Mobile phase effects in reversed-phase liquid chromatography: a comparison of acetonitrile/water and methanol/water solvents as studied by molecular simulation

Molecular simulations of water/acetonitrile and water/methanol mobile phases in contact with a C(18) stationary phase were carried out to examine the molecular-level effects of mobile phase composition on structure and retention in reversed-phase liquid chromatography. The simulations indicate that increases in the fraction of organic modifier increase the amount of solvent penetration into the stationary phase and that this intercalated solvent increases chain alignment. This effect is slightly more apparent for acetonitrile containing solvents. The retention mechanism of alkane solutes showed contributions from both partitioning and adsorption. Despite changes in chain structure and solvation, the molecular mechanism of retention for alkane solutes was not affected by solvent composition. The mechanism of retention for alcohol solutes was primarily adsorption at the interface between the mobile and stationary phase, but there were also contributions from interactions with surface silanols. The interaction between the solute and surface silanols become very important at high concentrations of acetonitrile.     

Surface modification of III–V semiconductors: chemical processes and electronic properties

The possibilities of controlling the surface electronic properties of III–V semiconductors by varying the adsorption chemistry are analyzed. Variations of the adsorption process parameters and the adsorbate reactivity are able to affect the surface atomic and electronic structure of the semiconductor. The adsorbate reactivity is considered within the framework of the density functional theory using the reactivity indices. The easiest way to affect the reactivity of a particular adsorbate is to create a solvation shell around it, as is possible in both liquid solutions and the gas phase (microsolvation). Solvation of ions before their adsorption by different solvents affects considerably the relative nucleophility of the central atom in the ion, which results in a different charge transfer mechanism from the surface states on adsorption, and thus, in a different modification of the surface electronic structure of the semiconductor. The effect of halogen, sulfur and metal atoms reactivity on the electronic structure of the resulting adsorbate-covered surface of III–V semiconductor is discussed.     

HPLC of synthetic polymers characterization of polystyrenes by high performance precipitation liquid chromatography (HPPLC)

Summary The advantages and disadvantages of high performance precipitation liquid chromatography have been demonstrated for polystyrene homopolymers. Depending on the mobile phase composition at the dissolution point of the polymeric sample and surface properties of the stationary phase, elution is governed either by a solution process or by adsorption. A contribution by adsorption was noticed on silica as well as on reversed phases based on silica with a normal phase gradient of increasing polarity (heptane to dichloromethane). Elution was solely governed by solubility of the polymers on both types of stationary phase for polystyrenes with a molecular weight above 35 000 and reversed phase gradient of decreasing polarity (methanol to dichloromethane). Under these conditions an identical dependence of elution solvent composition on sample size was found as for turbidity titrations. Due to differences in the velocity of the eluent front and the polymeric sample with porous stationary phases the polymers can be eluted as colloidal solutions Non-porous stationary phases are superior in this respect because the velocities of eluent and solutes are identical.     

Retention equilibrium and mass transfer characteristics in reversed-phase liquid chromatography using methanol-water mixtures

Pulse response experiments (i.e., elution chromatography) were made in reversed-phase liquid chromatography (RPLC) using a C18 silica gel column and methanol-water mixtures of different compositions (phi). The moment analysis of the elution peak profiles measured in the RPLC system provided some items of information about four parameters characterizing the retention equilibrium and the mass transfer kinetics in the column, i.e., adsorption equilibrium constant, isosteric heat of adsorption, surface diffusion coefficient and activation energy of surface diffusion. Characteristics of the chromatographic behavior were studied by analyzing the dependence of the four parameters on phi and the correlation between them. It was found that surface diffusion was one of the important processes of molecular migration having a significant contribution to the mass transfer kinetics in the column. Both the adsorption equilibrium constant and the surface diffusion coefficient varied depending on phi. The direction of their changes was approximately opposite, suggesting that the mass transfer in the manner of surface diffusion was restricted owing to the retention of the sample molecules on the stationary phase.     

