Author
Satoru Nakashima, Research Institute for Natural Environment, Science and Technology (RINEST)
Keywords
infrared (IR) spectroscopy, quar tz cr ystal microbalance (QCM), relative humidity (RH), water adsorption/desorption, water contents, IR OH band areas
Abstract
IR micro-spectroscopy equipped with a quartz crystal microbalance (QCM) and a relative humidity (RH) control system has been developed. This IR/QCM/RH method has been applied to various materials for evaluating quantitatively water adsorption/desorption processes. Pectin containing Ca was found to retain much more water than that without Ca, possibly among polysaccharaide network chains made by COO–..Ca2+..–OOC. Water contents of collagen, keratin, lecithin and ceramide were found to be 24.6, 17.0, 12.2 and 1.2 wt% around RH＝80%, respectively. Water adsorbed on a clay (Na-montmorillonite) could be evaluated quatitatively. Adsorbed forms of phtalic acid on goethite (α-FeOOH) surfaces were found to change from bidendate (two COO– bound to Fe3+) to monodentate (one COO– bound to Fe3+) with protonation of one COO– to COOH with increasing RH. In most of these cases, water contents (weight ratios or weight %) and OH stretching or bending band areas show quasi-linear relations, which can be used for calibrating water contents from IR bands. The IR/QCM/RH method does not require knowledge of sample thickness (d) nor absorption coefficient (ε), but determine directly masses of adsorbed or desorbed water down to ng level together with the sample weights upon drying from solutions or suspensions. Therefore, this metod can be very useful for evaluating water contents and IR signatures of various materials upon adsorption/desorption of water, which are difficult to be done by other conventional methods.