Nucleation of Molecular Crystals beneath Guanidinium

Publication Date (Web): January 22, 1997 ...... IBM 486 personal computer and is available on the World Wide Web at http://www.cems.umn.edu/research/w...
0 downloads 0 Views 326KB Size
Download PDF
+

+

330

Langmuir 1997, 13, 330-337

Nucleation of Molecular Crystals beneath Guanidinium Alkanesulfonate Monolayers Lynn M. Frostman and Michael D. Ward* Department of Chemical Engineering and Materials Science, University of Minnesota, Amundson Hall, 421 Washington Avenue SE, Minneapolis, Minnesota 55455 Received July 22, 1996X A novel Langmuir monolayer based on 1-octadecanesulfonate amphiphiles spread over an aqueous subphase containing guanidinium cations is described. The molecular area of this monolayer deduced from pressure-area isotherms agrees with that calculated for two-dimensional pseudohexagonal hydrogenbonded networks found in numerous guanidinium alkane- and arenesulfonates. The compressibility behavior of the monolayer suggests puckering of the hydrogen-bonded network due to the applied pressure and/or the existence of stable domains of two-dimensional crystallites which coalesce on compression. When benzenesulfonic acid is present in the subphase, these monolayers induce the oriented nucleation of crystals of diphenyl sulfone, the decomposition product of benzenesulfonic acid. The observation of oriented nucleation and growth in the absence of a specific stereochemical match between the monolayer and the (100) face of diphenyl sulfone suggests that the nucleation process may involve coincident epitaxy between the two-dimensional monolayer and the (100) face, which serves to maximize dispersive interactions at the interface and lower the free energy of incipient nuclei.

Introduction Several investigations within the last decade have demonstrated that Langmuir monolayers strongly influence the nucleation and growth of molecular and inorganic crystals, dramatically affecting nucleation rates, polymorphic selectivity, crystal morphology, and orientation with respect to the interface.1-5 Most of the systems examined rely on the design of Langmuir monolayers which present to the aqueous subphase an ordered twodimensional interface that structurally mimics a particular crystal plane of the crystallizing substance. This process is viewed generally as stereochemical matching, that is, a specific molecular recognition between twodimensional arrays of Langmuir monolayer functional groups extending into the subphase and molecular motifs in incipient nuclei of the crystallizing phase. Alternatively, this can be viewed as nucleation driven by structural mimicry by the monolayer of a specific crystal plane of the incipient nuclei. For example, Langmuir monolayers containing amino acid functional groups at the subphase interface were found to instigate oriented nucleation of amino acids in a manner that suggested that the compressed solid-phase monolayer possessed a two-dimensional structure mimicking that of a low-index plane of the bulk amino acid crystal.2 However, recent reports indicate that the nucleation can be induced at Langmuir monolayers whose structure and composition differ from * To whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) (a) Gavish, M.; Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L. Science 1990, 250, 973-975. (b) Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1991, 113, 8943-8944. (2) (a) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 318, 353-356. (b) Landau, E. M.; Wolf, S. G.; Levanon, M.; Lahav, M.; Sagiv, J. J. Am. Chem. Soc. 1989, 111, 1436-1445. (3) (a) Jacquemain, D.; Wolf, S. G.; Leveiller, F.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. J. Am. Chem. Soc. 1990, 112, 7724-7736. (b) Landau, E. M.; Popovitz-Biro, R.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Mol. Cryst. Liq. Cryst. 1986, 134, 323-335. (4) (a) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9-20 and references therein. (b) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A. J. Phys. D: Appl. Phys. 1991, 24, 154-164. (c) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681-4686. (5) (a) Zhao, X. K.; Fendler, J. H. Chem. Mater. 1991, 3, 168-174. (b) Zhao, X. K.; Yang, J.; McCormick, L. D.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933-9939.

