The Influence of Molecular Architecture on the Dynamics of H-Bonded Supramolecular Structures in Phenyl-Propanols

The relaxation behaviour of monohydroxy alcohols (monoalcohols) in broadband dielectric spectroscopy (BDS) is usually dominated by the Debye process. This process is regarded as a signature of the dynamics of transient supramolecular structures formed by H-bonding. In phenyl propanols the steric hindrance of the phenyl ring is assumed to influence chain formation and thereby to decrease or even suppress the intensity of the Debye process. In the present paper we study this effect in a systematic series of structural isomers of phenyl-1-propanol in comparison with 1-propanol. It turns out that by combining BDS, Photon Correlation Spectroscopy (PCS) and calorimetry the dynamics of supramolecular structures can be uncovered. While light scattering spectra show the same spectral shape of the main relaxation for all investigated monoalcohols, the dielectric spectra differ in the Debye contribution. Thus it becomes possible for the first time to unambiguously disentangle both relaxation modes in the dielectric spectra. It turns out that the Debye relaxation gets weaker the closer the position of phenyl ring is to the hydroxy group, in accordance with the analysis of the Kirkwood-Fr\"ohlich correlation factor. Even in 1-phenyl-1-propanol, which has the phenyl group attached at the closest position to the hydroxy group, we can separate a Debye-contribution in the dielectric spectrum. From this we conclude that hydrogen bonds are not generally suppressed by the increased steric hindrance of the phenyl ring, but rather an equilibrium of ring and chain-like structures is shifted towards ring-like shapes on shifting the phenyl ring closer to the hydroxy group. Moreover, the shape of the alpha-relaxation as monitored by PCS and BDS remains unaffected by the degree of hydrogen bonding and is the same among the investigated alcohols.Comment: 9 pages, 7 figure

On the Nature of the Debye-Process in Monohydroxy Alcohols: 5-Methyl-2-Hexanol Investigated by Depolarized Light Scattering and Dielectric Spectroscopy

The slow Debye-like relaxation in the dielectric spectra of monohydroxy alcohols is a matter of long standing debate. In the present work, we probe reorientational dynamics of 5-methyl-2-hexanol with dielectric spectroscopy and depolarized light scattering (DDLS) in the supercooled regime. While in a previous study of a primary alcohol no indication of the Debye peak in the DDLS spectra was found, we now for the first time report clear evidence of a Debye contribution in a monoalcohol in DDLS. A quantitative comparison between the dielectric and DDLS manifestation of the Debye peak reveals that while the dielectric Debye process represents fluctuations in the end-to-end vector dipole moment of the transient chains, its occurrence in DDLS shows a more local signature and is related to residual correlations which occur due to a slight anisotropy of the $\alpha$-relaxation caused by the chain formation.Comment: 5 pages, 5 figures; accepted in Phys. Rev. Let

Glassy dynamics in polyalcohols:intermolecular simplicity vs. intramolecular complexity

Using depolarized light scattering, we have recently shown that structural relaxation in a broad range of supercooled liquids follows, to good approximation, a generic line shape with high-frequency power law . We now continue this study by investigating a systematic series of polyalcohols (PAs), frequently used as model-systems in glass-science, , because the width of their respective dielectric loss spectra varies strongly along the series. Our results reveal that the microscopic origin of the observed relaxation behavior varies significantly between different PAs: while short-chained PAs like glycerol rotate as more or less rigid entities and their light scattering spectra follow the generic shape, long-chained PAs like sorbitol display pronounced intramolecular dynamic contributions on the time scale of structural relaxation, leading to systematic deviations from the generic shape. Based on these findings we discuss an important limitation for observing the generic shape in a supercooled liquid: the dynamics that is probed needs to reflect the intermolecular dynamic heterogeneity, and must not be superimposed by effects of intramolecular dynamic heterogeneity

Revealing complex relaxation behavior of monohydroxy alcohols in a series of octanol isomers

