Study On Fabrication And Growth Mechanism Of Cyclodextrins Self-assembled Monolayers Based On Its Host-guest Interaction

Cyclodextrins (CDs) can form inclusion complex with inorganic, organic and small or big biological molecules by its host-guest interaction, and it has aroused much attention in separation field. If CDs or CD derivatives self-assembled monolayers (SAMs) with oriented structure are assembled onto the surface of polymer membrane by electrostatic self-assembly, hydrophilic or hydrophobic interactions, it will positively impact CDs’selective separation performance. However, there are two critical challenges for preparing CDs SAMs by host-guest interaction with guest SAM:1) appropriate space between guest molecules in a homogeneous SAM is needed for CDs to form inclusion complex;2) second, a close approach for CDs to the guest molecule needs to be supplied before any kind of noncovalent interaction can occur.Firstly, the self-assembled processes and the obtained molecular layer structures of methylphenyldichlorosilane, phenyltrichlorosilane/trichlorosilane, methylphenyldichlorosilane/trimethylchlorosilane, phenyltriethoxysilane (PTES) were studied. The results showe that methylphenyldichlorosilane and phenyltrichlorosilane/trichlorosilane tend to form multilayer multilayer, there are much more trimethylchlorosilane molecules than methylphenyldichlorosilane molecules in methylphenyldichlorosilane/trimethylchlorosilane layer, however, PTES can form loose packed monolayer. PTES was selected as the objective molecule to fabricate guest molecular SAM, its phenyl group of PTES can be included by a-or β-CD, its three bulky ethoxyl groups can control the molecular distance in PTES SAM, and the hydrophilicity of PTES SAM will improve after ethoxyl groups hydrolyze.Secondly, the growth mechanism of PTES SAM was studied, it was revealed that there are two growth stages in the formation of closely packed PTES SAM. The first growth stage is from0to7.5h, the contact angle increases to33°rapidly, and then fluctuates around this value until the end of this stage. At this point substrate surface is almost completely covered by PTES molecules with lots of unhydrolyzed ethoxyl groups; the second growth stage is from7.5to24h, contact angle grows from33°to57°gradually, then keeps at this value, the self-assembled process of PTES finishs. The PTES molecules in PTES SAM are tilted to substrate surface, and molecular number in the PTES SAM obtained in second growth stage is about twice of the number obtained in first growth stage.Thirdly, it was found that PTES SAM which reacted for3h (PTES SAM-3h) can form inclusion complex with α-or β-CD. Compared with β-CD SAM, the inclusion complexes formed in a-CD SAM are more stable, and there are more inclusion complexes ina-CD SAM. The surfaces of PTES SAM-3h, α-and β-CD SAMs are all rather smooth, the tightnesses of these three SAMs rise in turn.Finally, The inclusion configurations in α-and β-CD SAMs are different. The thicknesses of αand β-CD SAMs are1.0±0.1nm and0.72±0.1nm respectively. In a-CD SAM, the mean diameter of a-CD cavity is similar to that of phenyl group, so a-CD molecule is tilted to the silica surface in order to include the phenyl group of PTES sufficiently; for P-CD SAM, it can include the phenyl group of PTES completely even its surface is parallel to the silica surface due to the large cavity of p-CD.