Ethyl-paraben and nicotinamide mixtures: Apparent solubility, thermal behavior and X-ray structure of the 1:1 co-crystal

Ethyl-Paraben and Nicotinamide Mixtures: Apparent Solubility, Thermal Behavior and X-Ray Structure of the 1:1 Co-Crystal S. NICOLI,1 S. BILZI,1 P. SANTI,1 M.R. CAIRA,2 J. LI,2 R. BETTINI1 1

Department of Pharmacy, University of Parma, Viale G.P. Usberti 27/A, 43100 Parma, Italy

2

Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa

Received 8 October 2007; revised 12 December 2007; accepted 4 January 2008 Published online 3 March 2008 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.21341

ABSTRACT: This work aims at investigating the nicotinamide (NA)–ethyl-paraben (EP) binary system both in solution and in the solid state. In particular, the apparent EP solubility in water was studied in the presence of different NA concentrations (between 0.28 and 1.64 M). It was found that the apparent EP solubility increase (nearly twofold) observed at the highest NA concentration tested can be ascribed to a change in the polarity of the solvent mixture, rather than to a direct effect of NA on EP. The effect of fusion and re-crystallization from water or ethanol solutions on EP and NA mixtures was investigated by means of differential scanning calorimetry, elemental analysis and X-ray diffraction both on powder and single crystal. It was discovered that EP and NA form a co-crystal having a 1:1 molar composition that can be easily crystallized from ethanol. Single crystal X-ray analysis of this species revealed that the NA and EP molecules form corrugated layers within which the two components are intimately associated by a dense network of hydrogen bonds. In the presence of an excess NA in solution, the EP-NA co-crystal has lower water solubility with respect to both the single co-crystal formers and precipitates in aqueous solutions at ambient temperature. ß 2008 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:4830–4839, 2008

Keywords: nicotinamide; ethyl-paraben; co-crystals; calorimetry (DSC); solubility; X-ray diffractometry; crystallography; interaction; precipitation; solid state

INTRODUCTION Parabens (alkyl esters of p-hydroxy benzoic acid) are antimicrobial agents widely used in cosmetics and pharmaceutical liquid or semisolid formulations. Although they have been successfully used for more than five decades, they are still a matter of debate and scientific discussion not only with respect to their potential toxicity1,2 but also as far

This article is dedicated to Professor Ferdinando Giordano on the occasion of his retirement. Correspondence to: R. Bettini (Telephone: þ39 0521905089; Fax: þ39 0521905006; E-mail: [email protected]) Journal of Pharmaceutical Sciences, Vol.97, 4830–4839 (2008) ß 2008 Wiley-Liss, Inc. and the American Pharmacists Association

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as their physical solid state characteristics are concerned.3,4 The relevance of the solid state properties for pharmaceutical products is very well established and has recently been highlighted by Hilfiker et al.5 Giordano et al.3 demonstrated that ethyl- and propyl-paraben are able to form an almost ideal solid solution in the 0–0.6 ethyl-paraben (EP) mole fraction range. This provided an explanation for the peculiar solubility behavior of propylparaben in water in the presence of 2-hydroxypropyl-b-cyclodextrin and other parabens.6 Among the physico-chemical properties potentially related to the solid state properties of the parabens, the solubility in water has attracted significant attention.7,8 This is a crucial factor for