those of the crystallizing substance. These observations may involve epitaxy driven by Coulombic interactions between ordered arrays of ions in the monolayers and ions in the incipient nuclei.5b Previous work in our laboratory also has demonstrated that organosulfur and organosilane monolayers immobilized on solid substrates influence nucleation, orientation, and polymorph selectivity of molecular crystals by chemical recognition (hydrogen bonding) between monolayer functional groups and complementary functionalities of incipient nuclei.6,7 In all of these systems, favorable interactions between the monolayer surface and individual molecules of the crystallizing phase facilitate the formation of incipient nuclei by lowering the surface free energy. Recently, we reported a series of guanidinium alkaneand arenesulfonates having the general formula [C(NH2)3]+[RSO3]- which share the common structural feature of a robust two-dimensional, quasihexagonal hydrogen bonding network (Figure 1).8,9 The robustness of these networks was attributed to the topological similarity of the guanidinium cations and sulfonate anions and the satisfaction of hydrogen bonding between all potential donors and acceptors. These salts crystallized into two different motifs with respect to stacking of the hydrogen-bonded sheets. In the case of small R groups bilayer motifs were observed in which the R groups interdigitate within the bilayers. “Single-layer” motifs were observed for large R groups in which the orientation of the R groups alternates between adjacent “ribbons” of guanidinium cations and sulfonate anions, with the sheets assembling in the third dimension by interdigitation of these motifs. In some of these salts, puckering of the nominally two-dimensional sheets about the one-dimensional axis joining these ribbons accommodated the steric requirements of the R groups. This behavior indicated that the hydrogen bonds joining the ribbons were relatively flexible. (6) Frostman, L. M.; Bader, M. M.; Ward, M. D. Langmuir 1994, 10, 576-582. (7) Carter, P. W.; Ward, M. D. J. Am. Chem. Soc. 1993, 115, 1152111535. (8) Russell, V. A.; Etter, M. C.; Ward, M. D. Chem. Mater. 1994, 6, 1206-1217. (9) Russell, V. A.; Etter, M. C.; Ward, M. D. J. Am. Chem. Soc. 1994, 116, 1941-1952.

+

Nucleation of Molecular Crystals

+

Langmuir, Vol. 13, No. 2, 1997 331

Figure 1. (a) Schematic representation of the two-dimensional hydrogen-bonding network present in guanidinium alkane- and arenesulfonate salts. The network can be described as one-dimensional ribbons, highlighted in bold, held together by hydrogen bonding between residual sulfonate oxygen lone pairs and guanidinium protons. Extensive single-crystal X-ray diffraction studies have revealed that the steric requirements of the R groups in many salts induce puckering of the sheetlike network about a “hydrogen-bonding hinge” formed by these residual hydrogen bonds. (b) Bilayer motif observed in guanidinium sulfonate salts for sterically undemanding R groups. (c) Single-layer motif observed in guanidinium sulfonate salts for sterically demanding R groups.

The robustness of the guanidinium sulfonate networks in which the alkane groups extend from the twodimensional network prompted us to examine Langmuir monolayers based on alkanesulfonates spread over an aqueous subphase containing guanidinium cations and to investigate the influence of these monolayers on nucleation of guanidinium alkane- and arenesulfonate salts. We anticipated that the planar two-dimensional structure of the monolayers might instigate the nucleation and growth of guanidinium sulfonates beneath the monolayer, with selectivity directed toward salts with the bilayer structure rather than the single-layer motifs. It also seemed feasible that selectivity toward specific guanidinium sulfonates could be directed by tuning the structure of the monolayer through adjustment of the surface pressure in the Langmuir trough. We report herein a Langmuir monolayer (GUAN-ODS), based on 1-octadecanesulfonate amphiphiles that assemble with subphase guanidinium ions, which exhibits pressure isotherm features consistent with the aforementioned two-dimensional hydrogen bonding network. Our studies were aimed at examining the influence of these monolayers on the nucleation of guanidinium alkane- and arenesulfonates (by recognition of the monolayer by the hydrogen-bonded guanidinium sulfonate networks in incipient nuclei). During studies of the nucleation of guanidinium benzenesulfonate beneath GUAN-ODS monolayers, which forms the bilayer motif in the crystalline state with the hydrogen-bonding network in the (010) plane, we discovered unexpectedly that these monolayers instigate the nucleation of oriented crystals of diphenyl sulfone (DPS) that form by decomposition of benzensulfonic acid in the subphase. An analysis of the crystallization interface suggests that nucleation does not involve specific stereochemical molecular recognition between functional groups or structural mimicry. Rather, the formation of DPS is driven by coincident epitaxy and intermolecular interactions at the nucleation interface.

