Linear Solvation Energy Relationship Analysis Research Articles

Preparation and evaluation of a pyridine sulfonate betaine-based zwitterionic stationary phase for hydrophilic interaction chromatography

A zwitterionic stationary phase comprising pyridinium cations and sulfonate anions was successfully developed through thiol-ene click chemistry. Using seven polar small molecules as probes, the zwitterionic stationary phase showed high separation selectivity and excellent column efficiency (35,200–54,800 plates/m) compared with two commercial columns. The influence of water proportion, salt concentration, and pH in the mobile phase, and column temperature, on the retention of six polar compounds was examined. The retention mechanism was explored by three hydrophilic retention models, Tanaka test and linear solvation energy relationship analysis. For the analysis of sample dairy products (milk powder, milk, and yogurt), the stationary phase was operated in hydrophilic interaction chromatography mode without the addition of buffer salts, facilitating rapid and efficient detection and quantification of melamine. The LOD and LOQ are 0.04 mg⋅g−1 and 0.13 mg⋅g−1, respectively, and the recovery rate is 90.3 − 102.8 %. The zwitterionic stationary phase has the advantages of simple preparation, good method reproducibility, good selectivity and high precision.

Contribution from non-ideality and preferential solvation to non-linear solvatochromism in binary mixtures

The knowledge of preferential solvation is of critical importance in description of the solute behavior in mixed solvents. The analysis of solvatochromic shifts as a function of solvent composition is popular in this regard, since it gives molecular information on solute-solvent interactions. However, due to the complexity of the interactions and possible non-ideality of mixed solvents, implementing an efficient approach for interpretation of solvent-solvent interactions is an opening major challenge in modeling solvatochromism to get reliable results on preferential solvation. Here, we proposed a new methodology to treat solvatochromism in solvent mixtures in order to reach a more reliable picture of the local composition around the solute. As the main idea, the effect of solvent-solvent interactions on spectral shift was considered by formulating the non-ideality of solvent in solvation shell of the solute as a function of the local composition by exploiting the results obtained from the analysis of the bulk mixture non-ideality. From that, the non-linear variation in spectral shift can be well separated into contributions from preferential solvation and the local non-ideality. To test this approach, solvatochroimsm of 2-methyl-4-nitroaniline was studied in some mono-solvents and binary mixtures. The spectral properties of 2-methyl-4-nitroaniline and its response to solvent were rationalized by quantum chemical calculation and linear solvation energy relationships analysis in mono-solvents. Then, the observed strong non-linear solvatochromism of 2-methyl-4-nitroaniline in aqueous mixtures of ethanol, DMF and DMSO was treated by the proposed model that provided both physically and statistically more reliable results on preferential solvation and molecular interactions, with respect to the other widely-used solvent exchange models. As an interesting and important result, non-ideality, but not preferential solvation, was responsible for non-linear solvatochromism in aqueous solutions of ethanol and DMF.

5-Nitrosalicylaldehyde in aqueous co-solvent mixtures of methanol, ethanol, isopropanol and acetonitrile: Solubility determination, solvent effect and preferential solvation analysis

The equilibrium 5-nitrosalicylaldehyde solubility in methanol (1) + water (2), ethanol (1) + water (2), isopropanol (1) + water (2) and acetonitrile (1) + water (2) mixtures was determined by a shake-flask method from 273.15 K to 313.15 K under local pressure of 101.2 kPa. At the same mass fraction of methanol (ethanol, isopropanol or acetonitrile) and temperature, the mole fraction solubility of 5-nitrosalicylaldehyde was greater in (acetonitrile + water) mixture than in ethanol (1) + water (2), isopropanol (1) + water (2) and methanol (1) + water (2) mixtures. The relative importance of solute–solvent and solvent–solvent interactions upon the solubility variation was valued by using the linear solvation energy relationship analysis of solvent effect. The method of inverse Kirkwood–Buff integrals was used to study the preferential solvation on the basis of thermodynamic solution properties. The preferential solvation parameters for methanol, ethanol, isopropanol and acetonitrile were positive in the four mixtures in co-solvent-rich and intermediate composition, indicating that 5-nitrosalicylaldehyde was solvated preferentially by the co-solvent. It is conjecturable that in these regions 5-nitrosalicylaldehyde acts as a Lewis acid with the alcohols’ molecules; while for the acetonitrile + water mixture, the preferential solvation could be attributed to polarization effects. Moreover, the drugs’ solubility was mathematically represented via the Jouyban-Acree model acquiring the average relative deviations<6.03%.

