ARTICLE pubs.acs.org/crystal
Asymmetric Growth of α-Resorcinol Crystals: In Situ Studies of Crystal Growth from the Vapor Phase K. Srinivasan‡ and J. N. Sherwood* WESTCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, Scotland, U.K. ABSTRACT: In situ studies of the growth of the acentric crystal α-resorcinol from the vapor phase confirm that the mechanism of growth differs on the faces presented at the opposite ends of the polar axis of this material. At medium supersaturations (σ = 0.76), the growth of the (011) and (011) faces proceeds by the cooperative development of strongly propagating growth sources. Under similar conditions, the (011) and (011) faces develop a mosaic of flat-topped growth centers. These develop individually and merge to form a continuous, curved, facet bearing a few residual, weakly propagating birth and spread growth sources. The degree to which this mosaic growth occurs depends on the damage done to the face of the seed crystal during preparation. This behavior accounts for the previously observed wide variation in the estimates of growth rate of the positive faces and the differential in growth rates compared with that of the negative faces. On more perfect (011) surfaces, growth is restricted to a small degree of nucleation at isolated random centers. These nuclei undergo a more limited localized growth to yield a nonpropagating macroscopic surface roughness. These observations provide further evidence for our previously expressed contention that the anisotropic growth of this and related highly polar acentric materials arises from intrinsic mechanistic causes.
’ INTRODUCTION Following the seminal work by Wells1 in the early 1950s on the anomalous growth of acentric materials from solution, it has become practice to ascribe the unequal growth of these materials along the polar directions to the differential absorption of solvent on these faces.2 7 Following a recent re-examination of the growth of the archetypical, acentric, organic material α-resorcinol and several other related materials,8 11 we have questioned the generality of this explanation.12 We have proposed that this behavior arises from causes other than the interaction of the solvent and that while an effect of solvent cannot be ruled out, this will be superimposed on a more fundamental influence on the growth process. Key to our conclusion has been the observation that growth of this and other acentric materials from the vapor phase, and hence in the absence of solvent from the systems, yields exactly similar gross kinetic behavior, morphological development, and differential polar growth as is observed in growth from solution. Additionally, we have noted that the difference in growth rates in the two polar directions is considerably greater and more variable than has been previously recognized. In fact, it appeared that, on the scale at which the growth was examined, there was little or no growth on clean, faceted, positive polar faces of the crystal compared with “normal growth” on the negative polar faces. The degree to which positive polar growth took place appeared to depend on the perfection of the surface examined and the imposed conditions of growth: growth under equilibrium conditions gave little or no growth, while extreme conditions of supersaturation and lack of agitation gave significant but highly imperfect growth. Our interpretation has been questioned.13 r 2011 American Chemical Society
Fundamental to our proposals is the possibility that the basic mechanisms of growth on the two polar faces are different. To examine whether or not this is the case and to determine why the differential growth can vary so widely, we have extended our previous studies to an examination of the growth mechanism at the polar faces and to a wider consideration of the potential role of solvent on the growth process. In the current manuscript, we present the results of a more detailed examination of the mechanism of the seeded growth of crystals of α-resorcinol from the vapor phase and of the influence of crystal surface imperfection on this process.
