device to terminate the reaction (vi). lites,s howing that they do not result from drying/irradiation ( Figure S5).
Thet ransformation mechanism of ACCi nto calcite has been the subject of much discussion, and dissolution/recrystallization, adirect solid-state transformation, or even acombination of mechanisms [15] have been suggested. ACCcan also be directly transformed into calcite or via avaterite intermediary phase. [5] Recent LP-TEM studies support the existence of multiple nucleation pathways, [8] and aragonite and vaterite crystals have been seen to form in direct contact with ACC particles,w ith nucleation of the crystalline phase occurring adjacent to the surface of an ACCp article.H owever,n o evidence for adirect transformation of ACCinto calcite had been obtained, which led to the suggestion that this mechanism is unlikely. [8] Ar ecent study of the transformation of ACCi np icoliter droplets on SAMs,i nc ontrast, indicated adirect transformation of ACCinto calcite. [4d] Thehypothesis that ACCc rystallization is initiated at the ACC/water interface was also supported by another recent study,w hich showed that ACCd ehydrates before crystallizing, even in solution, and that the loss of the final water fraction coincides with crystallization. [15a] Thehigh activation energy of the final step is in keeping with partial dissolution/recrystallization. Thef act that we here observed rhombohedral crystallites on the surfaces of the ACCp rovides intriguing support for the latter mechanism, while this is also consistent with simple arguments based on the relative magnitudes of the interfacial free energies.Contact between ahydrated calcite surface and water should be more favorable than that between ac alcite surface and ACC, where little water of hydration is available.
We then focused on the morphological development of the nascent calcite crystals and were surprised to observe that none of the morphological signatures associated with Mg 2+ or PSS were observed until the crystals were much larger in size (Figure 2e-h). Them ature crystals did, however, display morphologies comparable to those of their counterparts precipitated in bulk solution ( Figure S6). In the case of Mg 2+ ,t he mature crystals exhibited roughened surfaces and an elongation along the c axis,which is due to the interaction of the Mg 2+ ions with acute step edges. [16] At [Ca 2+ ] = 2.5 mm and [Mg 2+ ] = 1.25 mm,t he typical "transition size" at which morphological changes were first observed was 0.1-0.5 mm  Figure S6);t hese comprise rhombohedra with roughened surfaces and truncated edges that are flattened perpendicularly to the c axis. [17] In contrast to Mg 2+ , PSS preferentially interacts with the obtuse steps on calcite, causing the difference in morphology.Ca 2+ concentrations of ! 5mm or higher additive concentrations resulted in polycrystalline particles (Figures S6 and S7).
Theimpact of the reaction conditions on the crystal sizes at which morphological changes were observed was also explored. Ar eduction in the concentration of PSS from 500 mgmL À1 to 250 mgmL À1 while holding [Ca 2+ ]c onstant at 2.5 mm had little effect on the transition size or the final crystal morphology (Figure 2fvs.g), where this trend was also observed in bulk solution ( Figure S6). In contrast, an increase in the transition size from about 1 mmtoapproximately 2 mm was observed upon reducing [Ca 2+ ]from 2.5 mm to 1.25 mm at [PSS] = 250 mgmL À1 (Figure 2g vs.h ). These results are thus consistent with our assertion that the additives had little effect on the morphologies of the calcite crystals at sizes below 100 nm.
In interpreting our experimental data, it is important to note that as in the majority of studies of additive/crystal interactions,t he Crystal Hotel offers a" batch" system in which the solution composition varies as ions are consumed. We therefore performed experiments to confirm that this was not the origin of the effects seen in our study.During crystal growth, the concentration of Ca 2+ decreases significantly, while there is little change in the additive concentrations (as they are only sparingly incorporated into the calcite crystals). Thea dditive/Ca 2+ ratio therefore increases during crystal growth and reaches very high levels shortly before growth terminates. [18] Rough calculations showed how the solution composition changes during crystallization (Figure 3a). Assuming af inal population of 3 mmc rystals,o nly 1.6 %o f the Ca 2+ ions had been consumed when the crystals reached at ransition size of 0.5 mm, and for at ransition size of 1 mm, this value is still only 3.7 %. There is thus little change in the solution composition at the point of the first changes in morphology.W ea lso performed experiments under contin- uous-flow conditions to confirm that the initial additive concentrations were sufficient to induce changes in morphology.E xposure of 1-3 mmc alcite seed crystals to as upersaturated solution with constant composition yielded product morphologies consistent with those observed for micrometersized crystals in the Crystal Hotel (Figure 3b and c). Finally, we monitored the growth of crystals within the Crystal Hotel rooms by optical microscopy (Movie S1). This showed that the crystals form over as hort time period rather than over ap rolonged period of time as is often observed in the bulk. This highlights one of the key advantages of the Crystal Hotel over bulk solutions:i to ffers well-defined reaction environments in which au niformly supersaturated solution rapidly forms.