Thermodynamic solubility modelling, solvent effect and preferential solvation of p-nitrobenzamide in aqueous co-solvent mixtures of dimethyl sulfoxide, ethanol, isopropanol and ethylene glycol

The equilibrium p-nitrobenzamide solubility in mixtures of dimethyl sulfoxide (DMSO, 1) + water (2), ethanol (1) + water (2), isopropanol (1) + water (2) and ethylene glycol (EG, 1) + water (2) was determined with a shake-flask method from (293.15 to 328.15) K under atmospheric pressure (101.1 kPa). At the same temperature and mass fraction of DMSO (ethanol, isopropanol or EG), the mole fraction solubility of p-nitrobenzamide was greater in (DMSO + water) than in ethanol (1) + water (2), isopropanol (1) + water (2) and ethylene glycol (EG, 1) + water (2) mixtures. X-Ray power diffraction analysis of solid phase showed no polymorphic transformation or solvate formation in experiments. The degree of relative importance of solute–solvent and solvent–solvent interactions on the solubility variation was evaluated by the linear solvation energy relationship analysis of solvent effect. The inverse Kirkwood–Buff integrals was used to study the preferential solvation based on thermodynamic solution properties. The parameters of preferential solvation for ethanol and isopropanol were positive in the ethanol (1) + water (2) and isopropanol (1) + water (2) mixtures in co-solvent-rich and intermediate composition, indicating that p-nitrobenzamide was solvated preferentially by the co-solvent. p-Nitrobenzamide acted mainly as a Lewis acid interacting with proton-acceptor functional groups of the co-solvents. Moreover, the drugs’ solubility was represented mathematically via the Jouyban-Acree model attaining the average relative deviations smaller than 2.16%.

LSER analysis of reactive extraction of Zn and Cu with bis (2-ethylhexyl) phosphate (HDEHP) in six different diluents

ABSTRACTAn equilibrium study on the reactive extraction of Zn and Cu from aqueous solution using bis(2-ethylhexyl) phosphate (HDEHP) in six different diluents including n-hexane, n-heptane, benzene, toluene, chloroform and dichloromethane (DCM) was carried out. Additionally, the extraction equilibrium results were correlated with a linear solvation energy relationship (LSER) model. The extraction efficiency of HDEHP in terms of distribution coefficient was found to vary in the order of diluents as DCM> chloroform> toluene> benzene> n-heptane> n-hexane. The highest value of the distribution coefficient, KD, was observed with DCM as 21.55 (Zn) and 18.97 (Cu). The experimental data were correlated with the LSER model, and a close resemblance was observed between them.

Experimental solubility evaluation and thermodynamic analysis of biologically active D-tryptophan in aqueous mixtures of N,N-dimethylformamide and several alcohols

