Experimental artifacts in the surface pressure measurement for lignin monolayers in Langmuir troughs

The surface pressure measured with the Wilhelmy method may vary considerabl y with the experimental conditions adopted for isotherms of nonamphiphilic, macromolecular substances, in contrast to the majority of traditional amph iphilic compounds such as fatty acids. This is demonstrated here by analyzi ng monolayer characteristics of lignins, for which a variety of conditions were employed, including change in the mode of compression, positioning of the Wilhelmy plate, and compression speeds. A minimum in the decompression leg of a hysteresis experiment was found to arise from experimental artifac ts, which are explained in terms of the various forces experienced by the W ilhelmy plate. (C) 1999 American Institute of Physics. [S0034-6748(99)01009 -6].