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the applicative use of these molecules since their antimicrobial activity is directly related to their concentration in water. In fact, the water solubility decreases with the chain length of the alkyl moiety, while the antimicrobial activity increases. Furthermore, the chain length strongly influences the partitioning between aqueous and lipid phase, for instance, in dispersed multiphase systems such as emulsified formulations9,10 or the absorption through the skin, when dermal or transdermal formulations are concerned. Some attempts were made to increase the apparent water solubility of these lipophilic molecules, for instance by exploiting the formation of inclusion compounds with cyclodextrins11–13 or the interaction with methacrylic polymers.14 Nicotinamide (NA; vitamin B3) is a freely water soluble molecule widely used in cosmetics as moisturizing agent as well as in pharmaceutics for its well known hydrotropic action toward poorly water soluble drugs.15–17 Very recently, we have shown that NA is able to increase the solubility of parabens and to reduce their permeability coefficient through the skin (Nicoli et al.18). The solubility of methyl-, propyland butyl-paraben was progressively enhanced, by increasing the NA concentrations in solution. In the case of EP, however, NA showed only a small effect. In particular, in the presence of 20% (w/v) NA methyl-, propyl- and butylparaben solubility increased by about one order of magnitude while with EP only a twofold increase was observed. In some cases, modifications of the solubility in water can be the result of a change in the solid state characteristics of the compound under investigation, for instance the formation of a polymorph or hydrate. However, interaction phenomena in solution as well as in the solid state should be taken into consideration, especially when dealing with multicomponent systems.19,20 In this respect, the consequences, as well as the opportunities, afforded by co-crystallization have attracted a great deal of interest in recent years as a tool for obtaining improved medicines.21–25 The aim of this work was to investigate the NA– EP binary system both in solution and in the solid state. In particular, the apparent EP solubility in water was studied in the presence of different amounts of NA. Moreover, the effect of fusion and re-crystallization from water or ethanol solutions on EP and NA mixtures was investigated by means of differential scanning calorimetry (DSC) elemental analysis and X-ray diffraction both on DOI 10.1002/jps

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powder and single crystal. From previous studies of solid-solution formation involving the parabens,3 X-ray diffraction proved to be an invaluable tool for supporting findings derived from thermal analysis techniques. The present study confirmed the utility of X-ray diffraction methods in elucidating the nature of systems of this type.

EXPERIMENTAL Materials NA (MW 122.13 Da) and EP (MW 166.18 Da) were obtained from Sigma–Aldrich (St. Louis, MO, USA) and were used as received. Anhydrous ethanol, methanol, n-octanol and acetonitrile (Carlo Erba, Rodano, Italy) were of HPLC grade.

Phase Solubility Accurately weighed amounts of EP, in excess with respect to saturation, were added to 1 mL of distilled water or aqueous solutions of NA (0.28, 0.82, 1.23, or 1.64 M) and magnetically stirred in EppendorfTM vials at 25  0.58C for 48 h. Thereafter, the suspensions were filtered (0.45 mm, regenerated cellulose) and the filtrate was diluted with a 1:1 (v/v) mixture of acetonitrile–water. The EP concentration in each solution was determined by HPLC, while the solid residues were analyzed by DSC and X-ray diffraction on powder (PXRD).

Co-Crystal Solubility Accurately weighed amounts of EP–NA co-crystal (265 mg corresponding to about 0.9 moles), in excess with respect to saturation, were added to 0.5 mL of distilled water, methanol, ethanol or n-octanol and magnetically stirred in EppendorfTM vials at 25  0.58C for 48 h. Thereafter, EP and NA concentration in each solution was determined by HPLC after filtering the suspensions (0.45 mm, regenerated cellulose) and diluting the filtrate with a 1:1 (v/v) mixture of acetonitrile–water. Each measurement was replicated at least three times.

HPLC Analysis EP in solution was quantified by means of a validated HPLC method18 using a LC 250 pump (Perkin Elmer, Norwalk, CT, USA) equipped with

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a mBondapak C18 300 mm  3.9 mm column (Waters, Milford, MA, USA) and an automatic injector (50 mL) Prostar 410 (Varian, Paolo Alto, CA, USA). The UV-Vis detector (LC 290, Perkin Elmer) was set at 254 nm. As a mobile phase, a water:acetonitrile mixture (80:20, v/v) at the flux of 1.2 mL/min was used. NA quantification in solution was carried out by HPLC in similar conditions using as mobile phase a mixture composed by 92.5 volumes of methanol and 7.5 volumes of a KH2PO4 water solution (0.05 M) and by setting the UV detector at 261 nm.