The D-tryptophan solubility in aqueous solutions of methanol, ethanol, isopropanol and N,N-dimethylformamide (DMF) was measured by the saturation shake-flask method at temperatures ranging from 283.15 K to 333.15 K. The maximum solubility was found in water for alcohol (1) + water (2) mixtures, and in neat DMF for DMF (1) + water (2) mixture. When the same temperature and mass fraction of the water is given, the highest mole fraction solubility of D-tryptophan was found in (DMF + water). The solid phases were tested by X-ray powder diffraction (XPRD) showing no polymorphic transformation, solvate formation or crystaltransition. Three solubility models, i.e. the Jouyban-Acree, van’t Hoff-Jouyban-Acree and Apelblat-Jouyban-Acree were employed to correlate the experimental solubility values so as to obtain thermodynamic parameters. The RAD and RMSD data were no more than 1.65% and 9.40 × 10−5, respectively. Linear solvation energy relationship analysis based on Kamlet-Taft parameters indicated that the variation of D-tryptophan solubility depended on π∗ for methanol (ethanol, isopropanol) (1) + water (2), and cavity term for DMF (1) + water (2). The preferential solvation parameters (δx1,3) for methanol, ethanol, isopropanol and DMF derived by means of the inverse Kirkwood–Buff integrals were negative in the four solvent mixtures with intermediate and alcohol-rich compositions, which indicated that D-tryptophan was preferentially solvated by water. The higher solvation by water could be explained in terms of the higher acidic behaviour of the solvents interacting with the Lewis basic groups of the D-tryptophan. However D-tryptophan is preferentially solvated neither by water nor by DMF in DMF + water mixture.

Determination of lipid bilayer affinities and solvation characteristics by electrokinetic chromatography using polymer-bound lipid bilayer nanodiscs.

Styrene-maleic acid polymer-bound lipid bilayer nanodiscs have been investigated and characterized by electrokinetic chromatography. Linear solvation energy relationship analysis was employed to characterize the changes in solvation environment of nanodiscs of varied belt to lipid ratio, belt polymer chemistry and molecular weight, and lipid composition. Increases in the lipid to belt polymer ratio resulted in smaller, more cohesive nanodiscs with greater electrophoretic mobility. Nanodisc structures with belt polymers of different chemistry and molecular weight were compared and showed only minor changes in solvent characteristics and selectivity consistent with changes in structure of the lipid bilayer. Seven phospholipid and sphingomyelin nanodiscs of different lipid composition were characterized. Changes in lipid head group structure had a significant effect on bilayer-solute interactions. In most cases, changes in alkyl tail structure had no discernible effect on solvation environment aside from those explained by changes in the gel-liquid transition temperature. Comparison to vesicles of similar lipid composition show only minor differences in solvation environment, likely due to differences in lipid composition and bilayer curvature. Together these results provide evidence that the dominant solute-nanodisc interactions are with the lipid bilayer and that head group chemistry has a greater impact on bilayer-solute interactions than alkyl tail or belt polymer structure. Nanodisc electrokinetic chromatography is demonstrated to allow characterization of solute interactions with lipid bilayers of varied composition.

Solvent- and temperature-controlled inversion of π-facial selectivity in the 1,2-cycloaddition of singlet oxygen to hydroxyphenyl-substituted cyclohexadihydrofurans

Singlet-oxygenation of 3-hydroxyphenyl-substituted dihydrofurans fused with a cyclohexane 1a−c exclusively gave the corresponding syn/anti-stereoisomeric mixtures of dioxetanes 2a−c. The syn/anti-π-facial selectivity in the 1,2-cycloaddition of singlet oxygen (1O2) was found to be remarkably sensitive to the solvent as well as the reaction temperature. In fact, the solvent effect was so conspicuous that inversion of the syn/anti-π-facial selectivity was observed in different solvents, such as chloroform and toluene. An LSER (linear solvation energy relationships) analysis suggested that the Lewis-acidity/basicity and HBD (hydrogen-bond donor)/HBA (hydrogen-bond acceptor) ability as well as dipolarity/polarizability of the solvent played an important role in this change in syn/anti-π-facial selectivity. An investigation of the temperature-dependency of the singlet-oxygenation suggested that the syn/anti-π-facial-selective 1,2-cycloaddition of 1O2 to 1 was a conformationally-(entropy-) controlled process.

Phospholipid bilayer affinities and solvation characteristics by electrokinetic chromatography with a nanodisc pseudostationary phase.

