Molecular Dynamics Changes Induced by Hydrostatic Pressure in a

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Molecular Dynamics Changes Induced by Hydrostatic Pressure in a Supercooled Primary Alcohol Sebastian Pawlus,*,† Marian Paluch,† and Marzena Dzida‡ †

Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice Poland, and ‡Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland

ABSTRACT Hydrogen-bonded monohydroxy alcohols belong to unique group of materials that characterize with additional prominence the Debye relaxation peak occurring in dielectric loss spectra. This exponential relaxation process is accompanied by another faster, less pronounced and asymmetric structural relaxation process. The dominating, slower process can be classified as “strong”, while the faster one, commonly identified with structural relaxation, is “fragile”. Here we present direct observation of changes of fragility of the dominated process from strong-to-fragile temperature dependence of the relaxation times at elevated pressure in 2-ethyl-1-hexanol. For the highest pressure of 1.57 GPa, the fragility of the dominating process is almost the same as that for the structural one at ambient pressure. Our analysis of the dielectric spectra at different P-Tconditions also reveals that the response function of the dominating process transforms from exponential to nonexponential behavior. SECTION Macromolecules, Soft Matter

M

onohydroxy alcohols have one of the simplest chemical textures. However, paradoxically, as a result of the formation of hydrogen-bond structures, their molecular dynamics is complex.1-3 Because of the existence of hydroxyl groups, these materials can form different structures, involving hydrogen bonding of different strengths and geometries. Moreover, because of thermal liability of H-bonds, its strength decrease with temperature. This thermodynamic variable tends to be the dominant factor, governing the dynamics of associated liquids, whereas pressure exerts much smaller effects.4 Moreover, the effect of compression on hydrogen-bonding is more complicated than the effect of temperature and remains not clearly understood. Some experiments5,6 and molecular simulations7 indicate that pressure promotes the formation of H-bonds, while other studies8-10 suggest the opposite effect; e.g., in the case of glycerol, molecular dynamics simulations7 and NMR measurements11 indicate that pressure increases the hydrogen bonding. On the other hand, for water, the best-studied case of hydrogen-bonding materials, some studies indicate that pressure reduces the number of hydrogen bonds.12,13 Other scenarios are also possible, such as modification of H-bonds due to steric repulsion when hydrogen atoms come closer to other hydrogen atoms in H-bond. A repulsive interaction starts between them at a certain distance, and the structure is strained. Another possibility is the formation of centered H-bonds like in the case of Ice VII, where symmetric hydrogen-bonds are formed at elevated pressure.14 The commonly accepted scenario assumes that the concentration of hydrogen bonds at low temperatures and low pressures will be larger than that at high temperatures and high pressures. In light of these facts, monohydroxy alcohols

r 2010 American Chemical Society

are very attractive materials for investigations, mainly because of their simple molecular structure and affinity to water. However, in contrast to water, some primary alcohols can be easily supercooled to form glass and studied in this state by different experimental methods. In particular, an exceptionally useful experimental tool to study relaxation dynamics in this area is broadband dielectric spectroscopy because it allows following dynamics in a time range of 15 decades, much more than any other spectroscopy method.15 The universally observed structural relaxation process in glass-forming liquids is slowed down with decreasing temperature by many orders of magnitude and becomes nonexponential upon approaching the glass transition. The corresponding peak in the imaginary part of the complex dielectric permittivity, ε00 ( f ), becomes broader in frequency, f, than a single relaxation-time Debye peak. However, in dielectric spectra of monohydroxy alcohols, an additional, very pronounced exponential process is observed.1,3 Although dominating in dielectric spectra, this so-called Debye relaxation remains invisible for other techniques such as heat-capacity2 and optical1 or mechanical spectroscopy.3 Despite longlasting investigations, its microscopic origin remains a matter of debate, and various possible hydrogen-bonded structures, such as linear chains, cyclic multimers, and clusters as well as micelles aggregating through the polar hydroxyl groups have been recently considered to be responsible for the existence of the Debye relaxation process in dielectric spectra.15-21