Elemental Analysis for CHN Elemental analysis for CHN was carried out with a FlashEA 1112 (ThermoQuest, Milan, Italy) running at 9508C on powder samples of about 1 mg accurately weighed in Sn crucibles.

Powder X-Ray Diffractometry X-ray diffraction patterns on powder were recorded on a Miniflex (Rigaku, Tokyo, Japan) diffract˚) ometer using Cu Ka radiation (l ¼ 1.5418 A generated with 30 kV. The goniometer was set at a scan rate of 0.58 min1 over the 2u interval 2–408.

Re-Crystallization from Ethanol Re-crystallization from the binary was carried out by dissolving in ethanol fixed amounts of EP and NA in different molar ratios. After filtration (0.45 mm, regenerated cellulose) the obtained solutions were slowly evaporated at 258C to dryness. The obtained crystalline powders were gently ground in a China mortar and analyzed by DSC and PXRD. The melting temperatures recorded by DSC were plotted versus composition and the obtained phase diagram was compared with the theoretical liquidus curves calculated with the simplified form of the Schro¨ der–Van Laar equation20: ! DHAf 1 1 ln x ¼  f (1) T R TAf where x is the mole fraction of the component of the mixture melting at Tf, DHAf and TAf are the enthalpy and the temperature of fusion of the pure component, respectively, and R is the gas constant (8.314 J K1 mol1).

Thermal Analysis DSC was performed on an indium calibrated Mettler DSC 821e instrument (Mettler Toledo, Columbus, OH, USA) driven by STARe software (Mettler Toledo). DSC traces were recorded by placing accurately weighed quantities (5–7 mg) of powder sample in an Aluminium pan which was sealed and doubly pierced. Scans were performed between 25 and 1508C at 1 K min1 under a flux of dry nitrogen (100 mL min1). Each powder sample was analyzed at least in triplicate. Data are expressed as mean value  standard deviation.

Single Crystal X-Ray Diffraction Intensity data were collected from a single crystal coated in Paratone oil (Exxon Chemical Co., Houston, TX) and cooled in a stream of nitrogen vapor at 1608C on a Nonius Kappa CCD diffractometer using graphite-monochromated ˚ ). The dataMo Ka radiation (l ¼ 0.71073 A collection strategy indicated by the program COLLECT (Nonius26) involved suitable combinations of f- and v-scans. Program DENZO-SMN27 was used for cell refinement and data reduction. Laue symmetry 2/m indicated the monoclinic crystal system and the space group was deduced from systematic absences as P21/n (alternative setting of P21/c, No.14). The structure was solved by direct methods using program SHELXS-8628 and refined by full-matrix least-squares techniques using program SHELXL-97.29 All hydrogen atoms were located unambiguously in difference Fourier syntheses but were placed in idealized positions in a riding model with isotropic thermal parameters set at 1.2–1.3 times those of their parent atoms. All non-H atoms were refined anisotropically. Molecular parameters were calculated with program PLATON.30 Programs ORTEP31 and WebLab ViewerPro 3.732 were used to produce illustrations.

RESULTS AND DISCUSSION Phase solubility experiments on EP in NA aqueous solutions (Fig. 1) confirmed the peculiar solubility behavior highlighted by Nicoli et al.18: from 0.28 to 0.82 M NA concentration a decrease of EP solubility was observed, while at the NA

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Figure 1. Apparent EP solubility in water as a function of the NA concentration in solution. The bars represent the standard deviation (n  6).

