Organic modifiers for the separation of organic acids and bases by liquid chromatography

A straight-chain alcohol or diol additive in the mobile phase was used to modify and improve the HPLC separation of organic acids and bases. Incorporation of 2% 1-butanol, 1% 1,2-hexanediol, or 0.25% 1,2-octanediol into an aqueous mobile phase greatly improved the separation of alkane carboxylic acids on a silica C18 column, both in terms of separation time and peak shape. When 1.5% 1-hexanol, 0.09% 1-decanol or 0.01% 1-dodecanol was added to an acetonitrile-water (30:70) mobile phase, much sharper peaks and better resolution were obtained for aromatic bases separated on an underivatized polystyrene-divinylbenzene column. The mobile phase additive is believed to coat the stationary phase surface by a dynamic equilibrium. The coated surface is more hydrophilic and facilitates the efficient partitioning of analytes between the mobile and stationary phases.     

Solution crystallization of nylon 12

Electron microscopy and x-ray diffraction data have been obtained on nylon 12 crystallized from 1-hexanol, 1,6-hexanediol, and hexylene glycol. Ribbonlike lamellar crystals of the γ form are obtained by crystallization from all the solutions and elongated flat crystals of the α form by crystallization from the 1-hexanol and hexylene glycol solutions. The direction of the hydrogen bond in these crystals is almost parallel to that of maximum crystal elongation. α- and γ-form crystals both grow from 1-hexanol and hexylene glycol at appropriate crystallization temperatures. γ-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 7.6–10.6 nm. The melting behavior of the solution-grown crystals is examined and discussed. The melting temperatures of the γ form may be lower than that of the α form. An equilibrium melting temperature of 208.4°C for γ-form crystals is obtained by using a relation between thickness of lamellar crystals and their melting temperatures observed by differential scanning calorimeter measurements. Solvents affect the growth of the two crystalline forms in solution crystallization.     

Adjusting selectivity in micellar electrokinetic capillary chromatography with 1,2-hexanediol

In this report, we introduce a new micelle modifier useful to alter selectivity in micellar electrokinetic capillary chromatography (MECC). 1,2-Hexanediol acts as a class I organic modifier in that its effects are on the sodium dodecyl sulfate (SDS) micellar rather than the surrounding aqueous phase. This characteristic allows 1,2-hexanediol to improve resolution when applied at concentrations as low as 20 mM (0.25% v/v) by altering the selectivity observed with SDS alone. The effects of 1,2-hexanediol on the critical micelle concentration of SDS, electroosmotic flow, electrophoretic mobility of the SDS micelle, and reproducibility are presented. 1,2-Hexanediol had little impact on the migration time window at concentrations below 100 mM. Changes in selectivity induced by 1,2-hexanediol for a large set of model compounds are presented. Analytes capable of forming hydrogen bonds tend to decrease their interactions with the micellar phase while nonhydrogen bonding analytes increase their interactions. The usefulness of 1,2-hexanediol was demonstrated by examining its effects on the separation of dansylated amino acids. Eighteen of twenty amino acids could be separated with a resolution greater than 1.6 within 1600 s using a combination of 1,2-hexanediol and isopropanol.     

Heat capacity studies of formation of micelle-like structure in aqueous solutions of some alkane polyols

Some polyols show micellar behavior in aqueous solutions at concentrations greater than the critical micellar concentration (cmc). The 1,2-alkanediols (C(n)H(2n+2)O2 with n=5,6,7), the 1,2,3-alkanetriols (C(n)H(2n+2)O3 with n=7,8,9), and the geminated alkanetriols (C(n)H(2n+2)O3 with n=8 and 9) are investigated by microcalorimetric techniques. Only the 1,2-hexanediol (n=6), the 1,2,3-octanetriol (n=8), and the 2,2-dihydroxymethyl 1-heptanol (n=9) possess, in aqueous solutions, an organized structure above the critical micellar concentration (cmc). The 1,2-pentanediol (n=5) and the 1,2,3-heptanetriol (n=7) would form weak associations, whereas the 2,2-dihydroxymethyl 1-hexanol (n=8) does not form any associations even at large concentration. The 1,2-heptanediol (n=7), being only very slightly soluble even at a temperature of 30 degrees C, could not be studied. The 1,2,3-nonanetriol (n=9) is not soluble at temperatures between 20 and 35 degrees C. The critical micellar concentrations are determined by specific heat capacity methods. The passage from the dispersed environment to the organized environment gives a constant quantity of specific heat capacities (about 50 JK(-1)mol(-1)) only for the 1,2-hexanediol (n=6), the 1,2,3-octanetriol (n=8) and the 2,2-dihydroxymethyl 1-heptanol (n=9), which form true micelles. Structural effects of these systems are discussed.     

