J. Adhesion Sci. Technol., Vol. 18, No. 6, pp. 687–713 (2004)  VSP 2004. Also available online - www.vsppub.com Surface free energy of natural and surface-modified tropical and European wood species ILDIKÓ MOHAMMED-ZIEGLER 1,∗ , ÁGNES OSZLÁNCZI 2 , BARBARA SOMFAI 2 , ZOLTÁN HÓRVÖLGYI 2 , ISTVÁN PÁSZLI 3 , ALLAN HOLMGREN 4 and WILLIS FORSLING 4 1 Institute of Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, Pusztaszeri út 59–67, H-1025 Budapest, Hungary 2 Department of Physical Chemistry, Budapest University of Technology and Economics, Budafoki út 6–8, H-1111 Budapest, Hungary 3 Department of Colloid Chemistry, Loránd Eötvös University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary 4 Division of Inorganic Chemistry, Luleå University of Technology, S-97187 Luleå, Sweden Received in final form 9 March 2004 Abstract—To describe the wetting properties of various wood types from a practical point of view, the surface free energy of six tropical (guava, almond, teak, cloves, mango and neem) and six European wood species (English oak, Norway maple, hazel, ash, alder and Scots pine) were calculated using contact angles by the sessile-drop method. In order to provide water-repellent characteristics to the samples, they were silanized by a less used silylating reagent (using chloroform solution of trimethylsilyl N,N-dimethylcarbamate) and the results were compared with the effect of two commonly used reagents (chloroform solutions of chlorotrimethylsilane and octadecyltrichlorosilane). Since the Lifshitz–van der Waals/acid–base model is widely used in studies of biological surfaces, the energetics of the resultant wood surfaces were quantitatively described in terms of this model. For the mainly hydrophobic wood samples, anomalous surface behaviour (i.e. extremely high water contact angles (130–145 ◦ ) and in certain cases unreasonably low surface free energy values) was found. Since the Lifshitz–van der Waals/acid–base model did not yield numerical results in some cases and the calculated surface free energies depended on the test liquid triplet used, the limitations in the applicability of this model are also discussed. For comparison, we analyzed our data also in terms of the Chang model. Keywords: Contact angle; surface free energy; silylation; wood. ∗ To whom correspondence should be addressed. Tel.: (36-1) 438-4141/410. Fax: (36-1) 325-7554. E-mail: mohazihu@yahoo.com Present address: Gedeon Richter Ltd., Quality Control Department, H-2510 Dorog, Eszter- gomi út 27, Hungary.