concentration of 1.64 M EP solubility doubled with respect to pure EP. The DSC traces of the recovered solid phases (Fig. 2) presented two sharp endotherms whose melting temperature, Tm, and enthalpy of fusion, DHm are reported in Table 1 along with those relevant to pure EP and NA. It can be observed that the peak temperatures of the endothermic phenomena recorded from the recovered solid phases were significantly different from the melting temperatures of both pure EP and NA. Moreover, the intensity of the first peak (as expressed by the DHm) relative to the second decreased with the increase of the NA concentration in the original suspension. The composition (mole fraction) of the recovered solid phases was determined by HPLC: the solid recovered upon contact with the 0.28 M NA solution was composed by 0.17 NA and 0.83 EP,

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while both solids obtained from the 0.82 and 1.64 M NA solutions turned out to be 0.49 NA and 0.51 EP (the relative standard deviation in all cases was between 0.2% and 1.2%). The data reported in Table 1 clearly suggest the existence of an interaction between NA and EP in the solid phase, probably leading to the formation of an interaction compound that separates from the EP solution. It is worth emphasizing that Nicoli et al.18 reported that a single peak, corresponding to the pure paraben melting temperature, was instead obtained from DSC experiments carried out on solid phases recovered upon similar phase solubility experiments with methyl-, propyl- or butyl-paraben. While the apparent solubility decrease can be ascribed to the formation of a less soluble compound,33 the EP solubility increase observed at NA 1.64 M can be explained in terms of a change of the polarity of the solvent mixture (water þ NA).34,35 In this respect, the entire curve should be considered as the result of two opposing phenomena: formation of an insoluble complex and change in solvent polarity. At the highest NA concentration the second one becomes prevalent. This hypothesis is supported by the data reported by Nicoli et al.18 which indicate that the solubility enhancement induced by NA on the three parabens of the homologous series (methyl-, propyl-, butyl-) is directly proportional to the paraben’s lipophylicity, thus, suggesting that the solubilisation mechanism is likely related to the modification of the solvent polarity. Therefore, in the case of aqueous phase solubility experiments, disregarding the effect of the

Figure 2. DSC traces of pure NA (curve 1), EP (curve 2) and solids phases recovered upon suspension of EP in NA solution at 0.28 (curve 5), 0.82 (curve 4) and 1.64 (curve 3) molar concentration. DOI 10.1002/jps

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Table 1. Melting Temperature, Tm, and Enthalpy of Fusion, DHm of Pure EP and NA along with the Values Recorded from the Solid Phases Recovered from Suspensions Containing Different NA Concentrations: 1 ¼ from Solution 0.28 M; 2 ¼ from Solution 0.82 M; 3 ¼ from Solution 1.64 M

EP NA

Tm (8C)

DHm (J mol1)

DHm (J g1)

115.67 (0.4) 128.44 (0.3)

1.02 (0.04) 1.71 (0.01)

195.5 (6.6) 208.8 (1.2)

Tm (8C)

DHma (J g1)

First Peak Second Peak First Peak Second Peak 1 98.08 (0.8) 2 97.04 (0.3) 3 96.40 (0.2)

105.9 (0.1) 106.3 (0.4) 106.2 (0.1)

91.9 (7.3) 22.9 (3.5) 12.1 (3.8)

16.4 (0.1) 145.3 (5.7) 162.0 (2.6)

Standard Deviations in Parentheses (n ¼ 3). a Relative to the binary weight.

solvent polarity change, one should expect an immediate and progressive decrease in the EP apparent concentration, as observed at low NA concentration in the initial part of the curve reported in Figure 1.36 According to Higuchi and Connors33 in such a case the application of the phase rule with three degrees of freedom and three components would indicate that two phases should be present: one liquid solution and one solid solution. An alternative scenario would imply the existence of three phases: one solution and two solids. In that case the phase rule would indicate the existence at equilibrium of four components (the number of degrees of freedom still being three), namely EP, NA and water in solution and EP plus a newly formed interaction compound in the solid phase. The amount of EP and NA in solution would be governed by the solubility product of the latter.36 PXRD obtained from the solid phases recovered from water indicated in fact the presence of a different crystal form with respect to NA and EP: the diffraction pattern was only partly superimposable on those of pure EP and NA powders (Fig. 3). In order to clarify the nature of the possible interaction between EP and NA, namely the possible formation of a solid solution or an interaction compound, the relevant phase diagram was drawn by plotting the melting temperatures recorded by DSC versus composition. In order to assure better homogeneity of the specimens, these measurements were carried out on binaries

