# Densities and Viscosities of Ionic Liquid with Organic Solvents
## 1 Introduction  
Due to the depletion of fossil fuel and the continual emission of the harmful gas, biofuel has received reasonable attention and is considered a renewable energy. In a typical process for making biofuels from carbohydrates, the biomass (mainly containing the cellulose, hemicellulose, and lignin) has to be pretreated to break down the matrix and release the saccharides [1]. [PERSON] research group published the pioneer work on ionic liquids (ILs) for dissolving the biomass component [2]. Since then, more studies have focused on the use of ILs for bio-productions [3].  
Ionic liquids have always consisted of organic cations and organic or inorganic anions. Generally, the anions and the cations can be assembled in different combinations and that makes ILs more tunable and designable. ILs can be synthetized regarding the requirements for the industrial processes. Due to the favorable properties, e.g., lower vapor pressure, high thermal stability, and conductivity, ILs are recognized as promising alternatives for conventional solvents and are widely used.  
Among the huge number of ILs, imidazolium-based ILs are considered attractive solvents for biofuel and chemical processes [4; 5]. During the use of ILs, there are several disadvantages that have to be overcome. It is well known that most of the ILs possess high viscosity and that impedes the biomass dissolution in the ILs, affects the mass transfer rate, and increases the pump costs [6]. One of the possible ways to reduce the effects is the use of organic solvents as the additives to lower the viscosity and improve the dissolution of biomass in ILs. The organic solvents, namely N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), are widely selected as the diluents for the homogeneous derivatization of biomass component in ILs [7; 8; 9]. The acetate-basedimidazolium IL of 1-hexyl-3-methylimidazolium acetate ([C\({}_{6}\)mim] [OAc]) has attracted more attention and is chosen to study. More studies focus on the mechanism of making bio-productions using IL with solvents, however, little attention has been given to their thermophysical properties. [PERSON] et al. and [PERSON] et al. worked on the densities of [C\({}_{6}\)mim] [OAc] at 0.1 MPa in the temperature range of 283.15-338.15 K [10; 11]. [PERSON] et al. reported the viscosities at 278.15-343.15 K under 0.1 MPa with the relative standard uncertainty of 0.06 using a cone and plate ATS Viscoanalyzer [10]. Additionally, there is no publication concerning the properties of binary mixtures. Liquid density is important to engineers throughout chemical process industries. The knowledge of density is required in the design of storage and mass transfer. Viscosity is a measure of a fluid's internal friction and it can be considered as the resistance to flow. Fluid flow characteristics are valuable in predicting the parameters relevant to many chemical processes, such as the pumpability.  
In this work, the experimental studies on the viscosities and densities of [C\({}_{6}\)mim] [OAc] with organic solvents of DMA, DMF, and DMSO were conducted from 303.15-338.15 K at atmospheric pressure. The excess properties and energy barrier were calculated as well as the thermal expansion coefficient. Moreover, the microscopies for the binary mixtures of ionic liquid with organic solvents were determined to understand the interactions between ionic liquid with solvents. The hard-sphere model was employed to study the viscosities of pure substances and mixtures.