How Different Molecular Architectures Influence the Dynamics of H

Article pubs.acs.org/JPCB

How Different Molecular Architectures Influence the Dynamics of H‑Bonded Structures in Glass-Forming Monohydroxy Alcohols M. Wikarek,*,†,‡ S. Pawlus,†,‡ Satya N. Tripathy,†,‡ A. Szulc,† and M. Paluch†,‡ †

Institute of Physics, University of Silesia, ul. Uniwersytecka 4, 40-007 Katowice, Poland Silesian Center for Education and Interdisciplinary Research, ul. 75 Pułku Piechoty 1a, 41-500 Chorzów, Poland

‡

ABSTRACT: Primary alcohols have been an active area of research since the beginning of the 20th century. The main problem in studying monohydroxy alcohols is the molecular origin of the slower Debye relaxation, whereas the faster process, recognized as structural relaxation, remains much less investigated. This is because in many primary alcohols the structural process is strongly overlapped by the dominating Debye relaxation. Additionally, there is still no answer for many fundamental questions concerning the origin of the molecular characteristic properties of these materials. One of them is the role of molecular architecture in the formation of hydrogen-bonded structures and its potential connection to the relaxation dynamics of Debye and structural relaxation processes. In this article, we present the results of ambient and highpressure dielectric studies of monohydroxy alcohols with similar chemical structures but different carbon chain lengths (2-ethyl-1-butanol and 2-ethyl-1hexanol) and positions of the OH− group (2-methyl-2-hexanol and 2-methyl3-hexanol). New data are compared with previously collected results for 5-methyl-2-hexanol. We note that differences in molecular architecture have a significant influence on the formation of hydrogen-bonded structures, which is reflected in the behavior of the Debye and structural relaxation processes. Intriguingly, studying the relaxation dynamics in monohydroxy alcohols at high pressures of up to p = 1700 MPa delivers a fundamental bridge to understand the potential connection between molecular conformation and its response to the characteristic properties of these materials. Gainaru et al.11,18,19 The H-bonds inside the chains are stronger, thus the probability of the chain breaking in the middle is the least. In addition, the TCM postulates that the chain moves when molecules join and leave the chain ends, and the Debye process observed in the dielectric spectra is associated with motion of the end-to-end vector of the Hbonded chains, similar to the normal mode in type-A polymers.20 As a consequence, the motion of the entire chain requires more time than that of the free molecule, and the relaxation time of the Debye process is significantly longer compared to the structural relaxation time.11 However, the fundamental aspects of the molecular dynamics of both the relaxation processes are not fully understood, more concretely under high-pressure conditions, and the discussion on the nature of the complex molecular dynamics of primary alcohols remains open. As already mentioned, the Debye process observed in the dielectric spectra dominates over structural relaxation in many alcohols. However, in some materials, the amplitude of this relaxation is much lower and is close to the amplitude of the α process or even described by only a single stretch relaxation function.3,5,9,21−23 These observations were correlated with the

I. INTRODUCTION The molecular dynamics of supramolecuar hydrogen-bonded structures and associated liquids is an active area of current research.1−5 In this context, monohydroxy alcohols constitute a unique class of H-bonded materials due to their simple structure, the existence of different kinds of molecular architectures, and their outstanding affinity for supercooling. Intriguingly, the most essential aspect in many primary alcohols is the manifestation of a slow exponential relaxation process in the dielectric spectra. For a long time, this process was anticipated to be the structural relaxation process (α) because of its non-Arrhenius-type temperature dependence of relaxation times.6−8 Recently, Debye relaxation has been recognized in rheological experiments.9 A comparison of the results from different experimental techniques, such as photon correlation spectroscopy,10 calorimetric measurements,7 NMR,11−13 and light scattering,6 has revealed that the time scale of the α process observed using these methods agrees with that of a nonexponential relaxation, with a faster time scale and lower amplitude compared with those of the Debye process.7,14,15 In addition, the molecular origins of the Debye process have been connected to motion of the alkyl chains.14,16 Although several models have been proposed, the microscopic mechanism behind the unusual dielectric behavior of primary alcohols has remained controversial.11,17,18 One of the most commonly used models is the transient chain model (TCM) proposed by © XXXX American Chemical Society