Figure 3. Powder X-ray diffraction patterns obtained from pure EP and NA crystals and from the solid phase recovered after suspending EP in a water solution containing 0.28 M nicotinamide.

obtained after re-crystallization of the EP–NA mixtures from ethanol. The obtained data, reported in Figure 4, show the typical shape assumed by a binary melting point diagram relevant to the formation of an interaction compound (co-crystal) with stoichiometry 1:1.37 In fact at mole fraction 0.5 a single endothermic event was obtained at 106.7  0.128C associated with an enthalpy of 192.2  1.34 J g1 (relative to the mixture weight). This should be interpreted as the melting of a co-crystal formed by EP and NA in 1:1 molar ratio. This co-crystal,

Figure 4. Phase diagram of the EP–NA binary system. The solid circles represent the experimental points (peak temperatures from DSC measurements) and the lines the values calculated with Eq. (1) for EP, NA and their co-crystal with 1:1 molar composition.

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in turn, forms two eutectics with EP and NA, respectively. The temperatures of fusion, obtained by DSC on binary mixtures of different compositions, were in good agreement with the liquidus curves calculated with the simplified form of the Schro¨ der–Van Laar equation for each of the three components (solid lines in Fig. 4), namely EP, NA and the interaction compound. The compositions in mole fraction of the interaction compound of the two eutectics were 0.66 and 0.82 for the binary with EP and NA, respectively. In the case of the EP-interaction compound binary, both the measured and calculated eutectic fusions were at 98.18C, while in the case of the binary NA-interaction compound the measured value was 103.98C versus a calculated one of 102.58C. Finally, no, or very limited, NA/EP solid–solid miscibility can be deduced from the data reported in Figure 4. Based on these results the thermal behavior of the solid phases recovered upon phase solubility experiments (Fig. 2 and Tab. 1) can be interpreted in better detail: in the case of the samples recovered from the 0.82 and 1.64 M NA solutions the peaks at around 978C should be attributed to the eutectic melting, whereas the peaks at 1068C must be attributed to the melting of the co-crystal. In fact the above reported composition of these solids indicated that the precipitates were almost totally composed by the interaction compound with only a small amount (about 1%) EP, that justifies the presence of a small peak of eutectic fusion. On the other hand, in the case of the solid recovered upon contact with the 0.28 M NA solution, the first peak still corresponded to the eutectic melting, while the second one (105.98C) must be attributed to the melting of NA present in 0.83 mole fraction (relative to EP–NA mixture), namely largely in excess with respect to the eutectic composition of the EP-interaction compound. Serendipitously, the EP melting point recorded in these conditions was very close to that of 1:1 EP–NA interaction compound. To better characterize the molecular compound formed at mole fraction 0.5 an equimolar mixture of EP and NA was dissolved in ethanol. The crystalline solid obtained after solvent evaporation was carefully washed with water and investigated in terms of elemental composition, thermal behavior, solubility and crystal structure. DOI 10.1002/jps