Experimental Section Materials. Guanidine hydrochloride (99%, Aldrich), benzenesulfonic acid (90%, Aldrich), sodium 1-octadecanesulfonate (pure by IR and NMR, Lancaster), stearic acid (99+%, Aldrich), octadecaneamine (98%, Aldrich), diphenyl sulfone (97%, Aldrich), sodium sulfate (Merck), chloroform (99.9% with 0.8% C2H5OH preservative, Fisher), dichloroacetic acid (99+%, Aldrich), hexanes (99+%, Fisher), and hexadecane (99%, Sigma) were used as received. Water was deionized to 18 MΩ with a Barnstead E-Pure purification system. Isotherm Measurements. Surface pressure isotherms were measured in a laminar flow hood using a temperature-controlled KSV 5000 Langmuir trough (KSV Instruments LTD, Finland) equipped with a filter paper Wilhelmy balance. The trough was cleaned with ethanol and hexane and rinsed with deionized water prior to each experiment. The trough was then filled with the subphase solution. The surface of the solution was cleaned several times with an aspirator to remove any dust. An isotherm was taken of the bare surface to ensure the absence of surface active materials. The barrier was then reset and the spreading solution spread dropwise. Spreading solutions were prepared fresh prior to each experiment by dissolving the amphiphiles in a mixture of chloroform and dichloroacetic acid (49:1, v/v) to a concentration of 2.75 × 10-4 M (the addition of dichloroacetic acid was necessary to dissolve the sodium 1-octadecanesulfonate10). The spreading solution was allowed to evaporate and equilibrate for 15 min at 25 ( 1 °C before compression was initiated. Compressions typically were performed at a rate of 2 Å2/(molecule min), which was slow enough to ensure that isotherms were independent of compression rate. Crystallization. Crystallizations were performed in a custommade apparatus comprising a Teflon block with 25 circular Langmuir troughs, each 2.54 cm in diameter and 0.95 cm deep (Figure 2). The volume of each trough was approximately 5 mL. The Teflon block was embedded in a copper block. The temperature of the trough was controlled by circulating cooling water from a programmable circulating chiller through the copper block; this resulted in a 38 Å2/molecule), indicating that the monolayers were very robust under these conditions. Isotherms acquired by compression with a Teflon barrier were identical, for mean molecular areas >40 Å2/molecule, to isotherms constructed by adding sufficient monolayer spreading solution to a (11) Adam, N. K. The Physics and Chemistry of Surfaces; Oxford University Press: London, 1938; p 93. (12) CRC Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC Press, Inc.: Boca Raton, FL, 1981. (13) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966; and references therein.

+

Nucleation of Molecular Crystals

trough with a fixed barrier to give the desired surface concentration (Figure 3). This indicated that the monolayers formed by these two techniques were identical. The surface pressure at the collapse point is smaller than that observed when sodium sulfate or benzenesulfonic acid is present in the subphase. The monolayer is unstable in the plateau region, with the surface pressure decreasing slowly when compression was stopped. The observation of the plateau and the instability in this regime is consistent with solubilization of the amphiphile upon compression or with the buckling of the monolayer into a interdigitated bilayer or trilayer due to increasing surface pressure. Evidence for the latter has been observed recently in grazing angle incidence X-ray diffraction studies of dual-component Langmuir monolayers.14 The isotherm of stearic acid acquired with guanidinium sulfate in the subphase was identical to that obtained over deionized water, with limiting areas of approximately 20 Å2, indicating that guanidinium ions have little influence on the structure of stearic acid monolayers. Compression isotherms of octadecaneamine over guanidine sulfate solutions exhibited slowly changing surface pressure, with their steepest portion extrapolating to 4044 Å2/molecule, followed by a collapse at 25 Å2/molecule and a plateau region. This rather large molecular area suggests that the octadecaneamine monolayer is protonated and positively charged,13 which would allow anions from the subphase to incorporate into the monolayer. This is consistent with the pKa of octadecaneamine in solution (10.6)12 and in monolayers (10.1).15 The broad compression region observed for the GUANODS monolayer, rather than a steep rise near the solidphase region typically observed for conventional surfactant monolayers, can be attributed to the unique properties of the guanidinium sulfonate network. The slope of the isotherm in the region where the molecular areas exceed that of the collapse point is not very steep, making estimation of the limiting area/molecule difficult. However, extrapolation suggests an upper limit of 50 Å2/ molecule, while the collapse point suggests a lower limit of 40 Å2/molecule. The molecular areas observed in crystalline guanidinium sulfonate salts, based on a formula unit of one guanidinium ion and one sulfonate ion, range from an upper limit of 47 Å2 to a lower limit of 30 Å2. This range reflects a progression from networks which are rigorously planar to those which are highly puckered (Figure 1). The puckering is a consequence of the steric requirements of the intralayer R groups, which provide an internal “lattice pressure” that leads to puckering of the guanidinium sulfonate ribbons about a single hydrogen bonding axis. The degree of puckering reflects the balance between these forces and the energetic penalty associated with bending the hydrogen bonds of the network from their optimal linear arrangement. However, the latter is not expected to be energetically demanding given the large range of donor‚‚‚H‚‚‚acceptor bond angles observed in crystalline hydrogen-bonded solids.16 Consequently, it is reasonable to suggest that the application of external pressure with the Langmuir barrier can cause similar puckering of the two-dimensional hydrogen-bonded sheet at the air/water interface. An alternate, but not necessarily mutually exclusive, explanation for the broad isotherm is that robust twodimensional GUAN-ODS domains, instead of a disordered liquid phase, exist at low surface pressures. Misfit (14) Lahav, M.; et al. Science, in press. (15) Betts, J. J.; Pethica, B. A. Trans. Faraday Soc. 1956, 52, 15811589. (16) (a) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063. (b) Bernstein, J.; Etter, M. C.; Leiserowitz, L. In Structure Correlation; Burgi, H.-B., Dunitz, J. D., Eds.; VCH: New York, 1994; Vol. 2, p 431.

