다공성 유리구슬과 실리카입자 표면변형체로서

베타 사이크로 덱스트린의 활용

김만회, 더글라스 웨이*, 로버트 볼드윈*

공군사관학교 화학과

*콜로라도 스쿨 오브 마인스 화공과

Application of □-Cyclodextrin as a Surface Modifier

of Porous Glass Beads and Silica Particles

Manhoe Kim, J. Douglas Way*, Robert M. Baldwin*

Department of Chemistry, Air Force Academy (R.O.K.)

*Chemical Engineering Department, Colorado School of Mines

Introduction

Cyclodextrins (CDs) are cyclic oligopolysacharides containing from 6 to 13 glucose units bonded through 1,4-likages. Especially, □-CD, □-CD and □-CD industrially produced have abilities to form inclusion complexes with many substances. Beta-CD contains 7 glucose unit and has a central hydrophobic cavity and a hydrophilic outer shell (1). The most unique character of □-CD is its ability to include selectively various organic and inorganic molecules within hydrophobic cavities. Thus, CDs are good candidates as highly selective bonded phase materials for chromatographic separation.

Even though the difficulty of the immobilization of large size cyclodextrin to inorganic surface, Armstrong et al. made a success in immobilization of CD to silica gel by means of a specific silane linkage (2-4). Although a variety of linking spacers and synthesis procedures are well known, the effects of spacer groups between CDs and inorganic supports on the formation of inclusion complexes were rarely known.

In this study another possible reaction between end-capping agents, trimethylchlorosilane and hydroxyl groups will be presented through a simple chromatographic separation based on inclusion complex formation. Similarly, the effects of spacer groups between CD and solid surface, and cross-linking agents such as 1,8-trichlorosilyloctane on the formation of inclusion complexes are studied. The separation efficiency of the CD-modified silica particles were tested using column chromatography to separate a model mixture of naphthalene isomers, 1- and 2-hydroxy-naphthalene (naphthols) and ortho, meta, and para-xylenes.

Experimental Section

Materials. Silica (particle size 5□, pore size 8.0nm, and surface area 200m2/g, Altech) and glass beads (particle size 50-100□, pore size 31.6nm and surface area 97m2/g, Trisoperl, Schuller) were used as supports. Beta-CD (Wacker Biochem) was attached to the supports by a direct bonding method, using epoxy spacers, and cross-linking agent. 3-Glycidoxypropyltrimethoxysilane (Aldrich) was used as a spacer group. 1,8-Trichlorosilyloctane (Petrarch Systems) and epichlorohydrin (Aldrich) were used as cross-linking agents. HPLC grade of dimethylformamide (Aldrich), toluene (Aldrich), methanol (Aldrich), de-ionized water were used as solvents and washing agents. A10mL burette (diameter 0.55cm, Pyrex Brand, Fisher) was used as a chromatography column. The bed height was 37cm and the free volume percentage of the packed □-CD bonded glass beads was 53%. The flow rate was 0.25mL/min.

Methods. Epoxy spacer cyclodextrin bonded glass beads was prepared according to the method described by Armstrong (3). The amounts of organic loaded on the supports were estimated using a thermal gravimetric analyzer (TGA, Seiko) and a linear heating rate of 10K/min from room temperature to 873K. The direct bonding method was used and a typical □-CD weight loading on the glass beads and silica were 15.6 % and 23.8%, respectively.

In order to stabilize the directly bonded cyclodextrin on the surfaces. Twenty grams of directly bonded □-CD glass beads and 60mL of saturated □-CD salt solution was cross-linked with 1.5g of epichlorohydrin (or 2.5g of 1,8-trichlorosilyloctane) in 120mL of toluene. The mixture was maintained at 363K for 50minutes with stirring. The resulting slurry was put aside for a few minutes. The upper portion of the slurry was removed and the modified glass beads/silica were filtered with a filter paper, and washed with toluene and methyl alcohol.

Packing materials were packed into the column by slurry packing or dry powder packing. The column chromatographic experiments were carried out at the room temperature (295K). Xylene and naphthol isomers were tested as elute compounds.  Pure toluene and methyl alcohol were used isocratic mobile phases.  Eluting solvent compositions were analyzed with a HP GC-MS (model 5890/5970).