’ EXPERIMENTAL SECTION Materials. At room temperature and above, resorcinol exists in two acentric polymorphic forms designated α and β. The published values for the transition point between the two forms lie in the range 337 369 K.14 16 Most estimates group at the upper end of this range. The discrepancy in the published data can be ascribed to variations in the purity of the specimens used and the slow rate of the transition; described by Goworek et al16 as “taking days” to complete, that would influence the reliability of dynamic techniques such as differential scanning calorimetry (DSC). We assess that the most reliable assessments, carried out on carefully purified material, are those of Ebisuzaki et al.15 (369 ( 6 K) using spectroscopic techniques and Goworek et al16 (353 K) using positron annihilation techniques.17 Our own attempts using DSC yielded no evidence of a transition at all temperatures until Received: July 17, 2011 Revised: September 13, 2011 Published: September 14, 2011 5010
dx.doi.org/10.1021/cg200915d | Cryst. Growth Des. 2011, 11, 5010–5018
Crystal Growth & Design
Figure 1. Typical crystals of α-resorcinol grown by self-nucleation from solution in ethyl acetate and their morphology as viewed from the (a) [001] and (b) [001] directions. (Scale mark 1 mm). (c) The general shape and morphology of the mosaic blocks that nucleate on the {011} surfaces during growth (scale mark 20 μm). 380.9 K immediately prior to melting (388.5 K). Thermal analysis however revealed a distinct anomaly at 369 K. As noted in our previous study,12 all crystals grown by self-nucleation from the vapor phase in the range 340 355 K were shown by X-ray diffraction to be of the α-form. α-Resorcinol crystallizes in the orthorhombic system with space group Pna21 and point group mm2. The unit cell dimensions are a = 10.53 Å, b = 9.53 Å, and c = 5.66 Å. There are four formula units in a unit cell.18,19 Prior to use in the experiments, the source material (Sigma Ultra, Minimum purity 99%) was purified and dried using methods described in the first part of this series of papers.11 Using vapor phase chromatography, the final material was shown, independently,20 to contain less than 50 ppm of water (the lowest detectable limit). Our own assessment indicated considerably lower concentrations of other unidentified organic impurities. Vapor Growth. In situ growth experiments were carried out in a growth cell designed and used for the studies of the growth kinetics of nonlinear optical materials. The design and operation of the cell, which allowed the microscopic observation of the growing crystal under welldefined supersaturation conditions, is described in detail elsewhere.10,11 Supersaturations are expressed as σ = (ps/pc 1) where pc is the vapor pressure of the material at the deposition temperature and ps is that at the temperature of the source material. The vapor pressures were obtained from the recent publication of Verekin and Koslova.21 Experiments were restricted to the temperature range: source temperature 363 K and growth temperature 340 355 K, used in our previous study.12 These conditions lie well below the most reliable estimates of the α/β phase transition temperature (see above). Trials using source crystals prepared from various solvents (water, acetone, ethyl acetate, and toluene) yielded the same regrowth behavior irrespective of the source of the seed crystal. As a consequence, most experiments used crystals prepared by growth by self-nucleation from solution in ethyl acetate (Figure 1) which gave the largest {011} type faces (∼8 15 mm2). The polar orientations of the chosen surfaces could be identified readily from the morphology and asymmetry of growth of the source crystal. In the negative polar direction (emergent hydroxyl groups), the morphology was dominated by large (011) and
ARTICLE
Figure 2. Series of photomicrographs of the in situ development of growth centers on a {011} face of α-resorcinol. (a) 0 min, (b) 30 min, (c) 60 min, (d) 90 min, (e) 180 min, and (f) 240 min. (Supersaturation σ = 0.76, scale mark 1 cm = 125 μm.) (011) faces while in the positive polar direction (emergent benzene rings) the crystal showed four dominant {111} type faces and smaller but still well-defined (011) and (011) faces. (To save undue repetition in the text of the absolutely correct crystallographic designation of the {011} and {111} faces under consideration, we propose to identify those in the positive direction as {011} and {111} and those in the negative direction as {011} and {111}.) Full structural diagrams for this material will be found in our previous publication.12 Observations. The developing surface structure and its variation with time were observed using a Leitz Reichart Polyvar 2 microscope operated in interference contrast or interference microscopy modes.
