5. Dynamics of confined water 83
5.5. Influence of surface modification with APTES
Figure 5.19.:Measured SLR times of P21 (f) (blue circles) and theoretically calcu-lated〈T1〉from DS by assuming a weighted rate average of signal from hydroxyl groups and water molecules (squares). The contribution from the wall was as-sumed to range from 0 % to 100 % (pink to black) indicated by the arrow. Inset:
Average〈T1〉at 40 %.
length of the pore. The volume of a water molecule can be estimated using the average density of ρ = 1cm3/g to be VH20 = 0.03nm3. The number n of molecule in the pore is n=V/VH2O. The average area density g of OH groups necessary to amount for a signal of 40 % can be calculated using the surface of the pore AO=2πh:
g=0.62n AO = 3
5
4πhr2
VH2O2πh =42.8nm−2. (5.3) Here, the additional factor 2 is due to the two deuterons carried by one water molecule. The resulting value is unrealistic for MCM-41 samples. Usual values of the OH density range from 2 nm−2 to 6 nm−2 [103]. Thus, the OH groups at the walls cannot explain the behavior of the (f) relaxation component. By a similar calculation, it can be excluded that the wall contribution is responsi-ble for the slow (s) relaxation, since it exhibits up to 20 % of the total signal.
Therefore, it can be concluded that the interpretation given by Swenson can not explain our results, even if the OH groups at the MCM-41 surface are taken into account.
Figure 5.20.:(a) Mean relaxation times〈T1〉and (b) correlation timesτobtained from P21 and P22a. The line in (a) marks the temperature where the stretching β becomes smaller than unity in P22a. the dashed line in (b) is a VFT-fit using the high temperature data only, the dash-dotted line is an Arrhenius-fit to the STE data. Squares in (b) mark the position of the minima in T1 (green) and T2 (orange). The empty diamond is a partially relaxed STE experiment, pluses are DS measurements performed by M. Rosenstihl.
results found in MCM-41 are valid in diverse environments. In section 2.3.2, the investigation of water dynamics in different kinds of confinements were discussed briefly. Here, the influence of surface modification of MCM-41 is tested rather than using a different confinement material, see section 4.2. This approach combines the well defined pores found MCM-41 with the possibility of changing the specific guest-host interactions. The used MCM-41 was synthe-sized in the AK Buntkowsky and modified by the procedure explained in section 4.2. The APTES modified silica sample was filled with D2O and measured by F.
Dietrich [198] in his Bachelor’s thesis.
Figure 5.20 (a) reports the mean relaxation time 〈T1〉 of the modified sample P22a. Additionally, the results obtained from the unmodified sample P21 are shown. Both samples exhibit roughly the same pore diameter. The figure shows that at high temperatures above the T1 minimum the SLR times in both samples agree well. Below the minimum, the fast process (f) shows some deviations that indicate a minor slow down of dynamics in the P22a sample. This is in accordance to the slowing down on increasing the pores that was observed above. As in all water samples confined to MCM-41 samples a second relaxation process (s) is present. Within the uncertainties, it agrees well with the same process in the unmodified sample.
The correlation times τm are calculated by the BPP model using the CC SD.
Again, this is motivated by the findings of DS measured on the same sample (not shown). The results are reported in fig. 5.20 (b). The correlation times
coincide with the unmodified samples for temperatures above ca. 225 K. In this temperature range, they follow a VFT behavior. The corresponding fitting parameters are listed in table 5.2. As in the unmodified sample, a change in the temperature dependence is observed at T∗ ≈ 219 K. Both samples, P21 and P22a, show a similar temperature dependence, but the modified sample shows slightly slowed down dynamics compared to the unmodified one. The STE experiments show a more pronounced difference between the samples at low temperatures. They result in correlation times that follow an Arrhenius be-havior with an activation energy of Ea= 0.49 eV. The time constants are shifted by a factor of 3, thus the APTES modification seems to slow down dynamics in this temperature range. This slowdown is situated well within the differences observed for different pore sizes, see sec. 5.3.
Additionally to the STE data, the figure shows DS data measured by M. Rosen-stihl on water in the same APTES modified MCM-41. The data nicely connect to the high temperature data, but are slowed down by an additional factor of ca.
3 to 4 in comparison to the STE experiments at low temperatures. Similar devi-ations were found in the unmodified P21 sample, where the DS data [120] are shifted slightly with respect to the STE data. Interestingly, the DS experiments on the small samples P21 [120] and P22a do not show the slower process that was observed before in sample P25. The second process thus seems only to be observable in larger pores in DS. Possibly, this explains the shift of DS data compared to NMR STE data. In large pores, DS can resolve the two processes and therefore the shift is smaller, as e.g. evident in fig. 5.17 for sample P25. In the small pores, the slow process may be indistinguishable in DS and absorbed in the fit of the main process. Therefore, the DS time constants are shifted to larger values. Additionally this may explain the broader distribution observed in DS compared to NMR. Whether this is true should be investigated in more detail. In favor of this idea is the analysis of the SE signal minimum. It is ob-tained from the total signal and thus also takes the (s) process into account.
The correlation time obtained from the minimum (orange square) agrees well with the data from DS.
