Combination of ILs with other technologies. 3.4.1 Microwave-assisted extraction ILs can efficiently absorb and transfer microwave energy (Xxxxxxxx et al., 2003), and thus be employed to rapidly heat solvents and co-solvents in microwave-assisted extraction (MAE). MAE is rapid and effective compared to traditional extraction techniques (Zeng et al, 2010) and easy and cheap compared with other modern extraction techniques. Recently, microwave coupled with ILs aqueous solutions or pure ILs were evaluated for the extraction of some polyphenolic compounds (Du et al., 2007; 2009; Xxxx et al., 2010), alkaloids (Xx et al., 2008; Xx et al., 2010) and essential oils (Zhai et al., 2009) from medicinal plants. Compared to conventional heat-reflux extraction (HRE), MAE had the advantage of remarkably higher efficiency (20–50% higher) and shorter extraction time (from 2 h to 90 s) (Xx et al., 2008). The important parameters of the MAE procedure include the sample size, liquid-solid ratio, extraction time and temperature. The extraction rate of phenolic alkaloids with [Hmim][BF4] increased when the liquid-solid ratio was raised from 5:1 to 10:1 and decreases dramatically when the ratio further increases to 20:1. An extraction time of around 8-12 min was sufficient to obtain high extraction yields of polyphenolic compounds from 0.2-0.5 g of plant powder with 20-35 mL of a 1.5-2.0 mol/L [Bmim][Br] aqueous solution (Xxxx et al., 2010). Another aspect to be considered is thermal stability. Phenolic compounds were found to be stable up to 100 oC during MAE (Liazid et al., 2007), but with ILs as the solvent the stability temperature decreased to 60-80 oC and compounds with a larger number of hydroxyl substituents were more readily degraded under the extraction conditions (Du et al., 2009). MAE is highly efficient as an extraction method due to its unique mechanism. In MAE extraction, the direct interaction of the microwaves with the IL solution and free molecular water present in the cell, results in the rupture of the cells and release of intracellular products into the solvent. The surface of the plant material was observed to be greatly destroyed and the structure of the cell walls ruptured after IL-MAE (Xx et al., 2009; Xxxx et al, 2010). Moreover, ILs improved the transfer of energy from the microwaves to the sample, which increased the speed of energy transfer and thus extraction efficiency (Xxxxxxxxx and Xxxxxxxxx, 2000).