’ RESULTS In Situ Vapor Growth on Self-Nucleated Seeds Grown from Solutions in Ethyl Acetate. Growth on As-Grown {011} Surfaces. Drained {011} surfaces (Figure 2a) showed evidence of
the presence of numerous growth features over the full area of the surface. These are highly etched as might be expected following the retrieval of the seed crystal from the growth bath and the removal of residual solution. Initiation of vapor growth (Ts = 353 K, Tc = 347 K, σ = 0.76) led to both the healing of existing growth features and the nucleation of new centers across the surface (Figure 2b). The developing sources were of no specific crystallographic orientation either in shape or distribution. In the direction normal to the crystal surface, the photographs showed evidence that the growth centers were developing by two-dimensional layer growth nucleated at a central imperfection, potentially the emergent ends of dislocation sources. As time progressed, all centers developed continuously (Figure 2c f), to cover the full {011} surface. In passage, the growing units both coalesced with and flowed over adjacent units, bypassing or enclosing small pieces of surface debris in passage. From sequential photographs (Figures 2 and 3), the 5011
dx.doi.org/10.1021/cg200915d |Cryst. Growth Des. 2011, 11, 5010–5018
Crystal Growth & Design
Figure 3. Detail of Figure 2 showing the rate of development of growth steps on a {011} surface of α-resorcinol during growth from the vapor phase. Elapsed time (a) 0 h, (b) 4 h, (c) 7.5 h, (d) 9.5 h. (Supersaturation σ = 0.76, scale mark = 200 μm.)
Figure 4. Ex situ photomicrograph of a {011} surface of a crystal of α-resorcinol grown from the vapor phase at high supersaturation, σ > 1.46 (scale mark = 500 μm).
velocity (V) of the step advance was measured as V = 7 10 3 μm/s. In the experiment shown, the growth of the surface was observed for a total period of 4 days. During all of this time the growth continued to follow the above pattern of development. Estimation of the slopes of the more visible growth hillocks yielded a value of P = 4 10 2 in agreement with more accurate estimates from experiments10,22,23 on similar materials that have been proved to grow by dislocation-nucleated two-dimensional growth. Combining this value with the measured rate of step advance V yields a normal growth rate of R = PV = 2.8 10 4 μm/s for the growth hillocks. Gradual small increases in the supersaturation confirmed that the above process of crystal development continued for an initial increase in supersaturation. At much higher supersaturations, (σ = 0.93) features associable with the onset of a changed growth mechanism appeared (Figure 4). The basic growth process gave way to one dominated by the movement of high macrosteps across the crystal surface. For the most part, these macrosteps nucleated at the edges and vertices of the face, although, as can be seen from Figure 4, occasional large growth hillocks form at the face centers. The initiation of similar dual macrostep/hillock growth mechanisms has been observed previously22 in organic crystals growing under high supersaturation conditions. They have been attributed to the development of
ARTICLE
supersaturation gradients across the growing crystal surfaces due to the thinning of the diffusion layer at the edges of the growing crystal.24 The slow movement of such macrosteps as they progress across the surface can lead to the “piling up”, at the step edges, of microsteps that travel on the macrostep ledges and to overgrowth at the resulting macrostep edge. This process, in turn, is accompanied by acceleration in the growth rate of the face and a consequent deterioration in the perfection of the growing crystal. A consequence of this change is the development of defective, growth sectors. Reference to our previous examination12 will show that a similar defective growth was observed in αresorcinol crystals grown at higher supersaturations both from the vapor and solution phases. Such a change in mechanism accounts well for this observation and hence for the observed elongation of the habit, along [001], of crystals growing at higher supersaturations from both the vapor phase and from aqueous solution. Growth on As-Grown {011} Faces. The as-grown {011} surfaces of the crystal seeds presented a different, mosaic-like structure emphasized by the mild etching that inevitably occurs during the removal of the crystal from solution (Figure 5a). The surfaces were characteristically curved, leading to difficulty in imaging the full surface. The presence of residual debris deposited during the removal of the parent