Katrice A. Lippa, Analytical Chemistry Division, ACSL (Bldg. 227) Room B148, MS 8392, Tel: 301-975-3116, Fax: 301-977-0685, firstname.lastname@example.org, Sigma Xi member
Lane C. Sander (Mentor, Analytical Chemistry Division)
Raymond D. Mountain (Physical and Chemical Properties Division)
“Shape-selective” polymeric alkylsilane stationary phases (typically C18 chains) are routinely employed over the more common monomeric alkylsilane stationary phases in reversed-phase liquid chromatography (RPLC) to improve the separation of geometric isomers of constrained solutes, such as polycyclic aromatic hydrocarbons, carotenoids, steroids and polychlorinated biphenyls. Critical to describing such shape-selectivity processes in RPLC is the elucidation of the molecular-level structural features of the alkylsilane stationary phase that promote such separations. Previous molecular dynamic simulation models that illustrate the structure and dynamics of alkyl chromatographic stationary phases have been focused on only monomeric-type materials.1-4 Therefore, we have investigated the molecular dynamics of chromatographic models that represent both monomeric and polymeric stationary phases with alkylsilane lengths, surface coverages, bonding chemistries and temperature conditions that are typical of actual materials prepared in the laboratory.5,6 The resultant structural features of the alkylsilane chains (i.e., length, conformational order, gauche dihedral angle defects) of these simulation models were related to the observed shape-selectivity properties of corresponding LC stationary phases.
The structural characterization of these computational models are consistent with previous experimental observations: 1) alkyl chain order increases with increased surface coverage; 2) monomeric and polymeric phases with similar surface coverages give rise to similar degrees of alkyl chain order (although subtle differences exist); 3) longer chain length phases (e.g., C30 chains) exhibit significantly more order than C18 phases; and 4) alkyl chain ordering is significantly enhanced at lower temperature. In addition, a significant portion of the alkyl chain closest to the silica substrate surface is disordered (primarily gauche conformations) and the opposite end is most ordered (trans conformations). For simulation models that represent RPLC columns that have been characterized as highly shape-selective (through use of the SRM 869a Column Selectivity Test Mixture), a significant region of alkyl chain order with primarily trans dihedral angle conformations is observed. This is consistent with the view that the alkyl chains comprising the polymeric stationary phase contain a series of well-defined and rigid slots in which shape-constrained solutes can penetrate and hence be selectively retained.
1. Klatte, S. J.; Beck, T. L. Journal of Physical Chemistry 1995, 99, 16024-32.
2. Schure, M. R. Chemically Modified Surfaces, Pesek, J. J.; Leigh, I. E., Eds.; The Royal Society of Chemistry: Cambridge, 1994.
3. Slusher, J. T.; Mountain, R. D. Journal of Physical Chemistry B 1999, 103, 1354-62.
4. Yarovsky, I.; Aguilar, M. L.; Hearn, M. T. W. Analytical Chemistry 1995, 67, 2145-53.
5. Sander, L. C.; Pursch, M.; Wise, S. A. Analytical Chemistry 1999, 71, 4821-30.
6. Sander, L. C.; Wise, S. A. Analytical Chemistry 1984, 56, 504-10.