Langmuir 1995,11, 2187-2194
2187
Hexadecanol Microstructures of Crystallites in Aqueous Dispersions and of Langmuir-Blodgett Monolayers Sun Young Park and Elias Frames* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283 Received October 21, 1994. I n Final Form: March 10, 1995@ The effects of the dispersion preparation protocol on the microstructures of n-hexadecanol crystallites were examined. Dispersions of hexadecanol were prepared in saline at 25 "Cwith different thermal and mechanicalagitationhistories. The structures ofthe dispersed crystalliteswere probed by Fourier transform infrared spectroscopy(FTIR)and powder X-ray diffractometryon more concentratedsystems. Dry crystals, crystals previouslymelted without water present, and crystals simplymixed with saline had one crystalline structure (/?2), which is the stable structure for solid hexadecanol. By contrast, crystalswhich were previously melted in saline showed a second coexistingcrystalline structure (aor PI), which apparently contains some water of hydration, as indicated by the relative infrared absorbance of its OH bands. The structures of Langmuir-Blodgett (LB) monolayers at various surface pressures (10-50 mN1m) were also probed by FTIR. The monolayers, which were in the solid monolayer state, had a different (probably hexagonal) microstructure and a higher molecular mobility than the three-dimensional crystallites. Moreover, the LB monolayer surface density was quite independent of the surface pressure, as expected from the steepness of the surface pressure-surface area monolayer isotherm. The average molecular orientations of the hydrocarbon chains and dipole moments were calculatedfrom the dichroic ratios with a theory for ultrathin films. The hydrocarbon chains were oriented uniaxially, and the average chain tilt angle from the surface normal was -35". The results on the effects of dispersion preparation protocol on microstructures of crystallites can be used in further understanding the dynamic surface tension behavior of these dispersions (Park, et al. Langmuir 1993, 9, 3640).
1. Introduction
For hexadecanol dispersions as a model system, the effects of preparation protocol on the particle size, the surface Since the solubility of many surface active materials is area of the particle-water interface, and the rate of quite low in water, they are used in the form of dispersions dissolution of the dispersed particles have been elucidated. in many appli~ations.l-~One important application of Hexadecanol was chosen because it is chemically stable, dispersions is in the area of lung surfactants, which unlike the phospholipids. Moreover, it is one of the key regulate the dynamic surface tensions a t the alveolar ingredients of Exosurf, which is one of the two commercial ~ u r f a c e .The ~ lung surfactant is a complex mixture of lung surfactant replacement d r u g ~ . ~ - l Hexadecanol l phospholipids and proteins such as dipalmitoylphosphatidispersions were prepared either by mixing crystallites dylcholine (DPPC)and surfactant proteins (SP-Band SPas received (protocol l), or by melting the dispersed C), which have very low solubility in water.5 The lack of crystals, breaking up the emulsion droplets by stirring to lung surfactants causes the respiratory distress syndrome smaller droplets and recooling the suspension (protocol (RDS). There are two FDA-approved lung surfactant 2). The time scales for the surface tension equilibration replacement drugs in the market. Both drugs are differed signifi~antly.~J The tension amplitudes and the administered to patients as dispersions. Despite their minimum tensions under pulsating area conditions were widespread presence and importance, the tension behavior much lower for protocol 2 dispersions, which have much of surfactant dispersions has received little attention.6-8 smaller particle sizes. The results generally showed that Dispersions of DPPC and its mixtures with other the smaller the particle size, the faster the dissolution, phospholipids were prepared by various mixinglsonication and thus the faster the adsorption at the airlwater protocols a t different temperatures.6,8 The adsorption interface. We report here results of Fourier transform behavior of dispersions varied significantly with the infrared spectroscopy (FTIR) and X-ray diffraction (XRD) preparation protocol. Dispersions of n-hexadecanol also used to probe the particles microstructure at the different showed a strong dependence