Polyols In Aqueous Solutions Research Articles

Hydroxy protons as structural probes to reveal hydrogen bonding properties of polyols in aqueous solution by NMR spectroscopy

The solution properties of ethylene glycol (ethane-1,2-diol), glycerol (propane-1,2,3-triol), erythritol ((2R,3S)-butane-1,2,3,4-tetraol), D-xylitol ((2R,3r,4S)-pentane-1,2,3,4,5-pentaol), D-mannitol ((2R,3R,4R,5R)-hexane-1,2,3,4,5,6-hexaol), and D-sorbitol ((2S,3R,4R,5R)-hexane-1,2,3,4,5,6-hexaol), constituting a subgroup of polyalcohols/polyols of maximum six carbon atoms have been investigated using 1H NMR chemical shifts, coupling constants, temperature coefficients, and chemical exchange rates of hydroxy protons in aqueous medium.Relative within a molecule, minimum two-fold difference in rate of exchange values and higher temperature dependence of chemical shifts of the hydroxy protons on terminal carbon atoms confirm that sustainable hydrogen bonding interactions is accentuated for the hydroxyl groups on secondary carbons. Compared to the primary carbons i.e. terminal ones, the hydroxy protons on second and third carbon atoms exhibit much lower rate of exchange and smaller temperature coefficients, indicating that they are further involved in transient hydrogen bonding interactions. Scalar 3JOH,CH-couplings ranging between 3.9 and 7.2 Hz imply that the hydroxyl groups are practically in free rotation regime. Examination of the chemical shift differences with respect to the shift of glycol hydroxy proton reveals that the disparity between terminal and inner hydroxyl groups disclosed by the exchange rates and temperature coefficients is sustained with the exception of 0.003 and 0.053 ppm for O(3)H of mannitol and O(5)H of sorbitol respectively. The experimental findings have been augmented by quantum chemical calculations targeting theoretical NMR chemical shifts, as well as the conformational analysis of the structures.

Thermochemical analysis of intermolecular interactions between N-acetylglycine and polyols in aqueous solutions

The integral enthalpies of dissolution Δsol H m for N-acetylglycine in aqueous solutions of glycerol, ethylene glycol and 1,2-propylene glycol are measured via solution calorimetry. The standard enthalpies of dissolution (Δsol Н 0) and transfer (Δtr Н 0) for N-acetylglycine from water to aqueous solutions of polyhydric alcohols are calculated from experimental data. Positive values of enthalpy coefficients of pair interactions h xy for amino acids and polyol molecules are calculated using the McMillan–Mayer theory. The results are discussed using an approach for evaluating different types of interactions in ternary systems and the effect the structural features of interacting biomolecules have on the thermochemical characteristics of N-acetylglycine dissolution.

Effect of temperature on the volumetric and viscometric properties of polyols in aqueous solutions

In this work a systematic study of the effect of temperature on the apparent molar volumes and the viscosities of aqueous solutions of ethylene glycol, glycerol, meso-erythritol, xylitol, sorbitol and myo-inositol is presented. Experimental densities and viscosities of the polyol aqueous solutions were measured in the temperature range from 293.15 to 313.15K at intervals of 5K. The apparent molar volumes were determined as a function of composition at each temperature from experimental data. The partial volumes at infinite dilution of the polyols in aqueous solution were evaluated through extrapolation at each temperature.The relative viscosity values were adjusted by least-squares to a second order equation to obtain the viscosity B and D coefficients. The temperature dependence of the partial molar volume at infinite dilution and the viscosity B coefficient are discussed in terms of the relationship among polar and nonpolar groups and their effect on the structure of the aqueous solvent.

QM/MM simulations of polyols in aqueous solution

We present QM/MM molecular dynamics simulations of aqueous solutions of polyols. The solute is treated at the AM1 semiempirical level and the TIP3P potential is used for the water molecules. We focus our attention on the intramolecular/intermolecular hydrogen bond balance and try to analyse the role of water in the sweet taste mechanism of polyols.

Structural analysis of diols by electrospray mass spectrometry on boric acid complexes.

A method is presented to characterize diols using negative ion electrospray (ES) mass spectrometry in combination with collision-induced dissociation tandem mass spectrometry (MS/MS). The analyte diol is added to a solution containing an ethylene glycol/boric acid [2:1] complex and then subjected to infusion ES. The following boric acid complexes are formed: (i) a complex with two ethylene glycol molecules, (ii) a mixed ethylene glycol/analyte complex, and (iii) a complex with two analyte molecules. The first complex serves as a reference for the assessment of the extent of complex formation with the analyte. The ES mass spectra of acyclic vicinal diols all feature intense mixed complex signals, indicative of efficient complex formation. Chemical fine tuning is achieved by MS/MS experiments. Thus, although the (2R,3R)-(-)-2,3-butanediol and meso-2,3-butanediol stereo-isomers show the same complexation efficiency, MS/MS experiments reveal pronounced structure characteristic differences. By contrast, 1,3- and 1,4-diols are less prone to complex formation as they give only weak signals relative to the reference. For cyclic vicinal diols only the cis isomer produces an intense mixed complex, whose MS/MS spectrum is characteristically different from that of the trans form. The above procedure does not permit an unambiguous differentiation of acyclic polyhydroxy compounds like mannitol and sorbitol. However, structurally related methyl glycosides show characteristic MS/MS spectra. Our findings indicate that the above simple procedure may be useful to probe the presence and structure of diols and other polyols in aqueous solutions. Copyright 1999 John Wiley & Sons, Ltd.

