Journal of Colloid and Interface Science 291 (2005) 471–476 www.elsevier.com/locate/jcis Rapid adsorption and entrapment of benzoic acid molecules onto mesoporous silica (FSM-16) Yuichi Tozuka ∗ , Sara Sasaoka, Ayako Nagae, Kunikazu Moribe, Toshio Oguchi , Keiji Yamamoto Graduate School of Pharmaceutical Sciences, Chiba University, 1-33 Yayoicho, Inage-ku, Chiba 263-8522, Japan Received 14 March 2005; accepted May 2005 Available online 13 June 2005 Abstract Changes in the molecular state of benzoic acid (BA) in the presence of folded sheet mesoporous material (FSM-16), which has uniformly sized cylindrical mesopores and a large surface area, were assessed with several analyses When BA was blended with FSM-16 for (BA content = 30%), the X-ray diffraction peaks of BA crystals disappeared, suggesting an amorphous state Fluorescence analysis of the mixture showed a new fluorescence emission peak for BA at 386 nm after mixing with FSM-16 Fluorescence lifetime analysis of the BA component in the mixture at 386 nm showed a longer lifetime in comparison with that of BA crystals The solid-state 13 C CP/MAS and PST/MAS NMR spectra of the mixture with FSM-16 showed a significantly different spectral pattern from the mixture with nonporous glass, whose NMR spectra were identical to those of BA crystals These results indicate that BA molecules disperse quickly into the hexagonal channels of FSM-16 by a simple blending procedure and adsorbed BA molecules had clearly different physicochemical properties to BA crystals 2005 Elsevier Inc All rights reserved Keywords: Porous material; Adsorption; Solid-state NMR; Fluorescence spectra; Benzoic acid; FSM-16 Introduction To improve the dissolution profile of poorly water-soluble compounds, we have investigated the use of porous materials as pharmaceutical excipients, and have already reported that medicinal substances could be adsorbed onto the surface of porous material, resulting in the change of their physicochemical properties [1] A variety of organic compounds can be adsorbed onto the surface of porous materials due to their peculiar adsorptive properties [2,3] Folded sheet mesoporous material (FSM-16), mesoporous silica having one-dimensional pores with uniformly sized mesopores of cylindrical-like nature, has high porosity and * Corresponding author Fax: +81 43 290 2939 E-mail address: ytozuka@faculty.chiba-u.jp (Y Tozuka) Department of Pharmacy, University of Yamanashi, 1110 Tamaho-cho, Nakakoma-gun, Yamanashi 409-3898, Japan 0021-9797/$ – see front matter 2005 Elsevier Inc All rights reserved doi:10.1016/j.jcis.2005.05.009 large surface area (more than 1000 m2 /g) FSM-16 is composed of honeycomb-like hexagonal channels and is used as highly promising model adsorbents for fundamental adsorption studies [4] FSM-16 has been shown to maintain its texture at pressures up to 780 kg cm−2 [5] and this would allow the material to be used for grinding and tableting, unlike other porous materials These remarkable characteristics give rise to the possibility of FSM-16 for pharmaceutical use To regulate the quality of drug products in pharmaceutical dosage forms, the molecular states of the medicine and the molecular interactions between the medicine and any additives in the dispersed system should be investigated from the viewpoint of controlling the stability and safety of medicine Assessment using high-sensitivity and high-resolution analyses is necessary for the clarification of the molecular state in the solid dispersion Fluorescence spectroscopy is a useful tool for detecting molecules even in low concentra- 472 Y Tozuka et al / Journal of Colloid and Interface Science 291 (2005) 471–476 tions due to its high sensitivity [6] Fluorescence analysis has been applied to solid systems in order to study molecular interactions, such as inclusion phenomena of cyclodextrin and guest molecules [7], and to study physicochemical changes of organic compounds adsorbed on solid surfaces [8] Solidstate 13 C NMR spectroscopy is also a useful method for the study of materials in the solid state Combinational techniques of cross-polarization (CP) and magic-angle spinning (MAS) provide high-resolution 13 C spectra, showing molecular level information [9] Since solid-state 13 C NMR methods simply differentiate between mobile and immobile contributions, these methods provide detailed information about the mobility [10] In the present study, we