Solute Size Research Articles

Temperature, Pressure, and Length-Scale Dependence of Solvation in Water-like Solvents. I. Small Solvophobic Solutes.

We analyze the role of temperature, pressure, and solute's molecular size on the pattern of isochoric and isobaric solvation of small hard-sphere solutes in TIP4P/2005 water and in a water-like "Jagla" solvent exhibiting unusual thermodynamics. To this end, we employ molecular simulation to determine solvation free energies, isochoric solvation energies and entropies, isobaric solvation enthalpies and entropies, partial molecular volumes, and isothermal density derivatives of the solvation free energy along isobaric and isothermal paths covering solvent's stable liquid and supercritical states as well as supercooled and "stretched" liquid states. Results are found to be consistent with the most primitive scaled-particle theory and the Gaussian model of small-length-scale solvation. The temperature and pressure dependence of solvation quantities embraces solvent's water-like unusual thermodynamic behavior: its density is reflected in the solvation free energy; the isochoric solvation energy and entropy; and the isothermal density derivative of the solvation free energy, its isobaric thermal expansivity in the isobaric solvation enthalpy and entropy, and its isothermal compressibility in the partial molecular volume. The solute's size or length-scale dependence is found to combine with solvent's water-like behavior to produce the "convergence thermodynamics" picture characteristic of aqueous solutions of nonpolar solutes, which is unequivocally found here to be the mapping of the water-like density maximum into the isobaric solvation enthalpy and entropy versus temperature curves for a set of solutes of varying sizes.

Solution enthalpies of 1-bromoadamantane in monoalcohols at 298.15 K

Solution enthalpies of 1-bromoadamantane in 14 monoalcohols were measured at 298.15 K. Comparison with the solution enthalpies of 2-bromo-2-methylpropane was performed. The obtained data were discussed in terms of the effect of solute's size and solvent's structure.

Structural Properties Governing Retention Mechanisms on Immobilized Artificial Membrane (IAM) HPLC Columns

The aims of this study were to investigate whether three commercially available immobilized artificial membrane (IAM) HPLC columns yield collinear data for neutral compounds, and whether IAM scales are distinct from the log Poct (partition coefficient in the octanol/H2O system) scale. With these objectives, the retention mechanisms on the IAM HPLC columns were analysed by linear solvation free-energy relationships (LSERs). A set of 68 neutral model compounds with known solvatochromic parameters and log Poct values was investigated, allowing a regular and broad exploration of property space. The resulting solvatochromic equations clearly indicate that the three IAM stationary phases retain small neutral solutes by a balance of intermolecular forces closely resembling those underlying partitioning in octanol/H2O and retention on a reversed-phase LC-ABZ HPLC column. For all systems, the solute's size and hydrogen-bond-acceptor basicity are the two predominant factors, whereas dipolarity/polarisability and hydrogen-bond-donor acidity play only minor roles.

Characterization and selectivity optimization on diol, amino, and cyano normal phase columns based on linear solvation energy relationships

This paper describes the results of characterization and selectivity optimization on several normal phase columns by use of linear solvation energy relationships (LSERs). Retention data were obtained for a large set of judiciously selected test solutes on diol, amino, and cyano columns in a mobile phase of pure hexane. Based on the LSERs, the retention data were dissected into contributions from cavity formation/dispersive interactions, dipolar interactions, and hydrogen bond donor–acceptor interactions via both linear regression and principal component analysis. We find that the solute's hydrogen bond (HB) acceptor basicity is the predominant retention determining factors, particularly for diol and amino columns, and that its significance is directly related to the stationary phase acidity. However, the solute's size has a negative, but limited, influence on retention. Furthermore, the solute's hydrogen bond (HB) donor acidity and dipolarity also contribute significantly to retention, and cannot be neglected, as in the case of reversed-phase columns. Finally, the LSER results allow us to effectively predict the chromatographic selectivity on these normal phases, and it can be demonstrated that an increase in selectivity by about 10- to 1000-fold is expected when two solutes differ by only one polar functional group.

