Systematic study of the glass transition in polyhydric alcohols

Effect of Initial Seed Aerosol Acidity on the Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived Secondary Organic Aerosol

At fixed aerosol acidity, we recently demonstrated that dimers in isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) can heterogeneously react with hydroxyl radical (·OH) at faster rates than monomers. Aerosol acidity influences this aging process by enhancing the formation of oligomers in freshly generated IEPOX-SOA. Therefore, we systematically examined the role of aerosol acidity on kinetics and products resulting from heterogeneous ·OH oxidation of freshly generated IEPOX-SOA. IEPOX reacted with inorganic sulfate aerosol of varying initial pH (0.5, 1.5, and 2.5) in a steady-state smog chamber to yield a constant source of freshly generated IEPOX-SOA, which was aged in an oxidation flow reactor for 0-22 equiv days of atmospheric ·OH exposure. Molecular-level chemical analyses revealed that the most acidic sulfate aerosol (pH 0.5) formed the largest oligomeric mass fraction, causing the slowest IEPOX-SOA mass decay with aging. Reactive uptake coefficients of ·OH (γOH) were 0.24 ± 0.06, 0.40 ± 0.05, and 0.49 ± 0.20 for IEPOX-SOA generated at pH 0.5, 1.5, and 2.5, respectively. IEPOX-SOA became more liquid-like for pH 1.5 and 2.5, while exhibiting an irregular pattern for pH 0.5 with aging. Using kinetic and physicochemical data derived for a single aerosol pH in atmospheric models could inaccurately predict the fate of the IEPOX-SOA.

Predicting and parameterizing the glass transition temperature of atmospheric organic aerosol components via molecular dynamics simulations

Atmospheric aerosols contain thousands of organic compounds that exhibit an array of functionalities, structures and characteristics. Quantifying the role of these organic aerosols in climate and air quality requires an understanding of their physical properties. A key property determining their behavior is the glass transition temperature (Tg). Tg defines the phase state of aerosols, which in turn influences crucial aerosol processes. Molecular Dynamics (MD) simulations were implemented to predict Tg of a range of atmospheric organic compounds. The predictions were used to develop a Tg parameterization. The predictions and the parameterization link Tg with molecular characteristics such as the type and number of functional groups present in the molecule, its architecture, as well its carbon and oxygen content. The MD simulations suggest that Tg is sensitive to the functional groups in the organic molecule with the following order: -COOH > -OH > -CO. This trend is maintained even when more than one of these functional groups is present in a molecule. Molecular structure was also found to play a significant role. Cyclic structures exhibited consistently higher predicted Tg values compared to linear counterparts. Tg, as expected, increased as the number of carbon atoms increased. The parameterization was evaluated using a leave-one-out approach, providing insights into the contributions of various molecular features.

Structural Evolution in a Glass-Forming Liquid Alcohol by X-Ray Scattering: Contrasting Behaviors of Main Peak and Prepeak Structures

X-ray scattering of liquid 2-ethyl-1-hexanol (2E1H) has been measured from its liquid state to its glassy state with focus on the main scattering peak and the prepeak. The main peak, associated with the packing of the alkyl chains, shifts to higher angle and sharpens in a manner consistent with closely packed spheres, until kinetic arrest at the glass transition temperature Tg (146 K). In contrast, the prepeak, associated with the correlation of the hydroxyl groups separated by the hydrocarbon chains, shows a transition near 220 K, below which its width is nearly frozen and insensitive to the passage of Tg. This transition coincides with a similar transition in the Kirkwood factor gK which reports the orientational correlation of the OH dipoles, and with the transition reported previously as the "250 K anomaly" based on other observables. This transition arises from the increased hydrogen bonding between the hydroxyl groups and the resulting improvement of the regularity of the alcohol bilayers.

