Examination of water phase transitions in Loblolly pine and cell wall components by differential scanning calorimetry

This paper examines phase transformations of water in wood and isolated wood cell wall components using differential scanning calorimetry with the purpose of better understanding “Type II water” or “freezable bound water” that has been reported for cellulose and other hydrophilic polymers. Solid loblolly pine (Pinus taeda) milled pine, and holocellulose, cellulose, and lignin isolated from the same parent board were tested. For each sample preparation, the freezing and melting of water was examined at 10–20 different moisture contents. Only one freezing peak was observed for most sample preparations; in these cases, the phase transformation corresponded to the “Type I” or “free water” peak. The freezing and melting temperatures of this Type I peak depended on moisture content, and appreciable undercooling (−30 °C from bulk water) was observed at the lowest moisture contents. The isolated ball-milled cellulose exhibited a Type II peak, yet the Wiley-milled cellulose, from which the ball-milled cellulose was made, did not exhibit a Type II peak. When present, the Type II peak is consistent with the homogeneous nucleation temperature of bulk water. The results suggest that the detection of Type II or freezable bound water in wood may depend more on sample preparation than the chemical nature of the cell wall component in question; a point not discussed by previous researchers describing Type II water in hydrophilic polymers.