Our understanding of how additives affect the morphology of crystals such as calcite has mainly come from macroscopic specimens. [19] In situ atomic force microscopy (AFM) studies have revealed how calcite growth at the nanoscale is influenced by soluble additives,a nd have shown how Mg 2+ ions interact with the step edges on calcite. [16,20] Macroscopic calcite crystals are generally rhombohedral, bounded by lowenergy {104} faces.G rowth occurs via screw dislocations at supersaturations lower than about twice the saturation, and by surface nucleation and subsequent 2D growth at higher concentrations. [21] Both of these structures exhibit acute and obtuse step edges to which additives may selectively bind, [16,20] and it is recognized that the growth of low-solubility crystals such as calcite is limited by the availability of kink sites. [22] Strong additive binding to the kink sites leads to changes in the shape,s eparation, and velocity of the step edges,w hich can ultimately cause ap ile-up of steps,r esulting in rugged faces,w hich define the crystal habit. [19] Our experiments show the dominance of the {104} faces, even at small crystal sizes.T he crystal size at which dislocations form in calcite is not known, however. [23] The efficacyo fa na dditive in controlling crystal growth depends on its residence time at ak ink site compared to the step propagation rate.F or av ery small crystal, which necessarily exhibits short step lengths and hence few kink sites,t he probability of an impurity binding and affecting the completion of that growth layer is very small. As the crystal increases in size,the area of the faces,the length of the step edges,and the number of kink sites increase (Figures 4a-c). This increases the chance that an additive will retard the propagation of step edges and the completion of growth layers. Smaller transition sizes are therefore expected for more strongly binding additives (Figure 2e vs.f ), as observed. Higher supersaturations give rise to ag reater density of step edges, [20,22] and thus smaller transition sizes (Figure 2f vs.h ). Support for this model was obtained from simple calculations estimating the fraction of kink sites on acalcite crystal that are occupied by additive molecules rather than CaCO 3 units,asafunction of the crystal size.T his is calculated from the product of the molarity ratio of additive to CaCO 3 unit and the binding energy ratio of additive to CaCO 3 unit. Therefore,b inding to ak ink site increases with additive concentration and/or binding energy.F inally,weassume that amorphological change will occur when at least one kink site is occupied by an additive.These calculations were carried out for Mg 2+ and PSS based on the known frequencyofsteps and kink sites on the calcite {104} surface (see the Supporting Information). Figure 4d shows that as the concentration of additives in solution and their binding strength increases,the transition size associated with am orphology change decreases.M g 2+ has as imilar binding strength to Ca 2+ , [24] and both ions will have similar concentrations at the kink sites.Mg 2+ ions are therefore expected to affect the morphology as soon as sufficient kink sites are present. At a[ Mg 2+ ]/[Ca 2+ ]r atio of 1:2 ( Figure 4d (i), red circle), the model predicts am orphological change at crystal sizes of about 100 nm, in excellent agreement with our experimental data. [25] Thef act that Mg 2+ ions affect crystal morphology at smaller particle sizes than PSS is also consistent with the effects of these additives on bulk crystal growth, where Mg 2+ ions are strongly inhibitory owing to the combined effects of kink blocking and the enhanced solubility of Mg-substituted calcite. [16,22] Given the lack of data on the binding of PSS to calcite,itis more difficult to make predictions for PSS.H owever,r ough estimates can be made based on simulations of the binding of methanoic acid and poly(acrylic acid) (PAA) to calcite,where the free energy of binding for Ca 2+ and methanoic acid to a{ 104} face was estimated to be À7kJmol À1[25a] and À1.58 kJ mol À1 , [25b] respectively.T he fact that polymeric additives adsorb and modify crystal habits much more strongly than the corresponding monomers was considered using simulations that showed that about 15 %o ft he functional groups of PA Aw ere bound to calcite at any given time. [26] Hence,5 8o ft he 385 sulfonate groups in each of the PSS chains used here would be expected to bind. Concentrations of 250 mgmL À1 and 500 mgmL À1 PSS are approximately equivalent to 0.18 mm and 0.36 mm of styrene sulfonate (SS) side groups,r espectively.A dditionally,a sthe kink sites are far apart, each polymer chain can only bind to as ingle kink site.W eh ave indicated this region with ar ed circle in Figures 4d(ii) These results demonstrate that confinement provides an effective strategy for slowing down, and thus studying, crystallization processes.B yu sing aC rystal Hotel microfluidic device,wehave obtained strong evidence for the direct transformation of ACCi nto calcite in solution and have shown that calcite crystals growing in the presence of Mg 2+ and PSS are perfect rhombohedra until their size reaches at least 100 nm and 1 mm, respectively.T he size at which an additive begins to affect the morphology of calcite depends on the additive binding strength, the concentration, and the supersaturation, which was rationalized by considering additive binding to available kink sites.Our data also confirm that calcite grows by ac lassical ion-by-ion mechanism in the presence of PSS [18] rather than an on-classical assembly of nanoparticles. [27] These results provide insight into the growth mechanisms of sparingly soluble crystals such as calcite,a nd show that it is important to consider the action of additives on nucleation and growth to obtain product crystals with the desired properties.