Prediction of the zeta potentials and ionic descriptors of a silica hydride stationary phase with mobile phases of different pH and ionic strength

Solvent Influence on Zeta Potential of Stationary Phase-Mobile Phase Interface

Zeta potential is a surface characteristic formed on the solid surface and liquid interface. It is an interesting way to describe the surface properties of materials; thus, a series of four homemade polar embedded stationary phases that contain phosphate groups incorporated into hydrophobic ligands were investigated according to surface zeta potential. Measurements were carried out using Zetasizer Nano ZS for the stationary phases suspensions prepared in various solvent and solvent binary mixtures. The negative zeta potential values were obtained for most cases due to negatively charged residual silanols and phosphate groups. However, in some solvents: tetrahydrofuran, isopropanol, and toluene zeta potential are positive. Additionally, it was observed that the zeta potential seems to be independent of the type of silica gel used for the stationary phase synthesis.

Origin of the selectivity differences of aromatic alcohols and amines of different n-alkyl chain length separated with perfluorinated C8 and bidentated C8 modified silica hydride stationary phases

Perfluorinated C8-(PerfluoroC8) and bidentate anchored C8-(BDC8)-modified silica hydride stationary phases have been employed for the isocratic separation of homologous phenylalkanols and phenylalkylamines differing in their n-alkyl chain length, using aqueous-acetonitrile (ACN) mobile phases of different ACN contents from 10 to 90% (v/v) in 10% increments. These analytes showed reversed-phase (RP) retention behaviour with mobile phases of <40% (v/v) ACN content with both stationary phases but with the BDC8 stationary phase providing longer retention. The PerfluoroC8, but not the BDC8, stationary phase also exhibited significant retention of these analytes under conditions typical of an aqueous normal phase (ANP) mode (i.e. with mobile phases of >80% (v/v) ACN content), with the analytes exhibiting overall U-shape retention dependencies on the ACN content of the mobile phase. Further, these stationary phases showed differences in their selectivity behaviour with regard to the n-alkyl chain lengths of the different analytes. These observations could not be explained in terms of pK _a , log P, molecular mass or linear solvation energy concepts. However, density functional theory (DFT) simulations provided a possible explanation for the observed selectivity trends, namely differences in the molecular geometries and structural organisation of the immobilised ligands of these two stationary phases under different solvational conditions. For mobile phase conditions favouring the RP mode, these DFT simulations revealed that interactions between adjacent BDC8 ligands occur, leading to a stationary phase with a more hydrophobic surface. Moreover, under mobile phase conditions favouring retention of the analytes in an ANP mode, these interactions of the bidentate-anchored C8 ligands resulted in hindered analyte access to potential ANP binding sites on the BDC8 stationary phase surface. With the PerfluoroC8 stationary phase, the DFT simulations revealed strong repulsion of individual perfluoroC8 ligand chains, with the perfluoroC8 ligands of this stationary phase existing in a more open brush-like state (and with a less hydrophobic surface) compared to the BDC8 ligands. These DFT simulation results anticipated the chromatographic findings that the phenylalkanols and phenylalkylamines had reduced retention in the RP mode with the PerfluoroC8 stationary phase. Moreover, the more open ligand structure of the PerfluoroC8 stationary phase enabled greater accessibility of the analytes to water solvated binding sites on the stationary phase surface under mobile phase conditions favouring an ANP retention mode, leading to retention of the analytes, particularly the smaller phenylalkylamines, via hydrogen bonding and electrostatic effects.

Parametric study of a dyeing wastewater treatment by modified sericite

The aim of this study was to investigate color, suspended solids (SS), chemical oxygen demand (COD) and biological oxygen demand (BOD) removal using modified sericite with magnesium (Mg-Sericite) flocculants in dyeing wastewater. Mg-Sericite flocculants successfully removed >95% of color, SS. COD and BOD in dyeing wastewater at the following optimal conditions: Mg-Sericite dosage of 40 mg/L, pH of 11, Mg/Sericite ratio of 1.5, settling time of 20 min, mixing time of 10 min and mixing rate of 100 rpm. The bioflocculant, Mg-Sericite, can be a promising flocculants due to its high efficiency and low dose requirements in dyeing wastewater treatment. In addition, Mg-Sericite does not contaminate treated wastewater, which can be recycled to reduce not only the cost and the demand for water but also the extra operational costs for reusing wastewater.

Investigations into the separation behaviour of perfluorinated C8 and undecanoic acid modified silica hydride stationary phases

In this study, the surface charge properties of perfluorinated C8 (PerfluoroC8) and undecanoic acid (UDA) modified silica hydride stationary phases have been investigated. The zeta potential values of these stationary phases were measured in aqueous/acetonitrile mobile phases of different pH, buffer concentrations and acetonitrile contents. The retention behaviour of several basic, acidic and neutral compounds were then examined with these two stationary phases, with U-shaped retention dependencies evident with regard to the organic solvent content of the mobile phase. Plots of the logarithmic retention factor versus buffer concentration revealed slopes ≥ -0.41 for both stationary phases, indicating the involvement of mixed mode retention mechanisms with contributions from both ionic and non-ionic interactions. Using a linear solvation energy relationship approach, the origins of these interactions under different mobile phase conditions were differentiated and quantified. The PerfluoroC8 stationary phase exhibited stronger retention for basic compounds under high acetonitrile content mobile phase conditions, whilst stronger retention was observed for all compounds with the UDA stationary phase under high aqueous content mobile phase conditions. The more negative zeta potentials of the UDA stationary phase correlated with higher total charge density, surface charge density and charge density at the beta plane (the outer plane of the double layer) compared to the PerfluoroC8 stationary phase. With mobile phases of low buffer concentrations, more negative zeta potential values were unexpectedly observed for the PerfluoroC8 stationary phase with slight increases in the C descriptor value, reflecting also the greater accessibility of the analytes to the stationary phase surface. Comparison of the retention behaviours on these phases with other types of silica hydride stationary phases has revealed different patterns of selectivity.