Monohydroxy Alcohols Research Articles

Coupling of the electrical conductivity to the structural relaxation, absence of physical aging on the time scale of the Debye process, and number of correlated molecules in the supercooled monohydroxy alcohol 2-ethylhexanol

Using dielectric experiments the structural, the Debye-like, and the conductivity relaxation times of 2-ethyl-1-hexanol are measured in a wide temperature range. It is found that the electrical transport scales with the temperature dependence of the structural relaxation but that it is decoupled from that of the Debye-like response. Temperature-jump experiments were carried out, and signs of a physical aging on the time scale of the Debye-like response could not be detected. The minimum number of correlated molecules contributing to the structural and to the Debye-like responses was estimated on the basis of the temperature derivative of the susceptibility as expressed in terms of the fragility index and of the spectral line shape parameters.

Miscibility of Ionic Liquids with Polyhydric Alcohols

Liquid-liquid miscibility temperatures as a function of composition have been determined experimentally for the binary systems formed by ionic liquids ([bmim][BF(4)], [bmim][PF(6)], [emim][Tf(2)N], [bmim][Tf(2)N], [hmim][Tf(2)N]) and polyhydric alcohols (1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2,3-propanetriol, 1,2-butanediol). The impact of ionic liquid and di- or three-hydroxy alcohol characteristics focusing on the effect of the IL's anion nature, cation alkyl chain length, and alcohol structure (number of hydroxyl groups, position of the hydroxy groups in the molecule, and number of carbon atoms in the diols) is presented. It appears that all systems exhibit upper critical solution temperatures. For dihydroxy alcohols mentioned above, miscibility with 1-butyl-3-methylimidazolium ionic liquids follows the order [BF(4)](-) > [Tf(2)N](-) > [PF(6)](-) and is dependent on the hydrogen-bond basicity of the anion. Analysis of these findings leads us to conclude that the miscibility of ionic liquids is likely related to the hydrogen-bond acceptor strength of the anion. Comparing the miscibility of 1,2-ethanediol with 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides, it can be seen that surprisingly, T(c)([emim](+)) < T(c)([bmim](+)) < T(c)([hmim](+)). This arrangement of critical temperatures is opposite to that observed earlier for systems with monohydroxy alcohols. Analyzing the influence of the polyhydroxy alcohol structure, we also noticed that the miscibility of the polyhydroxy alcohols with [bmim][Tf(2)N] or [bmim][BF(4)] decreases when the polarity of the alcohol rises.

Dynamics of glass-forming liquids. XIII. Microwave heating in slow motion

Using time-resolved nonlinear dielectric relaxation measurements at fields as high as 450 kV/cm, the nonthermal effects of energy absorption are studied for simple and associating polar liquids in their supercooled state. The experiment is a low frequency analog of microwave heating and facilitates tracking the flow of energy in time, as it accumulates in slow degrees of freedom and transfers eventually to the vibrational heat bath of the liquid. Most findings agree with a phenomenological model of heterogeneous relaxation regarding structure and configurational temperature. The relevant thermal behavior of monohydroxy alcohols differs considerably from the cases of simple nonassociating liquids due to their distinct origins of the prominent dielectric absorption mode for the two classes of liquids. Nonthermal effects are observed as dynamics that are accelerated without increasing sample temperature, but for the present low frequencies the changes remain too small to explain the high efficiencies reported for microwave chemistry. Limitations as to how rapidly the faster relaxation time constants are able to adjust to temperature separate the modes of the dispersive alpha-relaxation into a "relaxation" and an "aging" regime, thereby explaining the incompatibility of heterogeneous dynamics with common physical aging observations.

Diluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane: A dielectric study

Glass-forming monohydroxy alcohols exhibit not only a structural relaxation but also a slower, single-exponential Debye-type relaxation process which already freezes in the liquid phase. By using dielectric spectroscopy, we study how these relaxations evolve when the aprotic alkyl halide n-bromobutane is added to n-butanol, thereby diluting the hydrogen-bond network. The structural relaxation times smoothly vary over the concentration range of this completely miscible binary system. The Debye process remains unaffected by the dilution of the OH groups up to n-bromobutane mole fractions of about 50%. For larger hydroxy dilutions, it turns rather abruptly into a feature which develops significant spectral broadening and it becomes faster. In the dilute limit, the decoupling between the time scale of the Debye process and that of the structural relaxation amounts to almost 6 decades when extrapolated to the glass transition temperature. This relatively large, strongly concentration dependent decoupling is interpreted in analogy to normal modes in polymers. The present results suggest that the structural and the Debye-like responses of monohydroxy alcohols are unrelated.

