Aqueous Precipitation of Calcium Carbonate Modified by Hydroxyl

Aqueous Precipitation of Calcium Carbonate Modified by Hydroxyl-Containing Compounds Steven R.

Dickinson†

and K. M.

McGrath*,‡

Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand, and School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Received May 12, 2004;

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1411-1418

Revised Manuscript Received August 27, 2004

ABSTRACT: Calcium carbonate was grown from aqueous solution in the presence of organic crystal modifiers of the saccharide and alcohol subclasses. The control crystallization procedure preferentially nucleates vaterite, the kinetically stabilized calcium carbonate polymorph. Addition of saccharides (mono-, di-, and polysaccharides) to the crystal mix drives the system to nucleate calcite, the thermodynamically preferred polymorph. However, the degree of calcite stabilization for the monosaccharides is modified by the number of equatorial -OH moieties. Disaccharide results were not a simple addition of the individual component behaviors. Deviation from rhombohedral calcite was only observed for amylose, producing crystals elongated along the c-axis. The mixed saccharide/alcohol series gave results comparable to the addition of saccharide alone. The alcohol acted only to displace the observed saccharide trend to lower saccharide concentrations. Hence, saccharide additives dominate both nucleation and growth of calcium carbonate and act principally to drive the system to thermodynamic nucleation. Introduction Calcium carbonate has elicited much interest due to its industrial importance, diversity as a biomineral, and crystalline complexity. Calcium carbonate has six polymorphic forms: three anhydrous: calcite, aragonite, and vaterite; two hydrated species: monohydrocalcite (CaCO3‚ H2O) and ikaite (CaCO3‚6H2O);1 and an amorphous form. Of the six calcium carbonate polymorphs, calcite is the thermodynamically most stable. Kinetic factors relevant to crystallization processes can influence both morphology and polymorph of a crystal. The large number of crystal habits identified for calcite reflects the role that kinetics play in crystal growth. As such, the possible role of kinetics in both the nucleation and growth of crystals should be considered when interpreting effects of crystal modifiers on a growing crystal. Calcium carbonate nucleation and growth can be manipulated through the addition of crystal modifiers to the crystal growing solution. Examples of additives include simple organic molecules,2-4 inorganic salts,5,6 and proteins.7-14 Other crystal modifiers investigated include organic molecules that possess complex functional groups in combination with specific stereochemistry, for example, glycoproteins.8,10,12-14 With respect to the presence of organic molecules, it is possible that the functional groups themselves and also the orientation in which they are held both play pivotal roles in controlling the growth of biominerals. Glycoproteins have been identified as holding an important role in many biomineralization events, and numerous studies have been performed investigating the effects extracted protein or a protein component have on growing calcium carbonate crystals. Studies involving extracted and purified sea urchin organic material have indicated that * Corresponding author. Phone: +64-4-463-5963 Fax: +64-4-4635237. E-mail: [email protected]. † University of Otago. ‡ Victoria University of Wellington.

the organic component acts as a morphological modifier to the growing calcium carbonate.10,12 Deglycosylated protein component stabilizes the {2 h 03} planes of calcite, leading to a crystal morphology that is quite different to the thermodynamic rhombohedral calcite.12 Moreover, it has been shown that the sugar component of the glycoproteins may be the major source of the observed crystal modification rather than the protein as a whole.10,12 It was these results that motivated the study reported here. To begin to investigate the role of saccharides in controlling calcium carbonate deposition, we have studied the affect that simple saccharides exert over calcium carbonate growth from aqueous solution (i.e., polymorphic ratio and resultant crystal morphologies). A number of mono- and disaccharides has been used as additives in addition to a single polysaccharide, amylose. This work is an extension to that previously published on simple straight chain alcohol crystal modifiers,15 where now we have a higher number of -OH groups per additive, and the functional groups are held in rigid conformations. Organisms generally utilize complex mixtures of crystal modifiers (rather than a simple additive) to generate the variety of biomaterials observed. To begin to address this, we have investigated the possible synergism that arises when both a sugar and an alcohol are present. Alcohol crystal modifiers led to calcite dominating the system and additionally stabilized hopper crystals as the dominant calcite morphology, acting by controlling the solution environment rather than through a specific chemical interaction with the growing crystal.15 In the first section of this paper, we present results for single saccharide additives. This is followed by a report on mixed systems (saccharides and alcohols), where additives that showed the greatest individual affects with respect to dominant polymorph nucleated and final crystal habit were used.

