Adsorption Mechanism in RPLC. Effect of the Nature of the Organic Modifier

Abstract

The adsorption isotherms of phenol and caffeine were acquired by frontal analysis on two different adsorbents, Kromasil-C 18 and Discovery-C 18 , with two different mobile phases, aqueous solutions of methanol (MeOH/H 2 O ) 40/60 and 30/70, v/v) and aqueous solutions of acetonitrile (MeCN/H 2 O ) 30/70 and 20/80, v/v). The adsorption isotherms are always strictly convex upward in methanol/water solutions. The calculations of the adsorption energy distribution confirm that the adsorption data for phenol are best modeled with the bi-Langmuir and the tri-Langmuir isotherm models for Kromasil-C 18 and Discovery-C 18 , respectively. Because its molecule is larger and excluded from the deepest sites buried in the bonded layer, the adsorption data of caffeine follow biLangmuir isotherm model behavior on both adsorbents. In contrast, with acetonitrile/water solutions, the adsorption data of both phenol and caffeine deviate far less from linear behavior. They were best modeled by the sum of a Langmuir and a BET isotherm models. The Langmuir term represents the adsorption of the analyte on the highenergy sites located within the C 18 layers and the BET term its adsorption on the low-energy sites and its accumulation in an adsorbed multilayer system of acetonitrile on the bonded alkyl chains. The formation of a complex adsorbed phase containing up to four layers of acetonitrile (with a thickness of 3.4 Å each) was confirmed by the excess adsorption isotherm data measured for acetonitrile on Discovery-C 18 . A simple interpretation of this change in the isotherm curvature at high concentrations when methanol is replaced with acetonitrile as the organic modifier is proposed, based on the structure of the interface between the C 18 chains and the bulk mobile phase. This new model accounts for all the experimental observations.Most chemical analyses, impurity detections, separations, and sample preparations are now performed by using RPLC techniques. The number of experimental parameters the analyst can adjust in order to achieve his goal is important. The degree of hydrophocicity of the RPLC column (length of the alkyl bonded chains attached to the solid support, density of these ligands on the surface, nature of the solid support), the temperature, the pressure, the composition of the mobile phase, and the elution mode (isocratic or gradient) are all experimental parameters that can be easily modified. To predict the analytical elution times or the shape of overloaded band profiles for single-component and multicomponent systems, it is crucial to be able to determine the adsorption equilibria (single and competitive isotherms) of the compounds between the mobile and stationary phases. 1 The effect of the experimental parameters on the nature and on the parameters of the adsorption isotherm has gained some attention. The effects of the temperature, 2-4 the pressure, 5-7 the ionic strength of the liquid phase, 8-11 the presence of a buffer, 12 the nature and concentration of the buffer, 13,14 and the quantity of organic modifie...

Peak Parking-Moment Analysis Method for the Measurement of Surface Diffusion Coefficients

“…14 They constitute a solvent layer, which contacts with the bonded ligand layer. 15,16 Gritti and Guiochon reported the thickness of the adsorbed monolayer of methanol and multilayer of acetonitrile as 0.25 and 1.36 nm, respectively, on C18-silica gels. 15 Similarly, Shibukawa et al reported that the thickness of the solvent layer is 1.6 nm or more in the RPLC system consisting of alkyl bonded silica and 20% (w/v) acetonitrile-water.…”

“…15,16 Gritti and Guiochon reported the thickness of the adsorbed monolayer of methanol and multilayer of acetonitrile as 0.25 and 1.36 nm, respectively, on C18-silica gels. 15 Similarly, Shibukawa et al reported that the thickness of the solvent layer is 1.6 nm or more in the RPLC system consisting of alkyl bonded silica and 20% (w/v) acetonitrile-water. 16 It seems that some physico-chemical properties of the solvent layer are different from those of the bulk mobile phase.…”

“…The concentration of the organic modifier must be higher near the stationary phase surface than in the bulk mobile phase. 15,16,[31][32][33] The structure-making effects due to the hydrophobic repulsion of the polar solvent molecules from the stationary phase surface and due to the hydrophobic attraction of the organic modifier molecules to the C18-silica gel surface are a cause of the restriction in their mobility. The intercept at Qst = 0 kJ mol -1 in Fig.…”

Zeta potential determination as a new way of stationary phases characterization for liquid chromatography

“…It is known that the adsorption of ACN on hydrophobic stationary phase is much stronger than the adsorption of methanol [38]. On the contrary, significant amount of methanol may be adsorbed near the silica surface through the hydrogen bonding with residual accessible silanols.…”

Influence of temperature and pressure on the preferential adsorption of component of hydroorganic mobile phase in liquid chromatography

“…As was mentioned by Gritti and Guiochon (2005a), the type of the organic modifier has a big influence on the adsorption and retention of the solute. The retention and the elution in the chromatographic system is a competitive play between solute and organic modifier.…”