Dependence of Elution Curve and Adsorption Isotherms of Insulin on composition of Mobile Phase of Frontal Analysis in Reversed Phase Liquid Chromatography

With frontal analysis(FA),the dependence of adsorption isotherms of insulin on the composition of mobile phase in reversed phase liquid chromatography (RPLC) has been investigated,This is also a good example to employ the stoichiometric displacement theory (SDT) for ivestigating solute adsorption in physical chemistry.Six kinds of mobile phase in RPLC were employed to study the effects on the elution curves and adsorption isotherms of insulin.the key points of this paper are:(1) the stability of insulin due to delay time after preparing,the organic solvent concentration,the kind and the concentration of ion-pairing agent in mobile phase were found to affect both elution curve and adsorption isotherm very seriously.(2)To obtain a valid and comparable result,the composition of the mobile phase employed in FA must be as same as possible to that in usual RPLC of either analytical scale or preparative purpose.(3)Langmuir Equation and the SDT were employed to imitate these obtained adsorption isotherms.The expression for solute adsorption from solution of the SDT was found to have a better elucidation to the insulin adsorption from mobile phase in RPLC.     

Solvent effects on the retention of oligosaccharides in porous graphitic carbon liquid chromatography

Porous graphitic carbon (PGC) is known as well suited adsorbent for liquid chromatography of carbohydrates. In this work we report on systematic investigations of solvent effects on the retention mechanism of fluorescence labeled malto-oligosaccharides on PGC. The adsorption mechanism was found to depend on the type of organic modifier used in the mobile phase. Positive adsorption enthalpies and entropies, which have already been reported in the literature, were solely produced using acetonitrile. Both alternative solvents (tetrahydrofuran, 2-propanol) yielded in contrast negative enthalpies. As plausible retention mechanism for oligosaccharides on PGC applying acetonitrile as mobile phase component we propose the formation of a dense and highly ordered solvation layer of the PGC surface with the linear acetonitrile molecules. Adsorption of analyte molecules requires a displacement of numerous acetonitrile molecules, which explains the positive enthalpy and entropy values measured. The interplay of enthalpic and entropic contributions to the overall adsorption phenomena results in strongly temperature dependent chromatographic selectivity values.     

Study of solvation processes on cholesterol bonded phases

Four cholesterol bonded phases with different structures were investigated. The columns studied were packed with stationary phase containing cholesterol attached to the silica surface using different types of linkage molecules. The presence of the polar amino and carboxyl groups in the structure of the bonded ligand strongly influence on the solvation process. The possibility of hydrogen bonding, dipole-dipole and π-π electron interactions lead to preferential solvation of bonded ligands. The coverage density of bonded ligands and length of the linkage strongly influence the adsorption of solvent from the mobile phase. The removal of residual silanols during the hydrosilation procedure significantly influences solvation of the bonded phase. Excess isotherms of the commonly used solvents in RP HPLC (methanol and acetonitrile) were obtained using the minor disturbance method. For comparison of the stationary phases prepared on different silica gels the excess adsorbed amounts were calculated per volume of the stationary phase in the column. The hydrosilated UDC Cholesterol bonded phase is preferentially solvated by methanol whereas the highest coverage Cosmosil Cholester phase exhibit high adsorption of acetonitrile. Polar groups in the Amino-cholesterol type bonded phase are solvated with both solvent but the mechanisms of these processes are different.     

A new method of thin-layer chromatography with controlled vapor phase

A new version of thin-layer chromatography with controlled vapor phase is described. It has no analogues in planar and liquid chromatography. The method is based on a dynamic change in the physicochemical properties of a chromatographic system as a result of the contact of the plate bearing analytes with a vapor phase of certain composition created in the chromatographic chamber. The active vapor phase is created either by pumping a gas through the chromatographic chamber or by introducing a solvent vapor from another part of the chamber. The composition and properties of the mobile phase are controlled immediately in the course of separation by means of absorption or adsorption of one or another gas with the mobile or stationary phase. By the examples of benzoic acids and nitroaniline isomers in the presence of carbon oxide and vapors of acetic acid, ammonia, and ethanol, it was shown that this procedure utilizes the difference in the protolytic and solvation properties of adsorbates for changing the adsorption equilibrium and the selectivity and efficiency of separation.     