+

Langmuir, Vol. 13, No. 2, 1997 333

between domains can lead to apparently larger molecular areas, which diminish upon coalescence of these domains upon compression into a solid-like monolayer. The nucleation behavior observed at the GUAN-ODS monolayer interface is consistent with the presence of such domains (vide infra). The absence of hysteresis in the compression isotherms between 40 and 60 Å2/molecule argues that coalescence of these domains into a single solid two-dimensional phase, and their disassembly into isolated domains during decompression, must be fast on the time scale of these measurements. There is growing evidence in the literature that uncompressed monolayers can exhibit coherence lengths over several hundred angstroms when examined with grazing incidence X-ray diffraction (GIXD) on a synchrotron line.17 GIXD experiments have also suggested an equilibrium between ordered and disordered phases at low surface pressures for fatty acids and their salts at the air/water interface.18 TEM analysis of Langmuir monolayers transferred to grids at a variety of surface pressures indicates that twodimensional crystallites are formed by many amphiphiles immediately upon spreading, provided the spreading temperature is below the monolayer melting temperature. These studies suggest that features observed in the compression isotherms correspond to a “gathering” of these crystallites, with fusion and recrystallization, rather than phase transitions, as has been traditionally assumed.19 If this interpretation is correct, the shape of the isotherm should give an indication of the crystallite size. At the one extreme, if the film exists as a single crystallite, the slope of the isotherm should not change on compression until the limiting area of the molecules on the surface is reached. At this point, the surface pressure should increase sharply upon further reduction of surface area. In contrast, if the film exists as numerous small crystallites, a rise in surface pressure should be detected sooner upon compression, as the crystallites will begin interacting with each other at larger mean molecular area, producing a gradually sloping isotherm. However, this is indistinguishable from compression-induced puckering of the twodimensional hydrogen-bonded sheet. Crystallization. The influence of the GUAN-ODS monolayer on the nucleation of bulk guanidinium sulfonate crystals was examined by spreading the sodium 1-octadecanesulfonate on the appropriate subphases, confined in fixed area Langmuir wells. The average molecular area of the surfactant was controlled by spreading the volume of surfactant solution necessary to give the desired surface concentration. The surface pressure was deduced from the compression isotherms described above. All monolayer solutions spread efficiently at low surface concentrations. However, at higher coverages (smaller mean molecular areas), lensing and migration of solvent to the trough edges (17) (a) Weinbach, S. P.; Kjaer, K.; Bouwman, W. G.; Grubel, G.; Legrand, J.-F.; Als-Nielson, J.; Lahav, M.; Leiserowitz, L. Science 1994, 264, 1566-1570. (b) Leveiller, F.; Jacquemain, D.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Alsnielsen, J. Science 1991, 252, 15321536. (c) Jacquemain, D.; Leveiller, F.; Weinbach, S. P.; Lahav, M.; Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J. J. Am. Chem. Soc. 1991, 113, 7684-7691. (d) Jacquemain, D.; Wolf, S. G.; Leveiller, F.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Alsnielsen, J. J. Am. Chem. Soc. 1990, 112, 7724-7736. (18) (a) Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulos, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58, 2228-2231. (b) Kjaer, K.; Als-Nielson, J.; Helm, C. A.; Tippman-Krayer, P.; Mohwald, H. J. Phys. Chem. 1989, 93, 3200-3206. (19) (a) Uyeda, N.; Takenaka, T.; Aoyama, K.; Matsumoto, M.; Fujiyoshi, Y. Nature 1987, 327, 319-321. (b) Kajiyama, T.; Umemura, K.; Uchida, M.; Oishi, Y.; Takei, R. Bull. Chem. Soc. Jpn. 1989, 62, 3004-3006. (c) Kajiyama, T.; Tanimoto, Y.; Uchida, M.; Oishi, Y.; Takei, R. Chem. Lett. 1989, 189-192. (d) Uchida, M.; Tanizaki, T.; Oda, T.; Kajiyama, T. Macromolecules 1991, 24, 3238-3243. (e) Kajiyama, T.; Oishi, Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H. Langmuir 1992, 8, 1563-1569. (f) Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, Y.; Takahara, A. Langmuir 1993, 9, 760-765.