10.1021/cg049843i CCC: $27.50 © 2004 American Chemical Society Published on Web 10/01/2004

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Dickinson and McGrath

Experimental Procedures Calcium carbonate crystals were grown in the presence of mono-, di-, and polysaccharides and in combinations of alcohol and saccharide. The monosaccharides investigated were D-(+)glucose (monohydrate, Scientific Supplies), D-(+)-mannose (Aldrich, 99%), D-(+)-galactose (BDH, AR grade), and dfructose (Riedel-de Haen, 98%). The disaccharides used were D-(+)-lactose (Fluka, 98%), D-(+)-sucrose (Gibco BRL, ACS grade), D-(+)-maltose (monohydrate, Phansfield, 90%), and d-cellobiose (Aldrich, 98%). The polysaccharide used was R-amylose (Sigma, type III: from potato). Saccharide choice was based purely on ease of availability of all possible mono-, di-, and soluble polysaccharides. The alcohols investigated were methanol (Aldrich, AR grade) and ethanol (Aldrich, absolute LR grade). The alcohols were selected as they are highly soluble in water, and their effects on growing calcium carbonate crystals were comparable. The crystallization method was adapted from Falini16 and was based upon infusion of gaseous CO2 into an aqueous calcium chloride solution. All solutions used had a calcium chloride concentration of 100 mM. A series of calcium chloride/ saccharide solutions were prepared. The concentration of saccharide was varied from 0.5 to 20 wt % of the total solution. Saccharides, which were insoluble at concentrations within this range, included D-(+)-galactose, D-cellobiose, and D-(+)lactose. In each case, the series was completed to maximum solubility (D-(+)-galactose, d-cellobiose, and D-(+)-lactose; 5, 8.3, and 10 wt %, respectively). It should be noted that these values might not correspond to the absolute solubility limit in 100 mM calcium chloride solution for the respective saccharide. Amylose was the only polysaccharide investigated due to the general insolubility of polysaccharides. R-Amylose is relatively insoluble in 100 mM calcium chloride solution. A 0.5 wt % R-amylose solution was prepared and left undisturbed at room temperature for 4 weeks. The supernatant solution was then used as the growth medium. For the multi-additive systems, a series of calcium chloride/saccharide/alcohol solutions were prepared. The concentration of saccharide varied from 0 to 7 wt % of the total solution, while the alcohol concentration varied from 0 to 30 wt % of the total solution. A round cover slip 10 mm in diameter was placed at the bottom of three wells in a 24-well tissue culture plate. In each of the three wells, 1 mL of CaCl2/crystal modifier solution was placed. The plate was sealed in a 1.5 L crystallization container with 0.5 g of solid ammonium carbonate (Ajax Chemicals, LR grade) sprinkled evenly over the bottom of the container. The container was left at room temperature for 6 h, after which the cover slips were removed from the wells, mounted on scanning electron microscope (SEM) stubs, and washed thoroughly with Milli-Q water. The stubs were air-dried in a fumehood, coated with a Pd/Au mixture, and observed by SEM (Cambridge Instruments S-360) using an accelerating voltage of 10 kV and a working distance of 10 mm. Each series was run either in duplicate or triplicate (each run comprised three separate glass slides), comprising a total of between six and nine separate measurements for each experimentally quoted value. For the multi-additive investigations, two alcohols and two saccharides were chosen. The additives were selected based upon their strong influence over a growing calcium carbonate system. Fructose and sucrose were chosen as the saccharides due to their similar behaviors as calcium carbonate crystal modifiers. Further, a point of difference for these two saccharides is their size; fructose is a monosaccharide, while sucrose is a disaccharide. XRD diffractograms were collected on a Philips PW 1830 generator with a Philips PW 2213/20 camera. The data were obtained with generator settings of 40 mV and 20 mA, a time constant of 2 s, and a resolution of 100 steps per degree between 10 and 50°. Correlation of crystal habit with polymorph type was determined by comparing SEM and XRD data: rhombohedra, hoppers, and hexagonal plate morphologies were identified as calcite, calcite, and vaterite, respectively.