Glass Transitions in Viscous Monohydroxy Alcohols: Calorimetry Versus Dielectric Relaxation

An extensive comparison of the calorimetric and dielectric measurements of the glass transitions in both monohydroxy alcohols and other molecular liquids is presented, with only these alcohols displaying an additional pronounced dielectric Debye peaks. It is found that the calorimetric T g’s of the non-Debye liquids with moderate cooling and heating rates (±20 K · min−1, for example) are slightly higher than the kinetic ones which are determined as the temperatures at which the relaxation time is 100 s. As the difference does not exceed 3 K, this relation of calorimetric and kinetic T g’s is used as ‘standard.’ Two kinetic glass transitions from the dielectric relaxation of Debye type monohydroxy alcohols are identified, and it is found that the Debye process is not the signature of the glass transition.

Calorimetric versus kinetic glass transitions in viscous monohydroxy alcohols

An extensive comparison of calorimetric and dielectric measurements is carried out for generic molecular liquids and monohydroxy alcohols with focus on the identification of the dielectric modes which are associated with the glass transition. For generic liquids, the calorimetric glass transition temperatures (T g-cal) are always greater than their kinetic counterparts (T g-kin), but the difference remains below 3 K. Also, the nonexponentiality parameters of the Tool-Narayanaswamy-Moynihan-Hodge model applied to the calorimetric data and the stretching exponents of the dielectric measurements show remarkable agreement. The same behavior is found for glass-forming monohydroxy alcohols, provided that the faster and smaller non-Debye relaxation rather than the large dielectric Debye process is assigned to the structural relaxation. The study emphasizes that the dielectric signature of the glass transition in monohydroxy alcohols is a dispersive loss peak that is faster and significantly smaller than the prominent Debye feature.

Direct Enrichment of Metallic Single‐Walled Carbon Nanotubes Induced by the Different Molecular Composition of Monohydroxy Alcohol Homologues

Direct Enrichment of Metallic Single‐Walled Carbon Nanotubes Induced by the Different Molecular Composition of Monohydroxy Alcohol Homologues

Comparing calorimetric and dielectric polarization modes in viscous 2-ethyl-1-hexanol

Dielectric relaxation and dynamic heat capacity measurements are compared for 2-ethyl-1-hexanol near its glass transition temperature Tg in order to further clarify the origin of the prominent Debye-type loss peak observed in many monohydroxy alcohols and other hydrogen-bonding liquids. While the dielectric spectrum epsilon" displays two distinct polarization processes that are separated by a factor of 2000 in terms of the peak frequency, the heat capacity cp" shows only a single peak. The dielectric process with lower amplitude and higher peak frequency coincides with the calorimetric signal, whereas the large dielectric Debye signal is not associated with calorimetric modes. The authors conclude that the Debye process corresponds to a transition among states which differ in energy only in the case of an external electric field.

Glass Transition Dynamics and Boiling Temperatures of Molecular Liquids and Their Isomers

The relation between a dynamic and a thermodynamic temperature, glass transition Tg and boiling point Tb, is investigated for various glass-forming liquids, with emphasis on monohydroxy alcohols. As is well known, Tb and Tg are positively correlated across a large variety of liquids. However, we found that the same quantities show a negative correlation within an isomeric series, i.e., Tb decreases with increasing Tg for different isomers of the same chemical formula. For the alcohol series, CnH2n+1OH with 3 < or = n < or = 10, a master curve of the negative Tg - Tb correlation is obtained if the temperatures are normalized to the respective values of the n-alkanols. This Tg - Tb dependence of isomeric liquids is linked to entropic effects and responsible for much of the scatter of the correlation observed for a large number of molecular organic glass-formers with 45 < Tg < 250 K. Dielectric relaxation is measured for three groups of isomers: (a) 3-methoxyl-1-butanol and 2-iso-propoxyethanol, (b) 1,4-, 1,2-, and 2,4-pentanediol, and (c) di-n- and di-iso-butyl phthalate. Two key parameters of the dynamics, fragility m and stretching exponent beta, are found to be indistinguishable within isomers of moderately different Tgs. Larger fragility differences are readily expected with pronounced structural change, but no systematic trend is observed within an isomer series. The results provide a useful tool for assessing Tg, m, and beta for marginal glass formers on the basis of their isomers.

