Properties of grass lignin layers at the air-water interface

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