Effect of temperature on the surface tension of diluted aqueous solutions of 1,2-hexanediol, 1,5-hexanediol, 1,6-hexanediol and 2,5-hexanediol

Surface tension of aqueous solutions of 1,2-hexanediol (1,2HD), 1,5-hexanediol (1,5HD), 1,6-hexanediol (1,6HD), and 2,5-hexanediol (2,5HD) was measured as a function of composition using the method of capillary rise at 283.15, 288.15, 293.15, 298.15, 303.15 and 308.15 K with emphasis in the very dilute region.     

Depolymerization behavior of thermoplastic poly(urethane) (TPU) and its dependence on initial molecular weight

The quantitative and qualitative depolymerization behavior of thermoplastic poly(urethane) (TPU) consisting of 4,4′-methylenediphenyldiisocyanate (MDI), 1,6-hexanediol (HD) and 1-hexanol (HEX) was described using temperature-modulated differential scanning calorimetry and Fourier transform infrared (FT-IR) spectroscopy. The depolymerization behavior could be altered by lowering the starting molecular weight. This resulted not only in a higher ceiling temperature, but also in a faster decrease of the molecular weight as a function of temperature once the depolymerization has started. The increase in the ceiling temperature for low molecular weight TPUs is attributed to the lower entropy of polymerization for these compounds compared to high molecular weight TPUs.     

Excess Volumes of 1-Alcohol + Heptane Mixtures at 298.15 and 308.15 K-Application of An Association Model with Flory Interaction Term

Abstract

The excess volumes of 1-pentanol+, 1-hexanol+, 1-heptanol+, 1-octanol+, 1-decanol+ and 1-dodecanol + heptane mixtures were evaluated at 298.15 and 308.15 K from the density data. The excess volumes were found to decrease with the increase in the carbon chain length of alcohol molecules. The temperature coefficient for the excess volumes was also observed to show a decreasing trend from 1-pentanol + to 1-dodecanol + heptane mixtures. The excess volumes were also calculated by combining the individual physical and chemical contributions, evaluated by combining an association model with Flory's free volume theory. Reasonable agreement between the experimental and calculated excess volumes for all the mixtures at 298.15 and 308.15 K was noted.     

Relaxations and nano-phase-separation in ultraviscous heptanol-alkyl halide mixture

To gain insight into the effects of liquid-liquid phase separation on molecular relaxation behavior we have studied an apparently homogeneous mixture of 5-methyl-2-hexanol and isoamylbromide by dielectric spectroscopy over a broad temperature range. It shows two relaxation regions, widely separated in frequency and temperature, with the low-frequency relaxation due to the alcohol and the high-frequency relaxation due to the halide. In the mixture, the equilibrium dielectric permittivity epsilon(s) of the alcohol is 41% of the pure state at 155.7 K and epsilon(s) of isoamylbromide is approximately 86% of the pure state at 128.7 K. The difference decreases for the alcohol component with decreasing temperature and increases for the isoamylbromide component. The relaxation time tau of 5-methyl-2-hexanol in the mixture at 155.7 K is over five orders of magnitude less than in the pure state, and this difference increases with decreasing temperature, but tau of isoamylbromide in the mixture is marginally higher than in the pure liquid. This shows that the mixture would have two T(g)'s corresponding to its tau of 10(3) s, with values of approximately 121 K for its 5-methyl-2-hexanol component and approximately 108 K for its isoamylbromide component. It is concluded that the mixture phase separates in submicron or nanometer-size aggregates of the alcohol in isoamylbromide, without affecting the latter's relaxation kinetics, while its own epsilon(s) and tau decrease markedly.     

