Liquid chromatography of polyoxyethylenes under critical conditions: a thermodynamic study

Polyethylene glycols (PEG), fatty alcohol ethoxylates (FAE) and fatty acid polyglycol esters were analyzed by liquid chromatography under critical conditions (LCCC) on typical reversed phase columns in mobile phases consisting of methanol-water and acetone-water in the region of the critical adsorption point (CAP) for the oxyethylene unit. Critical conditions can be adjusted by varying the mobile phase composition or temperature. In methanol-water the temperature dependence is almost negligible, while it is much more pronounced in acetone-water. Critical conditions for the oxyethylene unit were realized on octadecyl phases in methanol-water containing 85.8 wt.% methanol and in acetone-water at 89.6 wt.% acetone. In the van't Hoff plots the logarithm of the distribution coefficient was plotted versus the reciprocal temperature. In all plots straight lines were found, from slope and intercept of which the entropy and enthalpy changes were calculated. The relation of the thermodynamic parameters -deltaG/RT, -deltaH/RT, and deltaS/R to the numbers of A and B chain units in block copolymers AB and BAB at the CAP for A is studied both theoretically and experimentally. Various approaches for obtaining the critical temperature are discussed.     

α-氨基酸在水-乙醇中羧基质子化热力学

Enthalpy changes for the protonation of carboxyl group of four α-amino acids(glycine,L-α-alanine,L-valine and L-serine) were measured in water-ethanol mixtures (10－ 70wt％) at 298.15K using LKB-2277 Bioactivity Monitor.The corresponding entropy and Gibbs energy changes were also calculated.The results show that both enthalpy changes and entropy changes are favorable to the protonation of carboxyl groups of the investigated amino acids in water-ethanol mixtures.However,the influence of the composition of ethanol in the mixed solvents on the enthalpy change and entropy changes is complicated.Both sδ and sδ ,the differences of enthalpy changes and entropy changes in mixed solvents and in pure water respectively,show a minimum approximately at xEtOH=0.1.The effects of side chains on the enthalpy change and entropy changes were also investigated using the proton transfer process between glycine and the other three amino acids.The results demonstrate that the proton transfer processes for alanine and valine are spontaneous but not for serine,which could be interpreted in terms of the electrostatic interaction between amino group and carboxyl group within the molecule and the interaction between carboxyl group and the solvent.     

α-氨基酸在水-乙醇中羟基质子化热力学

α 氨基酸为重要的两性物质 ,其酸碱性质对于维持生物体内的酸碱平衡和蛋白质的生物活性起着十分重要的作用．α 氨基酸在纯水溶剂中的质子化反应热力学性质已进行了广泛的研究［１ -６］,但对球形蛋白来说 ,蛋白亚基间的作用是处在一个大量但不完全由水组成的非水环境中 ,因此 ,研究氨基酸在水 有机物混合溶剂中的质子化热力学性质具有重要意义．但这方面的工作 ,特别是用量热法直接研究溶剂对质子化焓变的影响开展得不多．本文选择乙醇和水组成的混合溶剂模拟蛋白质亚基所处的介质环境 ,利用精密微量流动热量计测定２９８．１５Ｋ时甘氨…     

Liquid exclusion-adsorption chromatography, a new technique for isocratic separation of nonionic surfactants. II. Quantitation in the analysis of fatty alcohol ethoxylates

A new technique of liquid chromatography, which allows baseline separation of fatty alcohol ethoxylates with up to 15-20 ethylene oxide units under isocratic conditions allows an accurate quantitative analysis of single hydrophobic chain surfactants. Using density and refractive index detection, the accurate weight fractions of the individual oligomers are obtained. Moreover, the contribution of preferential solvation can be determined. With refractive index detection alone, good accuracy can also be achieved.     

Characterization of complex copolymers by two-dimensional Liquid Chromatography

Complex polymers were characterized by combinations of different chromatographic separation mechanisms: liquid adsorption chromatography (LAC), liquid chromatography under critical conditions (LCCC), and liquid exclusion-adsorption chromatography (LEAC). These techniques were combined off-line and on-line in two-dimensional separations. Fatty acid ethoxylates, fatty esters of polyethylene glycol (PEG) and polysorbates were analyzed by two-dimensional liquid chromatography with normal phase LAC as the first and liquid chromatography at critical conditions (LCCC) or liquid exclusion adsorption chromatography (LEAC) as the second dimension. A full separation of all oligomers to the baseline could be achieved in both dimensions. In two-dimensional separations, the offline approach is compared to comprehensive chromatography, and the scope and limitations of both techniques are discussed.     