Phospholipid bilayer nanodiscs composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and synthetic maleic acid-styrene copolymer belts have been introduced as a pseudostationary phase (PSP) in electrokinetic chromatography and demonstrated good performance. The nanodiscs provide a suitable migration range and high theoretical plate counts. Using this nanodisc pseudostationary phase, the affinity of the bilayer structure for probe solutes was determined and characterized. Good correlation is observed between retention factors and octanol water partition coefficients for particular categories of solutes, but the general correlation is weak primarily because the nanodiscs show stronger affinity than octanol for hydrogen bond donors. This suggests that a more appropriate application of this technology is to measure and characterize interactions between solutes and lipid bilayers directly. Linear solvation energy relationship analysis of the nanodisc-solute interactions in this study demonstrates that the nanodiscs provide a solvation environment with low cohesivity and weak hydrogen bond donating ability, and provide relatively strong hydrogen bond acceptor strength.

Quantitative structure-retention relationship of selected imidazoline derivatives on α1-acid glycoprotein column

The retention behaviour of 22 selected imidazoline drugs and derivatives was investigated on α1-acid glycoprotein (AGP) column using Sørensen phosphate buffer (pH 7.0) and 2-propanol as organic modifier. Quantitative Structure-Retention Relationships (QSRR) models were built using extrapolated logkw values as well as isocratic retention factors (logk5, logk8, logk10, logk12, logk15 obtained for 5%, 8%, 10%, 12%, and 15%, of 2-propanol in mobile phase, respectively) as dependant variables and calculated physicochemical parameters as independant variables. The established QSRR models were built by stepwise multiple linear regression (MLR) and partial least squares regression (PLS). The performance of the stepwise and PLS models was tested by cross-validation and the external test set prediction. The validated QSRR models were compared and the optimal PLS-QSRR model for logkw and each isocratic retention factors (PLS-QSRR(logk5), PLS-QSRR(logk8), PLS-QSRR(logk10), MLR-QSRR(logk12), MLR-QSRR(logk15)) were selected. The QSRR results were further confirmed by Linear Solvation Energy Relationships (LSER). LSER analysis indicated on hydrogen bond basicity, McGowan volume and excess molar refraction as the most significant parameters for all AGP chromatographic retention factors and logkw values of 22 selected imidazoline drugs and derivatives.

Electrokinetic chromatographic properties of amphiphilic copolymer poly (styrene-co-methacrylic acid) self-assembled micelle pseudostationary phase

The amphiphilic copolymer poly (styrene-co-methacrylic acid) (P (St-co-MAA)) with molar ratios of 6:4 and 7:3 self-assembled to form micelles. The polymeric micelles were used as pseudostationary phase (PSP) in micellar electrokinetic chromatography ( MEKC). Their physicochemical properties and MEKC performance were investigated as well in the present work. The critical micelle concentration ( CMC) , polarity, surface charge density and hydrodynamic diameter were used to characterize the solution physicochemical properties, while the methylene group selectivity was evaluated with n-alkylphenone homologous series. The time window and linear solvation energy relationship (LSER) analysis were used to characterize the MEKC retention behavior and the selectivity. All of these were compared with poly (methyl methacrylate-co-methacrylic acid) (P (MMA-co-MAA)) with the molar ratio of 7:3 and sodium dodecyl sulfate (SDS) micellar systems. The results showed that P ( St-co-MAA) system had the minimum CMC, the widest time window and the best methylene group selectivity. LSER analysis results showed that the hydrophobic effect was the most important interaction between solutes and PSPs, and the hydrogen-bonding acidity was the second significant factor on selectivity and MEKC retention behavior. P (St-co-MAA) system, especially with the molar ratio of 7 :3, had the highest effective parameter in LSER and showed a high separation selectivity of PSP.

Electrokinetic chromatographic characterization of novel catanionic surfactants vesicle as pseudostationary phase.

