The investigation of hydration processes in horse chestnut (Aesculus hippocastanum, L.) and pine (Pinus silvestris, L.) bark and bast using proton magnetic relaxation

The proton free induction decay (FID) was shown to be effective in monitori ng 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 s ealed bark pores was detected. The liquid-to-solid (L/S) signal ratio as a function of relative mass increase (Delta m/m(0)) was proposed as a conveni ent method to evaluate the relative mass of water, necessary to dissolve th e whole soluble proton pool (Delta M/m(0)), the relative mass contribution of solid proton component (p(0) in absence of water soluble fraction or p(s 0) in presence of water soluble fraction), the saturation concentration of the water soluble fraction (c(s)) and the effective (scaled to water) proto n density of solid protons (beta(s)). The measurement of the absolute (in a rbitrary units) proton signal versus relative mass increase yielded additio nally the effective proton density water soluble proton fraction in solid ( beta(cu)) and in liquid (beta(cd)) phase. For the system containing the sol uble proton fraction (horse chestnut bast), the values of the parameters ob tained from L/S versus Delta m/m(0) dependence were compared with the calcu lated (or fitted) values obtained from the results of the absolute (in arbi trary units) NMR signal measurements. The effective proton densities (beta(s)) were: 0.33 for pine bark, 0.31 for horse chestnut bark and 0.20 for horse chestnut bast, which suggests the p resence of paramagnetic impurities in the solid matrix. The presence of var ious paramagnetic species was further confirmed by ESR spectroscopy. For horse chestnut bast, in which a water soluble extractive fraction is pr esent, the saturation concentration (c(s)) of the water soluble fraction wa s 0.64. The contribution of the water soluble fraction was 0.324 of the tot al dry mass and the relative proton density beta(cu) = 0.55 (in the solid p hase). The contribution of non-removable liquid fraction was 0.004 of the t otal dry mass. The proton relaxation experiment did not directly reveal a percolation thre shold, however, the dehydration extended below the removal of the loosely b ound mobile water fraction.