Received Date: September 14, 2010 Accepted Date: October 21, 2010 Published on Web Date: October 26, 2010

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DOI: 10.1021/jz101288v |J. Phys. Chem. Lett. 2010, 1, 3249–3253

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Until now, the only general conclusion about the Debye process was that “hydrogen bonding will need to be an essential ingredient of the explanation”.22 Although this process has been detected in only very few materials, apart from primary alcohols it can be also visible in water.23 Understanding of the molecular sources of the Debye process remains one of the hot topics in physics of glasses because of its importance for better understanding of the molecular dynamics of water. Moreover, knowledge of its origin provides an opportunity to study H-bond dynamics by direct investigations of the behavior of the Debye process in plenty of monohydroxy alcohols. Very recently, two papers devoted to the high-pressure behavior of relaxation dynamics in monohydroxy alcohols were published by Fragiadakis et al.24 and Reiser et al.25 On the basis of high-pressure investigation, Fragiadakis et al.24 hypothesized the existence of two relaxation peaks: the Debye and the structural one in primary alcohols, due to the strongly heterogeneous local organization of molecules that form nearly linear clusters of a small number of particles. The same structure was also postulated by Floriano and Angell to explain the behavior of the Debye process in n-propanol.21 It was pointed out by these authors that, with the addition of a very small amount of LiClO4, the amplitude and relaxation time of this process in n-propanol started to decrease, which was explained by progressive dismantlement of H-bonded clusters by addition of salt, whereas glass temperature remained unchanged. Similar behavior was observed by Goresy and B€ ohmer in the case of mixtures of n-butanol with n-bromobutane.26 In that case, time separation between the Debye and structural relaxation dramatically increased, and dielectric strength decreased with diminishing concentration of alcohol. Moreover, the authors of both articles noticed increased stretching of the Debye process with increasing concentration of solvent. As was already mentioned, compression can modify hydrogen bond structures.6 Pressure-induced decrease in the amplitude of the Debye peak was reported in both papers by Fragiadakis et al.24 and Reiser et al.25 However, the influence of elevated pressure on the relaxation dynamics of both structural and Debye processes was quite different from that observed in the mixtures and are indicative of the key role of pressure in controlling the molecular dynamics of alcohols. If we assume, that monohydroxy alcohols portray the behavior of water, one can expect that compression will modify the dynamics of both groups of materials in a similar way. Especially, modification of hydrogen bonds should be clearly visible by changes in the Debye process in primary alcohols and provide direct information on the possible changes of similar structures in supercooled water. However, noticeable changes of the dynamics in hydrogen-bonded materials can be observed if pressure of a few gigapascals is applied.4,14,27,28 In this Letter we present results of molecular studies of 2-ethyl-1-hexanol (2E1H), one of most studied primary alcohols.2,3,22,24 The behavior of both well-separated molecular processes, characteristic for monohydroxy alcohols, was monitored at elevated pressures up to 1.57 GPa. For the first time, we show that a “strong-to-fragile” change of relaxation


Figure 1. Dielectric loss spectrum measured at ambient pressure for 158.5 K.3 The solid line depicts the position of the Debye process. The inset presents the relaxation map of both the exponential (squares) and nonexponential (triangles) processes. Solid lines are fits to the data with the VFTH equation.