Dielectric Relaxation Study of Chloro Group with Associative Liquids. II. 1,2-Dichloroethane with Methanol, Ethanol, and 1-Propanol

Frequency spectra of the complex permittivity for 1,2-dichloroethane–alcohol binary mixtures have been determined over the frequency range 10 MHz to 20 GHz at 15, 25, 35, and 45°C, using the time-domain reflectometry (TDR) technique, for 11 compositions of each 1,2 dichloroethane–alcohol system. The alcohols used in the study were methanol, ethanol, and 1-propanol. The relaxation in these systems can be described by a single relaxation time using the Debye model. The static dielectric constant, relaxation time, the corresponding excess dielectric properties, Kirkwood correlation factor, and Bruggeman factor of the mixtures have been determined. The static dielectric constants for the mixtures have been fitted with the modified Bruggeman model.     

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.     

Comparaison des transformations polyreactionnelles induites par les chlorure et bromure de pyridinium : Cas de l'hexanediol-1,6

Pyridinium chloride and bromide react with 1,6-hexanediol through a complex multireactional chain process mostly furnishing unsaturated hydrocarbons, cyclic ethers, unsaturated halides and unsaturated alcohols. The similar transformations undergone by 5-hexene-1-ol and 1-hexanol under the same conditions suggest several hypotheses about the mechanism. Several original assignments from ¹³C NMR data were involved in determining the structures of the products.     

Dielectric relaxation of polymer networks built from macromolecules with dipole moment directed along the end-to-end chain vector

The dielectric relaxation properties are considered for polymer networks built from polar macromolecules with the dipole moment directed along the end-to-end chain vector. The viscoeleastic cubic model of a regular network is used. The fixed average volume of a polymer network is ensured by the effective internal pressure. The dynamic models of polymer networks with external and interchain friction are studied. Two cases are considered: (1) polar chains cross-linked in a network at their ends, and (2) a densely cross-linked network with many network junctions per polar chain. The expressions for the autocorrelation functions of the total dipole moment of a network, which determine the dielectric susceptibility, are calculated. The relaxation spectrum of the autocorrelation function consists of two regions: the high-frequency relaxation spectrum of a chain fragment between two neighbouring junctions (intrachain relaxation spectrum) and the lowfrequency interchain relaxation spectrum. The interchain relaxation spectrum is determined by cooperative motions of chains which form a network. The characteristic time of this spectrum for networks of type (1) is the relaxation time of a chain between junctions τ_min. For networks of type (2) a second time scale τ₁ exists, which corresponds to motions inside the volume occupied by a single long polar chain included in a network. It leads to different time behaviour of the autocorrelation functions for both network models. The existence of only interchain friction in the network model leads to a cut-off of the relaxation spectrum at the time τ_max depending on the volume of viscous interchain interactions.     

A mean field analysis of the static dielectric behavior of linear lower alcohols

The static dielectric responses of methanol, ethanol, and 1-propanol up to 1-hexanol are discussed in terms of a stiff-chain lattice model for the alcohol clusters. An analytical expression for the Kirkwood correlation factor gK is derived in terms of the canonical partition function associated to the configurational statistics of any of the dimers building up a chain. This allows for the estimate of the dipole moment mu0 of an alcohol molecule in the liquid phase from the temperature dependence of the dielectric constant. All alcohol species appear to be characterized by a dipole moment larger than in the vapor phase. The Kirkwood correlation factor is found to be an increasing function of the alkyl tail length.     

Dielectric relaxation studies of monoalcohol solutions with 1,2-diols

Dielectric relaxation processes in solutions of monoalcohols with ethanediol, 1,2-propanediol, and 1,2-butanediol have been studied using the dielectric time domain spectroscopy technique. The dielectric spectrum has been analyzed in terms of two dispersions. The influence of varying content of diol, and of chainlength of the alcohol, on the static dielectric constant _s and on the main dispersion is reported. For the long-chain alcohols _s is lowered and the main relaxation time successively shortened with the addition of diol to the solution. The results are compared to these found for aqueous alcohol solutions and discussed in terms of a structural model with rupturing of predominantly linear complexes on addition of water, diols, or on increasing the temperature.     