Comprehensive analysis of fatty alcohol ethoxylates by ultra high pressure hydrophilic interaction chromatography coupled with ion mobility spectrometry mass spectrometry using a custom‐designed sub‐2 μm column

Comprehensive analysis of fatty alcohol ethoxylates has been conducted by coupling ultra high pressure hydrophilic interaction chromatography and ion mobility spectrometry mass spectrometry. A custom‐designed sub‐2 μm column was used for the chromatographic separation of fatty alcohol ethoxylates by hydrophilic interaction chromatography. Ion mobility spectrometry provided a post‐ionization resolution during a very short period of 6.4 ms. Distinguishable families of singly, doubly, and triply charged fatty alcohol ethoxylates were clearly observed. By virtue of the combination of hydrophilic interaction chromatography and ion mobility spectrometry, comprehensive resolution based on both hydrophobicity difference and mobility disparity has been achieved for fatty alcohol ethoxylates. The orthogonality of the developed separation and analysis system was evaluated with the correlation coefficient and peak spreading angle of 0.0224 and 88.72°, respectively. The actual peak capacity obtained was individually 40 and 193 times than those when hydrophilic interaction chromatography and ion mobility spectrometry were used alone. The collision cross‐sections of fatty alcohol ethoxylates were calculated by calibrating the traveling wave ion mobility device with polyalanine.     

The role of entropy in initializing the aggregation of peptides: a first principle study on oligopeptide oligomerization

The initiation and progression of Alzheimer's disease is coupled to the oligo- and polymerization of amyloid peptides in the brain. Amyloid like aggregates of protein domains were found practically independent of their primary sequences. Thus, the driving force of the transformation from the original to a disordered amyloid fold is expected to lie in the protein backbone common to all proteins. In order to investigate the thermodynamics of oligomerization, full geometry optimizations and frequency calculations were performed both on parallel and antiparallel β-pleated sheet model structures of [HCO-(Ala)(1-6)-NH(2)](2) and (For-Ala(1-2)-NH(2))(1-6) peptides, both at the B3LYP and M05-2X/6-311++G(d,p)//M05-2X/6-31G(d) levels of theory, both in vacuum and in water. Our results show that relative entropy and enthalpy both show a hyperbolic decrease with increasing residue number and with increasing number of strands as well. Thus, di- and oligomerization are always thermodynamically favored. Antiparallel arrangements were found to have greater stability than parallel arrangements of the polypeptide backbones. During our study the relative changes in thermodynamic functions are found to be constant for long enough peptides, indicating that stability and entropy terms are predictable. All thermodynamic functions of antiparallel di- and oligomers show a staggered nature along the increasing residue number. By identifying and analyzing the 6 newly emerging dimer vibrational modes of the 10- and 14-membered building units, the staggered nature of the entropy function can be rationalized. Thus, the vanishing rotational and translational modes with respect to single strands are converted into entropy terms "holding tight" the dimers and oligomers formed, rationalizing the intrinsic adherence of natural polypeptide backbones to aggregate.     

Enthalpies of aqueous solution of noble gases at 25°C

The standard enthalpies of solution of rare gases (helium, neon, argon, krypton, and xenon) in water at 25°C have been measured by a high precision steady-state calorimetric method. The aqueous solvation process is energetically favorable at 25°C for the gases studied. Values of the standard free energy, enthalpy, and entropy changes are found to be well correlated with cavity surface areas and the number of water molecules in the first solvation shell. Also, the values of the standard enthalpy and entropy of solution for the rare gases are found to have the same dependence on the number of solvation shell water molecules as inorganic and hydrocarbon gases. These results imply that the dominant source of enthalpy and entropy change resides in the first solvation shell.     

Trace analysis of surfactants derived from fatty alcohols I. Optimization of derivatization reaction

Summary Pre-column derivatization of the primary hydroxyl group in fatty alcohols and fatty alcohol ethoxylates using carbazole-9-carbonyl chloride (CC−Cl) and FMOC-Cl is described and compared with derivatization with 1-naphthoyl chloride (N−Cl). As the excess of derivatization reagent leads to a broad and strongly tailing reagent peak, it hinders trace determination of fatty alcohols and fatty alcohol ethoxylates. Therefore, an off-line as well as an on-line solid-phase extraction (SPE) method for removal of excess reagent are described. The on-line method which is based on column switching, shows better reproducibility higher pre-concentration, lower risk of contamination and can be easily automated, while the off-line method is better suited for the analysis of derivatized, fatty alcohol ethoxylates. An example of the trace analysis of fatty alcohols with a concentration of 2 ppb is given.     