used benzoic acid as a model compound and FSM-16 (pore width: 2.1 nm) as a model porous material The molecular state of benzoic acid adsorbed onto the surface of porous material was investigated by using solid-state fluorescence spectroscopy, fluorescence lifetime analysis, and solid-state 13 C NMR spectroscopy 2.5 Fluorescence spectroscopy An FP-770F fluorescent spectrometer (Japan Spectroscopy Co., Ltd., Tokyo, Japan) was used for stationary fluorescence spectroscopy Powder samples were filled into a front-face reflectance cell (FP-1060) 2.6 Determination of the fluorescence lifetime and relative quantum yield Fluorescence decay profiles were measured by a nanosecond time-resolved single-photon counter with a pulse width of 1.5 ns (Horiba NAES-770, Tokyo, Japan) The exciting pulse and emission response functions were measured simultaneously, and the decay parameters were calculated from two or three exponential functions obtained by deconvolution of the excitation pulse profile using nonlinear least-squares fitting The goodness of fit was assessed by monitoring the value of χ and the distribution of residuals χ values below 1.4 indicate acceptable results for the fluorescence lifetime analysis Materials and methods 2.7 Solid-state NMR spectroscopy 2.1 Materials 13 C Benzoic acid (BA; Nacalai Tesque, Kyoto, Japan) of reagent grade was used without further purification Mesoporous silica FSM-16 (mean pore diameter of 2.1 nm, specific surface area of 1250 m2 /g) was kindly supplied from Toyota Central R&D Labs Inc., Japan FSM-16 was sieved using a 200-µm aperture size sieve and was used after drying under a reduced pressure at 110 ◦ C for h Glasperlen, a nonporous glass powder, from 0.25 to 0.35 mm in radius, was purchased from B Braun Melsungen AG, Germany Physical mixtures were prepared by blending BA and additives in a glass vial for definite intervals 2.2 Powder X-ray diffractometry Powder X-ray diffraction was performed using a Rigaku Miniflex diffractometer (Tokyo, Japan) The measurement conditions were as follows: target, Cu; filter, Ni; voltage, 30 kV; current, 15 mA; scanning speed, 2◦ /min NMR spectra were determined on a JNM-LA400 NMR spectrometer (JEOL, Japan) operating at 100.4 MHz with a CP/MAS (cross-polarization/magic-angle spinning) probe The sample (ca 190 mg) was contained in a cylindrical rotor made of ceramic materials, and spun at 6000 Hz A 90◦ pulse width was about 5.5 µs for both 13 C and H under CP conditions The contact time was ms, and the repetition times were 60 and s in the CP/MAS and PST/MAS (pulse saturation transfer/magic-angle spinning), respectively In the PST/MAS NMR technique, NOE enhancement is used to obtain the 13 C signal This technique enhances peak intensity for mobile carbon [11] 13 C chemical shifts were calibrated indirectly through external adamantane (29.5 ppm relative to TMS) The spectral width and the number of data points were 40 kHz and 16,000, respectively The number of accumulations was 2000 in the CP/MAS and PST/MAS experiments at room temperature Experimental conditions were as follows: temperature, 25.0 ◦ C; H decoupling field amplitude, 50 kHz; rf field amplitude for cross-polarization, 50 kHz 2.3 Thermal analysis A thermogravimetry-differential thermal analysis (TGDTA) was carried out using a MAC Science TG-DTA 2000S (Japan) at a heating rate of ◦ C min−1 under a nitrogen gas flow of 60 ml/min 2.4 Fourier transform infrared (FT-IR) spectroscopy Fourier transform infrared spectra were measured by the KBr disk method at a resolution of cm−1 for 32 scans using a JASCO 230 FT-IR spectrophotometer (Tokyo, Japan) Results and discussion The mixture of benzoic acid and FSM-16 (BA content: 30%) was blended in a glass vial for different periods of time The changes in powder X-ray diffraction (XRD) patterns of the mixture as a function of mixing time are presented in Fig The X-ray diffraction peaks of BA crystals decreased in intensity with duration of mixing time and disappeared completely after blending for min, indicating the disappearance of an ordered arrangement of molecules in the Y Tozuka et al / Journal of Colloid and Interface Science 291 (2005) 471–476 473 Table Fluorescence lifetime (τ ) and relative quantum yield (Q) of benzoic acid in various systems, λex = 262.7 nm BA crystals 30% BA–70% Glasperlen 30% BA–70% FSM-16 Fig Powder X-ray diffraction patterns of the 30% benzoic acid (BA)–70% FSM-16 