Polyhydric alcohols under high pressure: comparative ultrasonic study of elastic properties

We carried out an experimental ultrasonic study of polyhydric alcohols with the general chemical formula CnHn+2(OH)n with an increasing number of OH groups: glycerol (n = 3), erythritol (n = 4), xylitol (n = 5), sorbitol (n = 6). The baric and temperature dependences of the elastic characteristics of these substances in the crystalline and glassy states were studied both under isothermal compression up to 1 GPa and during the isobaric heating of 77-295 K. For glycerol, glasses were obtained at different cooling rates, glass-liquid transitions were studied at different pressures. All the studied glasses have lower elastic moduli than the same substances in the crystalline state at the same pressure-temperature conditions. We obtained a cascade of glass-supercooled liquid-crystal transitions during heating of glassy erythritol. In the series of substances with n = 3, 4, 5 the bulk moduli show a tendency to decrease with increasing n. However, sorbitol (n = 6) unexpectedly has the highest elastic moduli among the studied substances in both the glassy and crystalline states. The studied glassformers show a general tendency to increase the glass transition temperature Tg and the fragility coefficient m with increasing n.

The Role of Phase Separation and Local Mobility in the Stabilization of a Lyophilized IgG2 Formulation

The utility of employing solid-state NMR (SSNMR) to assess parameters governing the stability of a lyophilized IgG2 protein was the focus of the present work. Specifically, the interaction between the sugar stabilizer (sucrose) and protein component was measured using SSNMR and compared to physical and chemical stability data obtained from thermally stressed samples. 1H T1 and 1H T1⍴ relaxation times were measured by SSMNR for 5 different formulation conditions, and the resultant values were used to examine local mobility and phase separation, respectively. From the SSNMR measurements, it was found local mobility decreased as the sucrose to protein weight ratio increased. The decrease in local mobility corresponded to an increase in storage stability (both chemical and physical) of the lyophilized solids up to a critical weight ratio of sucrose to protein. Additionally, 1H T1⍴ measurements obtained on formulations having higher protein to sucrose weight ratios indicated phase separation of the protein and sucrose phases was occurring, at least on a small scale. Along with an increase in local mobility, phase separation in these specific formulations is thought to have played a role in their decreased storage stability in the solid state.

Glass Transition Temperatures of Organic Mixtures from Isoprene Epoxydiol-Derived Secondary Organic Aerosol

The phase states and glass transition temperatures (Tg) of secondary organic aerosol (SOA) particles are important to resolve for understanding the formation, growth, and fate of SOA as well as their cloud formation properties. Currently, there is a limited understanding of how Tg changes with the composition of organic and inorganic components of atmospheric aerosol. Using broadband dielectric spectroscopy, we measured the Tg of organic mixtures containing isoprene epoxydiol (IEPOX)-derived SOA components, including 2-methyltetrols (2-MT), 2-methyltetrol-sulfate (2-MTS), and 3-methyltetrol-sulfate (3-MTS). The results demonstrate that the Tg of mixtures depends on their composition. The Kwei equation, a modified Gordon-Taylor equation with an added quadratic term and a fitting parameter representing strong intermolecular interactions, provides a good fit for the Tg-composition relationship of complex mixtures. By combining Raman spectroscopy with geometry optimization simulations obtained using density functional theory, we demonstrate that the non-linear deviation of Tg as a function of composition may be caused by changes in the extent of hydrogen bonding in the mixture.

Structures of glass-forming liquids by x-ray scattering: Glycerol, xylitol, and D-sorbitol

Synchrotron x-ray scattering has been used to investigate three liquid polyalcohols of different sizes (glycerol, xylitol, and D-sorbitol) from above the glass transition temperatures T_g to below. We focus on two structural orders: the association of the polar OH groups by hydrogen bonds (HBs) and the packing of the non-polar hydrocarbon groups. We find that the two structural orders evolve very differently, reflecting the different natures of bonding. Upon cooling from 400 K, the O⋯O correlation at 2.8 Å increases significantly in all three systems, indicating more HBs, until kinetic arrests at T_g; the increase is well described by an equilibrium between bonded and non-bonded OH with ΔH = 9.1 kJ/mol and ΔS = 13.4 J/mol/K. When heated above T_g, glycerol loses the fewest HBs per OH for a given temperature rise scaled by T_g, followed by xylitol and by D-sorbitol, in the same order the number of OH groups per molecule increases (3, 5, and 6). The pair correlation functions of all three liquids show exponentially damped density modulations of wavelength 4.5 Å, which are associated with the main scattering peak and with the intermolecular C⋯C correlation. In this respect, glycerol is the most ordered with the most persistent density ripples, followed by D-sorbitol and by xylitol. Heating above T_g causes faster damping of the density ripples with the rate of change being the slowest in xylitol, followed by glycerol and by D-sorbitol. Given the different dynamic fragility of the three liquids (glycerol being the strongest and D-sorbitol being the most fragile), we relate our results to the current theories of the structural origin for the difference. We find that the fragility difference is better understood on the basis of the thermal stability of HB clusters than that of the structure associated with the main scattering peak.