Diluent Effects on the Debye-Type Dielectric Relaxation in Viscous Monohydroxy Alcohols

With the recognition that the Debye-type dielectric relaxation of liquid monohydroxy alcohols does not reflect the structural relaxation dynamics associated with the viscous flow and the glass transition, its behavior upon dilution is expected to differ from that of real alpha-processes. We have investigated the Debye-type dielectric relaxation of binary alcohol/alkane mixtures across the entire concentration range in the supercooled regimes. The focus is on 2-ethyl-1-hexanol in two nonpolar liquids, 3-methylpentane and squalane, which are more fluid and more viscous than the alcohol, respectively. The Debye relaxation is found to occur only for alcohol mole fractions x > 0.2 and is always accompanied by a non-Debye relaxation originating from the alcohol component. Prior to its complete disappearance, the Debye relaxation is subject to broadening. We observe that the Debye dynamics of 2-ethyl-1-hexanol is accelerated in the more fluid 3-methylpentane, while the more viscous squalane leads to longer Debye relaxation times. The present experiments also provide evidence that the breakdown of the Debye relaxation amplitude does not imply the absence of hydrogen-bonded structures.

Identification of dielectric and structural relaxations in glass-forming secondary amides

Dielectric relaxation dynamics of secondary amides is explored in their supercooled state near the glass transition temperature Tg by investigating N-ethylacetamide and its mixtures with N-methylformamide. All the samples are found to exhibit giant dielectric permittivities, reaching over 500 in N-methylformamide-rich mixtures around Tg. For both the neat and binary systems, the predominant relaxation peak is of the Debye-type throughout the viscous regime, which is an unexpected feature for a glass former with intermediate fragility. The present results combined with the earlier reported high-temperature data reveal that the dielectric strength delta epsilon(D) of the Debye relaxation extrapolates to zero at frequencies of 10(10)-10(11) Hz, which is about two orders of magnitude lower than the phonon frequency limit typical of the structural relaxation. This Debye process is remarkably similar to the dielectric behavior of many monohydroxy alcohols, which implies a common nature of purely exponential relaxation dynamics in these liquids. Based on the dielectric properties, we conclude that the Debye relaxation in the secondary amides is not a direct signature of the primary or alpha-relaxation, the latter being obscured at low temperatures due to the relatively low permittivity and close spectral proximity to the Debye peak. As in the case of monohydroxy alcohols, dielectric polarization and structure fluctuate on different time scales in secondary amides. The Kirkwood-Fröhlich correlation factors for Debye-type liquids are also discussed.

Ideal Mixing Behavior of the Debye Process in Supercooled Monohydroxy Alcohols

Glass-forming monohydroxy alcohols exhibit two dielectric relaxation signals with super-Arrhenius temperature dependence: a Debye peak and an asymmetrically broadened alpha-process. We explore the behavior of these distinct relaxation features in mixtures of such liquids by dielectric measurements. The study focuses on the viscous regime of two binary systems: 2-methyl-1-butanol with 2-ethyl-1-hexanol and 1-propanol with 3,7-dimethyl-1-octanol. We find that the logarithmic relaxation time, log(tau), of the Debye peak follows an ideal mixing law (linear change with mole fraction), even in the case of mixing structurally dissimilar components. By contrast, the log(tau) versus mole fraction curve for the alpha-process is nonlinear, indicative of slower structural relaxation relative to the expectation on the basis of ideal mixing behavior. The latter observation is analogous to the effect of composition on viscosity, heat of mixing, and glass-transition temperature, whereas the ideal mixing of log(tau) seen for the Debye peak is the exception. We conclude that the unusual ideal mixing behavior of dielectric relaxation in monohydroxy alcohols is not a result of structural similarity, but rather yet more evidence of the Debye process being decoupled from other dynamic and thermodynamic properties.

