ARTICLE pubs.acs.org/Langmuir
Water Structure and Dynamics at a Silica Surface: Pake Doublets in 1H NMR Spectra Christian Totland,‡ Signe Steinkopf,† Anne Marit Blokhus,‡,§ and Willy Nerdal*,‡ †
Bergen University College and ‡Department of Chemistry, University of Bergen, N-5007 Bergen, Norway § Centre for Integrated Petroleum Research (CIPR), Allegaten 41, N-5007 Bergen, Norway ABSTRACT: Detailed knowledge about the dynamics and structure of liquids in the vicinity of a solid surface is important in several fields of research. In this study a homogeneous model system of colloidal and nonporous silica particles with a narrow particle size distribution was used to examine such properties of adsorbed water and 1-heptanol. Doublet 1H water resonances (“Pake doublets”) indicate a preferred spatial orientation for the water molecules, as well as a lower molecular density in the surface-induced water structures compared to bulk water. These surface-induced structures are found to extend at least 8 nm from the silica surface. T1 relaxation measurements at several temperatures indicate weaker H-bonding in the adsorbed water compared to bulk water. T2 relaxation measurements at several temperatures reveal the presence of two water phases and give quantitative information on the mobility of water molecules and proton exchange processes. The presence of 1-heptanol changes the water characteristics, primarily in the water phase closer to the surface, where water molecules experience decreased translational and increased rotational freedom. In the absence of water, adsorbed 1-heptanol forms surface aggregates encompassing several molecular layers, where the first adsorbed layer shows severe restrictions in mobility and subsequent layers are more mobile.
1. INTRODUCTION Knowledge about the properties of water in the vicinity of a surface is useful in many different areas of research. In medicinal research such information can be valuable as cellular water is associated with the cell membrane, and also the stability and dynamics of biological macromolecules such as proteins largely depend on adsorbed water. Furthermore, information about liquid surface interactions is important to understand and improve the transport of fluids in porous media, such as oil and water in reservoir rocks. NMR relaxation measurements play an important role in the investigation of molecular dynamics due to the high sensitivity to changes in molecular mobility. However, employing high-field NMR on rock samples to obtain quantitative information about liquid/surface interactions is hampered by the presence of paramagnetic species on the solid surface. For this reason there have been numerous NMR studies employing silica model systems over the past 60 years.19 Despite this, uncertainty still exists about the properties of water in the vicinity of a solid surface. It is generally accepted that, in the absence of a solid surface, a water molecule is transiently networked through 34 hydrogen bonds with varying bond length, angle, and strength. In the presence of a solid surface, the opportunity for water molecules to form H-bonds is reduced, causing the water molecule to reorient itself so that it forms as many H-bonds as possible while, at the same time, balancing its dipolar interaction and density. This causes a significantly ordered structure in the layer of water molecules adjacent to the surface.10 As a r 2011 American Chemical Society
consequence, water molecules in the next layers reorient in a similar way, creating a cascade effect over some distance toward the bulk water phase. However, detailed knowledge about the structure and dynamics of water molecules in these surfaceinduced water structures, as well as their range, is still limited. Here we aim to further increase knowledge about water adsorbed on a solid surface and to see how addition of the amphiphile molecule 1-heptanol will affect the water properties at the silica surface, as well as to study the adsorption of 1-heptanol. Previous studies have focused on how amphiphile molecules such as alcohols affect water in bulk solutions,11,12 but less is known about the effects on water adsorbed on a solid surface. This knowledge is important as it has become generally accepted that one of the main factors governing, for example, the formation of surfactant aggregates or the interaction between biomolecules is the water structure.10,13 Conversely, the presence of certain molecules near a solid surface may, to some extent, dictate the local water structure in the vicinity of the surface. As a result, the interfacial water structure may also serve as a sensitive probe for adsorption events and adsorbed structure. Colloidal and nonporous silica particles with an average diameter of 40 nm were used in this study as a model rock carrying water and 1-heptanol. It was assumed that the Arrhenius equation describes the temperature dependence of the spinlattice (T1) Received: December 10, 2010 Revised: January 26, 2011 Published: March 17, 2011 4690
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Langmuir relaxation. Thus, employing a well-defined silica model system and high-field NMR, T1 relaxation measurements at several temperatures give information about molecular dynamics and the interaction of water and hydrocarbons with the silica surface by providing activation energies for molecular mobility. Characteristics of the liquid molecules and the solid surface produce an anisotropic environment for the molecular motion that influences the T1 relaxation and the corresponding Arrhenius relationship of the molecular correlation time.14 NMR spinspin relaxation (T2) measurements are sensitive to surface interactions of the liquid molecules and can give information on molecular dynamics of a shorter time scale than spinlattice relaxation. From previous studies it is known that the relaxation of water adsorbed on a solid surface decays multiexponentially, with two or more relaxation components corresponding to water in different phases. For the case of 0.12.0 layers of water adsorbed on various surfaces, this multiexponential behavior has been thoroughly investigated,14,15 and two T2 relaxation components are typically found in the relaxation data. With a temperature increase, the longer T2 value has been found to decrease along with a fractional population increase. This suggests proton transfer between two environments with distinct relaxation characteristics and is described by the theory proposed by Zimmerman and Brittin (ZB theory) for multiple-phase relaxation.3 O’Reilly et al.16 demonstrated that the shorter relaxation component is due to OH groups on the adsorbent surface and that the longer relaxation component will be due to water physically adsorbed to the surface. Overloop and Van Gerven5,6 addressed the nature of the multicomponent NMR relaxation in porous systems containing higher quantities of water. In their work the relaxation was monitored as a function of the water content. Porous systems with higher quantities of water display at least two water environments with different NMR relaxation rates: (1) water present in the adsorbed water layers close to the surface and (2) water further away from the surface. In addition to the two water phases, the OH protons of the adsorbent surface are also considered due to the occurrence of proton exchange with the adsorbed water. However, due to cross-relaxation between surface OH protons and water protons, a reduction of the signal corresponding to the surface OH protons occurs, making it hard to observe even with smaller amounts of water in the samples.6 In long-range-ordered media, where the dipolar interaction is not averaged out by fast molecular motions, the fine structure of the water proton resonance can be a doublet spaced around the water chemical shift.17 In this study observation of the proton resonances as Pake doublets revealed further information about the water properties. In addition to the necessary restrictions in molecular mobility, a Pake doublet also confirms the presence of somewhat isolated water molecules in the sample. In principle, the splitting of these two resonances is due to the orientation of the protonproton vectors in the bound water molecules with respect to the direction of the applied magnetic field as well as the motion the water molecule experiences.18 Such effects have previously been found in proton NMR studies on hydrated sodium β-alumina,19 water in clays,2022 and water on oriented collagen fibers,15 where motional averaging for all possible orientations yields the observed Pake doublet. In the study presented here we show that both the relaxation time characteristics and the appearance of NMR proton resonances give information about the liquid properties and interactions with the silica surface.