on the preparation p r o t o ~ o l . ~ , ~ protocols of preparation, since crystals of long chain alcohols are known to show p o l y m o r p h i ~ m . ' ~ - ~ ~ * Author to whom correspondence should be addressed. Phone, For solid hexadecanol, three different polymorphic (317)494-4078; F a ,(317)494-0805. structures were determined from XRD and thermal Abstract published in Advance A C S Abstracts, May 15,1995. analysis,16 infrared s p e c t r ~ s c o p yand , ~ ~ dielectric mea(1)Gaines, G. L. Insoluble Monolayers at Liquid-Gas interfaces surements.18 Following the notation of Chapman,17 the Wiley: New York, 1966;Chapters 4 and 9. structures will be referred to as a, PI, and P 2 forms. At (2)Ross, S.; Morrison, I. D. Colloidal Systems and Interfaces; Wiley: New York, 1988;Chapter 4. temperatures a few degrees (5-10 "C) below the melting (3)Evans, D. F.; Wennerstrtim, H. The Colloidal Domain: Where point (49 "C), hexadecanol exists in the a polymorphic Physics, Chemistry, Biology, and Technology Meet; VCH Publishers: @
New York, 1994;Chapters 1 and 10. (4) Shapiro, D. L. Surfactant Replacement Therapy; Shapiro, D. L.; Notter, R. H.,Eds.; Alan R. Liss: New York, 1989;Chapter 1. ( 5 ) Notter, R. H. Surfactant Replacement Therapy; Shapiro, D. L.; Notter, R. H., Eds.; Alan R. Liss: New York, 1989;Chapter 2. (6)Notter, R. H.; Smith, S.; Taubold, R. D.; Finkelstein,J. N. Pediatr. Res. 1982,16,515. (7)Park, S. Y.;Chang, C.-H.; Ahn, D. J.; Franses, E. I. Langmuir 1993,9, 3640. ( 8 ) Park, S. Y.; Peck, S. C.; Chang, C.-H.; Franses, E. I. Dynamic Properties of Interfaces and Association Structures; Shah, D., Ed.; American Oil Chemists Society Press, 1995,accepted for publication.
(9)Durand, D..J.; Clayman, R. I.; Heymann, M. A,; Clements, J. A. J. Pediatrics, 1988,107,775. (10)Clements, J . A. United States Patent No. 4,312,860,1982. (11)Clements, J. A.United States Patent No. 4,826,821,1989. (12)Malkin, J. Am. Chem. SOC.1930.52,3739. (13)Berna1,'J. D. 2.Krist. 1932,83, 153. (14)Meyer, J. D.; Reid, E. E. J . Am. Chem. SOC.1933,55, 1574. Smyth, C. P. J.Am. Chem. SOC.1938,60,1229. (15)Baker, W. 0.; (16)Kolp, D.G.; Lutton, E. S. J. Am. Chem. SOC.1951,73, 5593. (17)Chapman, D.Spectrochim. Acta 1967,11,609. (18)Hoffman, J. D.;Smyth, C. P. J. Am. Chem. SOC.1949,71,431.
0743-746319512411-2187$09.00/0 0 1995 American Chemical Society
2188 Langmuir, Vol. 11, No. 6, 1995
structure, which is hexagonal with the chains being perpendicular to the plane of the oxygen atoms and has some rotational motion. At lower temperatures, the @Istructure is present. It has lower chain mobility and an orthorhombic packing with the chains oriented normal to the plane of the oxygen atoms. This structure is quite unstable, however, and reverts to the more stable j3z structure, where the chains are tilted about 60" (or 120") from the plane of oxygen atoms (30"from the surface normal). Single-crystal XRD results on the BZhexadecanol structure indicated a similar tilt angle.lg In the presence of water, the a and @I forms could be stabilized and be present over a wider temperature range, apparently due to the association and hydrogen-bondingwith the dissolved water molecules.20 Since solid hexadecanol can have different structures, the first goal of this study was to elucidate the structures of dispersed crystallites prepared under W e r e n t protocols. The second goal was to examine the differences between the crystalline structures and the microstructures of the solid monolayer state. Langmuir or adsorbed monolayers of fatty acids, alcohols, phospholipids, etc. a t the aidwater interface have been recently studied with the surfacespecific X-ray diffraction technique21,22or by Brewster angle microscopy.23~24 A Langmuir monolayer of tetradecanol, which was at equilibrium with a pure alcohol drop, was reported to have a hexagonal structure.22 The surface pressure (n)-surface area (A) isotherm of a hexadecanol monolayer has been r e p ~ r t e d . The ~ ! ~equilibrium ~ spreading surface pressure, ne,of hexadecanol is about 40 mN/ m, obtained by sprinkling crystallites on the water s u r f a ~ e . The ~ , ~monolayer ~ collapse pressure, llc,ranges from 50 to 60 mN/m depending on the surface area compression rate.1,7 Using FTIR-attenuated total reflection (ATR) spectroscopy on Langmuir-Blodgett (LB) monolayers, we probe how the LB monolayer microstructure and density depend on the surface pressure and how they are linked to the monolayer isotherm.