Received: February 11, 2016 Revised: May 29, 2016

A

DOI: 10.1021/acs.jpcb.6b01458 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

monohydroxy alcohols were investigated isothermally and isobarically in the frequency range from 10−1 to 106 Hz using a Novocontrol α-impedance analyzer. For high-pressure studies, the sample capacitor was placed in a high-pressure chamber and compressed using silicone fluid. The sample was in contact with only Teflon and stainless steel. The temperature was stabilized within 0.5 K via a Tenney Junior compact temperature test chamber. Isobaric and isothermal investigations above p = 650 MPa were performed with another highpressure setup, and the temperature was controlled within 0.5 K. The pressure setup has been described elsewhere.38 For 5methyl-2-hexanol, all results were taken from a study reported elsewhere.28

location of the OH− group in the molecular architecture. Dannhauser showed that the location of the OH− group in the backbone of the alcohol at a peripheral or nonterminal position affects the amplitude of the dominating relaxation24 and thus divides monohydroxy alcohols into two classes.24−26 For materials from the first class, called type-I, because of the terminal position of the OH− group the formation of chain-like hydrogen-bonded structures is preferred. The characteristic feature of these alcohols is a large Debye process. Examples are propanol,4 2-ethyl-1-hexanol,3,7,15,17,27 5-methyl-2-hexanol,28 and 6-methyl-3-heptanol.29,30 On the other hand, monohydroxy alcohols that belong to the latter class, called type-II, have a hydroxyl group located at a nonterminal position. In this class, the most probable H-bonded configurations are ring-like structures and the number of chains is small.26,31 Here, a Debye-like process with a smaller amplitude, comparable to that of the structural relaxation, is observed in the dielectric spectra or even completely disappears. The corresponding examples are 3-methyl-3-heptanol,31 4-methyl-3-heptanol,26,32 and 5-methyl-3-heptanol.9,26 Moreover, if the large cyclic functional group is located in the molecular architecture, for example, the phenyl ring, it can limit the formation of Hbonded chainlike structures because of marked steric hindrance. In this case, the most possible associated structures are tree-like, in which only a single stretch structural relaxation in the dielectric spectra is evident.33 Until now, most of the previous studies on primary alcohols have been confined to only ambient pressure measurements at different temperatures.17,24,27,34−36 Note that during isobaric measurements not only the thermal energy but also the density of the sample changes. Both factors influence the dynamics of H-bonded structures simultaneously, and their effects cannot be distinguished easily. Thus, experiments at ambient pressure are not sufficient to understand the intrinsic features of alcohols. To solve this problem, it is necessary to perform isothermal measurements under elevated pressure conditions, at which only the density of the material changes. Clearly, a comparison of the results from isobaric and isothermal measurements is anticipated to provide a detailed understanding of the influence of thermal energy and density on the complex relaxation dynamics of alcohols.37 In this work, we study the role of variation of molecular architecture, mainly the alkyl chain length and position of the OH− group, in the dynamics of H-bonded structures. All alcohols were studied using dielectric spectroscopy under different thermodynamic conditions. The results obtained for type-I alcohol 2-ethyl-1-butanol are compared with data for 2ethyl-1-hexanol. Both materials have similar molecular structures, but with slightly different alkyl chain lengths, and are characterized by four decades in time scale separation between the Debye and structural processes at ambient pressure. High-pressure investigations (up to p = 1700 MPa) allowed us to determine the effect of compression on the relaxation dynamics and the time span between both processes, which reflects changes in the architecture of the associated structures. Similar investigations were carried out in the context of the position of OH− and methyl groups in the alcohol structure. For this, we compare the results for 2-methyl-2hexanol, 2-methyl-3-hexanol, and 5-methyl-2-hexanol, collected under different p−T conditions.

III. RESULTS AND DISCUSSION A. Impact of Different Alkyl Chain Lengths. The dielectric loss spectra of 2-ethyl-1-butanol (2E1B) and 2-ethyl1-hexanol (2E1H) for several selected thermodynamic conditions, but the same Debye relaxation time, are illustrated in Figure 1. In addition, the dielectric loss data of 2E1B have been described by the superposition of curves (solid lines) comprising the Debye and α processes for p = 0.1 and 220 MPa, respectively (see the inset). As can be seen, the time span between the prominent Debye relaxation and much lower structural process for both alcohols at ambient pressure is about four decades (larger for 2E1B). Both the processes are clearly

Figure 1. Comparison of the dielectric loss spectra at atmospheric pressure and high pressures (up to 1700 MPa) for both 2E1H and 2E1B. (Inset) Dielectric loss spectra of 2-ethyl-1-butanol under different selected p−T conditions.

