The Investigation of Hydration Processes in Horse Chestnut (Aesculus hippocastanum L.) and Pine (Pinus silvestris L.) Bark and Bast Using Proton Magnetic Relaxation

Abstract

Summary The proton free induction decay (FID) was shown to be effective in monitoring of hydration and the water soluble extractive fraction in horse chestnut (Aesculus hippocastanum, L.) and pine (Pinus silvestris, L.) bark and bast. The signal from the first bound water layers was separated from the whole liquid signal (L) and at low hydration the liquid signal from the stable sealed bark pores was detected. The liquid-to-solid (L/S) signal ratio as a function of relative mass increase (∆m/m0) was proposed as a convenient method to evaluate the relative mass of water, necessary to dissolve the whole soluble proton pool (∆M/m0), the relative mass contribution of solid proton component (p0 in absence of water soluble fraction or pS0 in presence of water soluble fraction), the saturation concentration of the water soluble fraction (cs) and the effective (scaled to water) proton density of solid protons (βs). The measurement of the absolute (in arbitrary units) proton signal versus relative mass increase yielded additionally the effective proton density water soluble proton fraction in solid (βcu) and in liquid (βcd) phase. For the system containing the soluble proton fraction (horse chestnut bast), the values of the parameters obtained from L/S versus ∆m/m0 dependence were compared with the calculated (or fitted) values obtained from the results of the absolute (in arbitrary units) NMR signal measurements. The effective proton densities (βs) were: 0.33 for pine bark, 0.31 for horse chestnut bark and 0.20 for horse chestnut bast, which suggests the presence of paramagnetic impurities in the solid matrix. The presence of various paramagnetic species was further confirmed by ESR spectroscopy. For horse chestnut bast, in which a water soluble extractive fraction is present, the saturation concentration (cs) of the water soluble fraction was 0.64. The contribution of the water soluble fraction was 0.324 of the total dry mass and the relative proton density βcu = 0.55 (in the solid phase). The contribution of non-removable liquid fraction was 0.004 of the total dry mass. The proton relaxation experiment did not directly reveal a percolation threshold, however, the dehydration extended below the removal of the loosely bound mobile water fraction.