Theoretical aspects of micellar liquid chromatography using C 12DAPS surfactant

Theoretical aspects of micellar liquid chromatography using a zwitterionic surfactant were investigated. The micellar mobile phase consisted of n-dodecyl- N, N-Dimethylamino-3-propane-1-sulfonate (better known as C 12DAPS) aqueous solutions. Chromatography was carried out on μ Bondapak C 18 column, UV detection was measured at 254 nm. Using the Armstrong equation, the partition equilibria constants of the solutes chromatographied were established between water and stationary phase; water and micelles; micelles and stationary phase. Compared to anionic or cationic surfactants, the zwitterionic surfactant gives the highest K SW values and a lowest K MW values. Therefore, for a constant surfactant concentration, capacity factors are greater on C 12DAPS. Hydrophobic interactions with the stationary phase, electrostatic effects of the surfactant from both the micelle and the surfactant modified stationary phase explain the retention of a solute. An other way to study molecular interactions is made via the linear solvation energy relationships (LSER). For the test solutes used, it seems that V 1/100 (solute's size) and β (basicity) are predominant to affect the retention. K SW LSER correlation express the binding of the solute with the micelles. The term s found is positive. C 12DAPS has a greater dipolar environment affecting considerably the MLC partition. The term a is positive, acidic solutes binds more easily with the zwitterionic surfactant than with SDS.

Viscoelastic Continuum Model of Nonpolar Solvation. 1. Implications for Multiple Time Scales in Liquid Dynamics

A continuum model of nonpolar solvation is presented. Coupling of the solute to the solvent is assumed to occur through a change in the solute's size or shape upon electronic excitation. Both spherical and nonspherical changes in the solute are treated. The time-dependent shear and longitudinal moduli of the solvent determine the solvation response function. Unlike prior continuum models of solvation, both a rapid, viscosity-independent inertial component and a slower, viscosity-dependent diffusive component emerge from the model, even when only one time scale is assumed in the moduli. The origin of multiple time scales in this model, which has a single solvent coordinate with complex dynamics, is contrasted with treatments such as the multimode Brownian oscillator models, which postulate multiple solvent coordinates, each with simple dynamics.

Characterization of polybutadiene-coated zirconia and comparison to conventional bonded phases by use of linear solvation energy relationships

This paper describes the use of linear solvation energy relationships (LSERs) to thermodynamically characterize retention on a reversed-phase type material based on polybutadiene (PBD)-coated zirconia. Retention data were obtained for a large set of judiciously selected test solutes on four PBD phases (carbon loads varying from 1.5 to 5.6% by weight) in various methanol-water mobile phases. Based on the LSERs, the free energy of retention was dissected into contributions from cavity formation/dispersive interactions, dipolar interactions, and hydrogen bond (HB) donor-acceptor interactions. The PBD-zirconia phases were compared to conventional silica-based bonded reversed phases. As is the case with conventional bonded phases, the solute's size and HB acceptor basicity are the predominant retention determining factors, and on the whole, PBD-zirconia phases closely resemble conventional chemically bonded reversed phase materials. Interestingly, the solute's basicity has a larger effect on its retention on the PBD-zirconia phase than on conventional bonded phases, so relative to their behavior on conventional phases, strong hydrogen bases and highly dipolar analytes, when compared to nonpolar solutes, are less strongly retained on PBD-zirconia than on conventional phases. PBD-zirconia and conventional phases are so similar that there should be little difficulty in transferring separation methods between phases.

Retention properties of a cyanopropylsiloxane‐bonded silica‐based sorbent for solid‐phase extraction

AbstractThe characteristic kinetic and retention properties of a silica‐based cyanopropylsiloxane‐bonded sorbent for solid‐phase extraction are described. Abraham′s solvation parameter model is used to characterize the contribution of individual intermolecular interactions to retention under liquid chromatographic and sample processing conditions with aqueous methanol mixtures as the mobile phase. The main features governing retention by the sorbent are the solute's size and hydrogen‐bond basicity; interactions of a dipole type are not significant when aqueous methanol solutions are employed as the mobile phase. Compared to typical silica‐based octadecylsioxane‐bonded sorbents the greater difficulty of forming a cavity in the solvated cyanopropylsiloxane‐bonded sorbent more than offsets the more favorable dipole‐type and solute hydrogenbond base interactions of the cyanopropylsiloxane‐bonded sorbent. It is shown that there are no practical circumstances for which a cyanopropylsiloxane‐bonded sorbent would be more useful than a typical ODS sorbent for the isolation of organic non‐electrolytes from water by solid‐phase extraction.