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Table 1. Specifications for the Aerosil OX50 Silica Particles and the Silica Samples Aerosil OX50 average diametera
40 nm
surface areaa
50 ( 15 m2/g
silanol densityb silanol density in superhydratedb OX50
2.0 OH groups/nm2 3.3 OH groups/nm2
H-bound H2O density in superhydratedb OX50
1.3 H2O molecules/nm2
mobile H2O density in superhydratedb OX50
2.7 H2O molecules/nm2
Samples SiO2/liquid ratio (w/w)
1.5 ( 0.2
silanol/water ratio (mol/mol)
7.4 103
a
Values obtained from the producer, Degussa AG. b Reference 25 (superhydrated means exposed to water vapor until equilibrium water levels are obtained (∼4 days)).
2. MATERIALS AND METHODS The Aerosil OX50 (Degussa AG, Germany) adsorbent is used in this study. These silica particles have a high chemical purity, making them suited for a homogeneous sample.23 The silica particles were hydrated with high-purity water, water with dissolved 1-heptanol, or 1-heptanol (98%, Sigma-Aldrich), referred to as the silica/water, silica/water/1heptanol, and silica/1-heptanol samples, respectively, in the succeeding text. The 1-heptanol solubility is about 1 g/L in water,24 giving a total of about 1% 1-heptanol in the water/1-heptanol solution used. 2.1. Sample Preparation. Samples were prepared by mixing silica particles with an excess of liquid in a sample tube. The samples were then equilibrated at 303 K for five days, and excess liquid was removed by high-speed centrifugation at 18 000 rpm. The centrifugation was repeated until no further liquid could be removed from the samples. Immediately after centrifugation, the samples were packed into 4 mm ZrO2 MAS (magic angle spinning) rotors and sealed with Kel-F rotor caps. By avoiding any significant evaporation of the sample fluid, the final sample consisted of a close-packed array of silica particles where the space between the particles is saturated with fluid. The amounts of fluid and silica in the samples were controlled by weighing, and the silica/fluid ratio did not vary significantly between samples. The specifications of the Aerosil OX50 particles and the samples are listed in Table 1. Initial NMR experiments were carried out immediately after sample preparation, as well as after six months of storage in the sealed MAS rotors at 277 K. The spinspin relaxation times, the fractional population of the relaxation components, and the 1H NMR spectra had to some degree changed compared to the data obtained on the freshly prepared samples. However, the spinlattice relaxation was not significantly affected by the six months of storage. In a study on sealed porous silica samples with adsorbed water that was kept for one year, it was found that both the T1 and T2 values had changed compared to the initial measurements,1 presumably due to a change of surface characteristics of the porous silica. The results presented here are from the experiments carried out after six months of sample storage. 2.2. NMR Experiments. All NMR experiments were carried out on a Bruker Advance 500 Ultrashield spectrometer operating at 500 MHz for protons, with a variable temperature control accurate to (0.2 K. For the silica-containing samples, an MAS probe head was used and spectra were recorded with a spinning speed of 5 kHz to identify the isotropic chemical shifts. For the pure liquid samples, a probe head for standard 5 mm NMR tubes was used. T1 relaxation times were obtained using an inversion recovery pulse sequence that compensates for radio frequency field inhomogeneity.26 The T2 relaxation data were recorded using a CarrPurcellMeiboomGill 4691
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Figure 1. Top: spectra of the silica/water sample at various temperatures. Δν1/2 is constant down to 258 K with an average value of 848 ( 7 Hz. The signal intensity is reduced gradually with reduced temperatures. Bottom: spectra of the silica/water/1-heptanol sample at various temperatures. Both the signal intensity and Δν1/2 are constant down to 263 K with an average value of Δν1/2 = 1160 ( 15 Hz. Enlargement of the water peak at 256 K (silica/water) and 258 K (silica/water/1-heptanol) reveals a Gaussian line shape. The position of the isotropic chemical shift was normalized to unity in all of the spectra. (CPMG) pulse sequence27 and then fitted to mono- or multiexponential decay curves using MATLAB software (version 7.5.0, The Mathworks, Natick, MA). T1 and T2 relaxation experiments were carried out every 5 K in the temperature ranges 278328 and 263333 K, respectively. Furthermore, the T1 and T2 relaxation experiments, as well as the onepulse 1H NMR experiments, on silica-containing samples were carried out on nonspinning (static) samples kept in MAS rotors to avoid the influence of sample spinning on the measured relaxation rates. In our experience water adsorbed on silica shows a T1 relaxation about 0.3 s lower at an MAS spinning rate of 5 kHz than without sample spinning, probably due to reduced spin diffusion efficiency at higher spinning rates.28 However, to get resolved spectra, relaxation measurements on the silica/1-heptanol sample were carried out using an MAS spinning rate of 5 kHz. The resolved spectra made it possible to measure T1 relaxation of each individual resonance from the 1-heptanol molecule. The pulse sequence used for the T2 measurements is not selective to any resonance and gives an average relaxation value for the molecule as a whole. To examine the nature of the 1-heptanol 1H resonances in the nonspinning spectra, simulated spectra were obtained using the spectral line shape analysis feature in Topspin 1.3 software (Bruker BioSpin 2005).
