Wood-water relationships: (I) Molecular sorption and capillary retention of water by Sitka spruce wood

An attempt is made to distinguish experimentally between the amount w a of water held by molecular sorption, and that, w c , held by capillary retention on finely ground flour of Sitka spruce wood. The method used consists in flooding the evacuated flour with an evacuated aqueous solution of sucrose and measuring the change in concentration of the solution with an interferometer. Since capillary retention requires for its manifestation free liquid surfaces in the interstices of the wood, it is claimed that, when the vacuum method here described is used, only true molecular sorption will cause a change in concentration of the solution. Since there are indications that a certain small amount of sucrose also is adsorbed by the wood, it is impossible to arrive at the exact value of w a , but a minimum possible value w a (min.) is given which varies with the vapour pressure of the solution used, and amounts, on extrapolation to infinite dilution, to some 23 per cent of the dry weight of the wood. The difference between the total sorption W measured on the vapour-pressure isothermal and the values of w a (min.) here found is taken to represent the maximum possible value for w c . The question of the sorption of sucrose is discussed, and the suggestion is made that the moisture-content hysteresis in w a is smaller than that in W ; thus a greater proportion of water is held by capillary forces on desorption than on adsorption.