dynamics occurs when pressure goes beyond ca. 0.5 GPa. These results present clear evidence that only one stretched relaxation process can be observed at highest pressure, and its temperature behavior is virtually the same as that for structural relaxation at ambient pressure. A representative dielectric relaxation spectrum of 2E1H at ambient pressure exhibits two relaxation processes (Figure 1). Spectral shape analysis reveals that the slower process has a Debye-like shape, while the faster process is broader, with a stretching parameter of ∼0.51-0.57. The relaxation map presented in the inset of Figure 1 shows that these two processes have different temperature dependences with almost Arrhenius-like behavior of relaxation times for the Debye process and clearly non-Arrhenius behavior in the case of the structural relaxation. Solid lines represent fit with the Vogel-Fulcher-Tamman-Hess (VFTH) relation that can satisfactorily describe the experimental results in the studied temperature range. This pattern of behavior changes with compression. In Figure 2 the dielectric spectra measured at different degrees of compression but for the same relaxation time of the dominant process were compared (vertical shift was necessary to superimpose on the spectrum at ambient pressure, chosen arbitrarily to be the reference spectrum) to estimated shape changes. Only for the ambient pressure reliable deconvolution of both processes is possible. As was already presented by Fragiadakis et al. (Figure 2 in ref 24), with isothermal pressurization, the structural relaxation process comes closer to the Debye one, and both processes merge into a single peak at elevated pressure. In that case, special analytical procedures based on some assumptions about the shape of relaxation processes are necessary to deconvolute both processes.24 For example, in the case presented in ref 24, the ansatz introduced by Williams and Watts was applied with the Debye process described by the exponential function and the structural relaxation described by the Kohlrausch-Williams-Watts

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Figure 3. Fragility plot of relaxation times for all isobars from ambient pressure up to 1.57 GPa.

Figure 2. Selected loss spectra measured for the same relaxation time of the dominating process at elevated pressures up to 1.57 GPa. Inset presents relaxation map of dominating processes (black circles - 0.53 GPa, green triangles - 0.96 GPa, blue stars 1.57 GPa). Solid lines are fits of the data with the VFTH equation.

function. On the other hand, in ref 26, authors showed that loss spectra can be described as a superposition of a symmetrically broadened structural relaxation peak with an asymmetrically broadened Debye peak. To avoid this problem of deconvolution, we analyzed only dominating relaxation process. It was found that a stretching parameter for this single peak is b ∼ 0.63-0.75. The relaxation map for the single process at elevated pressure is presented in the inset in Figure 2. The data was described with the VFTH equation. To address the problem of changes in relaxation dynamics at elevated pressure, we present the relaxation times for different isobars by scaling temperature by the arbitrarily chosen temperature at which the relaxation time of a given process reaches 100 s, Tref.29 Note, that Tref can be identified with Tg only for the structural relaxation process. Owing to this procedure, the reduction of all data into a comprehensible pattern was possible (Figure 3). At ambient pressure, two coexisting processes have two Tref-scaled runs: less steep for the Debye relaxation, and steeper for structural relaxation. Examination of the plot reveals that two extremes of the relaxation patterns can be distinguished, appointed by this relaxation picture observed at ambient pressure. With increasing pressure up to ca. 0.5 GPa, the temperature behavior of the relaxation times for the dominating process remains the same as that for Debye relaxation at ambient pressure. However, further compression starts to change the observed pattern of relaxation dynamics (intermediate behavior of data for 0.96 GPa), and at the highest pressure of 1.57 GPa, the relaxation pattern for this process is almost the same as that for the structural relaxation at ambient pressure. It becomes even more obvious from Figure 4. According to the concept of “fragility” introduced by Angell,30,31 the glassforming materials can be classified as “fragile” or “strong” due to value of the kinetic fragility parameter, m, defined as dlog τ m ¼ dðT . The m is an index of how fast the structural g =TÞ

Figure 4. Pressure dependence of fragility parameter, m of the dominating process. Arrows indicate the direction of changes in dynamics from strong to fragile behavior during compression. Black filled square represents the fragility of the nonexponential process at ambient pressure.

relaxation times increase while approaching the Tg. Consequently, fragility values typically range between m = 17 for systems classified as “strong” (with Arrhenius behavior of relaxation times), and m ∼ 150 for most fragile materials. Although strong-fragile classification is reserved to mainly describe the structural relaxation time behavior, we decide herein to use the same classification for the Debye-like process in order to emphasize the compression-induced changes in molecular dynamics. According to this scheme, for 2E1H at ambient pressure, the relaxation dynamics of the Debye process is strong, with m=30 ( 2 (empty square in Figure 4), whereas behavior of the faster, structural relaxation process is typical for medium fragile glasses with m = 60 ( 2 (solid square in Figure 4). At elevated pressure, the fragility parameter remains constant up to ca. 0.5 GPa (triangle) and then increases (circle) up to a value of m = 62 ( 2 (star) at the highest pressure of 1.57 GPa, which is close to the fragility of structural relaxation at ambient pressure. To the best of our knowledge, this is the first time this kind of transition from strong to fragile dynamics with compression was observed. One possible explanation of the observed phenomena is based on the model presented by Levin and Feldman.20 According to their explanation, the structure of 2E1H consists