3. RESULTS Figure 1 shows static 1H NMR spectra acquired on the silica/ water sample between 278 and 256 K and on the silica/water/1heptanol sample acquired between 278 and 258 K. In general, a
proton NMR spectrum of ice will give a Gaussian line shape, and at 273 K the line width has been found to be 72 kHz, corresponding to a T2 value of only 5.6 μs.29 Due to the large spectral line width of ice, water that freezes will not contribute significantly to the signal intensity of the resonances observed in Figure 1, and thus, the relative resonance intensity below 273 K corresponds to the amount of nonfrozen water in the samples. For the silica/water/1-heptanol sample the resonance intensity is constant down to 263 K (Figure 1, bottom), and at 260 K the resonance intensity is reduced by 89% and at 258 K by 99.6% compared to the resonance intensity at 278 K. This indicates that about 90% of the water molecules in the silica/water/1-heptanol sample have similar properties and freeze at about 260 K. The free surface energy of water molecules in the vicinity of the surface/water interface may be reduced further due to adsorption interactions and might explain why ∼10% of the water freezes at temperatures below 260 K. Contrasting this, the resonance intensity of the silica/water sample is gradually reduced between 273 and 258 K (Figure 1, top). For instance, at 265 K the resonance intensity is reduced by 27% and at 258 K by 46%, and at 256 K 99.4% of the resonance intensity has disappeared. These results indicate that the silica/ water sample may contain several water environments with different properties, causing the water in the sample to freeze at different temperatures. 4692
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Figure 2. Arrhenius plot of T1 as a function of temperature for the samples: water (0) and silica/water (2). The activation energies are calculated from the slope of the plots. The activation energy, 15.3 kJ/ mol, for the silica/water sample is consistent with a reduction in hydrogen bonding compared to that in pure water.
Figure 3. Arrhenius plot of T1 as a function of temperature for the samples: water/1-heptanol (0) and silica/water/1-heptanol (2). The activation energies are calculated from the slope of the plots. The activation energy, 14.2 kJ/mol, for the silica/water/1-heptanol sample is similar to the value, 15.3 kJ/mol, for the silica/water sample.
The spectra of the silica/water/1-heptanol (Figure 1, bottom) have the appearance of a Pake doublet, where the line width, Δν1/2, and doublet separation are constant over a temperature range of 263343 K (spectra with sample temperatures above 278 K are not shown) with average values of 1160 ( 10 and 744 ( 6 Hz, respectively. Between 260 and 258 K, a broadening of the doublet is observed in that Δν1/2 = 1520 Hz at 260 K and at 258 K the doublet feature disappears and the resonance is broadened to a line width of 2720 Hz. The amount of nonfrozen water molecules that give rise to the Gaussian peak at 258 K approximately corresponds to the amount of water molecules that cover the surface once (∼1% of the total water in the sample), indicating that this peak comes from molecules in the first adsorbed monolayer of water on the silica particles. Previous studies on porous silica systems show that water molecules in the first two or three monolayers remain unfrozen even at temperatures below 150 K and that water in these monolayers adopt a structure different from that of bulk water.8 The spectra of the silica/water sample display a resonance with a more rugged contour and the presence of at least two Pake doublets. This may suggest that water experiences more than one environment with molecular mobility restrictions as well as molecular separations consistent with smaller dipolar interactions. Inspection of the spectra shown in Figure 1, top, suggests that the spectrum mainly consists of two Pake doublets and that both are centered around the isotropic chemical shift of water. The peak separations in these doublets are found to be 475 and 140 Hz for the larger/wider Pake doublet and the smaller/less wide Pake doublet, respectively. Furthermore, the separation of 475 and 140 Hz of the two peaks in each of these doublets remained constant in spectra acquired at sample temperatures from 258 to 343 K. Similarly, the line width, Δν1/2, for the whole broad peak shown in Figure 1, top, remains constant over the 258343 K temperature range with an average Δν1/2 value of 850 ( 7 Hz. The spectrum acquired at 256 K displays only one broad resonance with a Gaussian line shape and a line width of 2170 Hz. Note that there is no visible peak at the isotropic chemical shift in the static (nonspinning) spectra shown in Figure 1. This
suggests that bulk water is not present in significant amounts in these silica-containing samples. It is worth noting that in samples with higher water content (data not shown) a central resonance at the isotropic chemical shift for water is visible in addition to the doublets shown in Figure 1. This suggests that some water in addition to the 40 wt % water in the silica/water sample will not disrupt the Pake doublet appearance of the water resonance. The shown differences in the line widths of the Pake doublets acquired at 278 K for the silica/water and silica/water/1-heptanol samples, of 850 and 1160 Hz, respectively, indicate that the dipolar interactions the water protons have with the 14 protons of the 1% 1-heptanol cause additional broadening of the water signal by these intermolecular interactions and that the overall water mobility is reduced in the presence of 1-heptanol. Figure 2 shows Arrhenius plots of ln T1 as a function of inverse temperature for the pure water (0) and silica/water (2) samples and the corresponding activation energies of 19.5 and 15.3 kJ/mol, respectively. Similarly, Figure 3 shows Arrhenius plots for the water/1-heptanol (0) and silica/water/1-heptanol (2) samples and the corresponding activation energies of 21.5 and 14.2 kJ/mol, respectively. These activation energies give information on the motional freedom of the water molecules and provide valuable information on how the water structure in the various samples differs from the water structure in bulk water. For the silica/water sample two T2 relaxation components were found. In the following the shortest T2 relaxation component in each sample is denoted T2b and describes the relaxation of nuclei in environmental state b, while the longest relaxation component is denoted T2a and describes the relaxation of nuclei in environmental state a. Similarly, the population fractions of water molecules that constitute the two relaxation components will be denoted Pa and Pb for the long and short relaxation components, respectively. Only one T2 component was deducible from the silica/water/1-heptanol sample data. Figure 4 displays Arrhenius plots of ln T2 as a function of inverse temperature for the T2 relaxation components in both the silica/water (Figure 4, middle and bottom (2)) and the silica/water/1heptanol (Figure 4, top (9)) samples, along with activation 4693
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Figure 5. 1H NMR spectra recorded at 298 K. Top: static spectrum of the silica/1-heptanol sample. Bottom: spectrum of liquid 1-heptanol. The static spectrum is enhanced 3.5-fold compared to the liquid spectrum for a better visual comparison. The OH peak at 5.7 ppm in the static spectrum is further enhanced for a better view of the line shape. Line shape analysis indicates that the sharper resonances at the positions of the isotropic chemical shifts include 40% of the sample 1-heptanol, while the remaining 60% of the 1-heptanol results in the broader peak/ shoulder and describes 1-heptanol significantly influenced by the surface.