Figure 1. Vaterite abundance for the four monosaccharides investigated. The remainder of the crystal mix was rhombohedral calcite. Galactose is not completely soluble at 10 wt %; hence, the series for this saccharide terminates at 5 wt % saccharide. The lines shown are for visual aid only. The control rhombohedral CaCO3 sample utilized for the crystal face intensity comparison (intensity data obtained from XRD analysis) with the crystals grown in the presence of amylose was sourced from Specialty Minerals, Inc. The calcite sample is Multifex-MM PCC and has a median particle size of 0.07 microns with a surface area of 19 m2 g-1.

Results For the results presented next, the values quoted for vaterite abundance are calculated means from six to nine data points. Percentage polymorphic nucleation was observed to fluctuate both within and between different crystal trials. In general, the standard deviation for vaterite levels was higher the lower the vaterite abundance. For vaterite levels e10%, the standard deviations rose to approximately (100% (e.g., for 20 wt %, maltose crystal trials the following vaterite abundances were measured: 0, 0, 0, 2, 2, 2, 2, 2, and 2% yielding a mean of 1.3% and a standard deviation of 1), whereas for vaterite abundances of ∼60-70% standard deviations as low as (10% were observed (e.g., for 2 wt % maltose crystal trials the following vaterite abundances were measured: 50, 55, 55, 60, 60, and 60% yielding a mean of 56.7% and a standard deviation of 4). Hence, comparing the relative uncertainties alone is dangerous because this would imply that the later stated vaterite abundance is more accurate than the former. However, comparison of absolute uncertainties would place more reliance on the former. This should be noted when considering the graphs presented next. Saccharides. A series of mono- and disaccharides was investigated as crystal modifiers in calcium carbonate deposition. Figures 1 and 2 represent graphically the polymorphic ratio of vaterite obtained as a function of additive concentration for the mono- and disaccharide systems, respectively. The remainder of the samples was calcite in all cases; no aragonite was observed. In the presence of no additive, the polymorphic ratio was 1:3 calcite to vaterite. Figure 3 shows SEM micrographs representative of calcium carbonate crystals obtained from the fructose series on increasing saccharide concentration. The micrographs shown for this saccharide correlate well with those obtained for all other sugars investigated. As the concentration of saccharide was varied, no significant change in calcite crystal size was observed. Some variation in the assembly of the vaterite

Aqueous Precipitation of Calcium Carbonate

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Figure 2. Vaterite abundance for the four disaccharides investigated. The remainder of the crystal mix was rhombohedral calcite. Lactose and cellobiose are not completely soluble at 20 wt %, so the series for these disaccharides terminates at 10 and 8 wt %, respectively. The lines shown are for visual aid only. Table 1. Intensities of Pure Calcite and Calcite Grown in the Presence of Amylosea

2θ

hkl assignment

amylose calcite intensity

pure calcite intensity

amylose calcite/ pure calcite

23.04 29.43 31.45 36.00 39.44 43.18 47.17 47.53 48.56

{01.2} {10.4} {00.6} {11.0} {11.3} {20.2} {02.4} {01.8} {11.6}

12.95 100 5.47 26.57 11.88 18.42 5.34 9.48 31.51

10 ( 1.1 100 3.9 ( 1.2 14 ( 1.6 21 ( 2.5 18 ( 1.3 8.0 ( 1.9 24 ( 4.5 24 ( 2.2

1.30 1 1.37 1.90 0.57 1.02 0.67 0.40 1.31

a Confidence intervals for pure calcite intensities are given to the 95% level. The hkl assignments are based upon the hexagonal unit cell of calcite. The intensity of the {10.4} peak was used as the reference point for all other peaks (i.e., it is set to 100); hence, no uncertainty is quoted for this value.