A novel catanionic surfactants vesicle system composed of octyltriethylammonium bromide/ sodium dodecyl benzene sulfonate (C8 NE3 Br/SDBS) has been developed as pseudostationary phase (PSP) in EKC. The C8 NE3 Br/SDBS system possesses a large vesicle phase region and none of agglomeration phenomena appeared while mixing cationic and anionic surfactants at any molar ratio. Electrophoretic and chromatographic parameters including elution window, hydrophobic selectivity, polar group selectivity, and shape selectivity were characterized using the vesicle at molar ratio of C8 NE3 Br to SDBS of 3:7 as PSP. Compared with SDS micelles, the vesicle PSP possessed a wider elution window and a better selectivity. The retention behavior and selectivity differences between the novel vesicle and SDS micelles were evaluated through linear solvation energy relationship (LSER) analysis. Though the cohesiveness and the hydrogen bond acidity have greatest influences on the solutes retention and selectivity in both the vesicle and SDS micelle, the vesicle PSP demonstrated a higher hydrophobicity and a lower hydrogen bonding donating capability owing to compact bilayer structure of vesicle. Additionally, the vesicle system had a stronger hydrogen bond accepting capability than SDS micelle. Consequently, according to LSER analysis, the bigger coefficients for v, b, and a revealed the vesicle PSP had a better separation selectivity than conventional SDS micelle.

Selectivity of Brij-35 in Micellar Liquid Chromatographic Separation of Positional Isomers

Implementation of Brij-35, a nonionic surfactant, as a mobile phase for separation of positional isomers is investigated. Chromolith C-18 SpeedROD is used as a stationary phase. The effect of surfactant and organic modifier (propanol) concentration on the separation of some selected isomers is studied and evaluated in terms of linear solvation energy relationship (LSER). Shape selectivity is assessed by α value of sorbic and benzoic acid, which is found to be 1.339 by using mobile phase composed of 0.5% aqueous solutions of Brij-35 and propanol in 9 : 1. Isomers of parabens, nitroanilines, nitrophenols, and quinolinols are successfully separated using mobile phases composed of various percentages of surfactant and propanol. System constants for nonionic MLC using LSER analysis show that hydrogen bond basicity and dipolarity may be major contributors to selectivity, while excess molar refraction helps fine-tuning the separation which also imparts unique selectivity to nonionic surfactants as compared to ionic ones.

Retention behavior and selectivity of a latex nanoparticle pseudostationary phase for electrokinetic chromatography

The retention characteristics and separation selectivity of a novel latex nanoparticle (NP) pseudostationary phase (PSP) for electrokinetic chromatography have been characterized. The anionic NPs have very low or no affinity for cationic solutes, but show significant interactions and retention based on hydrophobic interactions. Retention factors of alkyl-phenyl ketones increase linearly with the concentration of the NPs and have zero or near zero y-intercepts as expected for electrokinetic chromatography with non-micellar PSPs. The retention factors of these solutes and representative pharmaceuticals decrease logarithmically with increases in the concentration of ACN in the background electrolyte, as expected for reversed-phase retention. Linear solvation energy relationship analysis indicates that the NPs are less cohesive than would be expected for polymeric PSPs with similar structure but that the overall separation selectivity can be expected to be similar to polymer PSPs with similar backbone chemistry. The results indicate that the hydrophobic core of the NPs is non-cohesive and is highly accessible to solutes, whereas the ionic head groups are not as accessible and do not contribute substantially to retention or selectivity.

Relative strength of the intramolecular hydrogen bonding in 1‐phenylazo‐naphthalen‐2‐ol and 1‐phenyliminomethyl‐naphtahlen‐2‐ol

AbstractThe relative strength of the intramolecular hydrogen bonding (IHB) in the title compounds has been investigated by means of ab initio quantum chemical calculations, UV–Vis spectral study and linear solvation energy relationship analysis (LSER) of the obtained tautomeric constants. It has been found that in acetone (and in all used solvents with substantial proton acceptor abilities) the tautomeric constant in the azonaphthol compound is lower than could be expected. The fact is explained with the breakage of the IHB and the coexistence of closed and open enol tautomers. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Characterization of surface-confined ionic liquid stationary phases: impact of cation and anion identity on retention