Stabilization mechanism of amorphous carbamazepine by transglycosylated rutin, a non-polymeric amorphous additive with a high glass transition temperature

α-Glycosyl rutin (Rutin-G), composed of a flavonol skeleton and sugar groups, is a promising non-polymeric additive for stabilizing amorphous drug formulations. In this study, the mechanism of the stabilization of the amorphous state of carbamazepine (CBZ) by Rutin-G was investigated. In comparison with hypromellose (HPMC), which is commonly used as a crystallization inhibitor for amorphous drugs, Rutin-G significantly stabilized amorphous CBZ. Moreover, the dissolution rate and the resultant supersaturation level of CBZ were significantly improved in the CBZ/Rutin-G spray-dried samples (SPDs) owing to the rapid dissolution property of Rutin-G. Differential scanning calorimetry measurement demonstrated a high glass transition temperature (T_g) of 186.4°C corresponding to Rutin-G. The CBZ/Rutin-G SPDs with CBZ weight ratios up to 80% showed single glass transitions, indicating the homogeneity of CBZ and Rutin-G. A solid-state NMR experiment using ¹³C- and ¹⁵N-labeled CBZ demonstrated the interaction between the flavonol skeleton of Rutin-G and the amide group of CBZ. A ¹H-¹³C two-dimensional heteronuclear correlation NMR experiment and quantum mechanical calculations confirmed the presence of a possible hydrogen bond between the amino proton in CBZ and the carbonyl oxygen in the flavonol skeleton of Rutin-G. This specific hydrogen bond could contribute to the strong interaction between CBZ and Rutin-G, resulting in the high stability of amorphous CBZ in the CBZ/Rutin-G SPD. Hence, Rutin-G, a non-polymeric amorphous additive with high T_g, high miscibility with drugs, and rapid and pH-independent dissolution properties could be useful in the preparation of amorphous formulations.

Understanding functionality of sucrose in cake for reformulation purposes

We review the functionality of sucrose during the manufacture of cakes from the perspective of sugar replacement. Besides providing sweetness, sucrose has important functionalities concerning structure formation. These functionalities also need to be mimicked in reformulated cakes. First, we review the hypotheses, concerning the development of structure and texture of cakes during manufacturing, which are conveniently summarized in a qualitative way using the Complex Dispersed Systems methodology. Subsequently, we represent the changes of the state of the cake during manufacturing in a supplemented state diagram, which indicates the important phase transitions occurring during baking. From the analysis, we have learned that sucrose act both as a plasticizer and as a humectant, modifying the phase transitions of biopolymers, dough viscosity, and water activity. If sugar replacers exactly mimick this behavior of sucrose, similar textures in reformulated cakes can be obtained. Physical theories exist for characterizing the plasticizing and hygroscopic behavior of sugars and their replacers. We have shown that the starch gelatinization and egg white denaturation can be predicted by the volumetric density of hydrogen bonds present in the solvent, consisting of water, sugar or its replacers, such as polyols or amino-acids.