A correlation for the estimation of critical micellization concentrations and temperatures of polyols in aqueous solutions

AbstractAn empirical correlation is presented for the estimation of critical micellization concentrations (CMC) and critical micellization temperatures (CMT) for poly(ethylene oxide)‐block‐poly(propylene oxide)‐block‐poly(ethylene oxide) copolymers in aqueous solutions. The CMC and CMT are expressed as a function of the polyol molecular weight, composition, and temperature (for CMC determination) or concentration (in the case of CMT). The correlation was developed from experimental CMT data for a set of 12 polyols that covered a wide range of molecular weights (2900–14600) and poly(ethylene oxide) contents (30–80 wt%) and is based on a simple expression for the standard free energy of micellization. Such a correlation should be useful to practitioners of the field as it allows easy prediction of CMC and CMT for a wide range of polyols with a minimal number of input parameters.

Transesterification Reactions of Parabens (Alkyl 4‐Hydroxybenzoates) with Polyols in Aqueous Solution

Accelerated stability tests of aqueous solutions containing parabens and polyols were performed using concentrations similar to pharmaceutical and cosmetic formulations. Reaction products were detected in these solutions by HPLC and identified by chromatographic and spectroscopic means. Using xylitol and methylparaben as model reactants, three unknown peaks having the relation 1:2:4 were obtained together with the hydrolysis product 4‐hydroxybenzoic acid. Diode array detection gave identical UV spectra for each peak with a maximum at 255nm. The structures of the isomeric 1‐, 2‐, and 3‐xylityl 4‐hydroxy‐benzoic acid esters were proved by means of LC‐MS, GC–MS, and NMR and correlated to the peaks in the HPLC chromatograms. The rate of the transesterification was shown to be highest in strongly alkaline medium (ph 10–11), whereas equilibration of the reaction was optimally balanced at pH 8–9. An increase of polyol concentration enhanced the formation of the esters. The reactivity of different substituted parabens was higher in the case of parabens with a short alkyl ester function. Similar reaction profiles were observed with C3‐C6 polyois, but no transesterification took place when aldoses were used.

Deactivation of supported copper based catalysts during polyol conversion in aqueous phase

Degradations and cyclodehydration of polyols in aqueous solution were studied on copper deposited on charcoal unmodified or modified by addition of chloride and a second metal (Pt or Au). These catalysts had properties very close to those of Raney copper based catalysts. Their deactivation was studied in a dynamic flow three-phase reactor. Sintering of copper particles, surface oxidation and copper(I) hydroxide dehydration were responsible for the deactivation of the unmodified Cu/C. In the cases of modified Cu/C, decrease of the cyclodehydration selectivity was attributed to the loss of chloride. Addition of chloride ions to the polyol solution to be converted increased the cyclodehydration selectivity but strongly decreased the activity.

Komplexbildung von geradkettigen Polyolen in w��riger L�sung mit partiell hydratisierten K+ -und Ca2+ -Ionen in X- und Y-Zeolithen

Complex Formation of Straight-Chain Polyols in Aqueous Solution with Incompletely Hydrated K+ and Ca2+ Cations on X and Y Zeolites A systematic study was performed on the adsorption of alditole-water mixture with a chain length of C3 to C6 on KX, KY and Ca(83)NaY zeolites. From the adsorption excess isotherms the separation factors and the equilibrium diagrams xs vs x1 of the binary mixtures were calculated. The possible triangles of 0 atoms of polyols and the influence of the conformation on formation of adsorption complexes are discussed.

A comparative analysis of the interaction of borate ion with various polyols

Although it is now generally accepted that borate ion, B(OH) 4 −, reacts with suitable polyols in aqueous solution to form two types of complexes with strongly acidic properties (1), several investigators have presented evidence to show the formation of two types of complexes is stoichiometrically unsatisfactory (2,3). Aside from its usefulness in structural studies, the reaction has important implications in numerous biochemical applications. Many compounds of biological importance contain hydroxyl groups in positions favorable for reaction with borate ion. As an extension of previous work (4) we report here the determination of solution stability constants of borate ion complexes with several biologically important polyols by means of a pH method and in two instances a calorimetric method.

Stereospecific contact interactions in the nuclear magnetic resonance spectra of polyol–lanthanide complexes

Addition of lanthanide ions to polyols in aqueous solution causes shifts in their 1H n.m.r. spectra which are interpreted as being due to contact and pseudo-contact interactions; the contact interaction is greatest when the bonds connecting the proton and the cation form a planar zig-zag arrangement.

Equilibria between borate ion and some polyols in aqueous solution

The amount of complex formation between borate ion and some diols and polyols of known stereochemistry has been measured by a pH method at four temperatures, allowing determination of values of K, ΔG°, ΔH° and ΔS°. The major factors affecting complexing appear to be the ease of attaining coplanarity of the hydroxyl carbons and oxygens entering the complex, and the degree of substitution on the hydroxyl carbons.