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Data obtained from elemental analysis were in fairly good agreement with a theoretical 1:1 EP–NA molar composition (Tab. 2). The DSC trace of the co-crystal gave rise to only a sharp endothermic peak at 107.1  0.58C with a melting enthalpy of 0.64  0.03 J mol1. Table 3 reports the EP and NA solubility, as molar concentration, measured at the equilibrium at 258C from the co-crystal in water or three different alcohols in comparison to the EP solubility in the same conditions. The co-crystal solubility in the three alcohols can be expressed with the molar concentration of either co-crystal component, as the EP and NA measured concentration were equal (differences largely within the experimental error). Furthermore, in these solvents the solubility of the cocrystal was significantly lower than that relevant to the pure EP. In water the co-crystal gave rise to a nonstoichiometric solution composition with an excess NA. The EP and NA concentration in solution resulted 0.009 and 0.164 M, respectively. This means that part of the EP should be present in solid phase along with the co-crystal. Considering the total amount of co-crystal added to water and the concentration measured in solution for both components, it was calculated that the EP mole fraction in the solid phase should be 0.09. DSC analysis on this phase gave rise to two endothermic peaks at 97.78C (eutectic melting) and 106.78C (co-crystal melting) perfectly in agreement with the calculated composition (see Fig. 4). The co-crystal solubility, S, in water was, given by the EP concentration which resulted slightly higher than that obtained from the pure ethylparaben (Tab. 3), although the difference was not statistically significant ( p ¼ 0.16 by t-test). A similar situation was presented by Nehm et al.36 in the case of carbamazepine–NA co-crystal Table 2. Composition Percent of the Crystalline Solid Obtained after Re-Crystallization of an Equimolar Mixture of EP and NA from Ethanol, along with the Theoretical Values Calculated for a 1:1 Mole Composition

Element % C H N

Theoretical for 1:1 Mole Composition

Measured

62.5 5.6 9.7

62.7 5.6 9.5

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Table 3. EP and NA Concentration (M) Measured at the Equilibrium in Different Solvents Form EP-NA CoCrystal and Pure EP Co-Crystal Solvent Water Methanol Ethanol n-Octanol

EP 0.009 0.716 0.619 0.259

(0.0003) (0.026) (0.037) (0.027)

Ethyl-Paraben NA

0.164 0.715 0.616 0.232

(0.009) (0.101) (0.027) (0.038)

EP 0.007 1.842 1.909 0.817

(0.001) (0.076) (0.039) (0.047)

Standard Deviation in Parenthesis.

formation. These authors discussed also the fundamentals of the control on co-crystal solubility by the solubility product, predicting that the addition of either co-crystal component in excess to S decreases the co-crystal solubility. Accordingly, in the present work, the nicotiamide transition concentration, NAtr, was calculate as36 ½NA tr ¼ S þ

Ksp  S2EP SEP

(2)

where, Ksp is the solubility product, and SEP is the solubility of the crystalline EP. In the phase solubility diagram the parameter NAtr represents the NA concentration in solution at which the solubility of the co-crystal is equal to the solubility of the crystalline EP and above which o the co-crystal solubility became lower than that of the pure EP. The value obtained from Eq. (2) for a phase solubility diagram in water was 0.2 M, namely slightly lower than the smallest NA concentration used in the phase solubility measurement carried out in the present work (0.28 M, see Fig. 1). This allow to explain why in said experiment the increase of NA concentration in solution determined a progressive precipitation of the co-crystal even in the presence of an increase of the apparent EP solubility that, therefore, should be attributed essentially to a change in solvent polarity.

Table 4. Crystal Data and Refinement Parameters for the EP-NA Co-Crystal Parameter

EP–NA Co-Crystal

Chemical formula Formula weight Dcalc (g cm3) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) V (A Formula units, Z F(0 0 0) m (Mo Ka) (mm1) Crystal size (mm3) T (K) u-range (8) Total reflections Unique reflections Observed reflections Data/restraints/parameters R1 (on F > 4s( F)) wR2 (on F2, F > 4s( F)) Goodness-of fit, S ˚ 3) Dr max., min. (e A

C9H10O3C6H6N2O 288.30 1.393 Monoclinic P21/n 5.1106(2) 11.5175(5) 23.3547(9) 90.148(2) 1374.7(1) 4 608 0.102 0.14  0.21  0.24 113(2) 2.5–25.3 6355 2457 1696 2457/0/192 0.0388 0.0853 0.969 0.19, 0.25

X-Ray Structure of the EP–NA Co-Crystal Crystal data and refinement parameters for this phase are listed in Table 4. The asymmetric unit in the crystal (Fig. 5) consists of one molecule of ethyl-paraben and one molecule of NA. The strong H-bond linking the pyridine N atom of NA and the hydroxyl group of the partner molecule is indicated. In achieving this H-bond, the aromatic

Figure 5. The asymmetric unit in the 1:1 EP–NA co-crystal. Thermal ellipsoids are drawn at the 50% probability level.

