Successful Direct Observation of Special Bound Water on Cotton Surface Study of Mechanism of Hardening of Wet Cotton Fabric After Natural Drying

In daily life, it is often observed that a cotton towel becomes stiff as if glued when washed without fabric softener and naturally dried. Although common, the mechanism has not been well understood thus far. Kao Corporation has conducted investigations of this hardening mechanism for a long time as part of its research for development of fabric softeners, which led to focused attention given to a special type of water on cotton surfaces, so-called bound water. Bound water is that which binds with a material surface and known to have different molecular mobility from ordinary bulk water, as well as a lower freezing point.
Kao's previous studies and those performed by various measurement techniques^*6 have revealed that a certain amount of bound water is adsorbed on the surfaces of naturally-dried cotton,^{*7 *8} with the bound water showing unique properties different from ordinary water. Based on those findings, Kao has proposed the following model^*4 : Cotton exhibits stiffness when naturally dried after wetting as bound water adsorbed on the surface of single cotton fibers causes cross-linking by "capillary adhesion".^*9
However, it has not been clarified how bound water is formed on cotton surfaces, why capillary adhesion occurs, how it plays a role in cross-linkage, and whether it exhibits other properties distinct from ordinary water.With the involvement of Dr. Ken-ichiro Murata, Assistant Professor at the Institute of Low Temperature Science, Hokkaido University, who has been conducting cutting-edge studies concerning special-state water on surfaces and at interfaces with materials, a more direct and molecular level technique was used to examine the model.

In the present study, to obtain direct observations of bound water on cotton surfaces, AFM and AFM-IR, the latter a cutting-edge surface analytical technique that enables high-precision observations of the surface of a sample and acquisition of its chemical composition/state information (infrared absorption spectra), were employed.
1. AFM observations
A large force was observed on the cotton side with detachment when approaching and removing the probe from cotton surfaces. This phenomenon demonstrated the existence of "a viscous substance" on the cotton surface because such adhesion behavior could not be explained only by the presence of cellulose, a major component of cotton. This suggests that such viscous bound water is involved in capillary adhesion of single fibers.
2. AFM-IR observations
AFM-IR spectra of naturally dried cotton surfaces showed two peaks corresponding to the vibration mode of water molecules (Fig. 1). On the other hand, no peaks were observed on the cotton surface after completely removing water. This indicates that these peaks came from bound water existing on the surface. Furthermore, the spectra, showing two clear peaks, significantly differed from those of ordinary water and are thought to reveal bound water in two different states on the surface, possibly corresponding to bound water at the air-water interface and that at the water-cotton interface, respectively.
As shown by results noted in 1 and 2 above, it was experimentally clarified that bound water was evident on cotton surfaces and contributed to dynamic properties such as stiffness mediated by capillary adhesion. In addition, the bound water itself manifested a unique hydrogen bonding state different from that of ordinary water.

In the present study, by focusing on capillary adhesion of bound water, the origin of the phenomenon of cotton hardening when dried naturally after wetting was revealed. These findings clarified that the state of bound water (hydrogen bonding state) is different from that of ordinary water, due to the influence of the cotton surface. It was thus speculated that adhesion between single fibers and the viscosity of bound water are deeply involved not only in the hardening phenomenon, but also in the dynamic property of cotton related to feel against skin.
Furthermore, from the standpoint of application, the results of the present study obtained by elucidation of a part of the mechanism of the cotton hardening phenomenon suggest a new perspective regarding "how to improve the function of fabric softeners".
Recently, along with progress in a variety of surface analytical techniques, the specific structure and function of "water on the surface of a material" have attracted considerable attention. The findings obtained in the present study provide effective clues for better understanding of this phenomenon.

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