We investigate the reorientation dynamics of four octanol isomers with very different characteristics regarding the formation of hydrogen-bonded structures by means of photon-correlation spectroscopy (PCS) and broadband dielectric spectroscopy. PCS is largely insensitive to orientational cross-correlations and straightforwardly probes the α-process dynamics, thus allowing us to disentangle the complex dielectric relaxation spectra. The analysis reveals an additional dielectric relaxation contribution on time scales between the structural α-process and the Debye process. In line with nuclear magnetic resonance results from the literature and recent findings from rheology experiments, we attribute this intermediate contribution to the dielectric signature of the O–H bond reorientation. Due to being incorporated into hydrogen-bonded suprastructures, the O–H bond dynamically decouples from the rest of the molecule. The relative relaxation strength of the resulting intermediate contribution depends on the respective position of the hydroxy group within the molecule and seems to vanish at sufficiently high temperatures, i.e., exactly when the overall tendency to form hydrogen bonded structures decreases. Furthermore, the fact that different octanol isomers share the same dipole density allows us to perform an in-depth analysis of how dipolar cross-correlations appear in dielectric loss spectra. We find that dipolar cross-correlations are not solely manifested by the presence of the slow Debye process but also scale the relaxation strength of the self-correlation contribution depending on the Kirkwood factor

Universal Structural Relaxation in Supercooled Liquids

One of the hallmarks of molecular dynamics in deeply supercooled liquids is the non-exponential character of the relaxation functions. It has been a long standing issue if 'universal' features govern the lineshape of glassy dynamics independent of any particular molecular structure or interaction. In the paper, we elucidate this matter by giving a comprehensive comparison of the spectral shape of depolarized light scattering and dielectric data of deeply supercooled liquids. The light scattering spectra of very different systems, e.g. hydrogen bonding and van der Waals liquids but also ionic systems, almost perfectly superimpose and show a generic lineshape of the structural relaxation, approximately following a high frequency power law $\omega^{-1/2}$ . However, the dielectric loss peak shows a more individual shape. In systems with low dipole moment generic behavior is also observed in the dielectric spectra, while in strongly dipolar liquids additional crosscorrelation contributions mask the generic structural relaxation

Influence of intramolecular dynamics on the relaxation spectra of simple liquids

The shape of the structural relaxation peak in the susceptibility spectra of liquids is of great interest, as it promises to provide information about the distribution of molecular mobilities and dynamic heterogeneity. However, recent studies suggest a generic shape of this peak near the glass transition temperature irrespective of the liquid under investigation, which somehow reduces the information contained in the peak shape. By contrast, at higher temperatures, say, around the melting point, the situation is different and the peak shape varies strongly between different liquids. In this study, we investigate molecules with a ring-tail structure and address the question how intramolecular dynamics influences the peak shape at these temperatures. Using depolarized light scattering and dielectric spectroscopy, we observe a bimodal relaxation, which we attribute to the fact that the reorientation of the ring group to some extent decouples from the rest of the molecule. This shows that the relaxation spectra are sensitive to details of the molecular motions at high temperatures, whereas in the supercooled state this microscopic information seems to give way to a generic shape, probably due to the onset of cooperativity which extends across different intramolecular moieties

Glassy Dynamics in Polyalcohols: Intermolecular Simplicity vs. Intramolecular Complexity

Using depolarized light scattering, we have recently shown that structural relaxation in a broad range of supercooled liquids follows, to good approximation, a generic line shape with high-frequency power law $\omega^{-1/2}$. We now continue this study by investigating a systematic series of polyalcohols (PAs), frequently used as model systems in glass-science, i.a., because the width of their respective dielectric loss spectra varies strongly along the series. Our results reveal that the microscopic origin of the observed relaxation behavior varies significantly between different PAs: While short-chained PAs like glycerol rotate as more or less rigid entities and their light scattering spectra follow the generic shape, long-chained PAs like sorbitol display pronounced intramolecular dynamic contributions on the time scale of structural relaxation, leading to systematic deviations from the generic shape. Based on these findings we discuss an important limitation for observing the generic shape in a supercooled liquid: The dynamics that is probed needs to reflect the intermolecular dynamic heterogeneity, and must not be superimposed by effects of intramolecular dynamic heterogeneity