2. Experimental Section 2.1. Materials and Sample Preparation. Hexadecanol (99%) was purchased from Sigma Chemical Co., St. Louis, MO, and was used as received. Sodium chloride, analytical reagent (AR)grade, was purchased from Mallinckrodt, Inc., Paris, KY. HPLC grade hexane, from Aldrich, Milwaukee, WI, was used for making cast films and spread monolayers. Saline solution (0.9 w t % NaCl in Millipore water) was used in to prepare all the dispersions. Millipore water had a n initial resistivity of 18MS2 cm and was obtained with a Milli-Q four-bowl system which uses distilled water as input. All experiments were done at room temperature (23 f 1 "C). For fabrication of cast films and LB monolayers, hexadecanol was dissolved in hexane (1mg/mL). For a cast film, a known amount of hexadecanol solution from 50 to 350 pL was spread on a n ATR crystal. The expected average dry hexadecanol film thickness was ca.0.1-0.7pm. LB monolayers were made under different surface pressures of 10,20,40, or 50 mN/m. A 30 pL of hexadecanol solution was first spread on a clean saline surface of a Langmuir trough, and 5 min was allowed for solvent (19)Abrahamsson, S.;Larsson, G.; von Sydow, E. Acta Crystallogr. 1960,13,770. (20) Lawrence,A. S. C.; Al-Mamun, M. A,;McDonald,M. P. J. Chem. SOC.,Faraday Trans. 1 1968,63,2789. (21) Jacquemain, D.; Grayer, S.;Wolf, G.; Leveiller, F.; Deutsch, M.; Kjaer, K.; Als-Nielsen,J.;Lahav, M.; Leiserowitz, L.An.gew. Chem. Int. Ed. Engl. 1992,31, 130. (22) Renault, A.; Legrand. J. F.; Goldmann, M.; Berge, B. J . Phys. II France 1993,3,761. (23) Mdbius, D.; Mohwald, H. Adu. Mater. 1991,3,19. (24) Honig, D.; Mobius, D. Thin Solid Films 1992,210/211, 64. (25)Nutting, G. C.; Harkins, W. D. J. Am. Chem. SOC.1939,61, 1180. (26) Deo, A. V.; Kulkami, S. B.; Gharpurey, M. K.; Biswas, A. B. J . Phys. Chem. 1962,66,1361.
Park and Franses evaporation. The surface area was then compressed at 2 (A2/ molecule)/min until a preset surface pressure was reached. A clean ATR crystal was dipped into the surface and pulled out at a speed of 5 mmlmin. During depositions, the surface pressure was maintained at 10 f 0.05, 20 f 0.2, 40 f 0.3, or 50 f 0.4 mN/m for each monolayer. The transfer ratio (TR) during the dipping down of a ATR crystal was generally small (~0.21,and itwasnearone(-1.1) duringthepullingout stage. Thisindicated a good deposition of LB monolayers on both sides of the crystal, mostly on the way up. For the LB monolayer a t 50 mN/m (LB501, which is above the hexadecanol equilibrium spreading pressure and close to the monolayer collapse pressure of 58 mN/m at the deposition conditions, the TR was 0.5 during the dipping down and 1.5 during the pulling up stages. Hexadecanol dispersions were prepared with two different protocols as done p r e v i ~ u s l y . ~In protocol 1, hexadecanol crystallites were simply mixed with the saline solution to make a 1500ppm dispersion, and the dispersion was shaken vigorously for 10 min. The crystallites were nonspherical, platelike, and rough-edged. Particle sizes varied widely from 60 to 2000 pm. In protocol 2, the above dispersion was heated to 52 f 2 "C, which is above the melting point of hexadecanol(49 "C). The resulting hexadecanol emulsion was shaken to decrease the droplet sizes and then cooled to room temperature, at which the dispersion became more turbid than before. The particles were spherical and smaller ( > d,, the
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The average tilt angles for the CH2 dipole moment are about the, same (-67" and 69" for peaks 8 and 111, indicating effectively uniaxial ~ r i e n t a t i o n The . ~ ~ average chain tilt angles are a bit smaller (29") compared to the angle in the LB monolayers. The dichroic ratios of the protocol 1samples were different from those of the protocol 2 samples. They were not interpreted further, however, because the films are quite nonuniform, consisting of some very thick patches, a substantial area fraction of thin monolayers, and probably some patches at intermediate thickness. Of course, the orientation of the molecules in the crystallites depends on many factors, such as crystallite structure and crystallite orientation, and cannot be compared directly with the monolayer orientation distribution, which is better defined. The regions of the OH stretches of the protocol 1 and 2 crystallites are quite different. The integrated absorbance of peaks 1-4 is about 14 times (1430 vs 100) the reference intensity (the reader should notice the difference in units) for the protocol 1crystallite deposition and about 18 times the reference intensity for the protocol 2 crystallites (Table 2). The OH stretch bands ofthe protocol 1 deposition are similar to those of the cast films and larger than those ofthe LB monolayer films. The deposited