Figure 4. ln T2 as a function of inverse temperature recorded in the temperature range 263333 K. Top (9): water T2 relaxation component in the silica/water/1-heptanol sample. Middle/bottom (2): two water relaxation components, T2a (middle) and T2b (bottom), in the silica/water sample. Activation energies for rotation (Ea,rot) and exchange (Ea,exc) are indicated in the plots. In accordance with ZB theory, Ea,rot describes the barrier for rotation in environmental state b (water closer to the surface).
energies (Ea,rot and Ea,exc) calculated for each relaxation component. The U-shaped dependency of ln T2 versus 1000/T of two of these plots displayed in Figure 4 is clearly present in Figure 4, top and middle, and indicates that the observed T2 values are dominated by proton exchange processes in the entire temperature range. Straight stapled lines in Figure 4 are drawn in regions where an approximately linear ln T2 versus 1000/T relationship is possible. From these straight stapled lines the corresponding activation energies for molecular rotation, Ea,rot, and proton exchange, Ea,exc, can be found in the high-temperature and lowtemperature regions, respectively. Thus, from the T2b component of the silica/water sample shown in Figure 4, bottom, only the activation energy for molecular rotation, Ea,rot, can be determined as there is no temperature region where T2b decreases with increasing temperature. The observed T2 relaxation time depends on both intramolecular interactions, characterized by the rotational transverse relaxation time T2,rot, and the proton exchange process characterized by the exchange time τ, defined as the average lifetime of a water molecule.15 In this way both processes have exponential temperature dependencies with well-defined activation energies. The short T2b component presented in Figure 4, bottom, can probably be attributed to water in a hydration layer with close
proximity to the silica surface, whereas the longer T2a component presented in Figure 4, middle, represents water further away from the surface. The population fraction of the T2a component in the silica/water sample decreases with increasing temperature, from 63% at 263 K to 54% at 323 K, indicating that more water resides in environmental state b at higher temperatures. Contrasting this, only one T2 component (9) was deducible from the silica/water/1-heptanol data (Figure 4, top). This T2 component is similar to the T2a component from the silica/water sample in that they both show a U-shaped dependency on temperature. The temperature at which the minimum in T2 occurs (Tm) is different for the two samples. For the T2a component of the silica/water sample Tm = 291.0 K, while for the T2 component of the silica/water/1-heptanol sample Tm = 298.1 K. A nonspinning spectrum of the silica/1-heptanol sample is shown is Figure 5, top. The relatively broad resonances confirm that some of the 1-heptanol molecules experience reduced motional freedom. However, a sharp resonance at the position of the isotropic chemical shift for each of the proton groups demonstrates that a fraction of the 1-heptanol molecules exist in a bulklike phase. Assuming that the characteristic line shape of each resonance is due to the presence of two 1-heptanol environments, where one gives rise to a sharp isotropic peak while the other gives a broader resonance due to surface interaction, spectral line shape analysis reveals that the sharp resonance corresponds to ∼40% of the 1-heptanol molecules. This indicates that as much as ∼60% of the 1-heptanol molecules reside in an environment of anisotropic motion. Another feature apparent from Figure 5 is the slightly larger chemical shift (0.1 ppm) of the narrow resonance labeled “a” in Figure 5, top spectrum than the corresponding chemical shift of the resonance labeled “a” in Figure 5, bottom spectrum. 4694
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decreases from 11.5 to 8.5 kJ/mol, indicating increased tumbling/rotation of the methyl group when the molecule interacts with a silica surface. The remaining protons (bd) all experience a slight increase in activation energy. These results show that the 1-heptanol molecule has a significant interaction with the silica surface, doubling the overall activation energy for the hydroxyl proton in comparison with neat 1-heptanol. T2 measurements on the silica/1-heptanol sample at 298 K confirm that the 1-heptanol molecule is present in both an anisotropic environmental state and an isotropic (bulk) environmental state. In fact, the data are best fitted to a three-component model where the longest component has a 39% population fraction and a T2 relaxation time of 760 ms. These results support the spectral line shape analysis, where ∼40% of the 1-heptanol molecules are shown to be less affected by the silica surface. Furthermore, the second longest relaxation component has a 57% population fraction and a T2 relaxation time of 430 ms, while the fastest relaxing component has a relaxation time of only 3 ms and a population fraction of 4%.
4. DISCUSSION
Figure 6. Arrhenius plots of T1 as a function of inverse temperature for the 1-heptanol (0) and silica/1-heptanol (2) samples. Similarly to Figure 5, the five proton groups are denoted “a”“e”. The activation energies are calculated from the slopes of the plots and give an indication of the motional freedom of the protons.
