Structure−Function Relationships in High-Density Octadecylsilane Stationary Phases by Raman Spectroscopy. 3. Effects of Self-Associating Solvents

Abstract

Raman spectroscopy is used to examine the subtle effects of polar, hydrogen-bonding solvents; temperature; and the 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 frequencies at which these Raman bands are observed. Alkyl rotational and conformational order decreases with decreasing surface coverage in these polar solvents, consistent with the behavior of these materials in air. For homogeneously distributed, high surface coverage materials, these polar solvents induce rotational ordering that is proposed to be due to the self-association of these solvents through hydrogen bonding or other dipole interactions at the alkylsilane-solvent interface. From these observations, molecular pictures of these solvent-stationary-phase interfaces are proposed in which solvent interaction with the stationary phase occurs primarily at the distal methyl group of the alkyl chains.

Characterization of Zr(IV)–Phosphonate Thin Films Which Inhibit O[sub 2] Reduction on AA2024-T3

“…The methanol immersion was used to remove any residual octanol from the coating treatments based on previous reports by Orendorff et al in which surface-bound octadecyl silane molecules were shown to retain long-chain hydrophobic solvents at the solvent-adsorbate interface. 47 Experiments showed dramatically inferior inhibition characteristics for trials not using a methanol rinse ͑data not shown͒. Multiple treatment cycles were performed by repetition of the threestep immersion protocol described above.…”

Vibrational Spectroscopy of Chromatographic Interfaces

“…[4,5,7,8,[12][13][14][15] The majority of the past efforts were spent studying a series of high-density octadecylsilane stationary phase materials prepared on YMC 3-µm silica particles by Dr. Lane C. Sander at NIST. [12][13][14][15] These materials were prepared using either "surface polymerization" or "solution polymerization" approaches for attaching the octadecylsilane to the silica, and had very high octadecylsilane surface coverage ranging from 3.1 to 6.5 µmol/m 2 (conventional octadecylsilane phases are in the 2 to 3 µmol/m 2 range.) Previous work by Dr. Sander and coworkers had shown these phases to exhibit unique shape selectivity for certain polynuclear aromatic hydrocarbon (PAH) solutes.…”

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.…”