system after mixing for different intervals: (a) 30 s, (b) 60 s, (c) 300 s Fig Solid-state fluorescence emission spectra of the 30% BA–70% FSM-16 system after storage at 25 ◦ C, λex = 262.7 nm (a) min, (b) min, (c) 15 min, (d) h, (e) h, (f) 72 h crystals Since FSM-16 had large specific surface area and hydrophobic features [12], it was assumed that BA molecules showed a rapid adsorption on mixing with FSM-16 To investigate the adsorption profiles of BA on FSM-16, BA and FSM-16 (BA content: 30%) were mixed by a spatula for approximately s and then placed in the sample holder for fluorescence spectroscopy measurement Fig shows the solid-state fluorescence emission spectra of 30% BA and 70% FSM-16 system as a function of storage time Two emission peaks were observed at 317 and 386 nm The λobs (nm) τ1 (ns) Q1 (%) τ2 (ns) Q2 (%) χ2 317.0 317.0 386.0 0.454 0.120 0.099 15.2 29.4 44.6 2.58 2.76 6.57 84.8 70.6 55.4 1.30 1.24 1.36 emission peak at 317 nm, which was identical to that in BA crystals, gradually decreased in intensity with an increase in storage time On the other hand, the intensity of a new emission peak at 386 nm increased with storage time accompanied by the disappearance of the emission intensity of BA crystals at 317 nm The new emission at 386 nm was not observed when BA crystals were blended with nonporous glass or other pharmaceutical additives, i.e., microcrystalline cellulose, glucose, lactose, mannitol, or sucrose Such an anomalously large Stokes shift might be explained by either the excited state with intramolecular proton transfer or the excimer-like emission of benzene moieties of BA molecules Denisov et al reported that the anomalously large Stokes shift, found in fluorescence emission spectra of molecules like salicylic acid and its derivatives, was characterized by a strong intramolecular hydrogen bond and the low-frequency band attributed to the excited state with intramolecular proton transfer [13] As it is impossible for benzoic acid to form an intramolecular hydrogen bond due to its structure, the anomalously large Stokes shift is due to another reason Aromatic compounds like benzene, naphthalene, and pyrene are known to form van der Waals dimers and to show excimer emission with large Stokes shifts [14] For excimer emission, it was reported that excimer formation occurs more easily in small pores than that in large pores [15,16] The mean pore width of FSM-16 used was estimated as 2.1 nm, which might be suitable for BA molecules to form parallel or Tshaped van der Waals dimers The new emission peak might be due to the excimer emission resulting from van der Waals dimer-like contact of adjacent BA molecules on the FSM-16 surface Fluorescence-decay kinetics was investigated for BAFSM-16 mixtures of different dispersibility The lifetime and relative quantum yield are listed in Table The component of a short lifetime (τ1 ) was estimated as a noise component [8] The fluorescence lifetimes of BA observed in the Glasperlen (nonporous glass) system were similar to those observed in BA crystals When the observation wavelength was fixed at 317.0 nm, it was difficult to determine the fluorescence lifetime in the mixture of 30% BA and 70% FSM-16, as χ values could not be converged due to low fluorophore concentration On the other hand, when the observation wavelength was fixed at the new emission peak at 386 nm, the lifetime of the second component was estimated as 6.57 ns, which was long enough compared to that observed in BA crystals This result also supported that BA 474 Y Tozuka et al / Journal of Colloid and Interface Science 291 (2005) 471–476 Fig Thermogravimetric curves of (a) 30% BA–70% Glasperlen (nonporous glass) and (b) 30% BA–70% FSM-16 Fig FT-IR spectra of the 30% BA–70% FSM-16 system: (a) BA crystals, (b) physical mixture of 30% BA–70% FSM-16 mixed for min, (c) FSM-16 molecules change drastically their molecular states during a simple blending with FSM-16 To investigate the molecular interaction between BA and FSM-16, IR measurement was performed (Fig 3) The absorption band observed at 1685 cm−1 in the spectrum of BA crystals was assigned to the carbonyl-stretching vibration, where BA molecules formed a centrosymmetric hydrogenbonded dimer in the crystal [17] Infrared spectrum of the physical mixture of 30% BA and 70% FSM-16 showed a new peak at 1702 cm−1 , indicating a dramatic change in the molecular state of BA during a