Figure 6 displays the results from the T1 measurements in the temperature range 278328 K of the silica/1-heptanol sample. It is apparent that the biggest change in activation energy induced by the introduction of silica particles is for the hydroxyl proton (a) and the methyl protons (e). The activation energy for the hydroxyl proton increases from 4.9 to 9.8 kJ/mol upon introduction of a silica surface, indicating reduced motion for the hydroxyl group, while for the methyl protons the activation energy
4.1. Water-Containing Samples. The silica-containing samples used in this study are packed without air with a sample volume of ∼60% silica and 40% liquid. Thus, this implies a relatively close-packed arrangement of the silica particles, achieved by centrifuging the samples at 18 000 rpm. This can be deduced from a packing arrangement with a theoretical maximum of 76% sample space occupied by uniform and spherical silica particles such as in hexagonal cubic packing. In a regular array of close-packed monodisperse spheres, as in an idealized sample with 20 nm radius silica particles, both octahedral and tetrahedral holes could accommodate spheres of radii 8.2 and 4.5 nm, respectively. In our samples hydration layers adsorbed to the surface of the silica particles will contribute to separating the particles, making room for more water than the theoretical 24% minimum. In addition, any particle size distribution will also lead to some extent of aberration from close packing. The silanol/water ratio of 7.4 103 (Table 1) for our samples shows that for each square nanometer of surface there are about 400 water molecules. Thus, the amount of water in the samples greatly exceeds the amount necessary to cover the surface with one hydration layer. Even though the amounts of liquid in the samples give the possibility of a bulk phase, the presence of a Pake doublet in the spectra with no visible sharp signal at the isotropic chemical shift indicates that these samples are without bulk-phase water. This indicates that, with a surface silanol density of 3.3 OH groups/nm2 (Table 1), pores (or in this case vacancies between the silica particles) with a 8.2 nm radius are not sufficient to sustain a bulk water phase because the surface-induced ordering of the water molecule stretches longer than the radius of the pore. In general, it has been found that for more hydrophilic surfaces the ordered water structure reaches further from the surface.30 Thus, for surfaces with higher silanol density this effect could be present also in systems with larger pores. Furthermore, a sample freezing point at temperatures below 273 K can be attributed to the free surface energy of water molecules that in the vicinity of the surface/water interface is reduced due to adsorption interactions. Both the demonstrated freezing characteristics and the Pake doublet appearance of the 4695
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Langmuir water proton resonance suggest that there is mainly one environmental state for water in the silica/water/1-heptanol sample and at least two environments experienced by water in the silica/ water sample. It is reasonable to assume that these two environments in the silica/water sample are a water layer close to the silica surface and a water layer further away from the surface. An effect of the 1% 1-heptanol in the silica/water/1-heptanol sample is that some of the differences between the “bound” water layers and the outer layers of water are relaxed. This effect is pronounced in the spectra shown in Figure 1, top, where the rugged shape of the water resonance in the silica/water sample indicates the presence of at least two environments experienced by the water molecules. Conversely, the single Pake doublet in the silica/water/1-heptanol spectra shown in Figure 1, bottom, indicates one environment experienced by the water molecules. The low-intensity Gaussian peak recorded at 258 K— presumably corresponding to water in the first adsorbed layer— indicates that water in this phase behaves differently from water in higher hydration layers. However, water present in the hydration layers constituting environmental states a and b still behaves differently from bulk water, indicating that surfaceinduced water structures can exist beyond the first two or three adsorbed layers as previously suggested.8,31 The presence of a Pake doublet (Figure 1) demonstrates by the splitting of the two peaks in the doublet that proton exchange is not rapid enough to eliminate the doublet appearance.15,32 Furthermore, as previously mentioned the water molecules in these structures must have a preferential orientation in addition to being sufficiently isolated from each other to give rise to Pake doublet NMR spectra, a feature that strong dipolar interactions will eliminate.17 It is possible to attribute the preferential orientation of the water molecules to the interaction with the solid surface. The reduction in intermolecular interactions is possible if the separation between water molecules increases compared to that of bulk water. Generally, the strength of dipolar interactions, Rij, between two protons, i and j, decreases with a factor of sixth power with respect to the protonproton distance,33 rij, where Rij = 1/rij6. The observed splitting is due to the intramolecular dipolar interactions of the two water protons with a protonproton separation of 0.153 nm. To reduce the intermolecular dipolar interaction to 1% of the strength of the intramolecular contribution, the intermolecular proton distance, rjk, would have to be 0.329 nm. In bulk water, where the intermolecular dipolar interaction is strong enough to eliminate the splitting, the intermolecular proton distance is on average about 0.24 nm at 4 °C (estimated from data in ref 34). Hence, to reduce the intermolecular dipolar contribution to an insignificant amount, an increase of about 35% in the intermolecular proton distance compared to that of bulk water is required. Due to the presence of Pake doublets one might expect T1 relaxation times of water in the silica samples to be considerably reduced in comparison to bulk values due to restrictions in mobility. However, there is only a ∼20% reduction in T1 values for the silica/water and silica/water/1-heptanol samples compared to bulk water values. Previous studies show that only the T1 of the first adsorbed water layer will be greatly influenced due to interactions with the surface hydroxyl groups, while subsequent layers will have a T1 more similar to that of bulk water, explaining the observed T1 values.6,31 These results show that water molecules above the first adsorption layer both participate in forming a Pake doublet and have some motional freedom.
