NMR studies on supercritical fluids in nanoporous materials

129Xe NMR spectroscopy is applied to investigate the physical properties of Xe confined in porous glasses up to the supercritical region. The chemical shift of confined Xe is larger than that of bulk Xe, which is approximately proportional to density, and the upward deviation is more enhanced for smaller pore. The deviation is large in the gas phase, and has a maximum far below the critical density, while it gradually decreases and approaches zero in the liquid phase. Below the critical density the both of the size and surface effects are observed, while the surface effect becomes minor and the size effect persists above the critical density. The chemical shift at the dilute limit depends on the surface properties of the matrix, as well as the pore size. [DOI: 10.1380/ejssnt.2005.338]


I. INTRODUCTION
NMR spectroscopy, which is a good probe for microscopic environment of a specific nucleus, is a powerful tool to investigate molecules confined in porous solids, because various non-metallic and non-magnetic matrices are transparent to the radio-frequency wave. 129 Xe NMR spectroscopy is commonly utilized for characterization of the structure of porous materials, in particular, a variety of zeolites [1,2]. because the large polarizability causes wide variation of the chemical shift.
Physical properties of fluids, such as the phase behavior [3], are affected by confinement even at high densities.
Since NMR spectroscopy provides various information on fluid, such as the local density and the rotational and translational motions, the technique is also suitable for direct observation of confined fluids. Moreover, recent our achievements in high-pressure NMR measurements enable us to explore over a wide temperature and pressure range, which could cover the whole fluid phase from the dilute gas region to the dense liquid region including the supercritical state.
Xe is a simple van der Waals fluid, which is composed of spherical molecules with 0.4 nm diameter [4]. The critical constants for Xe are T c =289.7 K, P c =5.84 MPa and d c =1.10 g cm −3 [5]. The critical point is close to that of carbon dioxide, and the supercritical condition is experimentally accessible. Moreover, 129 Xe NMR spectroscopy has advantages of relatively high sensitivity and wide variation of the chemical shift. Then, confined Xe could be for a model system for micro-and nanofluidics, where supercritical carbon dioxide might be utilized instead of the conventional organic solvents.
In this work, 129 Xe NMR spectroscopy is applied to investigation of fluid Xe confined in porous glasses. The chemical shift of confined Xe is obtained in a wide range * This paper was presented at International Symposium on Surface Science and Nanotechnology (ISSS-4), Saitama, Japan, 14-17 November, 2005. † Corresponding author: m-kanakubo@aist.go.jp of density from the gas phase to the liquid phase including the supercritical region. By varying the pore diameter and the surface functional groups of the matrices, the size and the surface effects on confined Xe are discussed.

II. EXPERIMENTAL
A Controlled-Pore Glass (CPG), which is a porous silica glass, was used as the matrix. The CPG provided as 80-120 mesh powder was purchased from Sigma Aldrich Co. Ltd. The physical properties of the CPG were listed in Table I. The CPG and the surface-modified CPG (CPG-Me) were treated as follows. The CPG was washed in nitric acid and rinsed by distilled water for several times, and then, dried at 423 K for one hour. The surface of the purified CPG was modified by the silylation procedure with hexamethyldisilazane [6], where the surface hydroxyl groups were converted to trimethylsilyl groups. Note that the silylation procedure does not significantly affect the pore size distribution [6,7]. Each matrix (CPG or CPG-Me) was filled in a capillary tube, where one end of the tube was sealed and the other end was open and capped by filling pure quartz wool. The matrices filled in capillary tubes were dried again at 423 K for one hour, and three capillary tubes were installed in the sample cell. The chemical shifts are simultaneously obtained for Xe confined in three different matrices, as well as for bulk Xe.
129 Xe NMR spectra were obtained with a spectrometer Varian Inova 500. A high-pressure NMR sample cell [8] was made of a poly(etherether ketone) tube of 10 mm diameter and a titanium cap. The sample cell was connected to a syringe pump (ISCO260D), which was used for compression of pure Xe (99.995%, Japan Fine Products Corp.). The pressure was monitored by a digital pressure indicator (Druck DPI 145), where the uncertainty was less than ±0.1 MPa. The NMR measurements were carried out along the isotherm of 300 K up to 15 MPa. The sample temperature was determined in advance with a calibrated thermometer (Takara D641) with a precision of ±0.1 K.  [9] represented by the solid line. The critical density is also depicted by the arrow.

