Solvatochromic Studies of Solvated Chromatographic Stationary Phases

Abstract

A series of solvatochromic dyes has been used to probe the solid/solution interface of Cis bonded silica in mobile-phase mixtures consisting of methanol and water and acetonitrile and water. Chromatographic capacity factors (k) were measured in order to determine the fraction of dye sorbed at the surface. An adaptive Kalman filter has been used to resolve the spectrum of the surface-phase dye from that due to dye in the adjacent solution. Values for the Kamlet-Taft dipolarity/ polarizability x* parameter for octadecylsilica in contact with the methanol/water mobile phase ranged from 0.72 to 0.75, indicating that the stationary phase is considerably more polar than bulk alkane solutions. The interfacial values for x* and a (the Kamlet-Taft hydrogen bond acidity parameter) support the hypothesis that the chemistry and solvation of the interphase region is strongly influenced by the presence of residual surface silanol groups.Reversed-phase liquid chromatography (RPLC) is a commonly used method for the separation of a wide variety of analytes. The application of RPLC involves the use of a relatively nonpolar stationary phase such as octadecyl silica, and a more polar mobile phase, such as water mixed with various organic solvents. From a practical viewpoint, RPLC is an extremely valuable technique. At the same time, the chemistry of the stationary phase (the interphase region)* 1 23456is complex and has not been completely characterized.The characterization of the interphase region using spectroscopic probes has, thus far, been limited to the use of fluorescent probe molecules. These methods have revealed information regarding the polarity of the solvated surface2,3 and have provided some of the first evidence that the chemistry and properties of the stationary phase are dependent upon interactions with mobile-phase components.2-6 Some common fluorescent probes that have been used are pyrene, n-propanedansylamide, and n-decanedansylamide.2-6 One limitation of these fluorescent probes is that they are relatively nonpolar and may only sorb to specific regions of the stationary phase. In addition, they do not allow specific interactions, such as hydrogen bonding, dipolar, and dispersion, to be identified and only give an indication of the overall

Comparative Study of Hydrocarbon, Fluorocarbon, and Aromatic Bonded RP-HPLC Stationary Phases by Linear Solvation Energy Relationships

“…Both chromatographic and spectroscopic measurements show that the π s *, R s , and s values of bonded stationary phases are significantly greater than zero. [46][47][48][49][50][51][52][53] Typical values for π s * range from 0.7 to 1.1, between 0.8 and 1.1 for R s , and between 0.35 and 0.8 for s , depending on the mobilephase composition and type of organic solvent. 46, 52 All the above cited studies suggest that the stationary phase plays an important role in governing retention in RPLC.…”

Solvatochromic characterization of Silica-based stationary phases for liquid chromatography

“…A slurry spectrum is a mixture of contributions from the dye on the stationary phase and the dye in the solution that remains with the slurry. The relative concentrations of the dyes was established in previous studies for adequate assessment of the characteristics of the stationary phases [36,41]. Each sample of stationary phase and silica slurries was prepared by mixing stationary phase (150 mg) with the designated liquid (6 mL) such that the ratio of liquid to stationary phase was kept constant.…”

“…In contrast with the very low c~ value for bulk solvent, the ODS slurry has a very high e value. The residual silanol groups are believed to make a significant contribution to the HB-acidity of the stationary phase environment [36,41]. Because of the extremely low c~ value of pure acetonitrile, the dye molecules have hydrogen-bonding capacity that is quite unsaturated.…”

“…These authors indicated that when the effects of ordinary solvents on the silica surface were compared, its dipolarity/polarizability, as measured by the re* value, makes the most important contribution to the overall polarity of the silica surface. Rutan and coworkers used two re* dyes to measure the re* values of bare silica [35] and a C18-bonded silica [36,37] in methanol-water and acetonitrile-water mixtures, with compositions ranging from 100 to 50 % of the organic modifier. The re* values for Cls phase slurries were higher than those for bulk alkanes because of stationary phase solvation; values were less variable than those for the mobile phase because of the protection of the solute by the C18 chains.…”

“…The re* values for Cls phase slurries were higher than those for bulk alkanes because of stationary phase solvation; values were less variable than those for the mobile phase because of the protection of the solute by the C18 chains. The c~ values of the stationary phase slurries measured by use of ET-33 [38] are generally higher than those for bulk mixtures of acetonitrile-water and methanol-water in this range of composition [36,37]. Dallas used n-hexadecane as a model of the stationary phase to evaluate the feasibility of a bulk partition mechanism [39].…”