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However, the doublet resonances observed in the spectra in Figure 1 are narrower and have smaller peak separations (peak separation 0.03 and 0.11 G (silica/water) and 0.18 G (silica/ water/1-heptanol)) than doublets obtained from, e.g., singlecrystal studies on isolated water molecules (peak separation 21.6 G),17 samples of water present between clay platelets (peak separation ∼4 G),2022 or water on oriented collagen fibers (peak separation 0.51.2 G).15 This indicates some extent of motional averaging of the water molecules in the samples. Motion about an axis perpendicular to the internuclear axis causes a narrowing of the peak separation.18 Furthermore, Migchelsen and Berendsen15 found the peak separation to decrease with increasing water content. Thus, considering the larger amounts of water in the silica/water and silica/water/1heptanol samples, a small peak separation is expected. Authors studying porous silica systems often report that the magnetization recovery in T1 experiments deviates from a monoexponential recovery,35 and this phenomenon is explained as being the result of uneven hydration of the porous matrix. In our experiments the spinlattice magnetization recovery did not deviate from a monoexponential behavior in any of the watercontaining samples. Consequently, experiments on the silica/ water sample show two T2 components but only one T1 component, implying that the observed T1 value is in fact a weighted average of two relaxation components. This occurs when the exchange of nuclei between the two environments, each with its own spinlattice relaxation, is in the fast exchange limit. It is not unusual to observe this in systems where T1 . T2 because fast exchange with respect to T1 does not necessarily mean fast exchange with respect to T2. Activation energies were calculated from the temperature dependence of the T1 values (Figures 2 and 3). The calculated activation energy of 19.5 kJ/mol for pure water is in good agreement with that from previous studies and describes the motion of water molecules when they reside in a network of hydrogen bonds.36 Over a larger temperature range previous studies show that the proton T1 relaxation of water has a nonlinear dependency on temperature. This is explained by the weakening of the H-bonded water structure at higher temperatures.37,38 Thus, for a higher temperature range (313363 K) an activation energy of 15.5 kJ/mol is obtained, presumably from the motion of “monomeric” water molecules only loosely involved in H-bonding.36 For the silica/water sample (this study) the activation energy of 15.3 kJ/mol obtained from the T1 temperature dependency is similar to the value obtained for monomeric water molecules in solution.36 Milligan and Whitehurst39 concluded from magnetic susceptibility measurements that the separation of water molecules in the first monolayer on the surface does not permit hydrogen bonding. However, the temperature dependency of the T1 values found in the study presented here suggests that a similar separation of water molecules may exist also in the water layers further from the silica surface. The presence of a Pake doublet supports this as it depends on a reduction in the intermolecular interactions in comparison with those in bulk water. The activation energy of water with 1% dissolved 1-heptanol is 21.5 kJ/mol, slightly larger than the value of 19.5 kJ/mol found for pure bulk water. This can be explained by a more “rigid” water structure around the hydrophobic part of the 1-heptanol molecule, caused by an entropy loss of water molecules. The entropy loss in the process of “iceberg” formation11,40 is thought to go 4696
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Langmuir along with a reduced enthalpy when water molecules around the hydrophobic moiety lose hydrogen bonds. In the silica/water/1heptanol sample the activation energy decreases to 14.2 kJ/mol, indicating reduction or absence of the iceberg formation phenomenon. This value is slightly lower but in the same range as the activation energy of 15.3 kJ/mol found for the silica/water sample. The presence of at least two T2 relaxation components is consistent with previous studies of water adsorbed on a silica surface,1,2,5,7 where explanations of the multiexponential behavior of the relaxation decay depend on the assumptions made. Some authors describing porous systems assume that the “bound” water and “free” water within a pore exchange rapidly enough to give one average relaxation time and that exchange of water between the pores is slow, leading to a distribution of relaxation times depending on the pore size distribution. Using these assumptions, Hills and Quantin7 investigated saturated suspensions of water and nonporous silica particles by measuring T1 and T2 on nonspherical particles with diameters of 4060 μm. Furthermore, it was assumed that the nonporous particles were close-packed and hence that the space between particles creates vacancies equivalent to “pores” and “pore throats” in a porous system. Two T2 components (3.7 and 33 ms) and one T1 component (300 ms) were found. Thus, these T2 values are similar, but slightly shorter than the results from the study presented here, probably due to paramagnetic impurities in their samples.7 If this reported multiexponential relaxation decay is caused by a distribution of pore sizes, one would expect more relaxation components from the multidisperse system of Hills and Quantin7 than from the more monodisperse like system presented here. It is therefore reasonable to assume that the two T2 relaxation components found in the system presented here are in fact due to water present in two different surface environments. Thus, the shortest component (T2b) of the silica/water sample, with a 44% population fraction, is due to water molecules in a hydration layer interacting directly with the surface, and the longer component (T2a) is caused by more mobile water molecules in a hydration layer further away from the surface. Similarly, such a conclusion was also reached by Overloop and Van Gerven5 in a study on porous systems with both uniform and heterogeneous pore size distributions. For a system of water adsorbed in controlled pore glass with a uniform pore size distribution (pore diameter 11.9 nm), Overloop and Van Gerven typically found three T2 relaxation components (3, 13, and 39 ms) where the two longest T2 values are similar to the results presented in this study. It is not unlikely that the presence of an additional T2 component in this porous system could be due to water present in pore throats. This indicates that both porous and nonporous adsorbents can give correlated results about water properties, given a similar degree of homogeneity and pore size. Both the transverse relaxation components in the silica/water sample display a nonlinear dependency on temperature (Figure 4, middle and bottom) similar to that of physically adsorbed water molecules in a two-phase ZB system. The T2a curve displays a minimum at 291 K (Figure 4, middle), and T2b apparently approaches a minimum value that occurs at slightly lower temperatures than those measured here (Figure 4, bottom). Assuming that relaxation of protons in the silica/water system follows the theory proposed by Zimmerman and Brittin3 for two phases, then the low-temperature range of the T2a curve (T < 291 K) describes a two-phase relaxation behavior affected by the transfer of nuclei between the phases. A consequence of
