Binary Mutual Diffusion Coefficients of Aqueous Solutions of Sucrose, Lactose, Glucose, and Fructose in the Temperature Range from (298.15 to 328.15) K Ana C. F. Ribeiro,* ,² Ornella Ortona, ‡ Susana M. N. Simo ˜ es, ² Cecı ´lia I. A. V. Santos, ² Pedro M. R. A. Prazeres, ² Artur J. M. Valente, ² Victor M. M. Lobo, ² and Hugh D. Burrows ² Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal, and Department of Chemistry, University of Naples, Federico II, Monte Sant’Angelo, 80126 Naples, Italy Binary mutual diffusion coefficients measured by the Taylor dispersion method in two different laboratories (University of Naples, Federico II, Italy, and University of Coimbra, Portugal) are reported for aqueous solutions of lactose, sucrose, glucose, and fructose at various concentrations (0.001 to 0.1) mol‚dm -3 and temperatures (298.15 to 328.15) K. The hydrodynamic radius and activation energy for the diffusion of aqueous sugars are calculated from those results. In addition, the measured diffusion coefficients are used with the Hartley equation to estimate activity coefficients for aqueous lactose, sucrose, glucose, and fructose. Introduction Carbohydrates are not only technological important com- pounds but also enjoy biological relevance. 1-3 As typical non- electrolytes carrying hydrophilic hydroxyl groups capable of hydrogen bonding, their properties play a significant role in the reaction conditions of many current industrial processes such as enzymatic conversion of biomass to useful chemicals. Furthermore, they are important components in formulations for pharmaceutical, food, and biomedical applications (e.g., for stabilization of proteins and membranes). 1-3 While numerous studies have been carried out on the thermodynamic properties of binary aqueous sugar solutions (e.g., activity coefficients, excess enthalpies, etc.), 2,4-7 data are more limited on the transport behavior of these sugar systems in aqueous solutions. 8-12 Transport properties, particularly diffusion coefficients, provide a direct measure of molecular mobility, an important factor in the preservation of biological materials in sugar matrixes. Hopefully, the studies reported here will lead to an increase in know-how, which will allow a better understanding of the physical chemistry conditions underlining the diffusion phenomena occurring in different systems (e.g., human oral cavity). Diffusion coefficients for sucrose and glucose have been previously reported. 8-11 However, those studies mainly focused on sucrose concentrations greater than 0.05 mol ‚dm -3 at 298.15 K. A study of mutual diffusion coefficients (D) of glucose and sucrose, at (303.15 and 323.15) K, obtained by the capillary cell method in concentrated solutions (c > 1.0 mol‚dm -3 ) was reported by Sano and Yamamoto. 1 These authors establish an empirical linear relationship between log D and the mole fraction of the solute (carbohydrate). However, bearing in mind the empirical nature of those equations and considering that the above authors admit the possible error limits in these values of D are of the order up to 30 %, the efforts in our repeating experimental diffusion study of these sugars appears justified. In fact, comparison of our experimental results with those obtained in this work through the cited equations leads to deviations greater than 30 % for these two carbohydrates. As far as we are aware, no data are available in the literature for lactose and fructose. In the present study, mutual diffusion (interdiffusion) coef- ficients D, measured by the Taylor dispersion method, are reported for aqueous solutions of lactose, sucrose, glucose, and fructose at concentrations from (0.001 to 0.1) mol‚dm -3 and temperatures from (298.15 to 328.15) K. The accuracy of the Taylor diffusion measurements is assessed by measuring binary mutual diffusion coefficients for aqueous solutions of potassium chloride at 298.15 K and comparing them with previously reported D values measured by accurate optical interferometric and conductometric techniques. 13-15 The reproducibility of these results was usually within ( 1 %. Comparison of the results suggests an uncertainty of (1 to 2) % for the D values reported here, which is typical for Taylor dispersion measurements. Experimental mutual diffusion coefficients were used to estimate various parameters such as the hydrodynamic radii and activa- tion energy for the diffusion of those aqueous carbohydrates. In addition, the measured diffusion coefficients are used with the Hartley equation to estimate activity coefficients for aqueous carbohydrate solutions. Experimental Section Materials. The solutes used in this study were lactose (BDH Chemicals with a water content of 10.0 %), sucrose (Sigma, pro analysi > 99 %), D(+)-glucose (Fluka, pro analysi > 99.5 %) and D(-)-fructose (Riedel-de-Hae ¨n, Chem. pure). These were used without further purification. The solutions for the diffusion measurements were prepared in calibrated volumetric flasks using bi-distilled water. The solutions were freshly prepared and de-aerated for about 30 min before each set of runs. The uncertainty on their compositions was usually within ( 0.1 %. Procedure. Dispersion methods for diffusion measurements are based on the dispersion of small amounts of solution injected into laminar carrier streams of solvent or solution of different composition, flowing through a long capillary tube. 16-19 The length of the Teflon dispersion tube used in the present study * Corresponding author. Tel: +351-239-854460. Fax: +351-239-827703. E-mail: anacfrib@ci.uc.pt. ² University of Coimbra. ‡ University of Naples, Federico II. 1836 J. Chem. Eng. Data 2006, 51, 1836-1840 10.1021/je0602061 CCC: $30.25 © 2006 American Chemical Society Published on Web 07/20/2006 Downloaded by PORTUGAL CONSORTIA MASTER on July 10, 2009 Published on July 20, 2006 on http://pubs.acs.org | doi: 10.1021/je0602061