Structure of wood extract colloids and effect of CaCl2 on the molecular mobility

Electron paramagnetic resonance (EPR) was used to study the colloidal structure of model wood extractive colloids composed of a resin acid (abietic acid), a fatty acid (oleic acid) and a triglyceride (triolein). Two nitroxides were chosen as EPR probes to gain a greater understanding of the different regions of the colloid in order to assess the current proposed models of the structure of the wood extractive colloid. A non-polar nitroxide probed non-polar regions of the colloid, such as triglycerides, while a surfactant-type nitroxide probed regions occupied by fatty acids. The effect of varying the amounts of each of the model colloid components on the structure of the colloid and its interaction with the probe was investigated. Results of the EPR study confirm the existence of a hydrophobic core. However, surface tension and EPR results suggest that the outer layer of the colloid is composed of mostly resin acids. It is proposed that a fatty acid layer exists between the resin acids and triglycerides and is sufficiently mobile to move between them. The addition of salt (CaCl₂) was found to affect the mobility of molecules at the core of the colloid.