Peculiar relaxation dynamics of propylene carbonate derivatives

The aim of this work is to analyze in detail the effect of the alkyl chain length on the dynamics of glass-forming propylene carbonate (PC) derivatives. Examined samples are low-molecular weight derivatives of the PC structure, i.e., the 4-alkyl-1,3-dioxolan-2-one series, modified by changing the alkyl substituent from methyl to hexyl. The molecular dynamics (MD) has been analyzed based on experimental data collected from differential scanning calorimetry, broadband dielectric spectroscopy (BDS), X-ray diffraction (XRD), and nuclear magnetic resonance relaxometry measurements as well as MD simulations. The dielectric results show in samples with the propyl- or longer carbon chain the presence of slow Debye-like relaxation with features similar to those found in associative materials. Both XRD and MD reveal differences in the intermolecular structure between PC and 4-butyl-1,3-dioxolan-2-one liquids. Moreover, MD shows that the probability of finding one terminal carbon atom of the side chain of BPC in the vicinity of another carbon atom of the same type is much higher than in the case of PC. It suggests that there is a preference for longer hydrocarbon chains to set themselves close to each other. Consequently, the observed slow-mode peak may be caused by movement of aggregates maintained by van der Waals interactions. Reported herein, findings provide a new insight into the molecular origin of Debye-like relaxation.

Influence of Functional Groups on the Viscosity of Organic Aerosol

Organic aerosols can exist in highly viscous or glassy phase states. A viscosity database for organic compounds with atmospherically relevant functional groups is compiled and analyzed to quantify the influence of number and location of functional groups on viscosity. For weakly functionalized compounds the trend in viscosity sensitivity to functional group addition is carboxylic acid (COOH) ≈ hydroxyl (OH) > nitrate (ONO₂) > carbonyl (CO) ≈ ester (COO) > methylene (CH₂). Sensitivities to group addition increase with greater levels of prior functionalization and decreasing temperature. For carboxylic acids a sharp increase in sensitivity is likely present already at the second addition at room temperature. Ring structures increase viscosity relative to linear structures. Sensitivities are correlated with analogously derived sensitivities of vapor pressure reduction. This may be exploited in the future to predict viscosity in numerical models by piggybacking on schemes that track the evolution of organic aerosol volatility with age.

Contrasting dynamics of fragile and non-fragile polyalcohols through the glass, and dynamical, transitions: A comparison of neutron scattering and dielectric relaxation data for sorbitol and glycerol

BACKGROUND

Glycerol and sorbitol are glass-forming hydrogen-bonded systems characterized by intriguing properties which make these systems very interesting also from the applications point of view. The goal of this work is to relate the hydrogen-bonded features, relaxation dynamics, glass transition properties and fragility of these systems, in particular to seek insight into their very different liquid fragilities.

METHODS

The comparison between glycerol and sorbitol is carried out by collecting the elastic incoherent neutron scattering (EINS) intensity as a function of temperature and of the instrumental energy resolution.

RESULTS

Intensity data vs temperature and resolution are analyzed in terms of thermal restraint and Resolution Elastic Neutron Scattering (RENS) approaches.

CONCLUSIONS

The number of OH groups, which are related to the connecting sites, is a significant parameter both in the glass transition and in the dynamical transition. On the other hand, the disordered nature of sorbitol is confirmed by the existence of different relaxation processes.

GENERAL SIGNIFICANCE

From the applications point of view, glycerol and sorbitol have remarkable bioprotectant properties which make these systems useful in different technological and industrial fields. Furthermore, polyols are rich in glassforming liquid phenomenology and highly deserving of study in their own right. The comparison of EINS and calorimetric data on glycerol and sorbitol helps provide a connection between structural relaxation, dynamical transition, glass transition, and fragility. The evaluation of the inflection point in the elastic intensity behavior as a function of temperature and instrumental energy resolution provides a confirmation of the validity of the RENS approach. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Distinctly Different Glass Transition Behaviors of Trehalose Mixed with Na2HPO 4 or NaH 2PO 4: Evidence for its Molecular Origin

PURPOSE

The present study is aimed at understanding how the interactions between sugar molecules and phosphate ions affect the glass transition temperature of their mixtures, and the implications for pharmaceutical formulations.

METHODS

The glass transition temperature (Tg) and the α-relaxation temperature (Tα) of dehydrated trehalose/sodium phosphate mixtures (monobasic or dibasic) were determined by differential scanning calorimetry and dynamic mechanical analysis, respectively. Molecular dynamics simulations were also conducted to investigate the microscopic interactions between sugar molecules and phosphate ions. The hydrogen-bonding characteristics and the self-aggregation features of these mixtures were quantified and compared.