mixing Sinhal et al reported that BA showed a carbonyl-stretching band at 1712 cm−1 when BA molecules were dispersed and interacted with the “CH2 OH” group of ethyl cellulose [18] We propose that the centrosymmetric hydrogen-bonded dimer in the BA crystals breaks and a hydrogen bond may form between the carbonyl groups of the adsorbed BA molecules and the surface of FSM-16 The thermogravimetry curves of the 30% BA–70% Glasperlen and the 30% BA–70% FSM-16 systems are illustrated in Fig The weight loss from the mixture of 70% Glasperlen was observed from 90 to 150 ◦ C, which was due to the sublimation of BA molecules On the other hand, the weight loss of BA from the mixture with FSM-16 was significantly suppressed The suppression of BA sublimation indicated that BA molecules must be trapped in the pore of FSM-16 by the surface interaction between the surface of FSM-16 and the BA molecules Although strong evidence was not found for the interaction between the silanol groups Fig Solid-state 13 C CP/MAS spectra of (a) BA crystals, (b) 30% BA–70% Glasperlen, (c) 30% BA–70% FSM-16 and carbonyl groups in IR spectra, suppression of BA sublimation may be due to its interaction with the FSM-16 surface However, it should be pointed out that there must be many BA molecules that are not interacting with silanol groups It is known that hysteresis is observed in adsorption and desorption isotherms of organic gases on mesoporous material This is based on the affinity of the adsorbate to the surface of the mesoporous material and its surface geometry [19,20] Thermogravimetry indicates a weight loss of BA from FSM-16 during desorption Therefore, suppression of BA sublimation might be due to a similar mechanism Y Tozuka et al / Journal of Colloid and Interface Science 291 (2005) 471–476 475 Fig Solid-state 13 C CP/MAS and PST/MAS spectra of BA in the 5-min mixture with FSM-16: (a) BA crystals, (b) 30% BA–70% FSM-16, (c) 10% BA–90% FSM-16 The solid-state 13 C CP/MAS spectra of BA, 30% BA– 70% Glasperlen, and 30% BA–70% FSM-16 are shown in Fig All NMR peaks were attributed to the carbon atoms of BA molecules, as both Glasperlen and FSM-16 have no carbon atoms in their structure The chemical shift observed at 172 ppm (•) in (a) and (b) was derived from the carbonyl carbon of BA molecules Whilst the peak positions in 30% BA–70% Glasperlen were identical to those of BA crystals, the resonance shifted to a higher magnetic field at 165.5 ppm (•) after mixing with FSM-16 Although this broad resonance is not clear and the magnitude of this shift was considered to be small, since the relaxation time of carbonyl carbon was comparatively long, it might be related to the interactions of some BA molecules with the FSM-16 surface The peaks between 60 and 70 ppm were attributed to the spinning sidebands of benzene carbons of BA molecules due to the chemical-shift anisotropy The disappearance of spinning sidebands in the BA/FSM-16 sample indicate that BA crystals not exist in this sample Fig shows the solid-state 13 C CP/MAS and PST/MAS spectra of BA–FSM-16 system with different BA contents Pulse saturation transfer is a technique using a nuclear Overhauser effect to obtain the 13 C signal, resulting in an enhancement of the 13 C magnetizations by saturated carbons The PST/MAS method has been reported to be useful for emphasizing the signal intensity of carbons in the flexible regions [21] A negligible peak of BA crystals was observed in the PST/MAS spectrum, because a repetition times of s was insufficient to relax the carbon atoms in BA crystals The physical mixtures of BA and FSM-16 clearly showed the chemical shifts arising from both the carbonyl carbon at 172 ppm and the benzene carbons at around 130 ppm The signals of benzene carbons were very sharp and their spinning side bands were not observed, despite that the spinning side bands were generally observed due to the chemical shift anisotropy No spinning side bands in the spectra suggested that the chemical-shift anisotropy is being averaged by high molecular mobility This is similar to the fast motion of p-nitroaniline molecules after heat treatment with FSM-type mesoporous silica as reported by Komori and Hayashi [22] With respect to the signal from the carbonyl carbons, the mixture showed a sharp peak at 172 ppm in the 13 C PST/MAS spectrum The magnitude of signal intensity from benzene and carbonyl carbons increased with the