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this is that the observed T2a0 value in this region is not the true relaxation time of phase a (indicated by a prime), but describes the lifetime of nuclei in this phase, Ca, where T2a0 = Ca1 is the proton exchange rate. Thus, the activation energy for exchange of nuclei between the phases (Ea,exc) could be calculated from the slope of Figure 4, middle, at T < 291 K and also from the slope of Figure 4, top, at T < 298 K. However, water can also exchange with the surface OH groups, and thus, a three-phase model would be needed to further explain the data shown in Figure 4. Unfortunately, no model that explains the temperature dependency of T2 for more than two exchanging phases has been developed. For this reason the nature of the T2 values in Figure 4 is not fully determined. However, considering the amounts of water in the samples, the population fraction, Pc, of protons in the phase of surface silanols is very low (Pc ≈ 0.5%) compared to the population fractions of the two water environments. It is therefore assumed that the results mainly describe proton exchange occurring between the two water environments and that interpreting the results assuming a two-phase model is reasonable. Thus, the activation energies are denoted Ea,exc for the lowtemperature regions in Figure 4, top and middle, describing the activation energy for exchange between two water environments. In the high-temperature range in Figure 4, top and middle, the relaxation values increase with increasing temperature due to higher exchange rates, where the observed T2a0 values are related to the spinspin relaxation and fractional proton population of water in environmental state b, where T2a0 ≈ T2b/Pb.4,41 From this relation the actual T2b relaxation of molecules in environmental state b and an activation energy for rotational motions of the water molecules (Ea,rot) can be approximated. It is worth noting that the true T2 relaxation rate of protons in environmental state a cannot be determined from the data in Figure 4, top and middle, because the observed T2a0 values are dominated by proton exchange effects in the entire temperature range. To sufficiently reduce the proton exchange rate, so to measure T2a, lower temperatures than those used here are needed. This could not be done here due to the freezing of the sample water that constitutes environmental state a (and also environmental state b) at lower temperatures. The silica/water/1-heptanol sample displays one T2 component with relaxation characteristics similar to those of the T2a component of the silica/water sample. Below 298 K T2 increases with decreasing temperature (Figure 4, top), indicating proton exchange between two stable environments. Thus, it is possible that a second environment exists and that the T2 relaxation of water in the two environments is too similar for them to be separated. The freezing of the sample shows that ∼10% of the sample water has slightly different characteristics and can probably be attributed to water in environmental state b, i.e., water close to the surface. From the data in Figure 4 it is apparent that the activation energy labeled Ea,rot is larger for the water molecules in the silica/ water sample than in the silica/water/1-heptanol sample. This indicates that the presence of 1-heptanol increases the rotational freedom for the water molecules in environmental state b. The activation energy for proton exchange between the two water phases, on the other hand, is fairly similar, with values of 10.1 kJ/ mol for the silica/water sample and 10.3 kJ/mol for the silica/ water/1-heptanol sample. These values are close to 10.0 kJ/mol, the activation energy for proton exchange in water obtained from 17 O NMR measurements.42 In comparison, Woessner2 found an activation energy of 14.4 kJ/mol for proton exchange, 4697
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Figure 7. Possible orientations of the adsorbed 1-heptanol molecules. Left: The molecules align with the hydrocarbon group parallel to the silica surface. Right: The hydrocarbon chain aligns along the surface normal, with every other layer having an opposite polar orientation.
presumably between bound water and surface hydroxyls. This supports the assumption that the low-temperature T2 values mainly describe proton exchange between water molecules, and not proton exchange between water and surface silanols. Another feature apparent in Figure 4, top and middle, is the temperature at which the minimum T2 value occurs, Tm. It has been shown that Tm depends upon the translational motion of the water molecules in environmental state b. Morariu and Mills4 found that an increase in the enthalpy of activation for diffusion in environmental state b by 1 kcal/mol corresponds to an increase in temperature of about 18 °C for Tm for the T2a curve. This indicates that the barrier to translation for water molecules in environmental state b of the silica/water/1-heptanol sample is 0.4 kcal/mol higher than in the silica/water sample. In summary, the presence of 1-heptanol has a significant impact on the adsorbed water properties. The concentration of 1-heptanol is too small for the alcohol to form separate aggregates. Thus, it is likely that the alcohol is evenly distributed throughout the sample, affecting the water through hydrophobic interactions. Besides an increased barrier to translation and a decreased barrier to rotation, the larger peak separation for the silica/water/1-heptanol sample may indicate an alteration in the orientation of the water molecules. 4.2. Silica/1-Heptanol Sample. The found reduction in the methyl group motional resistance (rotation) indicates that the density of the adsorbed molecules is lower than in solution. This explains the line shape of the static NMR spectrum (Figure 5). In addition to a peak at the isotropic chemical shift, the resonances display a shoulder to the right of the isotropic chemical shift. The adsorbed molecules giving rise to a broader peak experience a lower molecular density than in the bulk phase of 1-heptanol molecules, and this will cause the shoulder/broad peak to appear to the right of the isotropic chemical shift. In general, an increased molecular density (and hence increased deshielding) or H-bonding will shift the resonances to higher parts per million values and vice versa.43 In this manner, increased H-bonding can explain why the hydroxyl resonance changes to slightly higher parts per million values (Figure 5, top). As mentioned, integrals calculated from simulated spectra of the two components forming each peak in Figure 5, as well as T2 measurements, reveal that as much as 60% of the 1-heptanol molecules exist in an anisotropic state. Considering the available silica surface area, this amount of molecules cannot all be in direct contact with the surface in that ∼40 1-heptanol molecules would then reside per square nanometer of the surface, indicating that only a few of the molecules making up the anisotropic state
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actually are H-bonded to surface silanols. A possible explanation is that the 1-heptanol molecules form aggregates on the surface involving several adsorbed layers. This has previously been found for methanol, ethanol, and 1-propanol, where the surface aggregates are referred to as surface molecular macroclusters.4446 In these cases it is suggested that the molecules align with the hydrocarbon group parallel to the silica surface (Figure 7, left). Another possibility is that the molecules in the first layer adsorb with the hydrocarbon chain pointing outward along the surface normal and that the second layer adsorbs with an opposite polar orientation with respect to the first layer (Figure 7, right).47 Considering the long-chain 1-heptanol molecule, the latter explanation is the most plausible, where subsequent layers have opposite polar orientations and increasing orientational distributions as the layers transition into the bulk liquid structure. The smallest T2 component, representing only 4% of the total 1-heptanol molecules in the sample, probably describes the 1-heptanol molecules directly adsorbed on the surface. A T2 value of only 3 ms for this component confirms that these molecules experience severe restrictions in mobility. However, the T2 component describing the remaining adsorbed 1-heptanol molecules indicates higher motional freedom for these molecules with a T2 value of 430 ms. As for the water-containing samples, only one T1 relaxation component is found, indicating a fast motional regime for proton exchange between free and bound 1-heptanol molecules with respect to T1. It is worth noting that the inversion recovery experiments did deviate slightly from a monoexponential fit in some of the T1 measurements for the silica/1-heptanol sample. However, the improvement when using multiexponential fits was insignificant and could not be justified.
