Rapid adsorption and entrapment of benzoic acid molecules onto

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 1 ,
Keiji Yamamoto
Graduate School of Pharmaceutical Sciences, Chiba University, 1-33 Yayoicho, Inage-ku, Chiba 263-8522, Japan
Received 14 March 2005; accepted 6 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 5 min
(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

1. 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: (Y. Tozuka).
1 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-


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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 2 and the distribution of residuals. χ 2 values below 1.4 indicate acceptable results for the
fluorescence lifetime analysis.

2. 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 3 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 1 H
under CP conditions. The contact time was 5 ms, and the repetition times were 60 and 5 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; 1 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 5 ◦ 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 2 cm−1 for 32 scans using
a JASCO 230 FT-IR spectrophotometer (Tokyo, Japan).

3. 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. 1. The X-ray diffraction peaks of BA crystals
decreased in intensity with duration of mixing time and disappeared completely after blending for 5 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 1
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. 1. 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. 2. Solid-state fluorescence emission spectra of the 30% BA–70%
FSM-16 system after storage at 25 ◦ C, λex = 262.7 nm. (a) 0 min, (b) 3 min,
(c) 15 min, (d) 1 h, (e) 9 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 5 s and then placed in the sample
holder for fluorescence spectroscopy measurement. Fig. 2
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 1. 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 2 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


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Fig. 4. Thermogravimetric curves of (a) 30% BA–70% Glasperlen (nonporous glass) and (b) 30% BA–70% FSM-16.

Fig. 3. 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 5 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. 4. 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. 5. 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. 6. 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. 5. 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 do not exist in this sample.
Fig. 6 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 5 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


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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.

4. 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).

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