Sorption properties of β-cyclodextrin ― citric acid derivatives

β-cyclodextrin―citric acid derivatives were synthesized trough heating of mixture of citric acid, cyclodextrin and Na2HPO4 as a catalyst. The degree of substitution of the derivatives was estimated by determination of citric acid by HPLC in the samples hydrolysates. Sorption properties of β-cyclodextrin―citric acid derivatives were determined using spectrophotometric method with methyl orange as a model sorbate. Sorption properties of β-cyclodextrin―citric acid derivatives were compared to native β-cyclodextrin. Introduction Cyclodextrins (CD) are cyclic oligosaccharides consisting of α-1,4glycosidically linked D-glucose units (Fig.1). Cyclodextrin molecule has a hydrophobic inner cavity, contrasting with the two hydrophilic openings. A wide range of organic and non-organic molecules form host-guest inclusion complexes with cyclodextrins. Moreover, formation of inclusion complexes is also a main mechanism of sorption by water insoluble CD polymers. Recently, polycarboxylic acids, especially citric acid (CTR), have become a popular crosslinking agent able to


Introduction
Cyclodextrins (CD) are cyclic oligosaccharides consisting of α-1,4glycosidically linked D-glucose units (Fig. 1).Cyclodextrin molecule has a hydrophobic inner cavity, contrasting with the two hydrophilic openings.A wide range of organic and non-organic molecules form host-guest inclusion complexes with cyclodextrins. 1 Moreover, formation of inclusion complexes is also a main mechanism of sorption by water insoluble CD polymers. 2Recently, polycarboxylic acids, especially citric acid (CTR), have become a popular crosslinking agent able to
Figure 1 Chemical structure of the three main types of cyclodextrins. 18

Results & discussion
Spectrophotometric method CD ANT proved that the degree of substitution in most samples of investigated derivatives was around 4, which means that 4 CTR molecules react with 1 molecule of CD.As a matter of fact, the addition of NaOH to a mixture of substrates don't significantly influence the degree of substitution.However, the amount of a product was decreasing within increasing amount of NaOH added to the mixture of substrates.Probably it was caused by formation of water-soluble (less crosslinked) products.

Time dependency of the absorbance of methyl orange solutions in the presence of CD-CTR samples (K1,K3,K4), citric acid solution (CTR), cyclodextrin solution (CD) and blank sample (MO).
Analysis of sorption properties confirmed that CD molecules, present in investigated derivatives, preserved their ability to form inclusion complexes.
However, CD-CTR samples complexed MO from its solution much slower than native CD molecules.Moreover, the CD content in CD-CTR samples was around 60% (CD ANT ), and 50% of the CDs were able to form inclusion complexes (CD MO ).The limited accessibility of the CD cavity in the polyester may have been the result of the relative overcrowding of the CD molecules and the presence of CD esters with sterically hindered cavities.Furthermore, in all samples an aggregate of CTR and CD content was around 95% instead of 100%.Probably residual ~5% refers to presence  Water insoluble polymers -poly(β-cyclodextrin-co-citric acid) (CD-CTR) (Fig. 3) were synthesized through heating of mixture of CTR,CD and Na 2 HPO 4 as a catalyst with 6:1:2 molar ratios at 170 o C for 20 minutes (Fig. 2).The crude product was powdered and purified by soaking with distilled water and centrifuged until supernatant was free of unreacted material and the catalyst.CD content was determined using standard anthrone protocol 19 products like unsaturated carboxylic acids.Presence of those acids could be a result of heating of citric acid above its melting point (153°C) cousing its dehydration and thermal decomposition.Unsaturated carboxylic acids can also take place in esterification of CD's20 (CD ANT ) and a spectrophotometric method based on decolorization of methyl orange (MO) upon complexation by CDs (CD MO ).Mixtures of 0.023 mM MO in 0.1 M HCl with different concentrations of CD were prepared and absorbance (A) at 508 nm were measured against 0.1M HCl as a blank sample.Calibration curve was plotted as a dependency of 1/(A 0 -A) against 1/[CD].A suspension of 100 mg of CD-CTR in 10 ml of 0.023 mM MO in 0.1M HCl was prepared.Absorbance measurements were conducted in specific periods of time and suspensions were centrifuged before every measurement.An absorbance of supernatant at 508 nm was measured against 0.1M HCl as a blank sample.The CD content was calculated based on the calibration curve.

Figure 3
Figure 3 Poly(β-cyclodextrin-co-citric acid) . Finally, it was noticed that absorbance of MO solutions in presence of CD and CTR samples was constant against time.