crystal from the growth solution is identifiable as the dark images in the figure. Attempts to remove the debris resulted in damage to the surface. The consequences of such damage are demonstrated in the following section. On initiating growth of the as-grown surfaces in the cell (Ts = 363 K, Tc = 353 K; supersaturation σ = 1.46) the underlying, eroded mosaic blocks rapidly developed facets (Figure 5b). As time continued, each mosaic block grew independently. The upper face of each block was (011). Lateral development occurred principally in the Æ100æ and [011] directions. No development took place in the [011] direction. Two distinct types of mosaic block could be identified (Figure 5g). Steepsided blocks with well terraced hillocks (Figure 5g, A). These were steep sided with high growth steps. The step patterns on their surfaces were concentric and oval in shape suggesting propagation by a birth and spread mechanism initiated by some central defect. These blocks were relatively few in number. Their location in regions of dark spots suggests that they were generated at the site of the debris and cannot be regarded as intrinsic growth sources. As studies proceeded, using less contaminated polished surfaces (see below), the absolute identification of the position of such sources at the fewer, residual pieces of debris confirmed this speculation. The concentric growth steps on these blocks developed in all directions in the (011) plane. The resulting growth rings showed an anisotropy consistent with the unit dimensions of the underlying lattice. Mosaic blocks of lower height than the above on which microsteps, also of low individual height, propagated rapidly in the [011] direction and considerably less rapidly in the Æ100æ directions. The thicker of these showed evidence of {111} type facets along the {100} edges (Figure 5g, B). We suggest that these represent intrinsic growth sources that adopt the morphology and growth behavior described previously12 for growth on self-nucleated vapor grown seeds. On the surfaces of these blocks were microsteps of low height, barely visible on the enlargements shown in Figure 6. As Figure 5b d shows, the latter mosaic blocks gradually increase in size laterally in the Æ100æ and [011] directions, overlap, and coalesce. Figure 5e is an interferogram of the full crystal surface, the upper half of which is shown in Figure 5a d, 5012
dx.doi.org/10.1021/cg200915d |Cryst. Growth Des. 2011, 11, 5010–5018
Crystal Growth & Design
ARTICLE
Figure 5. Growth of mosaic blocks on a {011} face of α-resorcinol (a) the as-grown face of the seed crystal, followed by continued development after (b), 0.5 h (c), 1.5 h, (d), 2.5 h, (e) 3.5 h, and (f) 20 h. Panels (g) and (h) are enlargements of the areas outlined in white on (c) and (d), respectively. Since this face of the crystal was curved, only one-half could be adequately imaged. Panels (a), (b), (c), and (d) are the upper half of the full surface a composite of which is given in (e). (f) An interferogram of the lower half of the full face presented in (e) after 20 h growth. Supersaturation σ = 1.46, scale mark (a f) = 1 mm. 5013
dx.doi.org/10.1021/cg200915d |Cryst. Growth Des. 2011, 11, 5010–5018
Crystal Growth & Design
Figure 6. Detail of Figure 5 showing the movement of growth steps on the {011} surfaces of the mosaic blocks as a function of time (a) and (b) 0 h, (c) 1 h, (d) 2 h, (e) 2.75 h, and (f) 3.75 h. Scale mark = 150 μm.
after 3.5 h growth. This still shows distinctly the intermosaic boundaries of the gradually integrating crystal face. It defines that the active growth hillocks remain localized in individual mosaic blocks and do not spread across the facet surface. Figure 6 shows in more detail both the progress of integration and microstep motion over the next 4 h. Blocks overgrow and “absorb” adjacent blocks. In this process, growth centers are lost, the initiating defect presumably terminating at the submerged inter-mosaic boundary. As time progressed, the surface reached an equilibrium state (Figure 5f) which persisted for the remainder of the experiment with little change. This shows only a few residual growth hillocks that, even now, are not obviously contributing to the growth of the full surface. Their range may still be limited by intersecting low angle boundaries, the residue of inter-mosaic boundaries, crossing the surface. The steep-sided hillocks are the residue of the debris-nucleated growth centers. The overall result is the formation of a sector with a cell-like substructure of highly misaligned inter-mosaic/low angle boundaries leading to the characteristic macroscopic