Direct Observation of Bound Water on Cotton Surfaces by Atomic Force Microscopy and Atomic Force Microscopy–Infrared Spectroscopy

A wet cotton rag becomes stiff after natural drying. We propose a model for this hardening phenomenon, which explains that the stiffness of cotton is caused by a cross-linked network between single fibers, mediated by capillary adhesion of bound water on the surface of cellulose. Here, with the aid of atomic force microscopy and atomic force microscopy–infrared spectroscopy, we reveal the existence of the bound water on the surface of a cotton single fiber under naturally dried conditions. We also find that the hydrogen bonding state of the bound water is distinct from that of the bulk water. Two stretching modes of OH groups are clearly decoupled from each other, which arise from the effects of the air–water (hydrophobic) and water–cellulose (hydrophilic) interfaces. This suggests a possible link between the microscopic nature of the bound water and the macroscopic mechanical behavior of cotton fabrics.