hexagonal plates was evident, but this was quite variable between crystal trials and different saccharides. A general observation, for all saccharides investigated, was the enhanced nucleation of calcite as compared with the control experiments. Superimposed upon this general trend of calcite stabilization was a second trend of either vaterite restabilization at 2-3 wt % saccharide or reduced calcite stabilization (this can be seen in the fructose series shown in Figure 3). Reversion to control levels of vaterite (∼75%) was not observed except for one saccharide, sucrose, which yielded a vaterite abundance of 85 ( 5% at 3 wt % saccharide. Within this vaterite restabilization range, generally 4060% vaterite nucleation was observed, whereas for lower and higher saccharide concentrations, 0-15% vaterite was normal. Only two saccharides investigated did not show vaterite restabilization, glucose and lactose, for any concentrations investigated. Lactose exhibited minor vaterite restabilization (8 ( 7% vaterite) at 3 wt %; however, when the uncertainty associated with this value is taken into consideration, this maximum is not statistically significant. The crystal morphology of the calcite crystals was predominately rhombohedral, as seen in Figure 3. Hopper crystals were observed sporadically in very limited numbers (Figure 4), but this was not significantly different from control levels. Previous work

investigating the effect of simple alcohols as crystal modifiers showed that the formation of deep multifaced hopper crystals could be directly correlated with the solution viscosity, with a transition leading to a dramatic increase in the formation of hopper crystals occurring for solution viscosities exceeding 1.6 mPa s.15 The viscosity of all saccharide solutions was measured. Solution viscosity exceeded 1.6 mPa s only for the highest saccharide concentrations (results not included), but this did not correlate to a significant increase in the number of crystals showing hopper defects or the size and depth of the defects. Additionally, no significant variation in the number of hoppers observed was found for the different sugars used. The series of micrographs shown in Figure 3 is representative of all sugars investigated. Extension of the simple mono- and disaccharides series led to the use of the most water soluble polysaccharide, amylose. The morphology of the resultant calcium carbonate crystals grown from the amylose solution is shown in Figure 5. The crystals in Figure 5 are significantly elongated in a direction parallel to the c-axis. A powder X-ray diffractogram of these crystals was obtained and is shown in Figure 6. Analysis of the diffractogram shows that all peaks can be assigned solely to calcite, that is, no trace of any other calcium carbonate polymorph is observed. This implies that no more than 5 and 8% of aragonite or vaterite can be present, respectively, using the experimental equipment available.17 A comparison between normalized peak intensities for calcium carbonate crystals grown in the presence of amylose and the expected peak intensities for the Multifex control sample (100% rhombohedral calcite) is given in Table 1. When comparing the intensities of the peaks in the XRD diffractogram of the calcium carbonate grown in the presence of amylose to those obtained from the pure rhombohedral calcite sample referenced to the {10.4} peak, significant differences were observed. The intensity of the {11.0} plane increased by 190% relative to the {10.4} plane. Further, the {01.2}, {11.6}, and {00.6} planes all increased in intensity by ∼130%. Decreasing in relative intensity were the {01.8}, {11.3}, and {02.4} planes, which were 40, 57, and 67%, respectively, of their expected size. Returning a similar intensity ratio to the {10.4} plane was the {20.2} plane. The {11.0} plane is parallel to the c-axis of the hexagonal unit cell of calcite. Considering that the {10.4} planes are 45° to the c-axis, the stabilization of the {11.0} planes is consistent with elongation along the c-axis, as observed in Figure 5. The relative intensities of the {00.6}, {01.2}, and {11.6} planes suggest that they also play prominent roles in producing the observed elongated morphology, being dominant in the fine structure of the crystal habit. Multi-Additive Systems. Calcium carbonate crystals were grown from aqueous solutions containing both ethanol and sucrose. Figure 7 summarizes the polymorphic ratios nucleated. For sucrose concentrations 5 wt % and above, calcite nucleation occurred at a level of ca. 90% or greater for all ethanol concentrations used, and at lower sucrose concentrations, as the concentration of ethanol is varied,

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Figure 3. Series of SEM micrograph as a function of increasing fructose concentration. (a) 0.5, (b) 1, (c) 2, (d) 3, (e) 5, and (f) 10 wt %.