II. SAMPLES AND EXPERIMENTS 2-Ethyl-1-butanol, 2-ethyl-1-hexanol, 2-methyl-2-hexanol, and 2-methyl-3-hexanol were purchased from Sigma-Aldrich. These B


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The Journal of Physical Chemistry B resolved at ambient pressure. However, for both samples, the time separation decreases at elevated pressures, which indicates different pressure sensitivities of the main relaxations and supports the previous observations for 2E1H.7,8,17 It has already been demonstrated that elevated pressure modifies the relaxation dynamics of the Debye process in a manner different from that of structural relaxation, irrespective of the molecular structure.17,27−29 For 2E1H, Fragiadakis et al. and Pawlus et al. have shown that the two relaxation processes have different pressure sensitivities. 17,39 Upon compression, both the processes gradually approach each other, and for very high pressures, the time scales of both relaxations are very close. For p = 1700 MPa (2E1B) and p = 1570 MPa (2E1H), both processes merge and only a single slightly stretched relaxation is visible. As suggested by Fragiadakis et al. for 2E1H, compression can reduce the length of hydrogen bonding structures and eventually decrease the size and number of Hbonded chain clusters. The same explanation remains valid for 2E1B as well. It has to be emphasized that although 2E1B exhibits a better time separation of the two main relaxation processes at ambient pressure than that for 2E1H, at a pressure higher than p = 1.5 GPa, the Debye-like relaxation process has almost the same shape for both alcohols. This indicates a similar time distance between the Debye and structural relaxations under these high-pressure thermodynamic conditions. It can be concluded that in the case of 2E1B the sensitivity of the structural relaxation to pressurization is larger than that in 2E1H. Additionally, differences in the amplitude of the Debye process at different pressures can be detected for both alcohols from Figure 1. It is obvious that the amplitude of the Debye process is markedly lower under high-pressure conditions for both alcohols, which is associated with changes in the H-bonded supramolecular structure. The literature supports that compression causes a reduction in the number of hydrogen bonds that form the chain structure of molecules.40,41 Note that although 2E1H and 2E1B have different backbone lengths this does not influence the intensity of the Debye process, which is the same for both materials at ambient pressure. Moreover, the pressurization induces a similar reduction in the amplitude of the Debye process for both materials. These two observations clearly indicate that the supramolecular structures responsible for the Debye relaxation for both alcohols must have not only the same origin but also the same shape and length under all investigated p−T conditions. Although both molecules have different backbone lengths, similar pressures exert almost the same effect on the amplitude of the Debye process for the same τDebye, whereas the peak position of the structural relaxation changes in a different manner. This observation clearly indicates that chain length is important for the dynamics of the α process. For 2E1B with smaller molecules, molecular reorganization is easier during compression and, consequently, molecular packing is better than that for larger molecules of 2E1H. This results in a larger pressure sensitivity of the structural relaxation in 2E1B. B. Meaning of Different Positions of the OH− and Methyl Groups in the Alkyl Chain. Figure 2A,B compares the dielectric loss spectra with the same frequency as the maximum of the Debye peak for 2-methyl-2-hexanol, 2-methyl3-hexanol, and 5-methyl-2-hexanol, collected under different p−T conditions. In Figure 2A, the results from ambient pressure measurements are depicted. For 2M3H, in which the OH− group is located at a nonterminal position, the amplitude of the Debye process is almost 1 order of magnitude lower in

Figure 2. Selected loss spectra measured for the same relaxation time of the Debye process under different p−T conditions ((A) ambient pressure and (B) high pressure) for 2M2H, 2M3H, and 5M2H and (C) for 2E1H and 2E1B at ambient pressure.