increase of the concentration of BA, showing a significant amount of BA molecules adsorbed on the FSM-16 surface with higher mobility For adsorption profiles of organic compounds on a mesoporous structure, monolayer adsorption, multilayer adsorption, and/or capillary condensation can occur depending on the amount of adsorbent and texture of the mesoporous material Previous work has shown that acetonitrile adsorbed onto mesoporous materials forms hydrogen bonds with the surface hydroxyl groups and further adsorption takes place through physisorption [23] In the case of mixtures with FSM-16, the resonance enhancement observed by PST/MAS might arise from weakly adsorbed BA molecules (i.e., hydrophobic interaction) or the formation of multiple layers As described in the fluorescence studies, a mixture of 30% BA and 70% FSM-16 shows the two emission peaks at 317 and 386 nm The anomalously large Stokes shift of BA observed at 386 nm was observed in the long lifetime component of the fluorescence decay curve as well The above new emission and the sharp carbon signals in 13 C PST/MAS spectra were especially strong for the mixture of 30% BA and 70% FSM-16, indicating that the BA molecules with 476 Y Tozuka et al / Journal of Colloid and Interface Science 291 (2005) 471–476 high molecular mobility could be related to the BA molecules that showed the new fluorescence emission peak Conclusion Blending with FSM-16 drastically changes the molecular state of BA BA molecules adsorbed onto the FSM-16 surface showed high molecular mobility of carbon in terms of the relaxation time, leading to a sharp carbon resonance in the 13 C PST/MAS spectra The state of adsorbed BA molecules could be determined by solid-state fluorescence spectroscopy, fluorescence lifetime analysis, and solid-state 13 C NMR spectroscopy For pharmaceutical formulation, it has been generally recognized that the molecular state of organic compounds strongly affects the dissolution properties or a stability of organic compound Hence the detection and evaluation of molecular states of pharmaceutical active ingredients are important for the control of their properties This study reveals that solid-state 13 C NMR spectroscopy is a useful technique for estimating molecular states of pharmaceutically active ingredients in mesoporous additives Acknowledgments The authors thank Toyota Central R&D Labs., Inc., Japan, for the kind provision of FSM-16 We also thank Dr H Seki for her valuable assistance with the solid-state NMR applications This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (17790029) References [1] Y Tozuka, T Oguchi, K Yamamoto, Pharm Res 20 (2003) 926 [2] P Pendleton, S.H Wu, J Colloid Interface Sci 266 (2003) 245 [3] H Rong, Z Ryu, J Zheng, Y Zhang, J Colloid Interface Sci 266 (2003) 245 [4] D Gao, Z Shen, J Chen, X Zhang, Micropor Mesopor Mater 67 (2004) 159 [5] T Ishikawa, M Matsuda, A Yasukawa, K Kandori, S Inagaki, T Fukushima, S Kondo, J Chem Soc Faraday Trans 92 (1996) 1985 [6] J.H Lee, S.W Jung, I.S Kim, Y.I Jeong, Y.H Kim, S.-H Kim, Int J Pharm 251 (2003) 23 [7] E Junquera, E Aicart, Int J Pharm 1761 (1999) 169 [8] Y Tozuka, E Yonemochi, T Oguchi, K Yamamoto, Bull Chem Soc Jpn 73 (2000) 1567 [9] G.V Mooter, M Wuyts, N Blaton, R Busson, P Grobet, P Augustijns, R Kinget, Eur J Pharm Sci 12 (2001) 261 [10] C Mayer, G Lukowski, Pharm Res 17 (2000) 486 [11] H Yoshimizu, H Mimura, I Ando, J Mol Struct 246 (1991) 367 [12] A Matsumoto, T Sasaki, N Nishimiya, K Tsutsumi, Colloids Surf A Physicochem Eng Aspects 203 (2002) 185 [13] G.S Denisov, N.S Golubev, V.M Schreiber, S.S Shajakhmedov, A.V Shurukhina, J Mol Struct 381 (1996) 73 [14] H Hirata, H Ikeda, H Saigusa, J Phys Chem A 95 (1999) 1014 [15] T Fujii, A Ishi, N Takusagawa, H Yamashita, M Anpo, J Photochem Photobiol 86 (1995) 219 [16] Y Tozuka, E Tashiro, E Yonemochi, T Oguchi, K Yamamoto, J Colloid Interface Sci 248 (2002) 239 [17] G.A Sim, J.M Robertson, T.H Goodwin, Acta Crystallogr (1955) 157 [18] R Singhal, A.K Nagpal, G.N Mathur, J Thermal Anal Cal 58 (1999) 29 [19] S Inagaki, Y Fukushima, Micropor Mesopor Mater 21 (1998) 667 [20] M McNall, R.L Laurence, W Curtis Conner, Micropor Mesopor Mater 44–45 (2001) 709 [21] H Yasunaga, I Ando, J Mol Struct 301 (1993) 129 [22] Y Komori, S Hayashi, Micropor Mesopor Mater 68 (2004) 111 [23] H Tanaka, T Iiyama, N Uekawa, T Suzuki, A Matsumoto, K.K Unger, K Kaneko, Chem Phys Lett 293 (1998) 541