Structure−Function Relationships in High-Density Octadecylsilane Stationary Phases by Raman Spectroscopy. 2. Effect of Common Mobile-Phase Solvents

Abstract

Raman spectroscopy is used to examine the effects of solvent, temperature, and surface grafting method (surface or solution polymerized) on alkyl chain rotational and conformational order in a series of high-density octadecylsilane stationary phases ranging in surface coverage from 3.09 to 6.45 micromol/m2. Rotational and conformational order is assessed using the intensity ratio of the antisymmetric to symmetric v(CH2) modes as well as the frequency at which these Raman bands are observed. Solvents studied include perdeuterated hexane, toluene, chloroform, tetrahydrofuran, benzene, methanol, acetone, acetonitrile, and water. Stationary-phase order was investigated at temperatures between 258 and 323 K. Alkyl chain rotational and conformational order, and hence, solvation of the stationary phase, is dependent on solvent parameters (polarity, size, etc.), temperature, and stationary-phase properties (polymerization method and surface coverage). Information on stationary-phase conformational order allows solvent-stationary-phase interactions to be described in terms of a combination of adsorption and partitioning models for reversed-phase liquid chromatography. Finally, a distinct interplay between solvent- and temperature-induced ordering of these stationary phases is documented that is also a function of solvent and stationary-phase properties.

Fundamentals of Reversed Phase Chromatography: Thermodynamic and Exothermodynamic Treatment

“…Since the original treatment of the solvophobic theory did not explicitly focus on the effect of the stationary phase property on RPC retention, it has led some chromatographers to believe that the solvophobic theory is either inadequate [71,164] because it does not adequately take into account the stationary phase effects in RPC or inaccurate [165] because it treats the stationary phase as a passive entity, i.e., playing a relatively minor role. The effects of stationary phase properties on eluite retention were not well understood then.…”

Thermodynamic and kinetic characterization of a bridged-ethylene hybrid C18 stationary phase

“…The proximal regions, where the alkyl group is bound to the silica surface, are highly ordered with all trans carbon-carbon bonds. As the distance from the surface increases, there are more gauche bonds and greater disorder [34][35][36]. The more condensed PAHs, such as pyrene, probe only the distal regions of the alkyl chain, while less condensed PAHs, such as benz[a]anthracene, penetrate more deeply into the ordered regions of the alkyl chain.…”

Characterization of the Microscopic Surface Structure of the Octadecylsilica Stationary Phase Using a Molecular-Dynamics Simulation

“…A number of attempts to understand the surface structure of the ODS stationary phase have been reported, in which spectroscopic techniques were used, such as nuclear magnetic resonance (NMR) spectroscopy, [1][2][3][4][5][6][7] Fourier-transform infrared spectroscopy (FT-IR), 5,8 Raman spectroscopy, [9][10][11][12][13][14][15][16] contact-angle measurements, 17 thermal analysis 5,18 and actual chromatographic methods. 19 Also, the influence of the temperature and bonding chemistry on the alkyl ligand conformation have also been well described in these reports.…”

“…However, most of these reports are described based on the structure of the alkyl bonded silica surface in air, or in a pure solvent, such as water, methanol or acetonitrile. Pemberton et al have recently studied the ODS ligand conformation systematically using Raman spectroscopy, [13][14][15][16] in which the bonding density, temperature, some kinds of solvents were considered to be the contributing factors. Sander et al investigated the relationship between the alkyl ligand conformation and the mobile-phase composition using FT-IR spectroscopy.…”