An Attempt Directed toward Enhanced Shape Selectivity in Reversed-Phase Liquid Chromatography: Preparation of the Dodecylaminated β-Cyclodextrin-Bonded Phase

Abstract

Soluteshape selectivity in reversed-phase liquid chromatography (RPLC) has been a topic of much study over the last decade.1,2 Several classes of closely related compounds, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, steroids, and carotenoids, can be separated in RPLC based on the molecular shape. 3 Although the contribution of the reversed-phase material to shape selectivity is not completely understood, there have been a few trends observed in RPLC separations. Shape selectivity varies with carbon loading of the stationary phase, the length of the alkyl-chains bonded to silica gel and the properties of the silica substrate. Also, it is well known that shape selectivity is enhanced by reducing the column temperature or by increasing the organic-modifier content in the mobile phase. Furthermore, shape selectivity differences are strongly influenced by the kind of surface-modification chemistry used. 4 It has been noted that polymeric octadecylsilane (ODS) silicas provide higher shape selectivity than monomeric counterparts in the separation of PAHs. [5][6][7] This has been ascribed to increased chain ordering in polymeric phases, which results from their alkyl chain organization as a result of higher chain densities. 4,6,[8][9][10][11][12] and simulation 13 studies have shown that chain ordering increases with increasing chain density.The purpose of this paper is to report on the synthesis of a new RPLC bonded phase that is designed so as to have regularly spaced, ordered chains on a local level. The possible structure of the prepared stationary phase is depicted in Fig. 1. A number of PAHs were separated with this stationary phase, and its selectivity was compared with those obtained with conventional stationary phases. The preliminary experiments revealed that the new stationary phase offers enhanced molecular shape-recognition performance.It may be noted that, by far, a large number of modified β-CD stationary phases have been synthesized, most of which were directed toward chiral separations, but not toward RPLC separations. An exception to this trend has been presented by i and Ishizuka. 14 They synthesized dodecylamino-β-CD by the reaction of heptakis(6-bromo-6-deoxy)-β-CD with dodecylamine in dimethylformamide. ODS silicas were coated with the produced material and used as a stationary phase for the separation of structural isomers. Experimental Synthesis of the stationary phaseThe preparation procedure of the stationary phase consisted of four separate stages (Fig. 2): (1) silylation of silica gel through reactions of 3-glycidoxypropyltrimethoxysilane with the surface silanol groups, 15 which provides a site for subsequent bonding with dodecylaminated β-CD; (2) sulfonation of the primary hydroxyl groups of β-CD with mesitylsulfonyl chloride, 16 (3) reaction of n-dodecylamine with mesitylenesulfonylated β-CD, yielding n-dodecylamino-derivatized β-CD (DA-β-CD) (the synthesis of heptakis(6-dodecylamino-6-deoxy)-β-CD has been already described by Takahashi et al. 17 ); and (4...

Sol-Gel Synthesis of Ordered β-Cyclodextrin-Containing Silicas

“…Postsynthesis modification of silica surfaces was successfully realized by attachment of β-cyclodextrin or its derivatives to the silica support preliminary functionalized with N -(2-aminoethyl)-3-aminopropyl [ 29 ], carboxylated cuccinyl [ 30 ], 3-aminopropyl [ 31 – 35 ], 3-glycidoxypropyl [ 36 , 37 ], hydrosilyl [ 38 ], ester [ 39 ], and 3 - mercaptopropyl [ 40 ] groups, whereas the taken attempts to introduce β-cyclodextrin moieties into the silica framework by sol-gel methods involve the condensation of silica alkoxides with β-cyclodextrin [ 41 ] or β-cyclodextrin-containing silanes [ 42 – 44 ]. The predominant majority of these works relates to the synthesis of functionalized silica materials with disordered porous structure.…”