Architecture and dynamics of C-22 bonded interphases

The relationship between alkyl phase structure and chromatographic performa nce is investigated for a series of docosyl (C-22)-modified silica surfaces . The results of solid-state nuclear magnetic resonance (NMR) spectroscopy and fluorescence spectroscopy are evaluated and correlated with liquid chro matographic retention for relevant shape-selective separations. A set of fo ur different stationary phases was prepared by solution and surface polymer ization approaches, yielding materials with surface coverages ranging from 3.6 to 7.0 mu mol/m(2). C-13 cross polarization magic angle spinning (CP/MA S) NMR spectra indicate that a predominance of trans conformations exists f or high-coverage C-22 phases (>4.0 mu mol/m(2)). Two-dimensional solid-stat e NMR spectroscopy (wide line separation, WISE) was utilized to evaluate th e mobility of the trans and gauche alkyl chain conformations. Temperature-d ependent C-13 CP/MAS NMR measurements of the bonded phases exhibit large di fferences in the dynamic behavior of the immobilized C-22 chains. Unusually high chain rigidity was found for the self-assembled monolayer C-22 phase (7.0 mu mol/m(2)). Fluorescence lifetime measurements of 1,6-diphenylhexatr iene (DPH) exhibit two different lifetimes of tau(F) approximate to I and 7 ns, which are ascribed to probe molecule populations in the mobile and bon ded phases, respectively. Quantitative evaluation of the fluorescence decay curves shows that the partitioning of DPH into the alkyl phase is favored at higher surface coverages, reaching a maximum at a ligand density of 4.9 mu mol/m(2). Time-resolved fluorescence anisotropy measurements also reveal ed that probe mobility was minimized at this surface coverage. With increas ing temperature, the mobility of DPH was found to increase and the fraction of sorbed molecules to decrease. A shape selectivity test mixture containi ng five polycyclic aromatic hydrocarbons including DPH was employed for tem perature-dependent chromatographic studies. In accord with the spectroscopi c results, shape selectivity is enhanced at low temperatures and at high su rface coverages. The combination of these spectroscopic and chromatographic tools provides a wealth of information on the surface morphology of system atically prepared C-22 sorbents and greater insight on the molecular recogn ition process in liquid chromatography.