A closer look on the spectra obtained from P22a reveals that larger differences compared to P21 exist. The line shape transition region between ca. 220 K and 180 K shows unexpected features. This is shown in figure 5.21 where fully and partially relaxed spectra are shown for selected temperatures. The follow-ing discussion focuses on the PR spectra. The FR spectra again show only an additional Pake-like contribution that, analogous to the discussion of P21, is attributed to the slow relaxation component (s). Upon cooling from high tem-peratures first a broadening of the Lorentzian line is observed. In the transition, region the line shape shows a non-Lorentzian feature in the center, c.f. the spec-trum at 203 K. This additional spectral feature can first be observed at 211 K (not shown). At the same time as the non-Lorentzian feature, a third Pake-like
Figure 5.21.:Solid echo spectra obtained from partially (blue) and fully (red) re-laxed measurements in sample P22a. In black partially rere-laxed spectra of sample P21.
spectral contribution starts to arise. On further cooling, the Lorentzian line vanishes, see e.g. at 193 K. At this temperature, the non-Lorentzian center component exhibits a FWHM of about 40 kHz. At very low temperatures, the central component decreases, e.g. at 151 K, only a Pake spectrum is left over.
Note that this Pake spectrum is not in static, but as in the unmodified samples same residual intensity is left over in the center.
A similar spectral shape was observed before by Lusceac et al. [88, 226] in 2H NMR measurements on myglobin hydration water. The authors found that the hydration water in this case performs a large angular motion with angle ampli-tudes of ψ = 85° to 130°. A fast rotation about a symmetry axis by those an-gles causes an effective averaging of the anisotropy δ according to eq. (3.38).
This results in an effective coupling constant of δ ≈40 kHz which is in good agreement to the width of the central features observed in the present work.
Geometries that can feature those jump amplitudes are for example strongly distorted π-flips and tetrahedral jumps. Lusceac et al. also found a possible agreement with three site jumps on a cone similar to methyl group rotation.
They stressed that none of those geometries was able to explain all the fea-tures found in hydration water of myoglobin alone. Distributions of geometries could resolve the issue, but are hard to be determined unambiguously [226].
It was argued that in myoglobin a crossover from an isotropic reorientation to
Figure 5.22.:PR spectra at T =193 K of samples P22a and P21 normalized to the height of the Pake contribution at ±71 kHz. Red is the difference between of spectra. In dashed black: Lorentzian fit to the difference spectrum.
the anisotropic reorientation occurs in the vicinity of ca. 220 K, explaining the crossover from the isotropic Lorentzian to a narrowed Pake-like spectral shape.
At this temperatures, the motion of hydration water is still fast, but slows down upon further cooling. This causes the transition to a solid Pake spectrum. The line-shape transition extends to temperatures as low as 150 K in myoglobin. The crossover from isotropic to anisotropic motion is not observed in the line shape of other proteins, e.g. elastin and collagen [88]. Thus, either in myoglobin the crossover happens at higher temperatures than in other proteins or it is due to specific motional features of myoglobin.
Following this argumentation, in the case of D2O confined to APTES modified MCM-41 at temperatures above ca. 211 K, the spectra are isotropically aver-aged and exhibit a Lorentzian shape. Due to the decreasing T2, the Lorentzian broadens and exhibits a FWHM of 5 kHz at 211 K.7 Below these temperatures, the spectra deviate from the Lorentzian form and the isotropic averaged motion becomes anisotropic. In the same temperature range, an additional broad Pake component starts to grow in the PR spectra, indicating the presence of slow molecules with (τ > δ−1) due to a distribution G
log(τ) .
Direct comparison of the PR spectra of P22a and P21 shown in fig. 5.21, shows an interesting difference between the two samples. At 201 K, both show ap-proximately equal contributions of the Pake like spectra, while the central component is anisotopically broadened in the modified MCM-41. The situa-tion changes on cooling: the relative contribusitua-tion of the Pake component is much larger at 193 K in the modified MCM-41 compared to the unmodified
7 This corresponds to aT2≈64 µs
sample. This can be seen in fig. 5.22, where the spectra were normalized to their height at ±71 kHz. A much larger Lorentzian line is visible for P21, in-dicating that a significantly larger fraction of molecules in this sample is still fast. Subtraction of the two spectra results in the difference spectrum shown in figure 5.22. It can be described with a Lorentzian shape resulting in a FWHM of 4.9 kHz. The purely Lorentzian shape of the difference spectrum suggests that the same anisotropic line shape observed in P22a is also be present in P21.
There, it is mostly concealed by the still isotropic parts. If this was the case, this would mean that the anisotropic reorientation clearly observable in P22a is not a feature of the APTES modification, but rather of the water close to (MCM-41-) surfaces.
A possible scenario is the following: the elementary water reorientation close to surfaces in general or at least close to MCM-41 surfaces is anisotropic. If the molecules are fast enough to average over many of those positions or can interchange to layers that are further away from the surfaces, this averages out most of the anisotropy. Only if the molecules become too slow, the anisotropy is detected in the experiment as for example in the STE experiments shown in section 5.3. In this scenario, the main effect of the APTES modification is to prevent the water molecules from effective averaging, e.g. by spatial constric-tion. The anisotropic nature of the reorientation becomes dominant at higher temperatures, while fast, isotropic motion is hindered. Whether this idea is true can not be answered conclusively in this work. Additional experiments utilizing different surfaces may help to clarify this questions.
In this short section, we have found that the water dynamics in small pores of ca. d = 2.2 nm are not influenced by the APTES modification of the confine-ment at high temperatures. In the surface modified sample dynamics, show a more anisotropic behavior in the line shape transition region around 200 K.
Comparison to the unmodified samples suggests that this is a general feature of water that is better observable in these samples, since the APTES modification suppresses efficient averaging in this temperature region.