appearance of the growth that forms on the {011} faces of the grown crystal.12 The very few, residual intrinsic growth hillocks are of very low power (average slope P = 2.4 10 3) compared with those of the type A hillocks (P = 2.2 10 2). A combination of direct and interference microscopy of the developing blocks (Figure 6) allowed us to monitor the velocity of step advance of the intrinsic growth hillocks on the growing (011) surface to be 3.34 10 2 μm s 1 and 2.90 10 2 μm s 1 in the [011] and Æ100æ directions, respectively. From the slopes of the hillocks (P) and the calculated velocity of step movement (V), we estimate the average growth rate (R) of the intrinsic growth hillocks on the {011} faces as 7.48 10 5 μm s 1, a factor of 4 lower than that measured for the {011} surfaces. During the course of this experiment, the density of growth hillocks falls from ∼104 /cm2 to ∼60/cm2. The overall growth rate of the face, being a combination of the individual hillock growth rates and their density contributions, will thus fall to much lower values than those prevalent at the {011} surfaces. We note with interest that along the vertical edges of the {011} face presented in Figure 5e can be seen the contiguous {111}
ARTICLE
faces. These faces show a mosaic block structure (detailed in Figure 5h) of a trapezoidal shape consistent with the {111} geometry. This observation suggests that all faces with a growth vector close to the +c direction may grow by a similar mosaic block mechanism and potentially suffer similar growth characteristics. Growth on Polished {011} Surfaces. An attempt was made to remove any interference of debris initiated growth centers by polishing the crystal faces before regrowth. In this respect, the attempt was successful and it was possible to remove virtually all potentially debris generated growth sources confirming our conclusions above. The cleaning procedure caused other extrinsic growth problems that shed more light on the growth process on the positive polar faces. As Figure 7a shows, the polishing procedure resulted in significant damage to the crystal surface. To remove this, the surface was sublimed under a controlled negative supersaturation (Ts = 346 K, Tc = 351 K; σ = 0.61). The resulting etched surface showed the presence of a high density of etch-pits (Figure 7b) reflecting residual damage. On reversing the supersaturation (Ts = 353 K, Tc = 348 K; supersaturation = 0.60), mosaic growth blocks initiated at these etch pit sites to yield a very high density of growth centers (Figure 7c,d). These developed in the previously described manner. Their initial wider separation than those on the as-grown crystal surfaces allowed a measurement of the relative growth rates of the mosaic blocks in the Æ100æ and [011] directions. As shown in Figure 8, the growth rates R are very high in the early stages of growth but rapidly reduce to much lower values ([011], R = 6.9 10 3 μm/s; [100], R = 3.8 10 4 μm/s). In due course, the mosaic blocks join and meld as before, but due to the much higher initial density of mosaic blocks this takes longer (16 h) than in the previous situation (4 h). The result is a thicker but very highly faulted growth surface which generates a very disordered “growth sector”. At this time, the residual growth centers on the massive macrosteps reflect both in power and number the situation eventually reached on the “as grown” surface. The face growth rate thus falls to a similarly low value. To complete the comparison with the behavior of the {011} faces, a boost in supersaturation at this time (Ts = 358 K, Tc = 348 K, σ = 1.53) did not resuscitate growth but led to nucleation of isolated secondary nuclei on the now much more perfect surface Figure 9a). These were similar in general shape and developmental characteristics to those identified on the pristine and hence “perfect” {011} surfaces of vapor grown seed crystals and described in our previous manuscript.12 Eventually, a stage was reached at which these crystallites reached a pyramidal shape bounded principally by {111} and {011} faces. In some cases, they continued to develop in the [011] direction to give ridges. These features did not meld together other than in the [011], “ridge”, direction (Figure 9b). We suggest that these features give rise to the characteristic, rough appearance of the {011} growth surfaces of this material. At even higher supersaturations, these features develop into the macroscopically rough surface (Figure 9c) observed in our previous manuscript to be the precursor of the 180 twinning, characteristic of the growth of these materials.