narrow regions of vaterite restabilization were observed. The generally low abundance of vaterite observed throughout the saccharide series indicated that the presence of sucrose dramatically enhanced the nucleation of calcite. It should be noted that the concentration of saccharide at which the vaterite restabilization occurred decreased as the alcohol concentration increased. For 0, 10, and 20 wt % ethanol, the vaterite restabilization occurs at approximately 3, 2, and 1 wt % sucrose, respectively. By 30 wt % ethanol, the vaterite restabilization region was absent. We cannot at this time explain the disparity in the strength of vaterite restabilization as alcohol concentration is varied. In general, the behavior of the mixed system more strongly resembles that of the pure sugar series than that of the pure alcohol series (e.g., the vaterite resta-

bilization region, present in the pure saccharide series, persists in the multi-additive series). To test this hypothesis, the data were replotted from the perspective of ethanol rather than sucrose. No general discernible trend was observed, that is, the sucrose did not act as a modifier of the ethanol behavior. Rather, the ethanol acts to modify the concentration at which vaterite restabilization occurs. This supports our hypothesis that the presence of saccharide in the growth solution dominates the nucleation of calcium carbonate. The polymorphic ratios nucleated in the ethanol/ fructose series are shown in Figure 8. Generally, the trends observed in the ethanol/sucrose series were mirrored here. That is, a broad preference for nucleation of calcite was observed, combined with narrow localized regions of vaterite restabilization modified by the etha-


Figure 4. SEM micrograph showing typical degree of pitting seen in each of the saccharide systems studied. The micrograph shown is from a 10 wt % sucrose crystal growth trial.


Figure 7. Vaterite abundance for the sucrose-ethanol series from the perspective of sucrose. The remainder of the sample was calcite. Lines are shown for visual aid only.

Figure 8. Vaterite abundance for the fructose-ethanol series. The remainder of the sample was calcite. Lines are shown for visual aid only. Figure 5. SEM micrograph of calcium carbonate crystals grown from a saturated amylose solution. The black arrows show the c-axis for two of the crystals.

Figure 6. Powder X-ray diffractogram of calcium carbonate crystals grown from a solution containing amylose (crystal morphology as shown in Figure 5). All peaks can be assigned as arising from calcite.

nol concentration. Again, for 0, 10, and 20 wt % ethanol, the high vaterite restabilization occurred at ca. 3, 2, and 1 wt % fructose, respectively. Calcite was the major polymorph nucleated (>95%) for all fructose concentrations used for 30 wt % ethanol. The polymorphic information obtained for the methanol/sucrose series is shown in Figure 9. Here, calcite nucleation is broadly stabilized. Both additives independently stabilize calcite nucleation to a degree, but

Figure 9. Observed vaterite abundance for the methanolsucrose series. The remainder of the sample was calcite. Lines are shown for visual aid only.

together they appear to be stabilizing this nucleation more than simple additivity of the two modifiers would imply. The significant vaterite restabilization regions seen in the other alcohol/saccharide series were not seen for this series. Hoppers were observed sporadically in all alcohol/ saccharide series investigated. The degree and number of hoppers varied, but in general, the size of the pits observed was significantly smaller than those seen in the pure alcohol series. The hoppers observed in these mixed systems were similar to those seen for the pure saccharide series (see Figure 4).