Changes in thermodynamic quantities upon contact of two solutes in solvent under isochoric and isobaric conditions

The changes in excess thermodynamic quantities upon the contact of two solutes immersed in a solvent are analyzed using the radial-symmetric and three-dimensional versions of the integral equation theory. A simple model mimicking a solute in water is employed. The solute-solute interaction energy is not included in the calculations. Under the isochoric condition, the solute contact always leads to a positive entropy change irrespective of the solute solvophobicity or solvophilicity. The energy change is negative for solvophobic solutes while it is positive for solvophilic ones. Under the isobaric condition, the contact of solvophobic solutes results in system-volume compression but that of solvophilic ones gives rise to expansion. Effects of the compression and expansion on the changes in enthalpy and entropy are enlarged with rising temperature. When the solute solvophobicity is sufficiently high, the entropy change (multiplied by the absolute temperature) can become negative due to the compression, except at low temperatures with the result of an even larger, negative enthalpy change. The expansion in the case of solvophilic solutes leads to a large, positive entropy change accompanied by an even larger, positive enthalpy change. The changes in enthalpy and entropy are strongly dependent on the temperature. However, the changes in enthalpy and entropy are largely cancelled out and the temperature dependency of the free-energy change is much weaker. The authors also discuss possible relevance to the enthalpy-entropy compensation experimentally known for a variety of physicochemical processes in aqueous solution such as protein folding.     

Retention of 3,5-dinitrobenzoyl derivatives of linear alcohol polyethoxylates in reversed-phase liquid chromatographic system with acetonitrile-water mobile phase

The influence of mobile phase composition (acetonitrile-water ratio) on the separation of derivatised linear alkyl polyethoxylates (LAEs) is evaluated using thermodynamic quantities (Gibbs energy, enthalpy and entropy). In comparison to homologue series of alcohols oligomers of LAEs show irregular chromatographic behaviour that is demonstrated in irregular changes of thermodynamic quantities. It might be explained considering an influence of some of the following processes or their combinations on the retention of LAEs. These processes are solvation of oxyethylene chains in mobile phase, their interaction with silanols on silica surface of stationary phase and possibly their conformation changes. The composition of a mobile phase affects the mentioned processes and that is why the retention of LAEs is strongly (for the reversed-phase system unusually) sensitive to this composition in the studied range (volume fraction of acetonitrile phi = 0.90, 0.96 and 0.98). The experimental data also support the idea of the active role of stationary phase in the reversed-phase system.     

A thermodynamic study of the complexation reaction for some amino acids with cerium(III) and yttrium(III)

The thermodynamic parameters for the complexation reaction of leucine, valine, proline and hydroxyproline with cerium(III) and yttrium(III) were determined potentiometrical1y in aqueous solution at 25, 35 °C and μ=0.1. The values for the formation constants have been reported. The values of enthalpy changes (ΔS) and entropy changes (ΔS) are positive for all systems. The chelation effect is believed to be essentially an entropy effect.     

Entropic and enthalpic contributions to the chair-boat conformational transformation in dextran under single molecule stretching

The contribution of entropy and enthalpy to the chair-boat conformational changes (clicks) occurring during the force-extension of single molecules of an axially linked polysaccharide, dextran, was investigated. Experimental single molecule force-extension measurements were carried out by atomic force microscopy over the temperature range of 5-70 degrees C. This enabled the separation of the entropy and enthalpy components of the conformational change. The contribution of entropy to the Gibbs energy of the conformational transformation was found to be small (<12 J mol(-1) K(-1)), demonstrating that the click is largely (>89%) enthalpic in nature.     

Characterization of poly(ethylene glycol) esters using low energy collision-induced dissociation in electrospray ionization mass spectrometry

A method of characterizing polyglycol esters, an important class of industrial polymer, has been developed using electrospray ionization ion trap mass spectrometry (ESI ITMS). The fragmentation behavior of polyglycol esters is found to be different from that of polyglycols whose functional end groups are linked to the polymer chain via ether bonds (i.e., polyglycol ethers). The fragmentation pattern of an oligomer ion generated by low-energy collision-induced dissociation is strongly dependent on the type of cation used for ionization. It is shown that structural information on the polymer chain and end groups is best obtained by examining the fragment ion spectra of oligomers ionized by ammonium, alkali, and transition metal ions. The application of this method is demonstrated in the analysis of two surfactants based on fatty acid methyl ester ethoxylates.     

Enthalpies of Dilution of Aqueous Solutions of LiCl, KCl, and CsCl at 300, 325, and 350°C

Dilution enthalpies, measured using isothermal flow calorimetry, are reported for aqueous solutions of LiCl, KCl, and CsCl at 300°C and 11.0 MPa, 325°C and 14.8 MPa, and 350°C and 17.6 MPa. The concentration range of the chloride solutions was 0.5 to 0.02m. Parameters for the Pitzer excess Gibbs ion-interaction equation were determined from the fits of the experimental heat data. Equilibrium constants, enthalpy changes, entropy changes, and heat capacity changes for ion association of the chloride salts were estimated from the heat data. For all systems, the enthalpy and entropy changes were positive and had accelerating increases with temperature. The resulting equilibrium constants show significant, but smaller, increases with temperature.     

