PROPERTIES OF GRASS LIGNIN LAYERS AT THE AIR-WATER-INTERFACE

In plant cell walls, lignins, the second most abundant biopolymers, fo rm 3D crosslinked molecular structures which interact with polysacchar ides to form lignin-carbohydrate complexes. The incorporation of these phenolic polymers into synthetic polysaccharide-based composites is c urrently under investigation. The properties of the final material (me chanical strength, degradability,...) strongly depend on the degree of compatibility between such hydrophilic and hydrophobic molecules. Thi s phenomenon is expected to be in part related to the surface properti es of lignins. Organosolve dioxane lignins were isolated from wheat st raw and characterized with respect to their HPSEC elution profile (M-n = 3.6 x 10(3)), total sugar and esterified phenolic acid content (3 a nd 3.5%, respectively) and to the degradation yield of thioacidorysis (750 mu moles g-l). These lignins were then dissolved in a dioxane-wat er mixture and used for Langmuir trough experiments at the air-water i nterface. Compression isotherms (pressure/concentration) were obtained at 20 degrees C by compressing the surface deposited layers at specif ied rates. A particular feature of the interfacial layers of lignin is that surface pressure relaxes strongly when compression is stopped. T hus, the compression isotherms exhibit a surface pressure which is rou ghly three times higher than the value reached after 48 h relaxation. These data suggest that previously reported isotherms have been determ ined in conditions far from equilibrium. Dilational moduli, measured a t several surface concentrations, range from 13.5 to 72 mN m(-1). Thes e special surface properties should determine a part of the behavior o f lignins in blends. (C) 1997 Elsevier Science B.V.