Tg


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of molecules linked into oligomer chains by intermolecular hydrogen bonds. The most probable mechanism of the structural relaxation in this material is that of consequent reorientation of molecules in chains. Because motions of the molecules inside the chains are only internal without involving cooperative rearrangement of many particles, the Debye process remains invisible to the mechanical or optical spectroscopies. However, because these internal motions could produce a change in the distance between chains ends, the total dipole moment changes markedly and is visible in dielectric relaxation spectra. Elevated pressure reduces the number of hydrogen bonds and, consequently, the length of hydrogen-bonded chains and decreases local ordering of the dipoles. The shorter chains relax faster and both Debye and structural relaxations start to merge. At high enough pressure, the formation of the chains is no longer favorable, only isolated molecules are possible, and only single, stretch relaxation is observed. An alternative explanation of the observed phenomena can be similar to that presented by Fragiadakis and co-workers.24 According to their explanation, the structure of 2E1H consists of micellar aggregates with a polar core of up to 10-15 hydroxyl groups. These are bonded together in linear structures with the shell of the alkyl chains arranged radially outward (left part of Figure 14 in ref 24). Because motions of the molecules inside the clusters are only internal without involving cooperative rearrangement of many particles, the Debye process remains invisible by mechanical or optical spectroscopy. However, because these internal motions could be rotations of molecules around their chain cluster axis, the total dipole moment changes markedly and is visible in dielectric relaxation spectra. Similarly to the model by Levin and Feldman, elevated pressure reduces the number of hydrogen bonds and the size of hydrogen-bonded clusters as well as the local ordering of the dipoles (right part of Figure 14 in ref 24). The smaller clusters relax faster, and both Debye and structural relaxations start to merge. At high enough pressure, formation of the clusters is no longer favorable, only isolated molecules are possible, and only single, stretched relaxation is observed. Thus, using dielectric spectroscopy to probe relaxation dynamics of 2E1H at high pressure up to 1.57 GPa, we have observed progressive merging of both Debye and structural relaxations. Even at 0.5 GPa we were unable to plausibly portray the dynamics of both processes separately. Above ca. 1 GPa, only the single relaxation process is observed. Change of hydrogen bonds with compression results in a progressive change of relaxation dynamics visible as an increase of fragility from strong behavior at the low pressure limit up to fragile behavior at the highest pressure, the same as that for the structural process at ambient pressure. This surprising result directly shows that in monohydroxy alcohols the dominating exponential relaxation is directly related to H-bond structure and can be dramatically modified at high enough pressure.

carried out using a Novocontrol Alpha Analyzer in the frequency range between 10 mHz and 3 MHz. The high-pressure technique used herein is very similar to that of Johari and Whalley.32 The pressuring system was constructed by UNIPRESS with a homemade special flat parallel capacitor. The pressure was exerted on the sample by a steel piston. The tested sample was in contact with only stainless steel and Teflon. During high-pressure measurements, the temperature was controlled to within 0.5 K by a Tenney thermostatic system.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed.

ACKNOWLEDGMENT M. Paluch is deeply thankful for the financial support within the framework of the project entitled/ From Study of Molecular Dynamics in Amorphous Medicines at Ambient and Elevated Pressure to Novel Applications in Pharmacy/, which is operated within the Foundation for Polish Science TEAM Programme cofinanced by the EU European Regional Development. S. Pawlus acknowledges financial assistance from FNP HOMING program (2008) supported by the European Economic Area Financial Mechanism.

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EXPERIMENTAL SECTION The 2E1H sample was purchased from Aldrich (98% purity) and used as obtained. Dielectric measurements were


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