Elsevier

Fluid Phase Equilibria

Volume 301, Issue 2, 25 February 2011, Pages 234-243
Fluid Phase Equilibria

Binary diffusion coefficients of phenolic compounds in subcritical water using a chromatographic peak broadening technique

https://doi.org/10.1016/j.fluid.2010.12.003Get rights and content

Abstract

Infinite dilution diffusion coefficients of certain phenolic compounds were measured as a function of temperature in water slightly acidified with formic acid using the Taylor dispersion method. The diffusion coefficients calculated using the chromatographic peak broadening technique were found to increase exponentially with an increase in the temperature. The diffusion coefficients of the selected phenolic compounds did not vary as a function of their molecular weights and the diffusion coefficients of the phenolic compounds increased as a function of temperature (from 2.16 × 10−10 m2 s−1 at 298 K to 5.79 × 10−10 m2 s−1 at 413 K for malvidin-3,5-diglucoside). However, for some phenolic compounds such as gallic acid monohydrate, quercetin-3-β-d-glucoside, protocatechuic acid and (−)-epicatechin, there were difficulties in making measurements above temperatures of 352 K, 372 K, 392 K and 413 K, respectively, due to thermal degradation of the phenolic compounds in water above these temperatures. The experimentally measured diffusion coefficients of the phenolic compounds were correlated as a function of temperature and solvent viscosity and were compared with those predicted using theoretical models. The validity of the Stokes–Einstein diffusion model in predicting the diffusion coefficients of the phenolic compounds in hot pressurized water was also evaluated.

Introduction

Extraction of value-added products from natural matrices using pressurized fluids is gaining widespread application in the food and nutraceutical industries. Phenolic compounds are compounds present in natural products and are known for their benefits to human health due to their antioxidant, anti-microbial, anti-viral and anti-proliferative properties [1]. However, the knowledge of fundamental solution properties such as solute solubility and binary diffusion coefficients of polyphenolic compounds is required to optimize their extraction from natural products. Such data are limited in the literature and are also quite difficult to measure due to the molecular complexity of such compounds and their sensitivity to heat and light. Studies have indicated that the prediction of binary diffusion coefficients of various compounds using theoretical models such as the Stokes–Einstein model and hydrodynamic theory can show a deviation up to 20% from that measured experimentally [2]. In this study, we have measured the binary diffusion coefficients of certain phenolic compounds in water at temperatures above its boiling point.

Water is an environmentally benign solvent, and at higher temperatures, exhibits properties such as a higher diffusivity and lower viscosity [3]. Subcritical water, at temperatures above its boiling point and kept liquefied under pressure, can be used as a solvent to extract polar organic compounds from natural product matrices [4]. Hot pressurized water has been used for example to extract antioxidants from rosemary plants [5], canola meal [6], Eucalyptus grandis [7], citrus [8] and grape pomaces [9].

Knowledge of the thermodynamic and mass transfer properties of the phenolic compounds in subcritical water is important in predicting optimized conditions for maximum recovery from natural products. The Hansen solubility parameter concept has been used to predict the optimal temperature range for the extraction of phenolic compounds from natural products [10]. However, studies have indicated that the extraction rates of certain subcritical water extraction processes, especially at lower solvent flow rates, are mass-transfer limited [11]. Mass transport properties such as binary diffusion coefficients are dependent on the chosen processing conditions such as temperature, pressure, pH, residence time, particle size and solvent flow rate.

Studies have indicated that moderate pressures (>40 bar) are required to maintain water in its subcritical state, and that pressure has a negligible effect on the extraction of solutes from natural matrices [2]. However, a number of studies have been performed that indicate a significant effect of temperature and pH [12], particle size [13] and solvent flow rate [14] on the subcritical water extraction of antioxidants from natural products. Studies performed by Cacae and Mazza [15] on the extraction of flavonoid compounds from milled frozen black currants using aqueous ethanol showed that the extraction rate was dependent on both temperature and ethanol concentrations. This study also indicated that thermal degradation of the phenolic compounds occurred at high temperatures resulting in lower flavonoid yield. Another study [16] by Palma et al. reported that phenolic compounds, especially aglycones, showed lower stabilities at temperatures greater than 372 K resulting in lower recoveries of flavonoids such as catechin and epicatechin, when methanol was used as solvent. Recent studies in our group have shown that even though the solubility of most flavonoids compounds increases exponentially with increasing temperature [17], [18], it is also critical to determine physicochemical data which influence the mass transfer of solutes in this temperature range.