comparison to that for the other two alcohols, which indicates a significantly lower number of associated chain-like structures. On the other hand, for the other two alcohols with the same terminal position of the hydroxyl group but different positions of the methyl group, that is, 5M2H and 2M2H, no difference in the shape and amplitude of the Debye process was observed. These observations suggest that in both alcohols the shape and length of the H-bonded chains are the same despite differences in backbone structures. On the other hand, a shift in the hydroxyl group to a more nonterminal position in the molecular structure results in a marked reduction in the amplitude of the Debye relaxation. It is worth noting that the position of the α process seems to be similar for all alcohols, which suggests that the structural relaxation also remains slightly sensitive to the location of the methyl group for the same alkyl chain length. Figure 2C presents the spectrum of 2E1B and 2E1H for the same relaxation time for the Debye process as that for 2M2H, 5M2H, and 2M3H. It turns out that for these two alcohols the amplitude is similar to that for alcohols with a terminal position of the OH− group (i.e., 2M2H and 5M2H). This indicates that the same shape and length of the H-bonded structures are responsible for this relaxation, whereas the position of the α process markedly depends on the alkyl chain length. However, the situation varies significantly for the elevated pressure case, as shown in Figure 2B. The amplitude of the Debye-like process becomes similar for all examined alcohols, but it changes with pressurization in a different manner for different samples. Note that the intensity of the dominating process for 2M3H increased significantly under compression in comparison with the results obtained at C


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determines the shape of the supramolecular structures formed by hydrogen bonding and, consequently, the amplitude of the Debye process. For both materials, although the H-bonded structures are, at least in part, chain-like, the strength of the Debye process is strongly dependent on the location of the hydroxyl group. Moreover, this behavior of this relaxation in the case of 2M2H has never been reported before. It is the first time that the strengthening (visible as a rise in the amplitude) and then diminishing, above some pressure limit, of the dominating process could be observed during isothermal pressurization. It can be explained by the initial increase in the population of H-bonded chains due to breaking of the ring structures, which reaches a maximum at some pressure value. With further densification, the number of newly formed chains starts to be compensated by a decrease in size, or even destruction, of the already existing ones, which is reflected in the lower intensity of the Debye process in comparison with that measured at ambient pressure. To describe the relaxation processes analytically, dielectric data of 2-ethyl-1-butanol, 2-ethyl-1-hexanol, 2-methyl-2-hexanol, and 2-methyl-3-hexanol under isothermal and isobaric conditions were fitted by the sum of the conductivity, Debye equation, and Havriliak−Negami (HN) equation

atmospheric pressure. On the other hand, for 5M2H and 2M2H, the amplitude of the process decreased. In addition, the difference appears in the broadening of the Debye-like relaxation, reflected by the half-width: about 1.3 decades for 5M2H (p = 780 MPa), about 1.5 decades for 2M2H (p = 722 MPa), and about 1.4 decades for 2M3H (p = 955 MPa). It is well established that the H-bonded architecture is rebuilt during compression; then, it can be concluded that two opposite behaviors of the evolution of the Debye process at elevated pressures indicate various changes of the associated structures in different alcohols. These changes are less related to the position of the methyl group than to that of the hydroxyl group. Figure 3A,B compares the dielectric loss spectra collected during the isobaric cooling at atmospheric pressure and

ε″(ω) = Im(ε∞ +

∑ i

Δεi αi βi

(1 + (iωτi) )

)+

σdc ε0ω

(1)

where ε∞ is the high-frequency dielectric constant, Δεi is the dielectric strength, τi is the dielectric relaxation time, α and β are the shape parameters, and σdc is the dc conductivity; i stands for the Debye or structural process. For high-pressure data, in which only the Debye-like process is present, the conductivity part and single HN functions were used to parameterize the spectra. The isothermal pressure dependences of the Debyeand α-relaxation times in 2-ethyl-1-butanol and 2-ethyl-1hexanol are presented in Figure 4A,B. The relaxation time, τ, for both relaxations and both alcohols increases with compression in a linear manner and was parameterized by a linear function.37 In the case of Debye relaxation, the trend for both alcohols was similar. On the other hand, the structural relaxation was much more sensitive to compression in the case of 2E1B. The parameter commonly used to characterize the pressure sensitivity of the relaxation dynamics of the materials is activation volume ΔV

Figure 3. Isochronous pairs of dielectric loss spectra for 2M3H (A) and 2M2H (B), each corresponding to a different Debye relaxation time.