Hexadecanol Microstructures of Crystallites protocol 1 crystallites can absorb (or retain) water vapor and increase the OH intensities as in the case of the cast films. Since the OH intensities for the protocol 2 deposition are much larger than the other types of films, they suggest that in addition to the increase by the absorbed water vapor, other sources for hydroxyl bands should be present for the protocol 2 crystallites. At room temperature, the stable solid hexadecanol crystallites have the monoclinic structure.16-18 However, if the crystallites have been previously melted in the presence of water, other (perhaps metastable) structures, which contain more water, could be present in the protocol 2 dispersions. Since we are interested in crystalline structuresin aqueous dispersions, it would be more appropriate to examine the degree of hydration of crystals in the presence of water. This was not done because the spectra of hexadecanol crystallites in aqueous dispersions are not easily determined by FTIR spectroscopy because of the large water signal. 3.4. X-rayDiffractometry Results. To further probe for differences in the crystal structure of protocol 1 and protocol 2 dispersions in the presence of bulk water, X-ray powder diffractograms were obtained (Figure 6 and Table 4). Protocol 1 samples show a typical crystal pattern of long Bragg spacings (in A), 35, 3512, 3513, etc. Protocol 2 samples show clear evidence of a second crystal pattern with higher spacing (peaks l', 2', 3', etc.). We conclude that after melting a dispersion in the presence of saline, a second crystal structure appears. Although no detailed crystallographic calculations were done, we infer from these and the literature (reviewed below) data, that the first structure is p 2 and the second is either a or pl. Kolp reported the long spacings of the X-ray powder diffraction for different polymorphic structures of solid he?adecanol.l6 The long spacings were 37.1,44.9,and 44.3 A, for the p 2 , ,!?I,and a structures, respectively. The Bragg spacing calculated from single-crystal XRD for the p 2 structure was 74.4 A for the (001) plane.lg From the single-crystal pz structure results, the Bragg reflection from the (001) plane is systematically absent if 1 is an odd number.19 Thus, the first observed peak corresponds to the (002) plane, the next peak is to the (004) plane, and so on. Peaks 9, 10, 11, and others a t higher angles of 28 probably correspond to other planes of (hkl) with h or k being different from zero. The relative intensities of X-ray diffraction peaks are determined from the followingfactors: (a)Thecalculated (FAand observed structure factors (F,)for n-hexadecanol single monoclinic (Pz) crystalslg show that the absolute structure factors of the first few peaks oscillate with a n overall decreasing trend. Since the intensity of a diffraction peak is proportional to IF12,the intensity pattern observed in Figure 6 is supported by the literature data and calculations. (b) The volume probed by the X-rays may decrease with increasing angle 28. (c) Whereas the sample in Figure 7 is completely dry, the sample in Figure 6 contains water between the crystals. This may affect the penetration depth of the X-rays and the absorption losses. (d) Orientation effects and Lorentz and polarization effects. Because of all these factors, the relative intensities of the various peaks were not interpreted in detail. Finally, to test whether the appearance of the second structure is due to the prior melting per se of the crystals or to the melting in the presence of water, two other samples were examined: (i) dry crystal, as received, and (ii)dry crystals after prior melting (Figure 7). The powder diffraction data in the X-ray diffractogram database for (41)Schulz,D.;McCarthy,G. Joint CommitteeforPowderDiffraction Standards -International Centre for Difraction Data (JCPDS-ICDD) 37-1611, 1991.