A number of methods have been used for measuring binary diffusion coefficients of organic compounds in solvents such as light scattering [19], nuclear magnetic resonance spectroscopy [20], the diaphragm-cell method [21], a membrane-based technique [22], interferometry [23], capillary evaporation [24] and the Taylor dispersion method [25] and its well known variant, the chromatographic impulse response method [26]. There are few studies on the measurement of diffusion coefficients of flavonoids in the literature. Mantell et al. [27] measured the infinite dilution binary diffusion coefficient of malvidin-3,5-diglucoside in supercritical carbon dioxide at different temperature, pressure and methanol (co-solvent) conditions using the Taylor dispersion method. Diffusion coefficients of catechin and epicatechin in water have also been measured using NMR pulsed-field gradient spin echo technique [28]. The diffusion coefficient of catechin at infinite dilution in water by this technique was found to be about 7.9 × 10−10 m2 s−1. The Taylor dispersion technique was also used to measure the diffusion coefficients of various flavonoids in alcoholic solvents at 298 K to study the effect of the hydrogen bonding between the solute and the solvent on mass transfer [29]. However, there are no data available in the literature on the diffusion coefficients of the phenolic compounds in water above its boiling point.

In this study, the binary diffusion coefficients at infinite dilution of the phenolic compounds in hot pressurized water were measured using a chromatographic peak-broadening method. The experimentally measured diffusion coefficients as a function of temperature were compared with those predicted using theoretical calculations based on the Stokes–Einstein model.

Section snippets

Theoretical background

The binary diffusion coefficient of the phenolic compounds can be measured using a method developed by Taylor [30] and extended by Aris [31]. The Taylor–Aris dispersion method involves the injection of a pulse of solute into a continuous flow of solvent. The solute flows through a capillary column placed in a constant temperature oven and the concentration profile of the solute is recorded at the outlet from the oven using an absorbance detector. The dispersion of the solute pulse through the

Chemicals

The description and product information on the phenolic compounds used in this study are shown in Table 1. Ultrapure water (18.2 Ω cm; 1–5 ppb TOC and <0.001 EU/mL pyrogen levels) was obtained from a Milli-Q Synthesis A10 system (Millipore, Bellerica, MA, USA). Formic acid (CAS# 64-18-6; ACS grade) was purchased from VWR (Batavia, IL, USA). The solvent (0.5% (v/v) formic acid in water) was degassed using a nitrogen purge. For convenience, the molar volume at the boiling point of the phenolic

Effect of temperature on the experimentally measured diffusion coefficients

The diffusion coefficient of the phenolic compounds as a function of temperature are reported in Table 2 and plotted in Fig. 2. It can be seen from Fig. 2 that the experimentally measured diffusion coefficients of the phenolic compounds in water increase exponentially with an increase in temperature. This is also shown in Fig. 2 by imposing an exponential trend line for the diffusion coefficient of malvidin-3,5-diglucoside chloride plotted as a function of temperature. This trend line shows

Conclusions

The binary diffusion coefficients of the phenolic compounds in water at infinite dilution were measured using Taylor dispersion method between 298 K and 413 K. The measured diffusion coefficients of the phenolic compounds increased exponentially with an increase in temperature. The measured diffusion coefficients of the phenolic compounds in water were correlated as a function of temperature and solvent viscosity. It was found that the values of /T did not remain constant as a function of

Acknowledgment

This study was supported by the United States Department of Agriculture (grant number 2006-35503-17618) under the CSREES National Research Initiative (NRI).

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