Dynamics of glass-forming liquids. IX. Structural versus dielectric relaxation in monohydroxy alcohols

The prominent Debye-type but non-Arrhenius dielectric relaxation is a feature common to many monohydroxy alcohols in their liquid state. Although this exponential process is often considered to reflect the primary structural relaxation, only a faster, smaller, and nonexponential relaxation peak correlates with viscous flow and mechanical relaxation. We provide dielectric relaxation data for 2-methyl-1-butanol, 2-ethyl-1-hexanol, and 3,7-dimethyl-1-octanol across ten decades in time. Based on these and previous results, we show that there exists a variety of dielectric to mechanical relaxation time ratios in the viscous regime, but a universal value of 100 for that ratio appears to evolve in the high temperature limit. The temperature dependence for both the relaxation time and strength of the Debye peak differs from the typical behavior of structural dynamics in terms of the alpha process. The implications of these findings for rationalizing the Debye-type dielectric process of hydrogen-bonded liquids are discussed.

FA monoalkylesters from rice bran oil by in situ esterification

AbstractExtraction and in situ esterification of rice bran oil with ethanol were investigated by studying the effects of rice bran oil FFA content and water content of ethanol. Ethyl ester formation in the ethanol phase increased as FFA content increased. Neutral oil solubility in this phase fell considerably, resulting in a high ethyl ester content. The decrease of the water content in ethanol led to an increase in neutral oil solubility in ethanol and promoted the equilibrium of reaction to ethyl‐ester formation, resulting in lower FFA content of the product. The main factor that affected yield and monoester content when using high‐acidity bran and various monohydroxy alcohols was the solubility of neutral oil in alcohol. The highest monoester content was obtained with methanol.

Experimental study of the high frequency relaxation process in monohydroxy alcohols

We have critically examined the possibility of the existence of yet another process (often referred to as process II in the literature) of much smaller magnitude on the high frequency side in the case of 2-ethyl-1-hexanol and 4-methyl-3-heptanol using the dielectric relaxation technique. We have also studied the mixtures of 2-ethyl-1-hexanol with nonpolar methylcyclohexane and a mixture of 1-bromobutane in 4-methyl-3-heptanol for this purpose. In addition, the differential scanning calorimetry (DSC) has been used to study the structural relaxation in the vicinity of the glass transition temperature using the Tool–Narayanaswami–Moynihan procedure as discussed by Sartor et al. [J. Phys. Chem. 100, 6801 (1996)]. Our results show the presence of a clear process II on the higher frequency side whose freezing-out corresponds to the glass transition event at Tg in the DSC studies. It is suggested that process II is associated with free alcohol molecules and is connected to the structural relaxation in alcohols. The dielectric data have been analyzed to understand the thermodynamics of hydrogen bonding using the model of Dannhauser [J. Chem. Phys. 48, 1911 (1968)]. An attempt has been made to correlate the ratio of relaxation rates of processes I and II and the observed deviation from Debye behavior in the case of alcohols.

Effects of induced steric hindrance on the dielectric behavior and H bonding in the supercooled liquid and vitreous alcohol

The extent of H bonding in alcohols may be reduced by sterically hindering its OH group. This technique is used here for investigating the reasons for the prominent Debye-type dielectric relaxation observed in monohydroxy alcohols [Kudlik et al., Europhys. Lett. 40, 549 (1997); Hansen et al., J. Chem. Phys. 107, 1086 (1997); Kalinovskaya and Vij, ibid. 112, 3262 (2000)], and broadband dielectric spectroscopy of supercooled liquid and glassy states of 1-phenyl-1-propanol is performed over the 165–238 K range. In its molecule, the steric hindrance from the phenyl group and the existence of optical isomers reduce the extent of intermolecular H bonding. The equilibrium permittivity data show that H-bonded chains do not form in the supercooled liquid, and the total polarization decays by three discrete relaxation processes, of which only the slower two could be resolved. The first is described by the Cole–Davidson-type distribution of relaxation times and a Vogel–Fulcher–Tammann-type temperature dependence of its average rate, which are characteristics of the α-relaxation process as in molecular liquids. The second is described by a Havriliak–Negami-type equation, and an Arrhenius temperature dependence, which are the characteristics of the Johari–Goldstein process of localized molecular motions. The relaxation rate’s non-Arrhenius temperature dependence has been examined qualitatively in terms of the Dyre theory, which considers that the apparent Arrhenius energy itself is temperature dependent, as in the classical interpretations, and quantitatively in terms of the cooperatively rearranging region’s size, without implying that there is an underlying thermodynamic transition in its equilibrium liquid. The relaxation rate also fits the power law with the critical exponent of 13.4, instead of 2–4, required by the mode-coupling theory, thereby indicating the ambiguity of the power-law equations.