Results and Discussion

(Effects of Pore Sizes of Supports.) In the packed column chromatography experiments, two major factors controlling the separations were observed. The first was the possibility of formation of inclusion complexes with the □-CD pores or the pore structures induced by linking between □-CD and supports. The other was the steric hindrance effect between the eluting compounds and the stationary phases. The steric hindrance factor was confirmed from the packed column chromatography tests with unmodified 5□m silica particles having 8nm pores and unmodified 50-100□m glass beads having 31.6nm pores as well. Molecules having smaller cross sectional areas have less steric hindrances in the columns packed with unmodified silica or glass beads. The elution orders from unmodified packing were generally ortho>meta>pare-xylene among xylene isomers and 1-naphthol>2-naphthol between naphthol isomers as shown in Figure 1. However, the column packed with the directly bonded □-CD silica (its loading 23.8%) has a different elution order para>ortho>meta-xylene among xylene isomers, and 2-naphthol>1-naphthol between naphthol isomers. These results suggest that p-xylene forms strong inclusion complexes with the □-CD pore, as a result, p-xylene have the longest retention period among xylene isomers. The separation between meta and oroth-xylene are determined by differences in the capability of inclusion complex formation and in the steric hindrance effect of two isomers as well. Adsorption capacities of the □-CD and unmodified silica for the vapor of para, meta, and ortho-xylene in the stream of helium are 1.49%, 0.63%, 0.83% and 9.42%, 6.73%, 9.21%, respectively. Molecular simulation has shown that 2-naphthol form more stable inclusion complexes than 1-naphthol which is consistent with the chromatographic data. Naphthol isomer separation is controlled by the formation of inclusion complexes with the □-CD.

When the □-CD directly bonded glass beads (its loading 15.6%) were used, the xylene elution order was ortho>para>meta-xylene. This result is interpreted that the portion of larger pores of the □-CD bonded glass beads bed is much larger than that of the □-CD bonded silica bed because of the larger pores (31.6nm) and particles sizes (5-100□) of the glass beads. Thus, ortho-xylene is more strongly bound rather than para-xylene to the □-CD bonded glass beads. This trend was conformed in the xylene adsorption experiment to the glass beads. The adsorption capacities of the glass beads for the vapor of para, meta, and ortho-xylene in the stream of helium are 3.30%, 3.07%, and 3.59%, respectively. It is known that separation selectivity is determined by bonded phase and support material as well.

(Effect of Spacer Group) The □-CD directly bonded silica has a strong binding force for p-xylene. From the column chromatography data, we speculate that the pore openings of the cyclodextrin host can be narrowed or blocked by spacer groups, 3-glycidoxypropyltrimethoxysilane. The spacer □-CD silica (its loading 10.7%) has a greater affinity for o-xylene rather than p-xylene possibly due to weaker inclusion complex formation by pore narrowing and pore blocking, which could be offset by a greater steric hindrance effect from the spacer groups. In addition, binding forces for o-xylene are increased by the pores induced by the spacer groups between the surfaces of silica and □-CD. When glass beads were used as a supporting, the spacer group effect was negligible in the separation of xylene isomers between the □-CD directly bonded glass beads and the spacer □-CD glass beads because of the large pores of the glass beads. However, a large difference in the separation of naphthol isomers was observed between the □-CD directly bonded glass beads and spacer □-CD glass beads. The □-CD directly bonded glass beads having a strong binding force for 2-naphthol due to inclusion complex formation mechanism. In comparison, the inclusion complex formation of 2-naphthol was lowered because of increasing steric hindrance contribution. Thus, selectivity of the spacer □-CD glass beads for naphthol isomers was decreased.

(Effect of Cross Linking) The □-CD direct bonded glass bead packing gives a good selectivity between xylene isomers (elution order ortho>para>meta-xylene) and a high selectivity to naphthol isomers (elution order 2-naphthol>1-naphthol). These results show inclusion complex formations of 2-naphthol with the □-CD pores. By comparison, the cross-linked □-CD direct bonded glass beads by 1,8-trichlorosilyloctane lose the selectivity between naphthol isomers due to pore narrowing and blocking by the cross-linking agent. In contrast, the selectivity between xylene isomers is clearly increased by cross-linking maintaining the same elution order (ortho>para>meta-xylene) as shown in Figure 2. In particular, the binding force for o-xylene is clearly increased and the one for p-xylene is slightly decreased. A possible explanation for this result is that cross-linking narrows the □-CD pore openings, thus, inclusion complex formation of p-xylene is limited. Conversely, large pores induced by cross-linking agent among □-CDs and glass beads give a better binding force for ortho-xylene. The cross-linked □-CD direct bonded glass beads by epichlorohydrin lose the selectivities between naphthol isomers and among xylene isomers. This result means that three atoms chain is too short to cross-link between □-CDs, and the pore opening of □-CD is narrowed or blocked by the reaction of epichlorohydrin with hydroxyl groups of □-CD in an alkaline medium.

Literature Cited

1. Szejtli, J. Chem. Rev. 1998, 98, 1743-1753

2. Armstrong, D. W.; DeMond, W. J. Chromatogr. Sci. 1984, 22, 411-415

3. Armstrong, D. W. U.S. Patent 1985, No.4,539,399

4. Armstrong, D. W.; DeMond, W.; Alak, A. Anal. Chem. 1985, 57, 234-237.

Figure 1. The compositional plots of eluting solutes from the column packed with unmodified glass beads with methanol mobile phase versus time (minutes)

Figure 2. The compositional plots of eluting solutes from the column packed with □-CD glass beads cross-linked by 1,8-trichlorosilyloctane with methanol mobile phase versus time (minutes)