Article pubs.acs.org/JPCC
Behavior of Polyhydric Alcohols at Ice/Liquid Interface Makoto Uyama,† Makoto Harada,† Takehiko Tsukahara,‡ and Tetsuo Okada*,† †
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan
‡
S Supporting Information *
ABSTRACT: Glycerol (Gly) and 1,3-butanediol (13BD) are widely used as a cryoprotective reagent. Although these compounds have similar chemical structures, their interfacial behavior is not necessarily the same. Frozen aqueous 13BD gives discrete liquid inclusions dispersed in an ice matrix, whereas frozen aqueous Gly forms the film-like liquid phase spread over the entire ice crystal surface. Nuclear magnetic resonance (NMR) measurements of Gly/water at 253 K indicate that the chemical shifts of both water and alcohol OH protons move upfield with increasing concentration of Gly regardless of whether the solution is frozen ( 60°; otherwise, the liquid phase intrudes into the grain boundary. For pure ice prepared in a laboratory or natural ice, the dihedral angle is reported to be ϕ = 20−30°; in these cases, the liquid phase must be pure water or possibly salt solution.44−47 This dihedral angle strongly implies that the liquid phase forms veins along the grain boundaries rather than forming isolated inclusions. Most of these ϕ measurements
Figure 4. Raman spectra measured at different temperatures for (a) 10 wt % Gly/water, (b) 50 wt % Gly/water, (c) 10 wt % 13BD/water, and (d) 50 wt % 13BD/water. 24876
dx.doi.org/10.1021/jp408722x | J. Phys. Chem. C 2013, 117, 24873−24882
The Journal of Physical Chemistry C
Article
samples. For both Gly and 13BD, αIL increases and αLL decreases with decreasing temperature. Although Gly and 13BD gave almost the same αIL value at 298 K, αIL for 13BD becomes larger than that for Gly as the temperature decreases; at 253 K, αIL becomes almost equal to αLL for 13BD, while αIL is still ca. 60% of αLL for Gly. It is understandable that the water structuring is enhanced at lower temperature because the hydrogen bond becomes stronger. However, interestingly, this tendency is obviously more marked for 13BD than for Gly, suggesting that 13BD facilitates the formation of ice-like water to a greater extent than Gly at lower temperature. As noted above, it has been indicated that Gly forms a homogeneous mixture with water.14 The increase in αIL for Gly with decreasing temperature therefore implies that the hydrogen bonds are strengthened at lower temperature, while the same liquid phase structure is maintained. In contrast, the more obvious increase in αIL for 13BD/water suggests that the microscopic structure may also be changed as the temperature decreases. Temperature dependence of the viscosity of Gly/water and 13BD/water mixture supports this inference. The viscosity for 50 wt % Gly and 13BD are 5.4 and 7.8 mPa·s at 298 K, respectively,12 and increases to 60 and 171 mPa·s at 253 K, respectively.52 The larger temperature dependence of viscosity for 13BD should come from the progressive structuring in 13BD/water mixture at the lower temperature. This aspect will be discussed again for the interpretation of the NMR data. NMR Measurements. 1H NMR spectra were measured for the aqueous alcohol solutions at 253 K to discuss the interactions of alcohol and water molecules on the ice surface from molecular viewpoints. Figure 6 shows the NMR spectra for Gly/water and 13BD/water mixtures. A sample was frozen for cor < 48 wt %, whereas the solution remained unfrozen for cor ≥ 48 wt %. Although signals become broad and fine structures are largely lost for frozen samples, the resolution is still high enough to allow us to specify the chemical shifts and to measure the relaxation times. In these spectra, only the liquid phase components contribute to the signals while ice does not. The spectra measured for 13PD/water and isopropanol (IPA)/ water at 253 K are illustrated in Figure S4, Supporting Information, for comparison. The former mixture is frozen at the concentration lower than ca. 40 wt % and the latter at the concentrations lower than ca. 36 wt % according to the corresponding phase diagram.53,54 For unfrozen Gly solution (cor ≥ 48 wt %), the chemical shifts of the water proton (δw) and the alcohol hydroxyl proton (δalc) move downfield as the Gly concentration decreases, and in turn, the water concentration increases. The gradual changes in the chemical shifts support the formation of molecularly homogeneous mixtures of Gly/water. Thus, the molecular environments of Gly and water are continuously modified by Gly concentration changes. In contrast, both δw and δalc are constant and independent of the 13BD concentration, suggesting that the molecular circumstance in 13BD/water is not sensitive to the concentration change. The constant chemical shifts for 13BD/water can be explained, assuming that these components are microscopically separated and are possibly assembled into individual clusters. The concentration change influences the size and/or number of clusters but does not affect the molecular circumstances for the liquid phase components. Although there have been no structural studies on the cluster formation of 13BD/water to the best of our knowledge, clustering has been found for various simple