A series of surface-confined ionic liquid (SCIL) stationary phases for high-performance liquid chromatography were synthesized in-house. The synthesized phases were characterized by the linear solvation energy relationship (LSER) method to determine the effect of residual linking ligands and the role of the cation and the anion on retention. Statistical analysis was utilized to determine whether the system coefficients returned from multiple linear regression analysis of chromatographic retention data for a set of 28 neutral aromatic probe solutes were significantly different. Examination of the energetics of retention via kappa-kappa plots agrees with the results obtained from the LSER analysis. Residual linking ligands were determined to contribute reversed-phase-type retention character to the chromatographic system. Furthermore, retention on the SCIL phases was observed to be more profoundly affected by the identity of the anion than by that of the cation.

Evaluating coal tar–water partitioning coefficient estimation methods and solute–solvent molecular interactions in tar phase

Equilibrium partitioning coefficients between an industrial coal tar sample and water ( K CT/w) were determined for 41 polar and nonpolar solutes in batch systems. Together with literature values, 69 K CT/w data were analyzed using the following model approaches: Raoult’s law, the single parameter linear free energy relationship (SPLFER) with octanol–water partitioning coefficients ( K ow), the linear solvation energy relationships (LSERs), SPARC and COSMOtherm. Estimations by Raoult’s law and the SPLFER agreed well with the experimental log K CT/w values for the investigated coal tar, with root mean square errors (RMSE) of 0.31 and 0.33, respectively. LSER resulted in as good estimations (RMSE = 0.29) as the previous two. The LSER analysis revealed significant hydrogen (H)-bond acceptor properties of the studied coal tar phase. Using naphthalene as a surrogate solvent for the coal tar phase, SPARC and COSMOtherm provided fairly good predictions (RMSE of 0.63 and 0.65, respectively) of log K CT/w, without any additional empirical parameter. Further calculations using SPARC and COSMOtherm for partitioning between water and other tar-components (e.g., benzofuran, phenol and quinoline) suggested that minor components in coal tar do not significantly influence K CT/w of nonpolar solutes, and that Raoult’s law and the SPLFER thus may be generally applied to these types of solutes, e.g., polycyclic aromatic hydrocarbons and alkylbenzenes, regardless of coal tar compositions. In contrast, partitioning of H-bonding solutes (e.g., phenols) can significantly vary depending on the amount of polar tar-components such as N-heterocyclic aromatic compounds. Therefore, the presented successful applications of Raoult’s law and SPLFER to the studied coal tar could be a special case, and these simple approaches may not provide reasonable estimations for partitioning of H-bonding solutes from compositionally different coal tars.

Characterization of surfactant and phospholipid vesicles for use as pseudostationary phases in electrokinetic chromatography