5. CONCLUSIONS We have shown that when employing water- and 1-heptanolsaturated colloidal and nonporous silica particles, details of the liquid properties are present in the 1H NMR spectra. Furthermore, relaxation measurements at several temperatures reveal quantitative information on both molecular motion and proton exchange not previously reported for this type of system. In the case of adsorbed water the system apparently consists of water in two different environmental states, one “bound” water layer and another layer further from the silica surface. Water molecules in both environments give rise to a Pake doublet, thus implying reduced intermolecular interactions and a nonrandom orientation of the water molecules in both states. Activation energies calculated from the temperature dependency of the longitudinal relaxation values confirm reduced intermolecular interactions between water molecules in the sample, while the presence of two T2 relaxation components confirms that water exists in two different environmental states. Dissolving 1% 1-heptanol in the water prior to adsorption on the silica surface reduces the differences in relaxation rates between the two environments. Furthermore, the presence of 1-heptanol seems to primarily affect the water molecules in the adsorption layer closer to the surface, where the water molecules experience reduced translational freedom accompanied by increased rotational freedom. However, a 35% increase in the line width compared to that of the silica/water sample resonance indicates an overall reduced mobility of the sample water when 1% 1-heptanol is present. The activation energy for proton exchange 4698
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Langmuir is similar in both the silica/water (10.1 kJ mol1) and the silica/ water/1-heptanol (10.3 kJ mol1) samples. In the case of 1-heptanol adsorbed to the silica particles, the 1H spectra revealed the presence of at least two environments for the 1-heptanol molecules. T2 measurements indicate the presence of three environments for the 1-heptanol molecules in the silica/1heptanol sample. A narrow resonance at the isotropic chemical shift indicates the presence of a bulklike phase (39 mol %), and a broad shoulder to the right of the isotropic resonance is due to 1-heptanol molecules in a hydration layer close to the surface (57 mol %). This broad resonance appearing at lower parts per million values than the isotropic chemical shift indicates a lower density of the 1-heptanol molecules adsorbed to the silica surface compared to 1-heptanol molecules in the bulk phase. The 1-heptanol molecules directly adsorbed to the silica surface (4 mol %) experience severe restrictions in mobility. Activation energies show that the 1-heptanol hydroxyl proton experiences overall restrictions in motion in the silica/1-heptanol sample compared to in neat 1-heptanol. However, the 1-heptanol methyl group experiences increased rotational freedom in the silica/1heptanol sample, probably due to a lower molecular density in the adsorbed layers. This study shows that a homogeneous model system has the potential to reveal detailed information about adsorption of liquids on a silica surface.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 47-55-583353. Fax: 47-55-589400. E-mail: Willy.Nerdal@ kj.uib.no.
’ ACKNOWLEDGMENT We thank Dr. Scient. Ketil Djurhuus for assistance in the preparation of the samples and Dr. Scient. John Georg Seland for assistance with the NMR spinspin relaxation experiments. ’ REFERENCES (1) Woessner, D. E.; Zimmerman, J. R. J. Phys. Chem. 1963, 67, 1590–1600. (2) Woessner, D. E. J. Chem. Phys. 1963, 39, 2783–2787. (3) Zimmerman, J. R.; Brittin, W. E. J. Phys. Chem. 1957, 61, 1328–1333. (4) Morariu, V. V.; Mills, R. Z. Phys. Chem. 1973, 83, 41–53. (5) Overloop, K.; Van Gerven, L. J. Magn. Reson. 1992, 100, 303–315. (6) Overloop, K.; Van Gerven, L. J. Magn. Reson. 1993, 101, 147–156. (7) Hills, B. P.; Quantin, V. M. Mol. Phys. 1993, 79, 77–93. (8) Overloop, K.; Van Gerven, L. J. Magn. Reson. 1993, 101, 179–187. (9) Holly, R.; Peemoeller, H.; Choi, C.; Pintar, M. M. J. Chem. Phys. 1998, 108, 4183–4188. (10) Kailash, C. J.; Hore, D. K. Phys. Chem. Chem. Phys. 2010, 12, 14383–14404. (11) Corsaro, C.; Spooren, J.; Branca, C.; Leone, N.; Broccio, M.; Kim, C.; Chen, S. -H.; Stanley, H. E.; Mallamace, F. J. Phys. Chem. B 2008, 112, 10449–10454. (12) Harvey, J. M.; Jackson, S. E.; Symons, M. C. R. Chem. Phys. Lett. 1977, 47, 440–441. (13) Chang, N. J.; Kaler, E. W. J. Phys. Chem. 1985, 89, 2996–3000. (14) Tsiao, C.; Corbin, D. R.; Dybowski, C. J. Am. Chem. Soc. 1990, 112, 7140–7144.
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