Limitations of SC-CO 2 and modifier or co-solvent

The main disadvantage of SC-CO₂ as a solvent is its non-polar nature. It has good solvent properties for the extraction of non-polar compounds, and

a large quadrupole moment also enables it to dissolve some moderately polar compounds like alcohols, esters, aldehydes and ketones (Lang and Wai, 2001). However, this can be overcome by adding modifiers or co- solvents. A modifier can be added with the solvent or together with the sample in the extraction chamber. The first method can be applied to both countercurrent SFE and static SFE. The second method is always used for static SFE. These modifiers can range from ethanol, acetonitrile and methanol to water. It is assumed that the effect of a modifier depends on the nature of solute to be extracted (Walsh et al., 1987). The first basis of modifier selection is the increased solubility of the target compounds in the modified SCF (Pourmortazavi and Hajimirsadeghi, 2007). The ability to distort and swell the matrix as a consequence, favouring the penetra-tion of CO₂ into the matrix for extracting the analyte, is another basis for selecting the modifier (Casas et al., 2007). The effect of the modifier is determined by the phase behaviour of the mixture under operating con-ditions and is dependent on the concentration of co-solvent in the super-critical phase (Shi et al., 2009). At least 17 modifiers have been studied in the SFE of natural products (Modey et al., 1996). The selection of a modi-fier becomes a great challenge. Water and ethanol become attractive alter-natives over other organic-based solvents because of their relatively safe nature and the advantages of cleanness (Shi et al., 2009). But, in practice, methanol and acetone are the most popular among the different modi-fiers (Engelhaqrdt and Hass, 1993; Taylor et al., 1993; Phelps et al., 1996;

Jeong and Chesney, 1999). Vegetable oils have also been used as a modifier for SC-CO₂ (Vasapollo et al., 2004; Sun and Temeli, 2006; Krichnavaruk et al., 2008). Up to 20% methanol is miscible with CO₂ and it is an ef-fective polar modifier. Ethanol, although not as polar as methanol, may be a better choice in the SFE of natural products because of its lower toxicity.

Generally, their concentration ranges between 1 and 15%. The addition of a small percentage of polar organic modifiers to supercritical carbon dioxide can increase the extractability of target analytes remarkably. To summarize, modifier is added to an extraction process mainly for two reasons: (i) to increase the polarity of supercritical carbon dioxide; and (ii) to facilitate the desorption of analytes from the plant matrix (Onuska and Terry, 1989; Wright et al., 1989).

When a modifier is added, viscosity also increases. The addition of a large amount of modifier will change the critical parameters of the mix-ture (Pourmortazavi and Hajimirsadeghi, 2007). The addition of ethanol increases the bulk density of SC-CO₂ due to the higher density of the modifier and clustering of SC-CO₂ molecules around the modifier (Guclu-Ustundag and Temeli, 2004). Water has also been used as a modifier in a number of SC-CO₂ applications, although water is only about 0.3% sol-uble in SC-CO₂ (Lehoty, 1997). Water could enhance the interaction of the analyte–modifier matrix because water can open pores and swell the matrix, thereby allowing the fluid better access to analytes to draw the analytes out of the matrix. Caffeine and epigallocatechin gallate were extracted from green tea using SC-CO₂ with water as the modifier and the yield increased significantly with an increase in water (Shi et al., 2009).

Several mechanisms that might explain the activity of the modifier have been suggested. The existence of a solute–modifier interaction in which the modifier interacts chemically with the solute is one theory (Pereira and Meireles, 2010). The formation of a solvation shell around the solute is another theory (Yonker et al., 1986). The polar modi-fier molecules accelerate desorption processes by competing with the analytes for active binding sites, as well as by disrupting the matrix structures (Turner et al., 2001). A higher percentage of methanol could disrupt the bonding between solutes and matrices (Bicchi et al., 1991).

Vegetable oils have also been used as modifiers to increase the extrac-tion of carotenoids and lycopenes in CO₂-SFE (Sun and Temeli, 2006;

John et al., 2009). Depending on the properties of the samples and de-sired compounds, the best modifier is usually determined based on the results of preliminary experiments. In the SFE of phenolic acids, methanol was a more effective modifier than acetonitrile, acetone, water or dichloromethane (DCM) (Ashraf-Khorassani et al., 1995). In the SFE of paclitaxel and baccatin III, DCM was the most effective among methanol, ethyl acetate, DCM and diethyl ether for paclitaxel;

however, diethyl ether was the best for baccatin III (Chun et al., 1994).

Smith and Burford (1992) reported that 4% methanol or chloroform did not result in any improvement in the recovery of santonin (a ses-quiterpene lactone), but 4% acetonitrile could increase recovery from 38% to 85%, while water-saturated CO₂ could further increase re-covery to 92%. Wang et al. (2001) compared the efficiency of (sub- and supercritical carbon dioxide) extraction with Soxhlet and ultrasound extraction of ginsenosides (Rb₁, Rb₂, Rc, Rd and Rg₁) and crude oil from ginseng root hair. Extraction with SC-CO₂ supplemented with ethanol (6 mole percentage) as the modifier produced significantly higher ex-traction yield than SFE without a modifier. Further, with former condi-tions at 31.2 MPa and 333 K, it was as effective as hot water extraction, but yield was lower when compared with a Soxhlet extraction using ethanol. Wood et al. (2006) tested methanol and dimethylsulfoxide as modifiers for their effect on extraction efficiency. With relatively higher  modifier concentration (27–30 mole percentage) and conse-quently higher operating conditions (e.g. temperature, pressure), it was feasible to extract up to 90% of the total ginsenosides content, which was comparable to Soxhlet extraction with methanol. It was also re-ported that with increasing mole fraction of the modifier and with in-creasing pressure and temperature, solubility increased enormously, with increase in solubility 100–150 times more for eflucimibe with modifier (Sauceau et al., 2004). It was observed that increasing the concentration of ethanol from 2 to 10% in SCF showed a 70–80 times increase in the solubility of gallic acid, (+) catechin and (−) epicatechin in grape seed extraction (Murga et al., 2000). The observed modi-fier effects can be explained not only by density effects but by the effect of molecular interactions on the basis of compound solubility parameters.