III. RESULTS AND DISCUSSION
In Figure 1, the chemical shifts of bulk and confined Xe are plotted as a function of density. Here we set the chemical shift of bulk Xe at the dilute limit to zero. The error bars estimated from the spectral widths are also shown. The observed chemical shift, δ bulk obs , of bulk Xe is approximately linear to density, and the density dependence agrees well with the reported one [9]. The observed chemical shift, δ conf obs , of Xe confined in the CPG is appreciably larger than that of δ bulk obs . In the gas phase, δ conf obs shows significant upward deviation from δ bulk obs . Above the critical density, the upward deviation becomes smaller, and approaches zero at the high density limit. The upward deviation of δ conf obs from δ bulk obs is more clearly observed for smaller pore, and it decreases rapidly with increasing the pore diameter. These results are consistent with the chemical shifts of Xe confined in the FSM-16 with 1.9-4.1 nm pore [10].
To discuss the effect of confinement, we define the chemical shift, δ conf , of confinement by In Figure 2, δ conf for the CPG and the CPG-Me is shown as a function of density. In the dilute gas region, δ conf increases with density, and has a maximum below d c . At high densities, δ conf gradually decreases with increasing density, and approaches zero at the high density limit. These behaviors of δ conf may be explained by the interactions of Xe, that is, the intermolecular interaction (fluidfluid interaction) and the interaction to the wall (fluidwall interaction). In the gas phase, where the fluid-wall interaction is more important, the chemical shift is much enhanced for the CPG than the CPG-Me, because the surface hydroxyl groups interact with Xe molecules more strongly than the trimethylsilyl groups. Above d c , where the fluid-fluid interaction is more important, the surface effect becomes very small, while the size effect persists even in the liquid state. The maximum of δ conf is located at ∼0.2 g cm −3 for the CPG, and at ∼0.5 g cm −3 for the CPG-Me. This apparent shift of the maximum due to the surface modification is a consequence of the change in the fluid-wall interaction. The fluid-fluid interaction is also affected by confinement, because the local density of Xe is somewhat different from the bulk density. For gaseous Xe in various zeolites, the chemical shift of confined Xe, which is also significantly larger than that of bulk Xe, is a linear function of the amount of adsorption [1]. Even at high densities near the supercritical region, chemical shift of Xe confined in zeolites is described in terms of the Langmuir's http://www.sssj.org/ejssnt (J-Stage: http://ejssnt.jstage.jst.go.jp) The chemical shift, δ0, at the dilute limit for various porous materials as a function of the pore diameter σ. δ0 is determined for the CPG, the CPG-Me and the Vycor glass [13] at 300K, and for the FSM-16 at 298 K [10]. The dotted line denotes the empirical relation for zeolites with cylindrical pore with infinite length at 299 K [15].
adsorption isotherm [11]. We have found that the mean density in nanopore of SF 6 confined in the mesoporous Vycor glass shows a similar maximum below the critical density, and that the mean density well accounts for the 19 F NMR chemical shift of confined SF 6 [12]. These facts suggest that the density enhancement in nanopore plays an important role in determining δ conf over a wide range of density not only in the gas phase.
At the dilute limit, δ conf is expected to be independent of the fluid-fluid interaction. Then, we define the chemical shift, δ 0 , at the dilute limit by In Figure 3, δ 0 is plotted against the pore diameter in a double logarithmic plot together with those for the FSM-16 [10] and the Vycor glass [13]. Although the porous structures are different between porous glasses and mesoporous silica, a single curve is obtained for these silica materials. These results are consistent with the correlation for Xe at ∼0.1 MPa confined in various silica materials [14]. In Figure 3, the dotted line represents δ 0 for zeolite with a shape of cylinder of infinite length, which is estimated from the empirical relation [15]. Although the line does not coincide with that for the silica materials, the gradient of the curve below 3 nm is similar. By replacing the surface hydroxyl groups by the trimethylsilyl groups, δ 0 decreases by ∼10, which depends little on the pore size. One might think that the pore size of the CPG-Me is smaller than the CPG, however, such reduction of the pore diameter should increase the chemical shift. Then, the lower shift of δ 0 is explained by the surface effect. For the Vycor glass [13], which has strongly adsorptive surface, δ 0 is larger by 25 than other mesoporous silica materials. Note that we have also found that δ 0 for the Vycor glass before the activation process, by which adsorbed substances are removed, is smaller by ∼30% than that for the Vycor glass. Then, δ 0 is a measure of the surface condition of the porous materials, as well as the pore diameter.

IV. CONCLUSION
129 Xe NMR spectroscopy is applied to investigation of Xe confined in the Controlled-Pore Glass (CPG) and the surface-modified CPG. The chemical shifts of bulk and confined Xe are obtained simultaneously up to the supercritical region. The density dependence of the chemical shift for confined Xe is affected by the matrix. Below the critical density the both of the size and surface effects are observed, while the surface effect becomes minor and the size effect persists above the critical density. The chemical shift of confined Xe at the dilute limit depends on the surface potential of the matrix, as well as the pore size.