The sensorial potentials of the OH stretching mode

The OH stretching band of 57 selected saturated alcohols has been investigated in two different apolar solvents, CCl 4 and CS 2, as an initial step to recognise and understand weak hydrogen bonding phenomena. Based on rotamer classification and mathematical curve fitting, complex OH bandshapes have been unraveled successfully. A detailed analysis shows that ten different classes of rotamers, AK, suffice to correlate molecular structure and band parameters. The sensing properties for variation in the local molecular structure are shown to be reflected in the band maximum, v ̃ OH , the half band with, HBW CCl4 OH and the solvent shift value, v ̃ d . A scheme is presented to classify saturated mono-hydroxy alcohols. For bicyclic alcohols and adamantanol-2 derivatives a sterical lever effect is established. No experimental evidence is found to support the involvement of an oxygen lone pair·C αH of primary and secondary alcohols in the band maximum nor in the relative intensity of the rotamers.

Linear and non-linear modelling techniques for the investigation of spectrum-structure relationships

The hydroxyl stretching vibration is very sensitive to the structural environment of the OH group. The effect of a number of structure descriptors on the absorption maximum is investigated systematically using advanced linear and non-linear regression techniques. The band maximum of saturated aliphatic mono-hydroxy alcohols and cyclohexanols is mainly affected by the number of α- and β-C atoms, whereas for hydrogen-bonded systems the aromaticity of donor and acceptor oxygen and the ringsize of the H-bonded system are of major importance. For prediction of full OH stretching profiles best results are obtained when using rotamer distributions as structure descriptors. The assumption of a statistical rotamer distribution seems to provide contradictory effects compared to experimental data for those rotamers where sterical repulsion is dominant.

Solvation Dynamics in Monohydroxy Alcohols: Agreement between Theory and Different Experiments

Recently three different experimental studies on ultrafast solvation dynamics in monohydroxy straight-chain alcohols (C-1-C-4) have been carried out, with an aim to quantify the time constant (and the amplitude) of the ultrafast component. The results reported are, however, rather different from different experiments. In order to understand the reason for these differences, we have carried out a detailed theoretical study to investigate the time dependent progress of solvation of both an ionic and a dipolar solute probe in these alcohols. For methanol, the agreement between the theoretical predictions and the experimental results [Bingemann and Ernsting J. Chem. Phys. 1995, 102, 2691 and Horng et al. J: Phys, Chern, 1995, 99, 17311] is excellent. For ethanol, propanol, and butanol, we find no ultrafast component of the time constant of 70 fs or so. For these three liquids, the theoretical results are in almost complete agreement with the experimental results of Horng et al. For ethanol and propanol, the theoretical prediction for ionic solvation is not significantly different from that of dipolar solvation. Thus, the theory suggests that the experiments of Bingemann and Ernsting and those of Horng et al. studied essentially the polar solvation dynamics. The theoretical studies also suggest that the experimental investigations of Joo et al. which report a much faster and larger ultrafast component in the same series of solvents (J. Chem. Phys. 1996, 104, 6089) might have been more sensitive to the nonpolar part of solvation dynamics than the polar part. In addition, a discussion on the validity of the present theoretical approach is presented. In this theory the ultrafast component arises from almost frictionless inertial motion of the individual solvent molecules in the force field of its neighbors.

Ester base stocks for synthetic lubricants

AbstractThis paper reports on work on the synthesis and characterisation of mixtures of aliphatic adipates with complex esters based on polyols, adipic acid, and monohydroxy alcohols, in order to formulate new synthetic base stocks. The viscosity and pour point values of these mixtures are higher than those of adipates. Both of these physical properties become greater with the increasing molecular weight of the complex ester or of the ester's concentration in the mixture.