Surface Morphology and Molecular Organization of Lignins in Langmuir-Blodgett Films Daniel Pasquini, ‡ D. T. Balogh,* ,† P. A. Antunes, | C. J. L. Constantino, § A. A. S. Curvelo, ‡ R. F. Aroca, | and O. N. Oliveira, Jr. † Instituto de Fı ´sica de Sa ˜ o Carlos, Universidade de Sa ˜ o Paulo, CP 369, 13560-970 Sa ˜ o Carlos, SP, Brazil, Instituto de Quı ´mica de Sa ˜ o Carlos, Universidade de Sa ˜ o Paulo, CP 780, 13560-970, Sa ˜ o Carlos, SP, Brazil, Faculdade de Cie ˆ ncias e Tecnologia, Universidade Estadual Paulista, CP 467, 19060-900 Presidente Prudente, SP, Brazil, and Materials and Surface Science Group, School of Physical Science, University of Windsor, N9B 3P4 Windsor, Ontario, Canada Received March 13, 2002. In Final Form: May 17, 2002 Atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) are used to investigate molecular organization in Langmuir-Blodgett (LB) films of two kinds of lignins. The lignins were extracted from sugar cane bagasse using distinct extraction processes and are referred to here as ethanol lignin (EL) and saccharification lignin (SAC). AFM images show that LB films from EL have a flat surface in comparison with those from SAC. For the latter, ellipsoidal aggregates are seen oriented perpendicularly to the substrate. This result is confirmed by a combination of transmission and reflection FTIR measurements, which also point to lignin aggregates preferentially oriented perpendicularly to the substrate. For LB films from EL, on the other hand, aggregates are preferentially oriented parallel to the substrate, again consistent with the flat surface observed in AFM data. The vibrational spectroscopy data for cast films from both lignins show random molecular organization, as one should expect. Introduction The surface properties, including molecular arrange- ments and rheological properties, of lignin macromolecules have been investigated mostly employing the Langmuir- Blodgett (LB) technology and microscopy methods such as scanning electron microscopy (SEM), environmental scanning electron microscopy (ESEM), and atomic force microscopy (AFM). 1-15 The use of spectroscopic methods is less common, and only a few reports are encountered in the lignin literature. Raman images of the cell wall of wood tissues indicated a certain degree of organization of the lignin molecules with the phenyl rings oriented preferentially along the cell wall surface. 15 Theoretical modeling using computer simulations showed that ex- tracted lignin molecules also form planes. 16 On the other hand, in condensed Langmuir films, the phenylpropane units do not lie flat on the plane but rather tend to form an arrangement in which the aromatic rings are somehow vertical to the air-water interface. 5,7-9,12 The “picture” for lignin emerging from these studies resembles an almost spherical or disklike molecule that behaves as a sphere in solution. 6 The lignin molecules have a tendency to self-organize on substrates, and the aggregates formed depend on the type of substrate and its interactions with the sample. 4 The molecular ag- gregates have some mobility and may grow to form spherical or ellipsoidal structures (“onionlike”) in multi- layers or in thin cast films. 4 In the films, some “space” between the layers may exist, which is necessary to accommodate the functional groups, and may be filled with gas or solvent. 2 Such porous structures were denoted in ellipsometric experiments performed on lignin LB films, for which low refraction indexes were measured. 10 On one hand, the tendency to form planes makes the lignin macromolecules capable of “covering” rough surfaces, which can be useful in nanotechnology applications where a very smooth surface is required. 16 On the other hand, rough structures formed for other lignins may also be useful in cases where large effective areas are of interest. 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