+

334

+

Langmuir, Vol. 13, No. 2, 1997

Frostman and Ward

Figure 4. Optical micrographs of diphenyl sulfone crystals grown from guanidinium benzenesulfonate solution under GUAN-ODS monolayers spread at different mean molecular areas: (A) 20 Å2; (B) 30 Å2; (C) 40 Å2; (D) 50 Å2. The molecular areas are based on the formula unit [guanidinium]+[1-octanedecanesulfonate]-, assuming a planar quasihexagonal lattice.

were evident, suggesting uneven spreading. This is consistent with collapse of the monolayer at small mean molecular areas, which is evident from the surface pressure isotherms. When the subphase contained benzensulfonic acid and the molecular area of the GUAN-ODS monolayer was greater than 40 Å2, large, flat, diamond-shaped crystals (exceeding several millimeters on a side) were observed (Figure 4). These crystals were extremely thin, with thicknesses < 25 µm. Infrared and X-ray diffraction revealed that these crystals were diphenyl sulfone single crystals oriented with their (100) faces parallel to the monolayer interface.20 The (100) face is terminated with close-packed benzene residues, suggesting a relatively low surface energy (Figure 5). The rhombus shape of the crystals is attributed to the presence of low-energy {011} or {111} faces on the edges, as deduced by inspection of the crystal structure and morphology calculations.21 The nucleation characteristics under the GUAN-ODS monolayers were indistinguishable for molecular areas in the range 40-100 Å2/molecule, which suggested that the (20) Diphenyl sulfone crystallizes in the P21/c space group, with a ) 12.225 Å, b ) 7.830 Å, c ) 11.328 Å, β ) 98.32°. Cambridge Crystallographic Database refcode: DPSULO. Sime, J. G.; Woodhous, D. I. J. Cryst. Mol. Struct. 1974, 4, 269. (21) BFDH morphology calculations, performed with the molecular modeling program Cerius2, suggest that the morphology of DPS is described by an eight-sided plate, with the plate face assigned to (100). Plate edges are formed by the {011}, {111}, {002}, {110}, and {102} families of planes, but the {002}, {110}, and {002} planes are not observed experimentally.

existence and the structure of the monolayer responsible for nucleation was independent of surface area. In contrast, nucleation was neglible at molecular areas below the collapse point of 40 Å2/molecule and the crystals that were observed were small and randomly oriented. The unique role of the monolayer was evident from several control experiments performed in parallel with crystallization under the GUAN-ODS monolayers. Crystallization was not observed under octadecaneamine monolayers nor on clean surfaces, i.e., where no monolayer had been spread. A moderate amount of nucleation was observed under stearic acid monolayers, but the crystals were much smaller and were randomly oriented. The origin of diphenyl sulfone stems from the decomposition of benzensulfonic acid (or the sulfonate) in water, which is thermodynamically favorable. One possible mechanism involves ipso protonation and elimination of SO3 from benzenesulfonate, the formation of a sulfate sulfone ester, and electrophilic substitution.22 We note that randomly oriented crystals of diphenyl sulfone were observed at the air/water interface when pure (surfactantfree) spreading solutions were applied, which can be attributed to supercooling of the air/water interface associated with rapid evaporation in the absence of (22) (a) Moreno, A.; Bravo, J.; Berna, J. L. J. Am. Oil Chem. Soc. 1988, 65, 1000-1006. (b) Gilbert, E. E. Sulfonation and Related Reactions; Olah, G. A., Ed.; Interscience Publishers: New York, 1965. (c) Cerfontain, H. In Mechanistic Aspect in Aromatic Sulfonation and Desulfonation; Olah, G. A., Ed.; Interscience Publishers: New York, 1968.