independent of the temperature. Thus, the contribution from the alcohol to the entire Raman band at 3050−3750 cm−1 was neglected for the following discussion on the spectral changes with the temperature; i.e., a change in the water structure largely determines the band shape. When the temperature of the 10% alcohol solution decreased down to 263 K or lower, the sharp peak emerged around 3150 cm−1 (Figure 4a,d), which originates from ice. In the absence of ice, the band at 3050−3750 cm−1 is broad with two apexes. The Raman band was deconvoluted to facilitate the discussion of the water structural change with the temperature. However, the deconvolution of the spectra for frozen samples was not possible because of too strong influence of ice. Therefore, the following discussions are confined on the water structures for unfrozen samples. Sun deconvoluted the Raman OH stretching band into five sub-bands, which were assigned DDAA (double donor double acceptor), DDA (double donor single acceptor), DAA (single donor double acceptor), DA (single donor single acceptor), and free OH.50 However, DDAA and DA are dominant, which give the bands at 3250 and 3400 cm−1, respectively. In most of the other studies, the OH stretching vibration band was deconvoluted into these two components.22−24,51 In our spectra for unfrozen samples, the band at 3050−3750 cm−1 was successfully deconvoluted into two bands at ca. 3200 and 3400 cm−1 as shown in Figure S3 in the Supporting Information. These bands should correspond to the DDAA and DA configurations, respectively. The band at 3200 cm−1 (assigned DDAA) comes from structurally arranged water molecules possibly engaged in the tetrahedral hydrogen bonding similar to that in ice; it is referred to as ice-like (IL) water. The band at 3400 cm−1 is attributed to partially structured water molecules, namely, liquid-like (LL) water. Figure 5 illustrates the temperature dependence of the relative contributions from IL and LL water (the contributions are defined as αIL and αLL, respectively) in 50 wt % alcohol
Figure 5. Temperature dependence of relative contributions of two Raman bands on the temperature for 50 wt % Gly/water and 13BD/ water. Open and closed symbols represent ice-like (3200 cm−1) and liquid-like (3400 cm−1) water, respectively, which were deconvoluted from the Raman OH stretching bands. Error bars are equal to the relative standard deviations of α. 24877
dx.doi.org/10.1021/jp408722x | J. Phys. Chem. C 2013, 117, 24873−24882
The Journal of Physical Chemistry C
Article
Figure 6. NMR spectra of Gly/water and 13BD/water mixtures of various concentrations at 253 K.
mixtures also show the downfield shift as the alcohol concentration decreases similar to Gly as shown in Figure S4, Supporting Information. Although δw similarly shifts for both 13PD and Gly, the shift of δalc is larger for Gly (Δδalc ≈ 0.4 ppm between 5 and 90 wt %) than for 13PD (Δδalc ≈ 0.2 ppm). The smaller shift of δalc obviously reflects the less hydrophilic nature and suggests the weaker interactions of 13PD with water and ice. In contrast, the chemical shifts for IPA/water shows no concentration dependence similar to 13BD. Since IPA appears more hydrophobic than 13BD, the interaction of IPA with ice is also very weak. Thus, it can be concluded that the interaction of alcohols with the ice surface decreases in the order of Gly > 13PD > 13BD ≈ IPA. The dynamic features of frozen alcohol/water mixtures can be discussed on the bases of the self-diffusion coefficient (D) and the spin−spin relaxation times (T2). Figure S5, Supporting Information, shows the plots of the echo attenuation for water/