ΔV = −RT

∂ log(τ ) ∂P

(2)

where R is the gas constant, equal to 8.31 J/(mol K). Figure 5 presents the temperature dependences of ΔV (0.1 MPa) for 2B1H and 2E1H. In the case of 2E1H the activation volume for both relaxations decreases with increasing temperature, and the values are the same in the range of error. It can be concluded that at a low pressure/low temperature range both relaxations in this alcohol have the same sensitivity to compression. Moreover, ΔV for the Debye process in 2E1B has a value and T-dependence similar to those of the relaxations in 2E1H. On the other hand, higher values were recorded for the α process in 2E1B, which confirms a higher pressure sensitivity of this process in comparison to that of the Debye process. This finding explains why the time-scale separation between the Debye and α relaxations in this alcohol decreases more than that for 2E1H (see Figure 1) under high-pressure conditions;

isothermal compression for 2M3H and 2M2H, respectively. For 2M3H (Figure 3A), it is obvious that during pressurization the amplitude of the Debye process increases. A similar type of behavior in the Debye process was observed for 4-methyl-3heptanol and 5-methyl-3-heptanol.9 In the case of 2M2H (Figure 3B), the picture is more complex. For lower values of pressure, the amplitude of the dominating relaxation also increases and becomes markedly larger than that measured at ambient pressure (for the same τDebye). But this trend changes with increasing pressure; at 700 MPa, the intensity of the Debye process is the same as that at ambient pressure, and with further pressurization, the value becomes lower than that at atmospheric pressure. These observations provide evidence that the location of the OH− group significantly affects the observed properties of the examined alcohols because it D


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where D is the material constant and T0 is the so-called ideal glass-transition temperature (in the case of structural relaxation). From the fit parameters, we can calculate the glass-transition temperature. Typically, Tg of the dielectric measurement is defined as being equal to 100 s for the αrelaxation time. For convenience, to avoid extrapolation of the fit functions considerably above the experimental data set, we use reference temperature Tr for different relaxation processes estimated for relaxation time equal to 1 s. To estimate the degree of departure from the activation-like behavior, the concept of kinetic “steepness index” (parameter m) can be used.45 m=

d log τ d

() T

|Tg (4)

This concept was developed to investigate the properties of the structural process close to the glass-transition temperature. The value of m parameterizes the degree of departure of the temperature dependence of τα from the linear activation behavior during the approach to the glass-transition temperature. It is well established that substances with m = 17 (almost Arrhenius-like temperature dependence) are classified as “strong”, whereas materials with an m of ∼150 are called “f ragile”.45 On the basis of the concept of steepness index to parameterize the thermal evolution of the Debye and structural relaxation times under ambient- and high-pressure conditions, Pawlus et al.28,46 showed that for 2E1H and 5M2H the Tr-scaled relaxation pattern for the Debye process at the highest pressures of 1570 and 1750 MPa, respectively, is nearly the same as that for the α relaxation at ambient pressure (similar values of m). The same pictures emerge from the high-pressure studies of 2-ethyl-1-butanol presented in this article. From the analysis, parameter values of m = 24 ± 2 for the Debye process and m = 35 ± 2 for the structural process were obtained for 2E1B at atmospheric pressure. On comparing with the values obtained for 2-ethyl-1-hexanol (m = 30 ± 2 for the Debye relaxation and m = 60 ± 2 for the α relaxation), it can be concluded that for both alcohols m varies slightly for the Debye process, whereas it differs markedly for the structural one. When the sample of 2B1H was compressed to 1700 MPa, m increased to up to 38 ± 2 for the Debye process, which was close to the value for the α process at ambient pressure. This finding agrees with the pattern reported for 2E1H and 5M2H.28,46 A description of the above behavior was proposed for the first time by Pawlus and co-workers for 2-ethyl-1hexanol.46 The elevated pressure reduces the number of hydrogen bonds and, consequently, the length of the hydrogen-bonded transient chains. Shorter chains relax faster, and both the Debye and structural relaxations start to merge, and the amplitude of the Debye process decreases. At a sufficiently high pressure, the transient chains are so short that their relaxation dynamics responsible for the Debye process becomes similar to the dynamics of the α relaxation. As a result, mDebye under high-pressure conditions approaches mα. This explanation is also valid for 2E1B because of the similar architectures of both alcohols. It has to be emphasized that a lower mα for 2E1B indicates a marked influence of the molecular size on the α-relaxation dynamics. For larger molecules of 2E1H, the energy necessary for reorientation during isobaric cooling is higher than that for smaller molecules of 2E1B. Consequently, the steepness index of the former is

Figure 4. Pressure dependence of the relaxation times for the Debye process (A) and α process (B) at different temperatures for 2-ethyl-1butanol and 2-ethyl-1-hexanol.