+

Nucleation of Molecular Crystals

+

Langmuir, Vol. 13, No. 2, 1997 335

Figure 5. Crystal structure of diphenyl sulfone illustrating the bilayer structure viewed (a) normal to the (100) plane and (b) parallel to the (100) plane. Only the C6H5SO2 residues at the surface of the (100) plane are depicted in part b for reasons of clarity. The area per molecule in the (100) plane is 44 Å2.

surfactant.23 This argues against the GUAN-ODS monolayer having a catalytic role in DPS formation, as has been reported for other reactions at Langmuir monolayers.24 It is more reasonable to suggest that the GUANODS monolayer is facilitating the nucleation of diphenyl sulfone from an otherwise undersaturated solution, removing diphenyl sulfone from solution by crystallization and shifting the equilibrium further toward its formation. The traditional interpretation of the guanidinium sulfonate monolayer isotherm would describe the monolayer as an “expanded liquid” in the region of gradually rising surface pressure and a two-dimensional “gas” in the zero pressure region.13 Both of these regions would be considered fluid-like, with a homogeneous distribution of amphiphiles and no long-range ordering. However, the observation of nucleation up to molecular areas of 100 Å2/molecule strongly supports either nucleation on liquidlike layers or the presence of discrete, two-dimensional monolayer-like domains of GUAN-ODS that are capable of nucleating the bulk crystals. Enhanced nucleation under “fluid-like” monolayers in which the “state” of the (23) Control studies using spreading solutions without surfactants (chloroform, chloroform with dichloroacetic acid, and hexanes) resulted in fast nucleation at the interface, producing a powder that typically lacked well-defined crystals and often appeared as an amorphous film. In contrast, spreading of hexadecane, which is significantly less volatile, inhibits nucleation at the interface. We ascribe the volatile-solventinduced nucleation to concentrated interfacial cooling effects: lensing of solvent due to the absence of surfactant, coupled with fast evaporation, results in a lowering of the temperature at the interface, thus leading to rapid supersaturation and nucleation. Nucleation under ODS and stearic acid monolayers, while relatively fast, is typically slower than nucleation under the surfactant-free volatile solvents, most likely because the surfactants slow the solvent evaporation, allowing more time for the interfacial cooling to be dissipated. (24) Monolayers have been previously shown to catalyze interfacial reactions: (a) Ahuja, R.; Caruso, P.-L.; Mobius, D.; Paulus, W.; Ringsdorf, H.; Wildburg, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 1033-1036. (b) Bohanon, T. M.; Denzinger, S.; Fink, R.; Paulus, W.; Ringsdorf, H.; Weck, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 58-60.

monolayer had little or no effect on nucleating properties of the monolayer has been reported previously. Zhao et al.5b reported that arachidic acid (C19COOH) monolayers at zero surface pressure and 40 Å2/molecule induced nucleation of PbS crystals which were oriented with their [111] axis normal to the interface. It was also noted that, at zero surface pressure, the arachidic acid molecules appeared to aggregate into disconnected domains which served as the sites for oriented nucleation. Mann et al.4b reported that “liquid-phase” (15-20 mN/m) and “solidphase” (35-45 mN/m) octadecaneamine monolayers both induced oriented CaCO3 nucleation. Nucleation densities of calcite were higher under “solid-phase” stearate monolayers than under “liquid-phase” monolayers, but crystals of vaterite and calcite were slightly larger and more uniform in size under “liquid” monolayers. This behavior was attributed to the increased mobility of the negatively charged stearate molecules in the “liquid” monolayer that enabled the monolayer to adapt to the structural requirements of the incipient nucleus, thereby lowering its surface free energy. Similar behavior may be responsible for the observed nucleating influence of the GUAN-ODS monolayer; that is, liquid domains of GUAN-ODS may assemble into a motif that is capable of favorable interaction with an incipient nucleus. The enhancement of nucleation and orientation of diphenyl sulfone by the GUAN-ODS monolayer can thus be viewed as a cooperative phenomenon in which the contacting planes of the monolayer and the incipient nucleus adapt to form a commensurate interface. Similar cooperative effects have recently been observed during calcite growth on polymerized polydiacetylene films.25 Alternatively, the nucleation at low surface pressures may be due to the presence of ordered crystalline two-dimensional islands of GUAN-ODS. (25) Berman, A.; Ahn, D. J.; Lio, A.; Salmeron, M.; Reichert, A.; Charych, D. Science 1995, 269, 515-518.