’ DISCUSSION From the above results, it is obvious that the mechanism and kinetics of growth differ considerably on the two sets of polar faces. Growth on the {011} faces takes place by a well-defined 5014
dx.doi.org/10.1021/cg200915d |Cryst. Growth Des. 2011, 11, 5010–5018
Crystal Growth & Design
ARTICLE
Figure 7. Growth on a polished {011} surface of α-resorcinol. (a) The seed surface after polishing. Scale mark = 1.7 mm. (b) The same surface after etching to remove mechanical damage (supersaturation σ = 0.61), (c, d) initial growth on the etched surface after zero and 2 h, respectively (scale bar = 200 μm), (e g) the whole surface of the seed after 16, 22, and 96 h, respectively. Supersaturation σ = 0.60, scale mark = 2 mm. (h) An interferogram of the area outlined in (g) showing the residual growth hillocks on the crystal surface after this period of growth.
5015
dx.doi.org/10.1021/cg200915d |Cryst. Growth Des. 2011, 11, 5010–5018
Crystal Growth & Design
ARTICLE
Figure 8. The rate of lateral development of mosaic blocks (Figure 7c,d) during the early stage of vapor growth on the (011) type polished surfaces of α-resorcinol.
birth and spread mechanism. The large number of continuously nucleating and cooperating growth sources yield relatively rapid growth. This results in the formation of a well-integrated, planar growth facet bounding a crystallographically perfect growth sector. From a kinetic viewpoint, the velocity of growth step advance on the growing surface is V = 7 10 3 μm s 1, and the rate of growth of the hillocks normal to the face is R = 2.8 10 4 μm s 1. Since the hillock density is high (2 104 /cm2), the growth rate of the face will be significantly greater than this. As the supersaturation is increased, the mechanism changes to a mixed mechanism of macrostep and hillock growth dominated by the former. Potentially, this change is initiated by changes in the hydrodynamics of the system.6,12 The overall consequence to the growth process is to lead to the development of imperfections in the growing {011} sector and its even more rapid growth. The exactly similar effect that occurs in solution growth can be overcome by increasing the agitation rate of the growing crystal,5,6,12 thus improving the hydrodynamics of the system and prolonging the range of satisfactory growth. Such procedures are not easily instituted in vapor growth systems.10,11 This general behavior is characteristic of the normal growth of molecular crystals, for example, paracetamol,22 and thus the {011} faces of resorcinol can be said to be growing normally. In contrast and under similar conditions as examined for the {011} faces, the {011} faces develop by a completely different mechanism. One may deduce from the above experiments that the growth is extrinsic in character, with nucleation taking place predominantly at centers pre-existing at the seed crystal surface or defects resulting from surface damage or debris. These nuclei develop into a mosaic of individually growing crystallites which mimic the underlying crystallography and micromorphology of the seed. The mosaic blocks meld in short periods of time to yield
Figure 9. (a, b) Small growth centers nucleated on the more perfect {011} faces of α-resorcinol following full integration of the initial mosaic structure showing their more isolated distribution and development to pyramidal and ridge-like features. Supersaturation σ = 0.61 increasing to σ = 1.53. Scale mark = 200 μm. (c) The micromorphology of a {011} surface developing after continued growth at high supersaturations. Scale mark = 160 μm.