Adsorption of naproxen enantiomers from solutions on chemically modified cellulose. The effect of a polar component of a liquid phase

The influence of mobile phase composition (hexane-ethanol with the addition of 0.1 vol.% of trifluoroacetic acid) on the adsorption of the naproxen enantiomers on a chiral adsorbent Chiralcel OJ-H was investigated. Methods of linear and nonlinear chromatography were used to investigate adsorption thermodynamics at infinite dilution and to measure adsorption isotherms for solutions containing 10 and 20 vol.% of ethanol. It was found that two groups of adsorption sites coexist on the surface of the adsorbent in contact with the mentioned solvent, strong and weak ones, with the strong adsorption sites contributing mainly to the adsorption of naproxen. The effect of ethanol content on the retention of naproxen consists of the change in the number of the strong adsorption sites and regulating solvation property of a solvent.     

Naphthalene adsorption on activated carbons using solvents of different polarity

The hydrophobic-hydrophilic character of a series of microporous activated carbons was explored as a key factor in competitive adsorption of a non-polar compound from liquid phase. The selectivity of the carbon surface towards naphthalene was explored by performing the adsorption isotherms in water, cyclohexane and heptane. Solvent polarity and adsorbent hydrophobic character were found to strongly influence the adsorption capacity of naphthalene. In aqueous media, despite the non-polar character of the adsorbate, surface acidity lowered adsorption capacity. This is attributed to the competition of water from the adsorption sites, via H-bonding with surface functionalities and the formation of hydration clusters that reduce the accessibility and affinity of naphthalene to the inner pore structure. In organic media the uptake decreased due to competition of the hydrophobic solvent for the active sites of the carbon and to solvation effects. This competitive effect of the solvent is minimized in oxidized carbons as opposed to the trend obtained in aqueous solutions. The results confirmed that although adsorption of naphthalene strongly depends on the narrow microporosity of the adsorbent, competitive adsorption of the solvent for the active sites becomes important.     

Segue between favorable and unfavorable solvation

Solvation of small and large clusters are studied by simulation, considering a range of solvent-solute attractive energy strengths. Over a wide range of conditions, both for solvation in the Lennard-Jones liquid and in the SPC model of water, it is shown that the mean solvent density varies linearly with changes in solvent-solute adhesion or attractive energy strength. This behavior is understood from the perspective of Weeks' theory of solvation (Annu. Rev. Phys. Chem. 2002, 53, 533) and supports theories based upon that perspective.     

Solvation of perfluorooctane and octane in water, methanol, acetonitrile, and aqueous mixtures of methanol and acetonitrile

Molecular dynamics simulations are used to examine the local solvation structure of single octane and perfluorooctane molecules in liquid water, methanol, acetonitrile, and aqueous mixtures of methanol and acetonitrile. The motivation is to obtain baseline information about the solvation of perfluorooctane by liquids used as the mobile phase in liquid chromatography and how it differs from the solvation of octane. While octane is uniformly solvated by both water and the second component, perfluorooctane is solvated by methanol and acetonitrile with the exclusion of water from the first solvation layer when the solvent is a mixture.     

Determination of the accessible volume and the interaction parameter in the adsorption mode of liquid chromatography

The main physical parameters in liquid chromatography of oligomers-the accessible volume and the adsorption interaction parameter-are discussed. It is shown, that in liquid adsorption chromatography (LAC) there is a linear relation between elution volume and the distance of two subsequent peaks of a homologous series. From the intercept of the regression lines in such a plot the accessible volume can be easily determined at any mobile phase in LAC (corresponding to conditions of weak or strong adsorption) without any information about the molar mass of the peaks. From the slope of this dependence the adsorption interaction parameter of a given repeat unit can be obtained. The accurate determination of the accessible volume and the adsorption interaction parameter in the LAC regime is presented for PEG, PPG and fatty alcohols on various reversed phase columns with different pore size in methanol-water or acetone-water mobile phases. The difference between the void volume, the dead volume or hold-up volume (from the solvent peak position) and the accessible volume (obtained by this procedure) is discussed.     