+

336

+

Langmuir, Vol. 13, No. 2, 1997

Regardless of whether nucleation occurs on monolayerlike islands of GUAN-ODS or by a process in which a two-dimensional GUAN-ODS plane and the (100) DPS plane assemble cooperatively at the interface, the absence of stereochemical similarity and structural mimicry between the (100) plane of diphenyl sulfone and the putative GUAN-ODS structure indicates that specific molecular recognition and structural mimicry of the DPS (100) plane is not responsible for the enhancement of nucleation and orientation. This suggests that the free energy lowering of incipient nuclei of diphenyl sulfone is provided by an epitaxial match between the GUAN-ODS monolayer and DPS (100) planes at the nucleation interface, with van der Waals interactions primarily responsible (however, longer range ion-dipole and dipoledipole interactions cannot be ruled out). It is interesting to consider a nucleation process which is governed by epitaxy of the DPS (100) face with a GUANODS layer that is compressible but retains the planar conformation and quasihexagonal symmetry. The planar conformation would be expected to maximize dispersive interactions between the monolayer and the (100) DPS plane of the incipient nucleus, thereby reducing the activation energy for nucleation. Dispersive interactions involving the DPS (100) plane are expected to be favorable, as this plane consists of densely packed phenyl groups whose molecular planes are parallel to (100). The difference in lattice constants and symmetry between the DPS (100) plane (b ) 7.830 Å, c ) 11.328 Å, β ) 98.32°) and an undistorted quasihexagonal GUAN-ODS network argues against a strictly commensurate epitaxy between these two planes, as commensurism would require all the lattice positions of the (100) DPS plane to sit on lattice positions of the quasihexagonal GUAN-ODS monolayer. Rather, it is more likely that the epitaxy involves coincident lattices, which can be defined as an epitaxy in which lattice rows in the contacting plane of the DPS nucleus repeat along a specific reciprocal lattice vector of the GUAN-ODS monolayer. Coincidence is favored by systems having strong interactions between molecules within the respective planes that form the interface and weak dispersive interactions between the opposing planes, as these conditions will tend to disfavor reconstruction that could lead to commensurism. Possible coincident lattices were examined using an algorithm developed in our laboratory in which the misfit between candidate lattices was calculated on the basis of a rigid DPS (100) plane and an elastic quasihexagonal GUAN-ODS monolayer lattice with lattice parameters in the range of those observed for bilayer-type crystalline guanidinium sulfonates.26 Most of the guanidinium sulfonate salts have monoclinic symmetry owing to symmetry lowering by the organic residues contained in the bilayer region. This results in a rigorously rectangular lattice in the twodimensional hydrogen-bonding plane, although the planar guanindium sulfonate networks themselves have hexagonal, or nearly hexagonal, symmetry. For example, the hydrogen-bonding network in guanidinium methane(26) Analyses of the epitaxial interface were performed with a program written in our laboratory (EpiCalc), which runs in the Windows v. 3.1 environment. EpiCalc is based on an analytical algorithm which calculates the misfit between two opposing lattices for different azimuthal angles, typically in increments of 0.1°. The program runs on an IBM 486 personal computer and is available on the World Wide Web at http://www.cems.umn.edu/research/ward. In this study, the program calculated the misfit between the (100) DPS plane and trial lattices of guanidinium sulfonate that were generated automatically by adjusting the hexagonal lattice constant of the guanidinium sulfonate plane within a defined range of lattice constants, based on crystallographic data obtained in our laboratories for over 30 guanidinium sulfonate salts. The program was designed to search and find the lattice constants for which coincidence exists, where the misfit tends toward a minimum. Details of this procedure are reported elsewhere: Hillier, A. C.; Ward, M. D. Phys. Rev. B, in press.

Frostman and Ward

sulfonate is crystallographically described by a rectangular lattice with lattice constants of a ) 12.778 Å and b ) 7.342 Å, but the hydrogen-bonding network itself can be considered as hexagonal with a lattice constant of a ) 7.342 Å. Trial lattices, with molecular areas ranging from 40 to 50 Å2 per guanidinium sulfonate pair, were generated, and their fit to the DPS (100) plane was calculated for all azimuthal rotation angles to search for coincident fits. These calculations revealed coincidence for a GUANODS lattice with threefold symmetry and a lattice constant of 7.44 Å (this lattice can also be expressed as a rectangular lattice with dimensions of 7.44 Å and 12.89 Å, which is near the mean of the lattice constants observed for alkaneand arenesulfonates). Coincidence was achieved when the [010] vector of DPS was rotated by an angle of θ ) 4.66° with respect to the a1 lattice vector of the GUANODS monolayer, the a1 lattice vector being defined as the ribbon direction in the guanidinium sulfonate layer (Figure 6). This can be expressed mathematically by indexing the DPS lattice vectors b1 and b2 (which are equivalent to [010] and [001], respectively) to the GUAN-ODS lattice vectors a1 and a2, which subtend an angle of 120°. These relationships are b1 ) 1.00a1 + 0.10a2 and b2 ) -1.00a1 + 1.75a2, in which the coefficients are elements of a matrix that defines the azimuthal orientation of the DPS (100) plane with respect to the GUAN-ODS lattice.27 The calculated orientation allows the [011] direction of the DPS (100) plane to be coincident with the a2 direction of the monolayer. That is, the [011] lattice rows of DPS repeat at regular intervals of reciprocal lattice constant a2* or, equivalently, the reciprocal lattice constants [011]* and a2* are coincident, in this case having identical magnitude and direction. The coincident epitaxy allows attractive interactions between the monolayer and the incipient nuclei to accumulate over the length scale of a critical aggregate size. This lowers the free energy of the incipient nucleus in the (100) orientation. It should be reiterated that presently there is no experimental evidence for this azimuthal orientation, but the determination that coincidence exists for the DPS (100) plane on a GUANODS lattice with reasonable lattice constants supports a nucleation mechanism driven by coincident epitaxy. Conclusions Monolayers prepared from 1-octadecasulfonate on subphases containing guanidinium ions exhibit compression isotherms that are consistent with both the anionic sulfonate head groups and the guanidinium cations lying in the plane of the air/water interface. The results support a hydrogen-bonded network that is similar to those observed in guanidinium sulfonate salts, which have been shown by single-crystal X-ray diffraction to possess (27) The latttice constants b1 and b2 are indexed to the GUAN-ODS lattice constants a1 and a2 according to the matrix relationship