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.     

DIELECTRIC PROPERTIES OF 1, 2-POLYBUTADIENES

The dielectric permittivity and loss tangent of 1,2-polybutadienes with different chain structures were determined as a function of temperature from-180℃ to 100℃ at different frequencies, and the frequency and structure dependence of the dielectric propertie of 1,2-polybutadienes have been investigated. It is found that a maximum of the permittivity occurs in the glass-transition region. The width of the glasstransition peak increases with increasing frequency while its height has little change. With a rise in the content of 1,2-units, the permittivity decreases and the height of the glass-transition peak slightly grows. A maximum of the width of the glass-transition peak appears when the content of 1,2-units is about 45%. Both the permittivity and dielectric loss drop down as the growth of the content of syndiotactic 1,2-units.     

Acid-base equilibria in nonpolar media. 3. Expanding the spectrophotometric acidity scale in heptane

The UV-vis spectrophotometric ion-pair acidity scale in heptane has been significantly expanded: it includes now 21 bulky CH and NH indicator acids and spans for about 10 pKip units. The phosphazene base t-Bu-P4 was used for deprotonating. The correlations between acidities in heptane versus gas-phase acidities or acidities in DMSO or 1,2-dimethoxyethane have been made for some compounds. It was demonstrated that the substituent effects on the acidity of the studied CH acids are attenuated ca. 1.24 times when the gas phase is substituted for the nonpolar solvent, heptane. In its turn, for the series of NH acids, the latter is found to be a somewhat more differentiating solvent than DMSO.     

Volumetric Properties of N, N-Dimethylformamide with 1,2-Alkanediols at 20°C

Dilatometric measurements of excess molar volumes and excess partial molar volumes have been made for binary mixtures of N, N-dimethylformamide with 1,2-ethanediol, 1,2-propanediol, 1,2-butanediol, 1,2- pentanediol, and 1,2-hexanediol at 20°C over the entire composition range. The results are explained in terms of dissociation of the self-associated 1,2-alkanediol molecules and the formation of aggregates between unlike molecules through C = O ... 3H-O hydrogen bonding. Further, the effects of difference in chain lengths and steric factors on molecular interactions are also examined. From the experimental results, excess molar volumes were calculated and correlated by a Redlich–Kister type function in terms of mole fractions.     

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.     

Explorations of alkyl polyols as "class I" organic modifiers to adjust selectivity in micellar electrokinetic capillary chromatography.

In this study, we investigated a novel series of micelle modifiers useful to alter selectivity in micellar electrokinetic capillary chromatography (MEKC). These modifiers were alkyl polyalcohols, including 1-octanol, 1,2-octanediol, 1,2,3-octanetriol, 1,2-hexanediol, and 1,2-butanediol, which act as class I organic modifiers in that their effects are on the sodium dodecyl sulfate (SDS) micelle rather than the surrounding aqueous phase. This characteristic allows the alkyl polyols to effect resolution when applied at concentrations as low as 20 mM (0.25% v/v) by altering the selectivity observed with SDS without a modifier. The effects of the alkyl polyols on the critical micelle concentration of SDS, electroosmotic flow, and electrophoretic mobility of the SDS micelle are presented. These modifiers had little impact on the migration time window at the concentrations explored. Changes in selectivity induced by the alkyl polyols for a large set of model compounds are presented. Trends indicate that solutes capable of forming hydrogen bonds tend to decrease their interactions with the micellar phase while nonhydrogen bonding solutes increase their interactions upon addition of the modifiers. The solvation parameter model was used to characterize the induced changes in selectivity. This model suggests that even though the modifiers are structurally similar, each produced a unique set of system constants. It was also demonstrated that the addition of alkyl polyols improved the correlation between the partition coefficients of SDS and water to 1-octanol and water. The usefulness of the alkyl polyols was demonstrated by examining their effects on the separation of 11 priority phenols.