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

Abstract

Liquid chromatography (LC) is one of the most powerful techniques for the separation of various nonvolatile organic compounds. Nevertheless, the separation mechanism in LC has not yet been completely clarified. It is well known that the retention in LC is generated by a dispersion of the analyte between the mobile phase and the stationary phase. Especially, the intermolecular interaction of the analyte and the stationary phase ligand molecule plays a very important role during the retention process in LC. Therefore, the characterization of the surface structure of the stationary phase at the molecular level is very important.Octadecylsilica (ODS) has been most widely used as a stationary phase in LC because of its high separation ability. 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, 1-7 Fourier-transform infrared spectroscopy (FT-IR), 5,8 Raman spectroscopy, 9-16 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.It is also experimentally known that a slight difference of the mobile-phase composition dramatically changes the LC retention. 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. 8 Molecular dynamics (MD) simulations represent a powerful method to estimate the time-series structural changes of a multimolecular system, and are widely used in biochemistry to characterize the higher structures of biomolecules. Although this technique is used by only a limited number of researchers, because it needs computational resources, the advances of computer technologies have made it possible to use this technique on personal computers, and also to easily handle calculations for a huge molecular system, such as the stationary phase in LC. Some articles or reviews have been reported to characterize the stationary-phase ligand conformation using this type MD simulation. [20][21][22][23][24][25][26][27][28] Klatte and Beck applied MD simulations to molecular models that consisted of tethered and randomly placed n-alkane monolayers, and investigated the influence of the temperature, alkyl chain length and density of alkane molecules on the physical properties, such as the alkyl chain conformation and phase transition. 20,21 Although these results interpreted the behavior of the surface struct...

An Abnormal Temperature Dependence of Alkylpyrazines’ Retention in Reversed-Phase Liquid Chromatography

“…The retention and separation mechanism of aromatic compounds including a variety of polycyclic aromatic hydrocarbons (PAHs) and pyrazines have been studied. [4][5][6][7][8][9][10][11][12][13][14][15][16][17] On the basis of actual retention measurements for pyrazines in several experimental conditions, a theoretical interpretation and the subsequent retention prediction were also studied in our previous work, 17 where the retention prediction models for pyrazine and alkylpyrazines were developed using multiple linear regression (MLR) and artificial neural networks (ANNs). During the study, the retention of these compounds was carefully determined in various conditions, including a variety of mobile phase conditions and column temperatures.…”

Influence of Residual Silanol Groups on Solvent and Ion Distribution at a Chemically Modified Silica Surface

“…Previous simulations of the more hydrophobic C 18 chains in neat water found the core bonded-phase region to be solvent-depleted, 14,15,18 probably because the highly disordered and partially backfolded C 18 chains form a hydrocarbon layer at the interface that is largely impenetrable to water. In a previous simulation of a C 8 -bonded phase, penetration of water molecules up to 0.6 nm into the bonded-phase in neat water and water/acetonitrile 70/30 (v/v) was observed, but this model did not include an explicit silica surface.…”

Miniaturization for the Development of High Performance Separation Systems

“…Numerous numbers of reports on the separation mechanism have been published on the basis of the retention analysis, however, there is still a long way to get the complete explanations of the whole separation mechanism. This is because many parameters are controlling the retention generated by a molecular recognition process in typical LC separations [5][6][7][8].…”

“…Development of novel fibrous extraction/separation media having different chemical structures and then different functionalities on the surface could be realized in the near future based on the concept of molecular shape recognition [3][4][5][6][7][8] and the systematic retention mechanism analysis with a help of various modern computational data calculations in LC [106][107][108][109][110][111][112][113][114][115][116][117][118]. Miniaturization of packed columns in GC [119][120][121][122][123] is also possible for high speed analysis with a unique selectivity but without a loss of sample loading capacity.…”