[][

m11 b1 ) b2 m21

m12 m22

][ ] a1 a2

The elements in the matrix can be described in terms of the azimuthal orientation by

m11 ) b1 sin(R - θ)/a1 sin(R) m22 ) b2 sin(θ - β)/a2 sin(R) m12 ) b1 sin(θ)/a2 sin(R) m21 ) b2 sin(R - θ - β)/a1 sin(R) where R is the angle subtended by the a1 and a2 vectors (R ) 60° for the GUAN-ODS monolayer lattice), β is the angle subtended by the b1 and b2 vectors (β ) 90° for the (100) plane of DPS), and θ is the angle subtended by a1 and b1, which defines the azimuthal orientation of the DPS (100) plane and the GUAN-ODS monolayer lattice.

+

+

Nucleation of Molecular Crystals

Langmuir, Vol. 13, No. 2, 1997 337

Figure 6. Schematic representation of coincident epitaxy between a DPS (100) plane (filled atoms) and a guanidinium sulfonate monolayer lattice (unfilled atoms) with a hexagonal lattice constant of a1 ) 7.74 Å. Only the C6H5 residues at the surface of the DPS (100) plane are illustrated here for reasons of clarity. The azimuthal orientation at coincidence is θ ) 4.66°, which is the angle subtended by a1 and the [011] direction (defined here as b1) in the DPS (100) plane. The equivalent hexagonal and rectangular units cells for the guanidinium sulfonate layer, with lattice constants of a1 ) 7.74 Å and a1 ) 7.44 Å, a2′ ) 12.89 Å, respectively, are both depicted. The sulfonate ions are contained with the open circles. This azimuthal orientation allows the [011] rows to be coincident with a2, which is equivalent to stating that the reciprocal lattice vectors [011]* and a2* are coincident and have equal magnitude and direction. The straight lines are intended to be a visual aid to illustrate the coincidence.

quasihexagonal two-dimensional hydrogen-bonded sheets composed of sulfonate anions and guanidinium cations. The compression isotherms are rather broad, suggesting puckering of the hydrogen-bonded monolayer and/or aggregation of monolayer-like domains with increasing pressure. The contributions of these structures to the compression behavior, as well as the possible existence of interdigitated bilayers and trilayers in the highly compressed regime, needs to be clarified further by structural characterization such as grazing angle incidence X-ray diffraction. The monolayers induce the nucleation of oriented diphenyl sulfone crystals from aqueous solutions even though molecular-level stereochemical recognition between the (100) face of incipient diphenyl sulfone nuclei and the monolayer is not possible. This argues for a mechanism in which the free energy of the incipient nuclei is lowered by an epitaxial match based on coincidence between the diphenyl sulfone (100) plane and a “flexible monolayer”. Furthermore, the observation of nucleation

enhancement at low surface pressures suggests the presence of discrete, two-dimensional domains capable of stabilizing incipient nuclei. These results suggest that nucleation at the air/water interface need not require precise stereochemical recognition, thereby expanding the types of monolayers that can be used for this purpose. Acknowledgment. The authors acknowledge Andrew Hillier and Christopher Yip for assistance with calculations, Victoria Russell for helpful discussions, M. Lahav for providing unpublished results, and Professor J. Doyle Britton (University of Minnesota) for helpful discussions. The authors gratefully acknowledge the support of the National Science Foundation (Grant NSF-DMR-9107179). L.M.F. also acknowledges the support of a University of Minnesota Doctoral Dissertation Fellowship during 1993 and 1994. LA960725+