Figure 5. Activation volume of 2E1H and 2E1B as a function of temperature.

because of the higher sensitivity of the α process to pressurization, both relaxations approach each other during compression. On the other hand, the temperature dependences of the relaxation times for the Debye (Debye-like process) and α processes and for all investigated alcohols (not presented) are clearly of the non-Arrhenius type and were described by a single Vogel−Fulcher−Tammann (VFT)42−44 relation in the entire investigated temperature range ⎛ D ⎞ τ(T ) = τ0 exp⎜ ⎟ ⎝ T − T0 ⎠

Tg

(3) E


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dynamics of the supramolecular structures responsible for the occurrence of the Debye process in this alcohol. Until now, because of the superposition of both Debye and α relaxations at pressures larger than ca. 1 GPa, it was impossible to draw any conclusions about the steepness index of the structural process under high-pressure conditions. However, because the Debyelike relaxation cannot become faster than the α process, for 2M3H, it can be concluded that not only does the steepness index of the H-bonded related process increase but also mα has to become larger under high-density conditions. This conclusion agrees with observations for other associated materials under high-pressure conditions.47,48 As has already been presented, if the hydroxyl group is located at the end of the molecular chain and alcohols differ in only the length of the alkyl chain, then their relaxation dynamics changes significantly. However, the picture becomes even more complex when the OH− group is at a nonterminal position in the molecule. In this case, not only does the time separation between the Debye and structural relaxations change but also marked differences in the amplitude of the Debye process, depending on the position of hydroxyl group, are observed. Comparison of the dielectric loss spectra measured under isobaric (points) and isothermal conditions for 2M3H (Figure 3A) reveals that the amplitude of the Debye relaxation increases significantly more with compression than with cooling. This dependence is opposite to the behavior noticed for 2E1H and 2E1B. These observations can be explained by the differences in H-bonded architecture between primary alcohols with terminal (2E1H, 2E1B) and nonterminal locations of the hydroxyl group (2M3H), first postulated by Dannhauser.24 To parameterize these changes, it is convenient to present the dependence of the dielectric strength as a function of the relaxation time. This representation was chosen to compare the results from both isothermal and isobaric experiments. The dielectric strength, Δε, of the Debye (or Debye-like) process is shown in Figure 7 for 2M3H (A) and 2E2H (B). It is observed that for both alcohols the dielectric strength of the Debye/Debye-like relaxation increases more strongly during isothermal pressurization than during ambient pressure cooling of the samples. Consequently, ΔεDebye at elevated pressures is larger than that at ambient pressure, at least for shorter relaxation times. However, for 2M2H, ΔεDebye at ambient pressure grows more with cooling than that for 2M3H and, for longer τDebye, their values becomes larger than for data collected under high-pressure conditions. This fact agrees with the discussed behavior of the amplitude of the spectra presented above in Figure 3B. Intriguingly, the pressure acts as the parameter controlling the molecular dynamics of this alcohol. At ambient pressure in alcohol, there exists a mixture of H-banded rings and chains; during compression, the rings are broken and the population of chains increases (e.g., in 4methyl-3-heptanol32). However, above some pressure limit, the transient chains begin to be disturbed by further compression, and the amplitude (dielectric strength) of the main process starts to decrease below the value observed for the same relaxation time at ambient pressure (as in 2E1H).

larger. On the other hand, during isothermal pressurization in the case of 2E1B the lower size of the molecules enables easier reorientation of the particles during densification; consequently, structural relaxation exhibits higher sensitivity to pressure changes than in the case of 2-ethyl-1-hexanol. A similar analysis was performed for the alcohol with a different position of the hydroxyl group (see Figure 6). For the

Figure 6. Dielectric relaxation time of the Debye relaxation and α relaxation in 2M3H, 2M2H, and 5M2H (inset) as a function of the reference temperature (Tr) compared with the temperature (T) measured isobarically for ambient and high pressures. The solid lines are fit using the VFT equation.