The physical, electrophoretic and chromatographic properties (mean diameter, electroosmotic flow, electrophoretic mobility, elution range, efficiency, retention, and hydrophobic, shape, and chemical selectivity) of three surfactant vesicles and one phospholipid vesicle were investigated and compared to a conventional micellar pseudostationary phase comprised of sodium dodecyl sulfate (SDS). Chemical selectivity (solute-pseudostationary phase interactions) was discussed from the perspective of linear solvation energy relationship (LSER) analysis. Two of the surfactant vesicles were formulated from nonstoichiometric aqueous mixtures of oppositely charged, single-tailed surfactants, either cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) in a 3:7 mole ratio or octyltrimethylammonium bromide (OTAB) and SDS in a 7:3 mole ratio. The remaining surfactant vesicle was comprised solely of bis(2-ethylhexyl)sodium sulfosuccinate (AOT) in 10% v/v methanol, and the phospholipid vesicle consisted of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) and phosphatidyl serine (PS) in 8:2 mole ratio. The mean diameters of the vesicles were 76.3 nm (AOT), 86.9 nm (CTAB/SOS), 90.1 nm (OTAB/SDS), and 108 nm (POPC/PS). Whereas the coefficient of electroosmotic flow (10(-4) cm2 V(-1) s(-1)) varied considerably (1.72 (OTAB/SDS), 3.77 (CTAB/SOS), 4.05 (AOT), 5.26 (POPC/PS), 5.31 (SDS)), the electrophoretic mobility was fairly consistent (-3.33 to -3.87 x 10(-4) cm2 V(-1) s(-1)), except for the OTAB/SDS vesicles (-1.68). This resulted in elution ranges that were slightly to significantly larger than that observed for SDS (3.12): 3.85 (POPC/PS), 8.6 (CTAB/SOS), 10.1 (AOT), 15.2 (OTAB/SDS). Significant differences were also noted in the efficiency (using propiophenone) and hydrophobic selectivity; the plate counts were lower with the OTAB/SDS and POPC/PS vesicles than the other pseudostationary phases (< or = 75,000/m vs. > 105,000/m), and the methylene selectivity was considerably higher with the CTAB/SOS and OTAB/SDS vesicles compared to the others (ca. 3.10 vs. < or = 2.6). In terms of shape selectivity, only the CTAB/SOS vesicles were able to separate all three positional isomers of nitrotoluene with near-baseline resolution. Finally, through LSER analysis, it was determined that the cohesiveness and hydrogen bond acidity of these pseudostationary phases have the greatest effect on solute retention and selectivity.

Retention behavior of hydrophobic organic chemicals as a function of temperature in soil leaching column chromatography.

To study the transport mechanism of hydrophobic organic chemicals (HOCs) and the energy change in soil/solvent system, a soil leaching column chromatographic (SLCC) experiment at an environmental temperature range of 20-40 degrees C was carried out, which utilized a reference soil (SP 14696) packed column and a methanol-water (1:4 by volume ratio) eluent. The transport process quickens with the increase of column temperature. The ratio of retention factors at 30 and 40 degrees C (k'30/k'40) ranged from 1.08 to 1.36. The lower enthalpy change of the solute transfer in SLCC (from eluent to soil) than in conventional reversed-phase liquid chromatography (e.g., from eluent to C18) is consistent with the hypothesis that HOCs were dominantly and physically partitioned between solvent and soil. The results were also verified by the linear solvation energy relationships analysis. The chief factor controlling the retention was found to be the solute solvophobic partition, and the second important factor was the solute hydrogen-bond basicity, while the least important factors were the solute polarizability-dipolarity and hydrogen-bond acidity. With the increase of temperature, the contributions of the solute solvophobic partition and hydrogen-bond basicity gradually decrease, and the latter decreases faster than the former.

Effect of class I and II organic modifiers on retention and selectivity in vesicle electrokinetic chromatography.

Vesicles are large aggregates of surfactant monomers consisting of a spherical bilayer surrounding an internal cavity of solvent. The bilayer structure allows vesicles to be attractive models for the study of various transmembrane and binding processes. The use of thermodynamically stable vesicles (TSV) formed from oppositely charged surfactants for use as a pseudostationary phase in electrokinetic chromatography (EKC) was first accomplished using dodecyltrimethylammonium bromide and sodium dodecyl sulfate (DTAB/SDS). Surfactant vesicles have demonstrated enhanced separation characteristics compared to conventional micelles in EKC, although only investigated in aqueous media. Organic modifiers have been widely studied and used in EKC to enhance separation conditions. In this study, vesicles formed from cetyltrimethylammonium bromide and sodium octyl sulfate (CTAB/SOS) were investigated in the presence of "class I and II" organic modifiers. Electrophoretic and chromatographic parameters were examined as well as linear solvation energy relationship analysis (LSER) to characterize the effects of the modifiers on retention and selectivity in EKC. LSER analysis is a useful way to quantitatively investigate solute/solvent interactions responsible for retention and selectivity.