Influence of potassium chloride on diffusion of 2-hydroxypropyl-β cyclodextrin and β-cyclodextrin at T = 298.15 K and T = 310.15 K

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Abstract

Ternary mutual diffusion coefficients measured by the Taylor dispersion method (D11, D22, D12 and D21) are reported for two aqueous systems, that is, potassium chloride and 2-hydroxypropyl-β-cyclodextrin (HP-βCD), and potassium chloride and β-cyclodextrin (βCD) at T = 298.15 K and T = 310.15 K, and at carrier concentrations from (0.000 to 0.010) mol·dm−3 for each solute. Based on these data, some conclusions about the influence of KCl on the diffusion of the cyclodextrins (HP-βCD and βCD) were made, and the association constants of the complexes formed between the two components for each system (KCl and βCD, and KCl and HP-βCD) were estimated.

Highlights

► Ternary diffusion coefficients for the systems KCl/βCD/H2O and KCl/HP-βCD/H2O. ► Influence of the KCl on diffusion of some cyclodextrins in the aqueous media. ► Estimation of the association constants of the complexes KCl/βCD and KCl/HP-βCD.

Introduction

Cyclodextrins (CDs) are cyclic oligosaccharides [1], [2], [3], [4], [5] having a truncated cone structure with a hydrophilic external surface and a relatively non-polar cavity. They are among the most important carriers used in drug release systems for local anaesthetics and other pharmaceuticals. The basis for this popularity is the ability of these materials to solubilize poorly soluble drugs and drug candidates, resulting in striking increases in their solubility in water and rates of diffusion. Taking into account their impact on biological systems and that most intra- and extra-cellular physiological fluids have a significant presence of ions (e.g., K+ and Cl), we strongly believe in the actual interest in studying the influence of this electrolyte on the diffusion of cyclodextrins in aqueous systems.

While some studies have been devoted to the structural aspects [4], [5], few have taken into account the multicomponent diffusion work on aqueous cyclodextrin solutions and the alteration of these properties as a result of the presence of different components in these solutions [6], [7], [8], [8], [10], [11], [12], [13], [14], [15]. The present work intends to fill this gap with the experimental determination of ternary diffusion coefficients D11, D22, D12 and D21 for the system containing potassium chloride (1) and 2-hydroxypropyl-β-cyclodextrin (HP-βCD) (2), and for the other one involving potassium chloride (1) and β-cyclodextrin (βCD) (2), already existing in the literature for the later some data we have published [13]. The measurements are made at T = 98.15 K and T = 310.15 K, and at carrier concentrations from (0.000 to 0.010) mol·dm−3 for each solute.

Comparing the main diffusion coefficients D11 and D22 and the binary diffusion coefficients of each component for the two systems (βCD and KCl) and (HP-βCD and KCl), and using the values of cross diffusion coefficients D12 and D21, we can conclude about the influence of the solutes in the diffusion behaviour of this salt and vice-versa. Also, from the ratios D21/D11 and D12/D22, it is possible to obtain information concerning the number of moles of each component transported per mole of the other component driven by its own concentration gradient. In addition, it will be possible to estimate some parameters, such as the association constants of the complexes formed between those components (KCl and βCD, and HP-βCD).

Section snippets

Materials

The β-cyclodextrin (β-CD) and 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) were purchased from Sigma, Germany, with water mass fraction of 0.131 and 0.0010, respectively (table 1), determined by drying to constant mass at T = 420 K in a nitrogen atmosphere. The solutions for the diffusion measurements were prepared in calibrated volumetric flasks using double distilled water. The solutions were freshly prepared and de-aerated for about 30 minutes before each set of runs.

Diffusion measurements

Having in mind that the Taylor

Results and discussion

Diffusion coefficient values, D11, D12, D21 and D22, for two ternary systems (that is, KCl (1) + βCD (2) + H2O, and KCl (1) + HP-βCD (2) + H2O) at T = 298.15 K and T = 310.15 K are summarized in TABLE 2, TABLE 3, TABLE 4, TABLE 5. These results are the average of six experiments. The main diffusion coefficients D11 and D22 were generally reproducible within ±0.035 × 10−9 m2·s−1. The cross diffusion coefficients were reproducible within ±0.050 × 10−9 m2·s−1.

As indicated in TABLE 2, TABLE 3, TABLE 4, TABLE 5, values

Conclusions

From our experimental results for the diffusion coefficients, Dij, and the values found for the association constants of both systems (i.e., KCl plus β-cyclodextrin, and KCl plus 2-hydroxypropyl-β-cyclodextrin), we can conclude that the potassium chloride species influences the diffusion of those cyclodextrins.

Acknowledgements

Financial support from FCT (FEDER)-PTDC/AAC-CLI/098308/2008 and PTDC/AAC-CLI/118092/2010 is gratefully acknowledged. C.I.A.V.S. is grateful for SFRH/BD/45669/2008 from “Fundação para a Ciência e Tecnologia”.

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