Debye-like process in 2M2H at ambient pressure, m = 59 ± 2, whereas for 2M3H, the obtained parameters were m = 41 ± 2 (Debye process) and m = 50 ± 2 (α process). At isobar p = 1700 MPa, the values of the steepness index of the Debye relaxation increase up to m = 65 ± 2 (2M2H) and m = 86 ± 2 (2M3H). We also present the dependence of relaxation times as a function of temperature for 5M2H (Figure 6 (inset)). In this case, scaled high-pressure dynamics of the Debye process is very similar to α relaxation under atmospheric pressure.46 For 2M2H, it is hard to discuss the value of the estimated parameters because we do not know the value of m for structural relaxation at atmospheric pressure, but the marked increase in m is obvious, as for other investigated alcohols. On comparing the results for all investigated samples, it is concluded that the increase in the steepness index of τDebye for pressures above ca. 500 MPa is a universal feature of these materials. However, the degree of this increase markedly depends on the material. Note that for 2M3H mDebye becomes much higher than the steepness index of the structural relaxation at ambient pressure under high-pressure conditions. This finding indicates marked changes in the architecture and

IV. CONCLUSIONS In conclusion, monohydroxy alcohols with different lengths of the alkyl chains and various positions of the hydroxyl and/or methyl group were studied using broadband dielectric spectroscopy under different p−T conditions. It is widely accepted that the supramolecular structure formed by the hydrogen bond F


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nonterminal) is also reflected by differences in the time scale between relaxation processes and relaxation strengths. Moreover, the relaxation dynamics of these alcohols is significantly different from that observed in an alcohol with the hydroxyl group at the terminal position of the backbone (e.g., 2E1H). 5. For 2M3H and 5M3H during pressurization, the amplitude of the Debye process increases and decreases, respectively, but for 2M2H, the situation is more complex. For a lower magnitude of pressure, the amplitude of the Debye relaxation increases to values markedly larger than those obtained at ambient pressure. This trend changes with increasing pressure, and for p = 700 MPa, the intensity of the Debye process becomes the same as that for p = 0.1 MPa. During further pressurization, the intensity of the peak has a value lower than that at atmospheric pressure. This behavior of relaxation has never been reported before. It can be explained by the initial increase in the population of Hbonded chains due to breaking of the ring structures, which reaches a maximum at some pressure value. With further compression, the numbers of newly created chains start to be compensated by the decreasing size or destruction of the already existing ones. 6. Pressure dependence of the steepness index (m) in 2M3H exhibits outstanding behavior. For this material, under pressure conditions, mDebye becomes significantly higher than the steepness index of the structural relaxation at ambient pressure. This result indicates the marked changes in the architecture and dynamics of the supramolecular structure are responsible for the occurrence of the Debye process and suggests that not only does the steepness index of this process increase but also mα has to become larger under high-pressure conditions, as was observed for other associated liquids. 7. Various positions of the methyl group do not significantly influence the relaxation behavior of the materials, which proves that it does not affect the hydrogen-bonded structures markedly. In this work, we show that the architecture of supramolecular structures in the studied alcohols critically depends on their molecular conformation. Furthermore, the properties of these structures formed by the hydrogen bonds cannot be known without testing at high pressures (as the parameter controlling the molecular dynamics).

Figure 7. Dielectric strength Δε of the Debye-like process of 2M3H (A) and 2M2H (B) as a function of the relaxation time under different p−T conditions.

determines the properties of associative materials. Therefore, we have compared the results for these alcohols to understand how the architecture (position of OH− and methyl groups) and length of the alkyl chains influence the properties of these materials. The key findings of our study are explained below. 1. Alcohols 2E1H and 2E1B differ in the size of their alkyl chains, and our results indicate that pressure dependence of the Debye process, relaxation times τ, relaxation strengths Δε, and steepness index m parameter exhibit similar behavior for both alcohols. 2. Structural relaxation of 2E1B is more sensitive to compression, which indicates that this relaxation is markedly governed by the length of the backbone. Under higher pressure conditions, the time separation between the Debye process and the α process for 2E1B decreases more than that for 2E1H, which is also reflected in the ΔV parameter. 3. Pressurization strongly modifies the supramolecular structure, which is reflected by a reduction in the amplitude of the Debye process in comparison with that at ambient pressure. 4. Similar pressures affect the amplitudes of the Debye process in the same manner for both alcohols, when compared with those under isochronous conditions, whereas the peak positions of the structural relaxation change in a different manner. This indicates that similar shapes and sizes of H-bonded structures are responsible for the Debye relaxation, even for molecules with different sizes. The results of 2M2H, 2M3H, and 5M2H were compared in terms of the location of the OH− and methyl groups. The difference in the properties of the examined alcohols with different positions of the OH− group (more or less

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support of the project by the Polish National Science Centre on the basis of Decision No. UMO-2012/05/B/ST4/00089.

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REFERENCES

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