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The viscosities of all the multi-additive systems were measured to determine if the correlation between hopper formation and solution viscosity seen for the pure alcohol additives could be applied here.15 While many of the alcohol/saccharide solutions yielded measured viscosities greater than 1.6 mPa s, no significant hopper formation was observed. This indicates that hopper formation is not solely controlled by solution viscosity and most importantly that the interaction of the sugars with the growing crystals overrides the effect of the increased solution viscosity. Discussion The addition of saccharides to the growth medium resulted in the stabilization of calcite. The appearance of the thermodynamically most stable phase of calcium carbonate could be due to a number of factors. In the control system, the energy barrier to nucleation for calcite is higher than the barrier for vaterite. This is probably an expression of Ostwald’s step rule (a metastable phase’s nucleation rate can be higher than those of the stable phase).18 For the addition of saccharides to the growth medium to reverse this trend implies that the saccharides are either reducing the barrier to calcite nucleation or increasing the barrier to vaterite nucleation (e.g., a lowering of the local supersaturation). The narrow region of vaterite restabilization observed for the saccharide series could be due to a lowering of the nucleation barrier relative to calcite or a stabilization of the polymorph once it has formed (i.e., the presence of the saccharide interferes with vaterite to calcite transformation pathways). It could also be due to a slight cessation of calcite stabilization. Generally, calcite nucleation continues to be preferred as compared with controls, but the degree of stabilization is much lower than in other regions of the series. The main dilemma here then is what is unique about this concentration range? Assuming a nucleation barrier argument, why should 2-3 wt % act significantly differently to 1 and 5 wt % saccharide? Also, why do glucose and lactose not display the vaterite restabilization seen in the other six saccharides? Further, the vaterite restabilization range is consistent over a range of mono- and disaccharides. This suggests that the effect is independent of molecular concentration, instead being dependent on the mass of saccharide in solution. With the vaterite restabilization being dependent on the weight of saccharide in solution, it could be either an environmental effect (e.g., viscosity) or a specific molecular effect. No correlation between viscosity and vaterite restabilization was found. Hence, it appears unlikely that viscosity is a control factor in determining polymorphic ratio, although we cannot rule out other environmental effects that might affect the system. It is pertinent to consider the stereochemistry of the saccharides investigated and their behavior in solution. In aqueous solution, mono- and disaccharides tautomerize into both R and β forms. This leads to at least two distinct isomers of a saccharide being present in solution. This complicates matters somewhat as it becomes difficult to assign an effect to a particular isomer. However, tautomerization only affects the stereochemistry of the anomeric -OH moiety. It has been shown that once equilibrium has been obtained (typically


around 100 minutes), the β/R ratio for the monosaccharides D-glucose, D-galactose, and D-mannose are 1.76, 2.38, and 0.453, respectively.19 Hence, over the time scale of the crystal growth experiments performed, the saccharide conformation in solution will have reached equilibrium. A comparison of the number of permanent axial -OH groups (discounting the -OH moiety that can undergo tautomerization) for the monosaccharide group yields an interesting trend. Glucose has no permanent axial -OH and displays no restabilization of vaterite. Mannose and galactose have one permanent axial -OH and display only mild vaterite restabilization at 2-3 wt % saccharide (yielding ∼40 ( 20% vaterite), while fructose possesses two permanent axial -OH groups and displays the strongest vaterite restabilization of any of the monosaccharides investigated (∼55 ( 20% vaterite). We were unable to further test this hypothesis due to the lack of ready availability of appropriate saccharides. The disaccharides require further consideration as they may display the effects of individual monosaccharide residues plus any potential influence of the stereochemistry of the linkage between the two monosaccharides. Of the four disaccharides investigated, two have an R-linkage (maltose and sucrose) and two a β-linkage (cellobiose and lactose). Sucrose (glucosefructose residues) and lactose (galactose-glucose residues) yielded vaterite abundances of approximately 85 ( 5 and 8 ( 7 wt %, respectively, for 3 wt % saccharide. When comparing these results to the monosaccharides from which the disaccharides are composed, sucrose (glucose-no vaterite stabilization and fructose-strong vaterite stabilization) should be expected to stabilize vaterite to a greater extent than lactose (galactose-mild vaterite stabilization and glucose-no vaterite stabilization). This was the case, but the variation in vaterite abundance was high, and this is not consistent with simple additive comparisons for the results for the constituent residues of the disaccharides investigated. Both maltose and cellobiose consist of two glucose residues, so a direct comparison can be made between which linkage (R or β) is present and the extent of vaterite restabilization. Approximately 55 ( 15% vaterite nucleation is observed when both cellobiose and maltose are present at approximately 2 wt %, although for maltose the restabilization region is broader than for cellobiose. Hence, the form of the disaccharide linkage does not control the level of vaterite restabilization or its location in this system. If the results seen for glucose translate into the disaccharide series, cellobiose and maltose would not be expected to stabilize vaterite significantly for any saccharide concentration used. This was not the case. Hence, again simple additivity cannot be applied; rather, the disaccharides elicit an effect distinct from their residues. For all mono- and disaccharides used, very high concentrations (on the order of tens of mg mL-1) were required to elicit any differentiation from control conditions. This indicates that these simple sugars, while manipulating crystal nucleation and growth to a greater extent than the single -OH moiety of a simple alcohol additive, only weakly moderate and control calcium carbonate nucleation and subsequent growth. In comparison, saccharides cleaved from glycoproteins occluded