FACTS: Fast analytical continuum treatment of solvation

An efficient method for calculating the free energy of solvation of a (macro)molecule embedded in a continuum solvent is presented. It is based on the fully analytical evaluation of the volume and spatial symmetry of the solvent that is displaced from around a solute atom by its neighboring atoms. The two measures of solvent displacement are combined in empirical equations to approximate the atomic (or self) electrostatic solvation energy and the solvent accessible surface area. The former directly yields the effective Born radius, which is used in the generalized Born (GB) formula to calculate the solvent-screened electrostatic interaction energy. A comparison with finite-difference Poisson data shows that atomic solvation energies, pair interaction energies, and their sums are evaluated with a precision comparable to the most accurate GB implementations. Furthermore, solvation energies of a large set of protein conformations have an error of only 1.5%. The solvent accessible surface area is used to approximate the nonpolar contribution to solvation. The empirical approach, called FACTS (Fast Analytical Continuum Treatment of Solvation), is only four times slower than using the vacuum energy in molecular dynamics simulations of proteins. Notably, the folded state of structured peptides and proteins is stable at room temperature in 100-ns molecular dynamics simulations using FACTS and the CHARMM force field.     

The effect of solvation processes on amino acid‐ and peptide‐silica stationary phases

The surface excess adsorption isotherms of water, acetonitrile, and methanol from binary hydro‐organic mobile phases were investigated on nine home‐made stationary phases with chemically bonded amino acids, dipeptides, and tripeptides using the dynamic minor disturbance method. The stationary phases were modified by the following amino acids: glycine, alanine, phenylalanine, leucine, and aspartic acid. We investigated the influence of the type of immobilized amino acids, in particular their different side chains, on the solvent adsorption. The interpretation of solvation phenomena shows significant accumulation of investigated solvents on the adsorbent surface according to their hydrophilic or hydrophobic properties. Moreover, the accumulated amount was dependent on the length and type of amino acid sequences bonded to the silica surface. Stationary phases with bonded amino acids and peptides show stronger water and acetonitrile adsorption in contrast to the stationary phase modified with aminopropyl groups—a support for the synthesis. The comparison of water and acetonitrile adsorption as well as a data obtained with the two‐site adsorption model reveal and confirm the heterogeneity of chemically bonded phases. As a consequence of performed investigations, the classification of tested stationary phases for the potential usage in particular high‐performance liquid chromatography mode was also accomplished.     

Elution mechanism of polypeptides in reversed-phase liquid chromatography based on the critical threshold of organic solvent to induce abrupt change in adsorption capacity to the column packing

Adsorption capacity of polypeptides to the column packing in a solution containing multiple organic solvents was found to be expressed by means of an fn value, which is the sum of the ratios of the content of each organic solvent in the solution to the critical content of each organic solvent to cause abrupt change in the adsorption capacity, and to change abruptly at the point where the fn value becomes 1. Additionally, our results indicate that each polypeptide is eluted by the eluent containing a specific organic solvent content regardless of gradient elution rate in reversed-phase liquid chromatography, and that total organic solvent content in the eluent containing polypeptides is equal to the critical content. Considering the power law relationship between the retention times and the gradient elution rates, our results suggest that the elution of each polypeptide in reversed-phase liquid chromatography is mainly controlled by abrupt change in the adsorption capacity induced by change in the organic solvent content of the eluent during a gradient elution process, and that the abrupt change repeats across the critical threshold while a polypeptide moves through the column, and as a result, each polypeptide is concentrated in the eluent with the critical threshold.     

Ionensolvatation in Ameisensäure. I Untersuchungen zur Solvatation von Alkaliionen mit 1H-NMR-Messungen

Ionic Solvation in Formic Acid. I. ¹HMR Studies on the Solvation of Alkali Ions Proton resonance spectra of solutions of alkali chlorides in anhydrous formic acid show the H? C proton signal to be shifted to lower fields by solvation effects. The investigations performed lead to some valuable conclusions about the structure of the cation solvates, and allow a first estimation of solvation numbers for alkali ions in this solvent.