alcohols, such as ethanol and propanol in a wide concentration range.55−57 When cor is lower than 48 wt %, the solution is frozen. The chemical shifts for 13BD/water remain constant at almost the same values as those in the unfrozen solution. This suggests that the molecular environments of both 13BD and liquid water are not affected by the presence of ice. The interaction between ice and the liquid phase components in the frozen 13BD/water is thus negligibly (or undetectably) weak. In contrast, both δw and δalc still move downfield with decreasing Gly concentration even in the frozen solution. The surface area of ice relative to the liquid phase volume increases as cor decreases. The ice interactions can thus be more significant at lower cor. The continuous downfield shifts of δw and δalc with decreasing Gly concentration under both unfrozen and frozen conditions indicate that ice is involved in the molecular interaction of the liquid phase components. Both δw and δalc for 13PD/water 24878
dx.doi.org/10.1021/jp408722x | J. Phys. Chem. C 2013, 117, 24873−24882
The Journal of Physical Chemistry C
Article
Table 1. Self-Diffusion Coefficients in Gly or 13BD Solutions D (m2 s−1 × 10−11) sample 10 10 48 48 48 48 a
wt wt wt wt wt wt
% % % % % %
Gly 13BD Gly 13BD Gly 13BD
T (K) 253 253 253 253 298 298
state f f u u u u
a
H2O 11.7 6.80 9.25 4.66 67.4 47.0
Dη/T (10−14 m2 Pa K−1)
alcohol b
(0.1) (0.04) (0.10) (0.02) (2.1) (0.7)
3.86 1.82 3.06 1.16 23.1 16.6
(0.01) (0.02) (0.05) (0.00) (0.7) (0.2)
η (mPa s)
H2O
alcohol
60 171 60 171 5.4 7.1
2.77 4.60 2.19 3.15 0.122 0.112
9.15 12.3 7.26 7.84 0.419 0.396
f = frozen; u = unfrozen. bStandard deviation in parentheses.
Gly and water/13BD binary systems.26,58 All of the plots were fitted by single exponential functions, indicating that the alcohol or water is located in a single molecular environment. Self-diffusion coefficients obtained from these plots are listed in Table 1. The D values of the alcohols were determined from the attenuation of the signal of the methylene group (−CH2). D’Errico et al.59 reported the diffusion coefficients of water and Gly in their mixtures by means of NMR. The interpolation of their data allows us to estimate the diffusion coefficients to be 5.6 × 10−10 m2 s−1 for water and 2.1 × 10−10 m2 s−1 for Gly in 48 wt % Gly solution at 298 K. Zhang et al.58 have also reported the molecular dynamics simulation of the molecular diffusion for Gly/water mixtures in the concentration range 1− 5 mol kg−1. Since 48 wt % Gly corresponds to 5.2 mol kg−1, the extrapolation of their data gives the diffusion coefficient for Gly of 4.4 × 10−10 m2 s−1. Although the value determined by molecular simulation is ca. twice as large as that reported in Table 1, the experimental values determined by NMR agree well with ours. In a usual liquid, D can be interpreted by the Stokes−Einstein (SE) equation.
D=
kBT 6πηr
the SE relationship was much smaller (Dη/T is larger) than other predictions for the dynamically heterogeneous system. The Dη/T values listed in Table 1 increases as the temperature decreases or when the solution is frozen. In addition, the increase is more obvious for the 13BD system than for Gly system. This result suggests that the regions of different structural relaxation grow when the temperature decrease or the solution is frozen and that this heterogeneity occurs to a greater extant for 13BD than for Gly. Raman spectra indicated that the water-rich and alcohol-rich regions are separated for the 13BD system. This should be related to the dynamic heterogeneity probed by NMR. Thus, both Raman and NMR strongly suggest the enhanced structural heterogeneity in 13BD/water mixtures. In order to confirm the validity of the above interpretation, the spin−spin relaxation time (T2) was examined for both Gly/ water and 13BD/water mixtures. Theoretically, the spin−lattice relaxation time (T1) and T2 are represented, respectively, as follows:
(4)
where kB, η, and r are the Boltzmann constant, the viscosity of a medium, and the molecular radius, respectively. This relationship suggests that Dη/T is constant for a given molecule. At 298 K, the values of Dη/T for water in Gly/water and 13BD/ water mixtures are identical as shown in Table 1. The corresponding values for Gly and 13BD are also identical. The values for both water and the alcohols increase from 298 to 253 K and further increase from the unfrozen to frozen state at 253 K. This tendency is more marked for the components in 13BD/water than for those in Gly/water. The validities of the SE and SE−Debye (SED) relationships have been discussed on the basis of computer simulation. Mezza et al.61 showed that the SE and SED relationships are inapplicable to supercooled water and that this breakdown comes from the dynamic heterogeneity. The spatially heterogeneous dynamics is caused by the presence of the regions, in which the structural relaxation is extremely different from the average of the entire system. There is a consensus that such heterogeneity occurs at the temperature of
