Temperature-Jump Investigation of Alkyl Chain Length Effects on Sorption/Desorption Kinetics at Reversed-Phase Chromatographic Interfaces

Abstract

The influence of the alkyl chain length on the kinetics of solute retention at reversed-phase chromatographic surfaces is examined. A Joule-discharge temperature-jump relaxation experiment was used to monitor reversible sorption/desorption kinetics at C4- and C8-modified silica/solution interfaces. Biexponential sorption/desorption relaxation kinetics were observed for a charged fluorescent probe, 1-anilino-8-naphthalenesulfonate (ANS), on both C4- and C8-silica surfaces. Both relaxation rates on C4 surfaces were sufficiently slow to be measured and increased linearly with solute concentration. One of the relaxations on a C8 surface is too fast to be resolved from the heating rate, similar to behavior of the solute on a longer chain C18-silica. These observations suggest that sorption kinetics on the intermediate chain length surfaace, C4-silica, are different from kinetics on longer chain length surfaces, C8- and C18-silica. From a fit of the data to a two-step kinetic model, the rates of both adsorption and partition of the ionic probe on the C4 chain are estimated; both rates exhibit significant influence over the equilibrium constant. The relaxation rate of a neutral probe, N-phenyl-1-naphthylamine, is also measured; the results indicate a fast (diffusion-controlled) adsorption step, followed by a detectable barrier to partition that is similar to the partition barrier for ANS on the C4-silica surface. These results show that the alkyl chain length of modified silica strongly influences retention kinetics.

Physical Aspects of Radiation-Induced Processes on SiO2, γ-Al2O3, Zeolites, and Clays

“…For the cation to be adsorbed or captured by the surface, the ion has to penetrate the solvent layer adsorbed at the surface and to reorganize its own solvation sphere. Other studies have reported finite and relatively low adsorption rates for ionic molecules − at surfaces. The adsorption rates of ionic probes are slower than a diffusion-controlled limit.…”

C18-Modified Metal-Colloid Substrates for Surface-Enhanced Raman Detection of Trace-Level Polycyclic Aromatic Hydrocarbons in Aqueous Solution

“…Based on the stronger adsorption of pyrene, it was surprising that phenanthrene exhibited a slower adsorption rate; this could be due to the small dipole moment of phenanthrene, which increas-es its interaction with the aqueous solution phase compared to the other two (non-polar) PAH compounds and slows its rate of transfer across the hydrophobic interface. 47 To determine whether the PAH response conforms to a simple isotherm, the solution concentration dependence of the pyrene SERS signal was measured and fit to a Langmuir isotherm: 48 FIG. 8.…”

Measuring Reversible Adsorption Kinetics of Small Molecules at Solid/Liquid Interfaces by Total Internal Reflection Fluorescence Correlation Spectroscopy

“…The probes used in these previous studies were anionic and neutral molecules smaller than R6G + , 1-anilino-8-naphthalenesulfonate (ANS - ) and its neutral analogue N -phenyl-1-naphthylamine (1-NPN). The temperature jump experiments were of two kinds: a Joule-heating experiment, where silica was heated by passing an electrical current through the solution surrounding the silica, − and a laser temperature jump experiment used with a silica colloidal dispersion . Because Joule heating required electrical current to be conducted through the sample, electrolyte concentrations were high so that the rate of heating was fast.…”

“…Adsorption and desorption kinetics at a C-4 interface appear to be unique, but the results also carry considerable uncertainty due to the biexponential form of the relaxation and the similarity of the two relaxation rates. The kinetics on a C-4 surface should differ from C-1 surfaces since solutes can partially partition into the alkyl chains, but the chain length is typically not long enough to fully solvate the molecule …”

“…The “relaxation” of the system to the new equilibrium position is followed by monitoring changes in concentrations of species that govern the equilibrium. Two approaches to perturbing adsorption/desorption equilibria have recently been developed for studying the kinetics of reversed-phase chromatographic systems: a pressure jump can produce a shift in an equilibrium constant that depends on the change in molar volume. , Alternatively, a temperature jump provided by Joule discharge − or laser heating of the sample can be used to perturb an adsorption/desorption equilibrium based on the temperature dependence of the equilibrium constant; the magnitude of the perturbation scales with the enthalpy of adsorption. These two relaxation methods have successfully revealed details of adsorption kinetics at reversed-phase chromatographic interfaces, but they are limited in their application to systems that exhibit a large change in molar volume or enthalpy upon adsorption.…”