RESULTS

Thermal analysis measurements demonstrated that the addition of NaH2PO4 decreased both the glass transition temperature and the α-relaxation temperature of the dehydrated trehalose/NaH2PO4 mixture compared to trehalose alone while both Tg and Tα were increased by adding Na2HPO4 to pure trehalose. The hydrogen-bonding interactions between trehalose and HPO4(2-) were found to be stronger than both the trehalose-trehalose hydrogen bonds and those formed between trehalose and H2PO4(-). The HPO4(2-) ions also aggregated into smaller clusters than H2PO4(-) ions.

CONCLUSIONS

The trehalose/Na2HPO4 mixture yielded a higher T g than pure trehalose because marginally self-aggregated HPO4(2-) ions established a strengthened hydrogen-bonding network with trehalose molecules. In contrast H2PO4(-) ions served only as plasticizers, resulting in a lower Tg of the mixtures than trehalose alone, creating large-sized ionic pockets, weakening interactions, and disrupting the original hydrogen-bonding network amongst trehalose molecules.

Thermal Decoupling of Molecular-Relaxation Processes from the Vibrational Density of States at Terahertz Frequencies in Supercooled Hydrogen-Bonded Liquids

At terahertz frequencies, the libration-vibration motions couple to the dielectric relaxations in disordered hydrogen-bonded solids. The interplay between these processes is still poorly understood, in particular at temperatures below the glass transition temperature, Tg, yet this behavior is of vital importance for the molecular mobility of such materials to remain in the amorphous phase. A series of polyhydric alcohols were studied at temperatures between 80 and 310 K in the frequency range of 0.2-3 THz using terahertz time-domain spectroscopy. Three universal features were observed in the dielectric losses, ϵ″(ν): (a) At temperatures well below the glass transition, ϵ″(ν) comprises a temperature-independent microscopic peak, which persists into the liquid phase and which is identified as being due to librational/torsional modes. For 0.65 Tg < T < Tg, additional thermally dependent contributions are observed, and we found strong evidence for its relation to the Johari-Goldstein secondary β-relaxation process. (b) Clear spectroscopic evidence is found for a secondary β glass transition at 0.65 Tg, which is not related to the fragility of the glasses. (c) At temperatures above Tg, the losses become dominated by primary α-relaxation processes. Our results show that the thermal changes in the losses seem to be underpinned by a universal change in the hydrogen bonding structure of the samples.

Role of hydrogen bonds and molecular structure in relaxation dynamics of pentiol isomers

Although the presence of hydrogen bonds determines most properties of associated materials, their role in relaxation dynamics of liquids remains unclear. Very recently Nakanishi and Nozaki [M. Nakanishi and R. Nozaki, Phys. Rev. E 84, 011503 (2011)] proposed a simplified model for the description of the molecular dynamics of H-bonding network and tested its validity for several polyols. The authors concluded that relaxation dynamics is controlled mainly by the number of hydroxyl groups, whereas the role of molecular architecture can be neglected. This conclusion, as demonstrated herein, fails in the case of pentiols. When we take into account the role of molecular architecture for development of H-bonded structures, it is still possible to satisfactorily describe molecular dynamics in polyols with the use of the Nakanishi and Nozaki model.

Model of the cooperative rearranging region for polyhydric alcohols

A simplified model of a hydrogen-bonding network is proposed in order to clarify the microscopic structure of the cooperative rearranging region (CRR) in Adam-Gibbs theory [G. Adam and J. H. Gibbs, J. Chem. Phys. 43, 139 (1965)]. Our model can be solved analytically, and it successfully explains the reported systematic features of the glass transition of polyhydric alcohols. In this model, hydrogen bonding is formulated based on binding free energy. Assuming a cluster of molecules connected by double hydrogen bonds is a CRR and approximating the hydrogen-bonding network as a Bethe lattice in percolation theory, the temperature dependence of the structural relaxation time can be obtained analytically. Reported data on relaxation times are well described by the obtained equation. By taking the lower limit of the binding free energy with this equation, the Vogel-Fulcher-Tammann equation can be derived. Consequently, the fragility index and glass transition temperature can be expressed as functions of the number of OH groups in a molecule, and this relation agrees well with the reported experimental data.