Langmuir, Vol. 11, No. 6, 1995 2193 20000
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hexadecano141are different from the data we obtained, which is probably due to the polymorphism of hexadecanol. Since Figure 7 shows that only one structure is present in both samples, the conclusionis that the second structure in the protocol 2 sample (Figure 6) is associated with the
2194 Langmuir, Vol. 11, No. 6, 1995
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86 A for P = 90" 17,20-is about 86ab A3. The unit cell volume of the second structure is probably 1.23 times larger than that of the PZ structure, if the a and b values are assumed to be unchanged. However, it does not seem likely that the second structure contains 20% or more water by volume. Probably, the a and b axes are smaller. More work is needed to precisely determine the water content. Nonetheless, the second structure can accommodate more water, consistent with the larger OH signal which was observed in the IR spectrum of protocol 2 samples (Table 2). Lawrence reported that the a and forms can be stabilized by the presence of water for a wider temperature range, because the structures are associated with water molecules.20 For example, in the a form, it is possible that up to one water molecule is associated with four molecules of the alcohol via hydrogen bonds. Therefore, the stability or metastability of hexadecanol crystallites depends not only on the temperature but also on the presence of water and perhaps on preparation history. One infers also that the crystals (cast, protocol 1, and protocol 2 samples) contain more water than the LB films, as discussed earlier (section 3.2). No attempt was made to quantify the amount of water. The results have important implications for several application areas of hexadecanol dispersions, such as the dynamic surface tensions, dissolution rates, and mass transfer rates.
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Figure 7. X-ray powder diffractograms for dry hexadecanol crystallites as received (A, C)and after they were melted and then cooled back to room temperature in the absence of liquid water (B,D).Inserts: expanded scale for the range between 28 = 4' and 14".
water of hydration. Comparisons of the unit cell volumes of the P 2 and the second structure support this inference, for the following reasons. The volume of the PZstructure with our data of d = 35 A - which implies a value of c = 83 A for the known angle ,B = 122" 19-is about 70ab A3, where a, b, and c are the unit cell axes and ,B is the angle between the a and c axes. The vplume of the second structure with our data of d = 43 A-which implies c =
The microstructures of dispersed crystallite of hexadecanol in saline a t room temperature depended on the thennaymixing protocol of preparation, as determined by X-ray diffraction and FTIR spectroscopy. This is important for understanding the effect of preparation protocol on the dynamic surface tension behavior. The results are consistent with previous reports on hexadecanol polymorphism. Dry crystals, previously melted crystals (melting point 49 "C), or crystals simply mixed with saline solution without heating have the monoclinic (Pz)structure. Crystals melted in the presence of aqueous NaCl and refrozen have a second coexisting structure [either hexagonal (a)or orthorhombic (PI)], which may well contain significant amounts of water-of-hydration. The IR spectra of LB monolayers deposited a t 10-50 mN/ m, at which the monolayers are in the solid-monolayer state with a hexagonal microstructure, are quite broader than the solid-crystallite spectra, implying higher molecular mobility. The water content is less (or nonexistent), probably because water cannot be dissolved in a single monolayer. Finally, the microstructure and surface density of the LB films depend little on the surface pressure, as expected from the steepness of the surface pressure-surface area isotherm. The hydrocarbon chains in the LB monolayers are oriented with a uniaxial orientation distribution with a chain tilt angle of about 35".
Acknowledgment. This research was supported in part by the National Science Foundation (Grants No. CBT 864904, CTS 9004147, BCS 91-12154,and CTS 93-04328), by a grant from the Showalter Trust, and by a Purdue Research Foundation graduate fellowship. We thank our former colleagues, Dr. Dong June Ahn and Mr. Peter Sutandar, for helpful discussions on FTIR and Professor Eric Kalerfor helpful discussions onXRD. We also thank one anonymous reviewer for some valuable suggestions on the interpretation of the X-ray diffraction results. LA940826C
