Fourier transform infrared spectrometric determination of alkyl chain conformation on chemically bonded reversed-phase liquid chromatography packings

Abstract

A series of dimethyl-n -alkyl bonded phase sorbents ranging in length from C, to C,, have been prepared and studied by use of Fourier transform infrared spectrometry (FT-IR). Spectra have been produced for each of the bonded phases, and comparisons are made to the Corresponding silane reagents. By use of assignments reported for nonplanar conformations of normal alkanes, the supramolecular structure of these bonded phase systems is examined. The presence of absorption bands at 1341, 1354, and 1367 cm-' In the bonded phase spectra confirms that bonded alkyl chains are bent, i.e., a significant fraction of the carbon-carbon bonds are in the "gauche" configuration. The degree of conformational disorder in chemically bonded statlonary phases Is shown to be comparable to that observed in the corresponding n -alkane liquids at room temperature and above. Phase transitions for the bonded phases are not observed at temperatures near or below the melting points of alkane and silane reagents of comparable length. Instead, the bonded phase chains are seen to retain a notable degree of disorder at low temperature. Finally, the effect of mobile phase environment on chain conformation Is examined for a range of deuterated water/methanoi mixtures varying from 70 % to 100% methanol. Evidence is presented that in this range the mobile phase effects a signlflcant increase in the order of the stationary phase.'The popularity of reversed-phase separations, particularly with CI8 bonded packing materials, can be attributed to the success of this technique in solving a diverse m a y of problems. Surprisingly though, a detailed understanding of the mechanisms of solute retention is lacking. The difficulty in describing the retention process originates in the nature of the bonded phase itself. Almost all current types of bonded phases for use in liquid chromatography are produced from the reaction of chloro-or alkoxyalkylsilanes with silanol groups at the silica surface. In most instances, reaction conditions are such that a bonded monomeric layer results, with an overall density of chains of about half that of the corresponding liquids (1-3). As noted by Guiochon (4), the properties of bonded phases are thus expected to differ from bulk liquids, not only because the distance between chains is greater than in liquids but also because translational and rotational motions are absent.The question of whether a bonded phase should be considered a surface or a thin liquid film has given rise to two theories of solute retention: adsorption and partitioning. Considerable effort has been expended by workers trying to 'Current address: National Bureau of Standards, Bldg. 222, Rm Current address: Department of Chemistry, Southern Methodist A113, Washington, DC 20234. University, Dallas, TX 75275.classify retention as one of the two categories (5-7), but evidence for such a classification remains inconclusive. Clearly, before an accurate picture of solute retention can be built up, the microscopic properties of the anchored hydrocarbons must be eluc...

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

“…First, it can be seen that when the temperature is increased, the gauche fraction becomes higher for all models. This trend is the same as experimentally observed trends by NMR, 5,7 FTIR, 5,8 and Raman spectroscopies. [11][12][13] Second, the contribution of the mobilephase composition to the gauche fraction is more clearly observed at 273 K and 300 K. A lower gauche population can be found for molecular models with a low methanol concentration (models B and C); in contrast, a relatively high gauche fraction is obtained for models A and F at these temperatures.…”

“…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.…”

“…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.…”

Peer Reviewed: Characterizing HPLC Stationary Phases

“…Berendsen and de Galan assigned characteristic absorption bands to the specific ligand functionalities (20). As the methods were developed and improved, the kinetics of modification and dynamics of bonded ligands were investigated (21,22). FTIR with signal averaging overcame the problems connected with high absorption of IR radiation by silica gel (2).…”

Physico-chemical methods for the chromatographic materials heterogeneity evaluation

“…[1,90] assigned characteristic absorption bands to the specific ligand functionalities. As the methods were developed and improved, the kinetics of modification and dynamics of bonded ligands were investigated [91,92]. FTIR with signal averaging overcame the problems connected with high absorption of IR radiation by silica gel [30].…”