Thermodynamical properties of reaction intermediates during apoplastocyanin folding in time domain

Two intermediates observed for the folding process of apoplastocyanin (apoPC) were investigated by using a photoinduced triggering system combined with the transient grating and transient lens methods. The thermodynamic quantities, enthalpy, heat capacity, partial volume, and thermal expansion volume changes during the protein folding reaction were measured in time domain for the first time. An interesting observation is the positive enthalpy changes during the folding process. This positive enthalpy change must be compensated by positive entropy changes, which could be originated from the dehydration effect of hydrophobic residues and/or the translational entropy gain of bulk water molecules. Observed negative heat capacity change was explained by the dehydration effect of hydrophilic residues and/or motional confinement of amino acid side chains and water molecules in apoPC. The signs of the volume change and thermal expansion volume were different for two processes and these changes were interpreted in terms of the different relative contributions of the hydration and the dehydration of the hydrophilic residues. These results indicated two-step hydrophobic collapses in the early stage of the apoPC folding, but the nature of the dynamics was different.     

醇系溶剂中冠醚与碱金属离子的配位反应

The enthalpy changes, entropy changes and equilibrium constants of coordination reactions of 18-crown-6 with the alkali ions Na+ and K+ at 298.15K were measured by using RD-496 microcalorimeter with methanol, ethanol, n-propanol and n-butanol as solvents respectively. The results indicated that the enthalpy changes △rHm, entropy changes △rSm, and the stability of the complexes are closely related with the polarity of the solvents, and the order of the stability is ethanol>methanol>n-propanol>n-butanol>H2O for the same complex in the solvents investigated. The stability order was reasonably explained by relating the stability constants of the complexes with the dielectric constants of these solvents, and by using the model of solvated-desolvated ions and ion pair in the coordination reactions.     

Microfluidic thermodynamics of the shift in thermal stability of DNA duplex in a microchannel laminar flow

This paper reports the shift in thermal stability of DNA duplex and its thermodynamics spectroscopically, caused by stretching and orientation of DNA strands in a microchannel laminar flow. For direct spectroscopic measurement of the microchannel, we prepared an in-house temperature-controllable microchannel-type flow cell. The melting curves of DNA oligomers in a microchannel laminar flow were measured. For DNA oligomers with more than 10 base pairs, the melting curve shifted to the high-temperature side with higher flow speed. However, for 8-base-pair DNA oligomers, a change in the melting profile was not observed in batchwise and microchannel flows. We undertook microfluidic thermodynamic analysis to elucidate details of the shift in thermal stability of the DNA duplex in a microchannel laminar flow. Enthalpy-entropy compensation is applicable to the microfluidic thermal stability shift. We studied the relationships between the enthalpy-entropy compensation and DNA strand length or flow speed. Results showed that the enthalpy-entropy compensation was influenced by both DNA strand length and flow speed, and the penalties of enthalpy were 2-12% greater than the benefits of entropy.     

Hydrophobic interaction in water-ethanol mixtures

The solubility, free energy, entropy, and enthalpy of solutions of methane and ethane were determined in various mixtures of water and ethanol. The hydrophobic interaction for the pair methane-methane was computed, in these mixtures, using an approximate expression derived previously. Also the entropy and the enthalpy associated with the hydrophobic interaction were evaluated from the experimental data. The main finding is the steep decrease in the strength of the hydrophobic interaction as the mixture becomes richer in alcohol. The variation of the corresponding entropy and enthalpy also shows an abrupt change at the composition ofx _ethanol0.2. Some interpretations of these findings in terms of structural changes in the solvent are discussed.     

Derivatization of fatty alcohol ethoxylate non‐ionic surfactants using 2‐sulfobenzoic anhydride for characterization by liquid chromatography/mass spectrometry

A derivatization procedure has been developed for the improved characterization of fatty alcohol ethoxylate non‐ionic surfactants by liquid chromatography/mass spectrometry. The end hydroxyl group of each surfactant species was converted into an oxycarbonylbenzene‐2‐sulfonic acid group with 2‐sulfobenzoic anhydride under mild conditions. The produced sulfonic acid group allows all species, including fatty alcohols and those with less than three ethoxylates, to be uniformly ionized by electrospray ionization (ESI) mass spectrometry. Both acid and base can be used as a mobile phase additive for liquid chromatography without affecting M_n and average ethoxylate values, although ion intensities are suppressed during the ESI process. The method was used to analyze seven commercial fatty alcohol ethoxylate non‐ionic surfactants, and the determined M_n and EO values were comparable with the results obtained by NMR. The relative ratio of different fatty alcohol based ethoxylates in a sample can also be determined using the summed mass spectral data. Copyright © 2009 The Dow Chemical Company