Origin of Apparent Slow Solvent Dynamics in Concentrated Polymer Solutions

Due to their size disparity, the dynamics of solvent molecules in concentrated polymer solutions can be faster by several orders of magnitude compared to the polymer matrix. Despite these observations, past research suggests that a fraction of the solvent contributes to the slow dynamics on the timescale of the macromolecules. By combining depolarized dynamic light scattering and molecular dynamics simulations, we show that long-lived solvent–solvent cross-correlations cause these slow solvent contributions. They originate from the energy landscape that the solute imprints onto the neighboring solvent, leading to enhanced correlations between the positions and orientations of different solvent molecules until the solute relaxes. This mechanism explains the discrepancies found between the results of techniques probing collective and single-particle dynamics, such as dielectric and nuclear magnetic resonance spectroscopy. Our findings shed new light on how solvents behave in the vicinity of macromolecules and are of relevance for materials ranging from polymer-plasticizer systems to hydrated proteins

Revealing complex relaxation behavior of monohydroxy alcohols in a series of octanol isomers

We investigate the reorientation dynamics of four octanol isomers with very different characteristics regarding the formation of hydrogen-bonded structures by means of photon-correlation spectroscopy (PCS) and broadband dielectric spectroscopy. PCS is largely insensitive to orientational cross-correlations and straightforwardly probes the α-process dynamics, thus allowing us to disentangle the complex dielectric relaxation spectra. The analysis reveals an additional dielectric relaxation contribution on time scales between the structural α-process and the Debye process. In line with nuclear magnetic resonance results from the literature and recent findings from rheology experiments, we attribute this intermediate contribution to the dielectric signature of the O-H bond reorientation. Due to being incorporated into hydrogen-bonded suprastructures, the O-H bond dynamically decouples from the rest of the molecule. The relative relaxation strength of the resulting intermediate contribution depends on the respective position of the hydroxy group within the molecule and seems to vanish at sufficiently high temperatures, i.e. exactly when the overall tendency to form hydrogen bonded structures decreases. Furthermore, the fact that different octanol isomers share the same dipole density allows us to perform an in-depth analysis of how dipolar cross-correlations appear in dielectric loss spectra. We find that dipolar cross-correlations are not solely manifested by the presence of the slow Debye process but also scale the relaxation strength of the self-correlation contribution depending on the Kirkwood factor.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Intermolecular Cross-Correlations in the Dielectric Response of Glycerol

We suggest a way to disentangle self- from cross-correlation contributions in the dielectric spectra of glycerol. Recently it was demonstrated for monohydroxy alcohols that a detailed comparison of the dynamic susceptibilities of photon correlation and broadband dielectric spectroscopy allows to unambiguously disentangle a collective relaxation mode known as the Debye process, which could arises due to supramolecular structures, and the $\alpha$-relaxation, which proves to be identical in both methods. In the present paper, we apply the same idea and analysis to the paradigmatic glass former glycerol. For that purpose we present new light scattering data from photon correlation spectroscopy measurements and combine these with literature data to obtain a data set covering a dynamic range from $10^{-4}-10^{13}\,$Hz. Then we apply the above mentioned analysis by comparing this data set with a corresponding set of broadband dielectric data. Our finding is that even in a polyalcohol self- and cross-correlation contributions can approximately be disentangled in that way and that the emerging picture is very similar to that in monohydroxy alcohols. This is further supported by comparing the data with fast field cycling NMR measurements and dynamic shear relaxation data from the literature, and it turns out that, within the described approach, the $\alpha$-process appears very similar in all methods, while the pronounced differences observed in the spectral density are due to a different expression of the slow collective relaxational contribution. In the dielectric spectra the strength of this peak is reasonably well estimated by the Kirkwood correlation factor, which supports the view that it arises due to dynamic cross-correlations, which were previously often assumed to be negligible in dielectric measurements