Figure 10. Schematic of the resultant calcite crystal faces expressed when amylose was present as a crystal modifier in a calcium carbonate crystal growth solution. Gray circles, {11.0} faces and open circles, {10.4} faces.

in sea urchin spines show substantial control of the calcite morphology at concentrations as low as 5 µg mL-1.8,10 This leads to the hypothesis that a higher degree of covalent connectivity between the residues may be required to elicit significant control over crystal nucleation and habit for low concentrations of additive. This hypothesis was supported by the results of the amylose growth trials. The concentration of amylose in solution was of the order of 10 µg mL-1 due to its low solubility. As such, it is unlikely that the observed 100% calcite nucleation and altered calcite morphology could be the result of just a global or solution effect. Almost certainly the morphology arises due to specific additive-crystal interactions. The enormous difference between the results obtained for the mono- and disaccharide additives and the amylose must be attributed to the increased covalent linkage between the glucose residues. This ensures a high local concentration of additive with respect to pre- and post-association of amylose with calcium and/or carbonate ions and the nucleated crystal. This high local concentration is required to (a) promote calcite nucleation at the exclusion of all other polymorphs and (b) to induce formation of a crystal habit that cannot be explained purely by diffusion arguments, as was the case for the formation of hopper crystals.15 These results therefore support the ideas that crystal morphology is manipulated by specific crystal/additive interactions, that increased local concentration of the additive with respect to the nucleation site is important in controlling both polymorph type and final crystal habit, and that polysaccharides can be successfully used to achieve such control. Combining the X-ray diffraction data (Figure 6) with the SEM micrograph of Figure 5, it is possible to generate a schematic depicting the possible crystal face alignment and abundance. This is shown in Figure 10. The ethanol-sucrose and ethanol-fructose series were broadly similar. The addition of additives to the growth solution generally led to decreased vaterite levels, that is, there was a general drive to thermodynamic nucleation. This could be explained by a decrease in the local concentration of material, leading to slower nucleation rates, which favor the more thermodynami-


cally stable polymorph. However, at this stage specific conclusions cannot be drawn. Within the ethanol/sucrose and ethanol/fructose series, there were distinct regions where calcite was not stabilized to the same degree as seen for other concentrations of saccharide. For example, for the 10 wt % ethanol series for both saccharides investigated, considerable vaterite restabilization was observed at ∼2 wt % saccharide. The trend of vaterite restabilization was similar to the control saccharides’ effect in isolation, with a slight modification of the position of the vaterite peak. The exact position of the vaterite restabilization region depended on the concentration of the ethanol, and hence, the multi-additive systems more generally mimicked the behavior of the saccharide rather than the alcohol component. The methanol/sucrose series exhibited en masse calcite stabilization, as seen in the other two multiadditive systems investigated, but here no vaterite restabilization region was observed. It has been reported that when simple alcohols are used as crystal modifiers, hopper formation is significantly enhanced when solution viscosity is greater than approximately 1.6 mPa s.15 When a single saccharide was added to the crystal mix, this viscosity was only achieved at the highest concentrations. However, even then, hopper formation significantly different to the control system was not observed. For the multi-additive systems investigated, hopper crystals were observed, but their appearance was erratic, their numbers were generally small, and their presence did not depend on solution viscosity, despite the 1.6 mPa s threshold being surpassed in many solutions. The types of hopper crystals formed were consistent with those seen for the pure saccharide series, that is, the saccharide additives determine the morphology of the calcite crystals as well as the polymorph type itself. Complete rhombohedra are thought to be a crystal morphology further along the continuum toward the thermodynamic end as compared with hopper crystals. The presence of the saccharide may be exerting a thermodynamic influence over the resultant crystal morphology, as well as the polymorphic mix that surpasses the observed increase in viscosity. That is, it is possible that hoppers will only be expressed in a viscous solution if no modifier with specific crystal interactions is present. A modifier that interacts with the growing crystal may increase the surface energy of the growing instability leading to the instability becoming increasingly unstable, with the hopper pits subsequently filling in. These observations, combined with the retention of vaterite restabilization at 2-3 wt % saccharide, indicate that the saccharides are the dominant of the two additives, in both polymorphic and morphological influence. Conclusions The addition of saccharide to a calcium carbonate growth solution in general facilitated calcite nucleation. However, the extent of calcite nucleation depended significantly on the concentration of saccharide. The number of axial -OH moieties present in a monosaccharide was linked to the degree of vaterite restabilization. As the number of axial -OH moieties increased, the degree of vaterite restabilization also increased. The

