# Alkylimidazolic Ionic Liquids Absorption and Diffusion in Wood
###### Abstract  
Ionic liquids represent a class of highly tunable organic compounds responsible for many applications in the domain of wood-based materials. It has often been emphasized that the use of ionic liquids derives from their high affinity for lignocellulosic and their good penetration ability into wood structures. This paper discusses the sorption ability of different types of 1-alkyl-3-methylimidazolium ionic liquids with lateral alkyl chains, ranging from ethyl to hexyl into spruce and beech wood, as a function of their cation molecular mass, anion type (chloride, tetrafluoroborate, acetate), and intrinsic properties (surface tension and kinematic viscosity) at room temperature. All the studied ionic liquids present high relative uptake values at the equilibrium, ranging from 11.2% to 69.7%. The bulk diffusion coefficients of the ionic liquids into the wood range from 2 \(\times\) 10\({}^{-3}\) to 28 \(\times\) 10\({}^{-3}\) mm\({}^{2}\)/min, being higher in the longitudinal direction for both types of wood. The value of the diffusion coefficients for 1-ethyl-3-methylimidazolic is only 25% lower than that for water, despite the obvious differences in viscosity and surface tension, demonstrating a good penetration ability and the potential for wood industry-related applications (as impregnation compound carriers and preservatives). M Date: 2021  
which do not interact with lignocellulosic (such as alkaline/alkaline earth metal halides) have a concentration-independent diffusion coefficient (0.2-0.4 cm\({}^{2}\)/day) [9; 10], while for inorganic compounds which interact with lignocellulosic (dichromates, copper salts, arsenic compounds, boric acid, and borates), substantial deviations are registered from the ideal Fickian behavior. Strong hydrogen bond-disruptors (usually compounds with large or aromatic side-chain constituents) present highly concentration-dependent diffusion coefficients values [11]. Ionic liquids (ILs) are a class of organic compounds with low melting points and negligible vapor pressures, linked since the late 1990s with various wood-related applications. Apart from their ability to dissolve wood, many ILs have been studied for their antifungal character, wood plasticizing effect, antistatic character, or thermal and UV stabilizers (especially those with long-chain alkylammonium and alkylimidazolium cations) [12; 13; 14; 15]. They are reported to present good penetration into the bulk of the wood, swelling it, while at the same time presenting a low leaching ability [16]. Apart from their use as preservatives per se, they can be used in wood-related applications as carriers [16], or as a means for transporting antifungal agents [13]. Even if many studies report (bio)polymers (including wood lignocellulosic) interaction, swelling, and dissolution in ILs [12; 17], there are very few studies that report the dependence of wood swelling on the structure of the ionic liquid, and virtually none that report the diffusion coefficients of commercial alkylimidazolium ionic liquids (in pure liquid form or as aqueous solutions) in the wood [14; 18]. This study, showing the potential of application, could provide the required encouragement for the application of ionic liquids in the wood industry. One can compile, for example, different information regarding the ILs sorption in wood from the reference literature. However, this information is not very useful, from a practical point of view, for comparison. Generally, the wood samples are prepared differently (only bulk sorption, or at most one or two sections, either tangential or radial, are taken into the discussion); expensive synthesized ILs with complex structures are discussed, or wood with questionable practical applications (tropical species) is used [19; 20]. The scope of this paper is to determine the absorbed amount and diffusion coefficients of several types of alkylimidazolium ionic liquids at room temperature in two typical wood types: Norway spruce (_Picea_ sp.) and beech (_Fagus_ sp.).