a substructure in which the intermosaic boundaries are, to a large extent, retained as subgrain boundaries. The misalignment of the boundaries offers opportunities for the termination of the single propagating defect centered in each mosaic block. As the mosaic blocks coalesce or are overgrown, the number of the growth centers decreases. The few intrinsic growth hillocks finally remaining on the integrated surface are not very powerful (P = 2.4 10 3, V = 3.12 10 2 μm s 1) and contribute little to face advance. With so few weak growth centers, the hillocks advance at a very slow rate, R = PV = 7.49 10 5 μm s 1. This distinction to the hillock growth rate on the {011} faces coupled with the considerably lower hillock density on {011} (∼60/cm2) will yield a significant 5016
dx.doi.org/10.1021/cg200915d |Cryst. Growth Des. 2011, 11, 5010–5018
Crystal Growth & Design difference in the growth rates of the two faces. A more perfect initial seed surface, for example, a self-nucleated seed growing under ideal conditions, should yield a thin (if it exists at all) growth sector. In contrast, an imperfect seed surface formed by surface damage or possibly prior growth under extreme conditions could propagate for longer periods before slowing to extremely low values, giving, as has been observed, an impression of continuous growth. In all cases however the growth rate of the face will slow to a relatively very low rate as the mosaics and their growth are integrated into a surface. A further consequence of this proposed behavior is that the latter growth could well favor the formation of voids in the growth sector that could, in solution growth, trap solvent or impurities, also a frequently observed phenomenon in the growth of the {011} faces of this material. This model accounts well for the (variable) growth behavior of imperfect {011} surfaces. The question that still remains is the nature and behavior of a perfect surface. The nearest that we can approach to this in the present study is the final integrated {011} surface achieved in the two experiments quoted above. Here the mosaic structure has to a great extent been eliminated. The residual intrinsic growth centers are weak, and the growth rates are low and virtually zero. This closely reflects the nature of the {011} surfaces of the self-nucleated crystals reported in our previous manuscript.12 There, at very low supersaturations (within what is often referred to as the kinetic “dead zone” since to low magnification assessment techniques there appears to be no measurable growth) the only growth features were steps of low height and hence low power on which were superimposed isolated hillocks and aligned ridge-like features. We speculate that in both cases, as the supersaturation is increased, it is not sufficiently depleted by the slow surface growth and rises rapidly to a point at which nucleation and growth of the secondary crystallites occur on the relatively perfect surface. These wellspaced, secondary nuclei develop independently for short periods to “cap-off” with (principally) {010}, {011}, and {111} faces, the ultimate form being {111} pyramids and [011] aligned ridges. These pyramidal and ridge features propagate slowly if at all. They do not meld together and probably give the {011} faces their characteristic macroscopically matte and rough appearance. To test this speculation, we have used atomic force microscopy (AFM) to assess the nature of growth and sublimation processes on crystals formed by self-nucleation at these lower supersaturations. This work will be reported in full elsewhere.25 For the present, we opine that the root cause of the differential in growth rate at the opposite poles of α-resorcinol seed crystals is an inhibition to nucleation at the positive polar surfaces. Such consequences as outlined above have their counterparts in other related systems. Studies of the potential of vapor growth for the preparation of large prismatic crystals of the acentric material methyl p-hydroxybenzoate (MHB)11 without the, then supposed, limiting effects of solvent adsorption, showed no continuous growth on one set of polar surfaces and normal growth on the other. The nongrowing surfaces always showed a matte appearance. Also, in interferometric studies of the growth kinetics of the acentric crystal N-(4-nitrophenyl)-prolinol) (NPP) in solution,23 the rough “non-propagating” surfaces initially showed no growth. At supersaturations well beyond that which supported growth on other normal faces, sporadic nucleation of weakly propagating hillocks commenced against a background of very slow growth of the underlying face. These processes mimic exactly the proposed current behavior. At the time of the NPP publication, this difference was ascribed to solvent influences. It
ARTICLE
now appears to have a more fundamental cause. More recently, exactly similar kinetic behavior to that noted for α-resorcinol12 has been reported for the growth of the acentric forms of lysozyme crystals from the solution phase.26 Additionally, recently published Monte Carlo calculations of Cuppens et al27 clearly show for crystals of the acentric form of L-aspartame that there is a structural rather than an impurity/solvent interaction basis for the differential in the growth rates of the opposite polar faces. What then restricts nucleation on the perfect {011} and {111} faces? Several possibilities present themselves: contamination by impurities, structural distortions of the surfaces, molecular effects at the surfaces, and the nature of emergent bulk defects at the surfaces. We believe that the first can be rejected. The material used was of high purity. Even though resorcinol would not be regarded as the most stable of materials, we do not believe that it would decompose to a significant extent in the solid state during the course of the above experiments. Neither do we accept the suggestions of Leiserowitz and Lahav13 that the impurity could be residual water. The water content of our material was independently assessed as