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vaterite restabilization seen in the disaccharide series showed that straight additivity of the affects of the two monosaccharides was not occurring. The strength of the crystal modifier amylose indicated the importance of polysaccharides as crystal modifiers for calcium carbonate growth. The absence of large numbers of deeply pitted rhombohedra, despite highly viscous solutions, indicated that solution viscosity could be over-ridden in the presence of an active crystal modifier. The presence of saccharides dominated both the polymorphic ratio and the morphology of calcium carbonate crystals grown from an aqueous solution. References (1) Gaines, R. V.; Skinner, H. C. W.; Foord, E. E.; Mason, B.; Rosenzqeig, A. Dana’s New Mineralogy; Wiley: New York, 1997. (2) Lopez-Macipe, A.; Gomez-Morales, J.; Rodriguez-Clemente, R. J. Cryst. Growth 1996, 166 (1-4), 1015. (3) Mann, S.; Didymus, J. M.; Sanderson, N. P.; Heywood, B. R.; Aso Samper, E. J. J. Chem. Soc., Faraday Trans. 1990, 86 (10), 1873. (4) Manoli, F.; Dalas, E. J. Cryst. Growth 2000, 218 (2-4), 359. (5) Loste, E.; Wilson, R. M.; Seshadri, R.; Meldrum, F. C. J. Cryst. Growth 2003, 254 (1-2), 206. (6) Meldrum, F. C.; Hyde, S. T. J. Cryst. Growth 2001, 231 (4), 544.

Dickinson and McGrath (7) Thompson, J. B.; Paloczi, G. T.; Kindt, J. H.; Michenfelder, M.; Smith, B. L.; Stucky, G.; Morse, D. E.; Hansma, P. K. Biophys. J. 2000, 79 (6), 3307. (8) Albeck, S.; Addadi, I.; Weiner, S. Connect. Tissue Res. 1996, 35 (1-4), 365. (9) Gerbaud, V.; Pignol, D.; Loret, E.; Bertrand, J. A.; Berland, Y.; Fontecilla-Camps, J.-C.; Canselier, J.-P.; Gabas, N.; Verdier, J.-M. J. Biol. Chem. 2000, 275 (2), 1057. (10) MacKenzie, C. R.; Wilbanks, S. M.; McGrath, K. M. J. Mater. Chem. 2004, 14 (8), 1238. (11) Belcher, A. M.; Wu, X. H.; Christensen, R. J.; Hansma, P. K.; Stucky, G. D.; Morse, D. E. Nature 1996, 381 (6577), 56. (12) Albeck, S.; Weiner, S.; Addadi, L. Chem.-Eur. J. 1996, 2 (3), 278. (13) Zaremba, C. M.; Morse, D. E.; Mann, S.; Hansma, P. K.; Stucky, G. D. Chem. Mater. 1998, 10 (12), 3813. (14) Zaremba, C. M.; Belcher, A. M.; Fritz, M.; Li, Y.; Mann, S.; Hansma, P. K.; Morse, D. E.; Speck, J. S.; Stucky, G. D. Chem. Mater. 1996, 8 (3), 679. (15) Dickinson, S. R.; McGrath, K. M. J. Mater. Chem. 2003, 13 (4), 928. (16) Falini, G.; Fermani, S.; Gazzano, M.; Ripamonti, A. J. Mater. Chem. 1998, 8 (4), 1061. (17) Dickinson, S. R.; McGrath, K. M. Analyst 2001, 126 (7), 1118. (18) Markov, I. V. Crystal Growth for Beginners; Fundamentals of Nucleation, Crystal Growth and Epitaxy; World Scientific: Singapore, 1995. (19) Isbell, H. S. Carbohydrates in Solution. Advances in Chemistry; ACS, Washington, DC, 1973.

CG049843I

Aqueous Precipitation of Calcium Carbonate Modified by Hydroxyl

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