Chromatographic approach to the measurement of the interstrand distance for some chiral bonded phases

Article abstract of DOI:10.1021/ac00001a003

A series of homologous N,N′-bis(2,4-dinitrophenyl)-α,ω-diaminoalkanes (bis-DNP's) was chromatographed at various temperatures on π-basic chiral stationary phases derived from N-(2-naphthyl)alanine in order to determine the enthalpy and entropy of adsorption. The number of methylene groups in the bis-DNP's influences the ability of the terminal π-acidic groups to interact simultaneously with neighboring strands of stationary phase, a process termed "bridging". When the number of methylene groups is optimal for bridging, the enthalpy of adsorption is most exothermic. The length of the bis-derivative required for optimal bridging is related to the interstrand distance. Optimal bridging occurs for the bis-DNP's having five methylene groups regardless of the extent of surface coverage of the silica for the surface coverage range investigated. This suggests that the strands are not randomly spaced on the silica, with interstrand distance being influenced only by surface coverage, but are instead clustered, the clusters having similar distributions of interstrand distances. Adsorption is more exothermic for phases of high surface coverages than for low. If Interstrand spacing is independent of surface coverage but surface coverage affects the enthalpy of adsorption, then surface coverage must influence the cluster size, which then influences the average extent of solvation of a strand of bonded phase.