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rings incline at 15.5(1)8 to one another to reduce steric repulsion between the ortho-H atoms on C6 and C14. The X-ray analysis thus confirms that this solid phase is indeed a co-crystal, as indicated by the phase solubility and thermal data reported above. Analysis of the crystal packing shows that in addition to the hydrogen bond shown in Figure 5, there is extensive hydrogen bonding involving the amide group of NA as well as the carbonyl oxygen atom of EP. The full extent of hydrogen bonding is evident in Figure 6, a projection of the crystal structure down the short a-axis that includes four unit cells. The NA molecules form centrosymmetric dimers at the corners of the unit cells and at their centers

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Table 5. Principal Hydrogen Bonds in the NA-EP Co-Crystal D–H  A

˚ D  A/A

˚ H  A/A

D–H  A/Deg.

O16–H16  N1s N9–H9A  O8i N9–H9B  O18ii

2.722 (2) 2.906 (2) 3.079 (2)

1.90 2.03 2.22

165.9 176.0 165.3

Symmetry codes: (i) 3  x, 1  y, 1  z (ii) 1/2  x, 1/2 þ y, 3/2  z.

via H-bonding (data in Tab. 5) involving their amide groups (HB–N–HA  O8 – C). Emanating from these dimers are four hydrogen bonds (two centrosymmetric pairs, one of type HA–N– HB  O18 – C, and one of type O16–H16  N1). This results in a dense network of hydrogen bonds that generates an infinite layer of molecules, seen in projection in Figure 6. The crystal comprises stacked layers of this type that are steeply inclined to the bc-plane and corrugated due to the noncoplanarity of the aromatic rings of the partners. No interlayer hydrogen bonding occurs. NA is well known as a co-crystal former. A recently reported example is the 1:1 co-crystal NA–carbamazepine.38 Co-crystals of parabens with bicyclic beta-lactams were reported earlier.39 The computed PXRD pattern of the NA–EP cocrystal based on the single crystal X-ray structure is shown in Figure 7. This is in good agreement with the PXRD pattern of the recovered solid shown in Figure 3, confirming the identity of these phases.

CONCLUSIONS We investigated the NA–EP binary system both in solution and in the solid state. It was found that

Figure 6. [1 0 0] Projection of the crystal structure of the EP–NA co-crystal. Four unit cells are included to illustrate the hydrogen bonded network. DOI 10.1002/jps

Figure 7. The computed PXRD pattern for the 1:1 EP–NA co-crystal based on the refined single crystal X-ray structure.

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EP and NA form a co-crystal having a 1:1 molar composition that can be easily crystallized from ethanol. Single crystal X-ray analysis of this species revealed that the NA and EP molecules form corrugated layers within which the two components are intimately associated by a dense network of hydrogen bonds. The EP–NA co-crystal has lower solubility (as molar concentration) in methanol, ethanol and n-octanol than the crystalline EP. In the presence of an excess NA in solution (more than 0.2 M), the EP–NA co-crystal shows lower water solubility with respect to both the single co-crystal formers and precipitates in aqueous solutions at ambient temperature. The apparent EP solubility increase observed at the highest NA concentration tested can be ascribed to a change in the polarity of the solvent mixture, rather than to a direct effect of NA on EP.

ACKNOWLEDGMENTS This work was partly supported by a PRIN grant from the Italian Ministry of Research. MRC and RB also wish to acknowledge financial support of the South Africa-Italy Program on Research Co-operation from the NRF (Pretoria).

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