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 bes… Show more
“…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.…”
The peak parking (PP)-moment analysis (MA) method was applied to the measurement of surface diffusion coefficients (Ds) in reversed-phase liquid chromatography using a C18-silica gel and an aqueous solution of methanol (70 vol%). The values of Ds measured by the PP-MA method were in agreement with those obtained by the pulse response (PR)-MA method, which is one of powerful strategies for the quantitative study on the mass transfer kinetics in columns and stationary phases. It was demonstrated that the PP-MA method is an alternative for the experimental measurement of Ds. The mechanism of surface diffusion was considered on the basis of molecular diffusion by introducing a restriction energy (Er) for surface diffusion. Physical meanings of Er were discussed by analyzing the correlation between the enthalpy change due to sample retention (Qst) and Ds. It seems that surface diffusion of a sample molecule is restricted by its retention strength and by the solvent structure formation due to the hydrophobic solvation of the stationary phase surface.
“…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.…”
The peak parking (PP)-moment analysis (MA) method was applied to the measurement of surface diffusion coefficients (Ds) in reversed-phase liquid chromatography using a C18-silica gel and an aqueous solution of methanol (70 vol%). The values of Ds measured by the PP-MA method were in agreement with those obtained by the pulse response (PR)-MA method, which is one of powerful strategies for the quantitative study on the mass transfer kinetics in columns and stationary phases. It was demonstrated that the PP-MA method is an alternative for the experimental measurement of Ds. The mechanism of surface diffusion was considered on the basis of molecular diffusion by introducing a restriction energy (Er) for surface diffusion. Physical meanings of Er were discussed by analyzing the correlation between the enthalpy change due to sample retention (Qst) and Ds. It seems that surface diffusion of a sample molecule is restricted by its retention strength and by the solvent structure formation due to the hydrophobic solvation of the stationary phase surface.
“…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.…”
Section: Influence Of the Organic Modifiermentioning
A set of seven homemade octadecyl silica-based bonded phases was investigated. Their zeta potential data in methanol and ACN as well as in methanol-water and ACN-water solution were obtained using Zetasizer. The influence of both the coverage density of bonded ligands and the end-capping of the modified surface on these data was investigated. Presented results may give useful information about the accessibility of the residual silanols in different mobile phases during the chromatographic analysis. Those measurements may be useful to choose chemically bonded stationary phases for CEC. The results also confirm the phenomena of anion exclusion from the pores of stationary-bonded phase.
“…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.…”
The excess isotherms of methanol and acetonitrile were measured on the series of C18 bonded phases. The measurements were done using the minor disturbance method. The goal of our work was to determine the influence of the temperature on the adsorption of two commonly used solvents. The influence on the mobile phase flow rate on the both organic solvent adsorption was also investigated. The effect of these two parameters was compared on the octadecyl packed columns with different coverage density and on the monolithic Chromolith column. Adsorption of both solvents decreases with the increase of the temperature. The increase of the pressure increases adsorption of methanol but decreases adsorption of acetonitrile.
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