1.1 Switchable Polarity Solvents SPS

The concept of “switchable compounds” and in particular of “switchable solvents” has been proposed for the first time by Jessop et al. few years ago [26]; this smart idea is based on the possibility to reversibly switch on and off some properties of a substance when a “trigger” is applied, from one version, with a specific set of properties, to another one, with very different properties. For solvents, such properties

can be polar/apolar, volatile/non-volatile, protic/aprotic. For the practical application of SPS, their switching solubility behaviour is a fundamental feature, correlated with their reversible polarity; an equimolar mixture of DBU (1,8-diazabicyclo-[5.4.0]- undec-7-ene) and an alcohol, for example, behaves as a slightly polar solvent, similar to chloroform, enabling to dissolve apolar compounds such as hydrocarbons, whereas the salt DBU alkylcarbonate, after CO2 treatment, is a polar liquid, very similar to dimethylformamide and immiscible with hydrocarbons (Fig. 5) [26].

The major “greenness” of SPS respect to volatile organic solvents as я-hexane relies mainly on the possibility to develop safer, more economic and more sustainable processes. Chemical processes often involve many steps which can require many specific solvents; after each step, the solvent has to be removed and replaced with a new one, more suitable for the following step, increasing the economic costs and environmental impact of the process itself. Switchable solvents represent a valid answer to these cumbersome procedures, because their properties can be adjusted for the following step, enabling the same solvent to be used for several consecutive reactions or separation steps. Moreover, the recyclability of a SPS is based on the principle of adding and removing CO2, and not on the recovery of the solvent by distillation; this means that the process of recycling is less expensive, less energy costly, and more environmentally friendly. Because of the peculiar recycle method, the solvents in the SPS formulation do not need to be volatile and this feature reduces the risks of fire and explosion, the release in the atmosphere, and the exposure for the operators.

Given the chemical nature of B. braunii lipid oil, the concept of “SPS” fits very well with the development of a new “greener” process of extraction [25]: the lipophi — licity of the non-ionic form of SPS is suitable to extract apolar materials as algal hydrocarbons, whereas the ionic form of SPS, obtainable by a simple bubbling of CO2, has a low affinity for these molecules, guaranteeing an easy recovery (Fig. 6).

The first important point in the development of the suitable SPS extraction process is the choice of the alcohol because it strictly influences the formation of the proper liquid carbonate anion. Previous works [40-42] have demonstrated that bicarbonate and methyl carbonate DBU salts have melting points lower than room temperature, whereas DBU salts with longer alkyl chains are liquids [43], therefore suitable for

hydrocarbons

the extraction process. For this reason in our extraction process of B. braunii, DBU ethyl carbonate (melting points of 35°C) and DBU octyl carbonate (melting points of 30°C) were investigated as suitable candidate solvents (Table 3). In particular, the choice of an alcohol as octanol, hydrophobic and low volatile, should guarantee a good affinity with the apolar matrix which has to be extracted, and a scarce solubility in water, useful in the case of liquid samples.

The kinetics of the extraction process (Fig. 7) clearly shows that DBU/octanol and DBU/ethanol exhibit approximately the same behaviour, with similar hydrocar­bons’ amount extracted after 240 min (14 and 13% yields, respectively). Moreover, the hydrocarbons extraction with DBU/octanol is quite efficient from the beginning, with a yield of 9% after 20 min (65% of the yield at the end of the extraction time after 240 min).

A second important factor for the development of the extraction process with SPS is related to the chemical features of the process itself, and specifically to the eventuality that the presence of specific chemical compounds could prevent the switching of the system and the formation of the right DBU alkylcarbonate salt, affecting the separation process. Free fatty acids in the algal oil for example could

of DBU/octanol and DBU/ethanol data, respectively form an ion pair with DBU, thus preventing the formation of a two-phases system when CO2 is added and altering the stoichiometry of the SPS. In the case of B. braunii, the percentage of free fatty acids is 0.6-0.7% on a dry weight basis (Table 1), thus an irrelevant amount (about 0.18 mg) if compared with the amount of DBU used for the extraction (1 g) eventually able to react with free fatty acids. However, this eventuality could be a problem for the extraction of other kinds of oil with a high content of free fatty acids (as waste cooking oils [44]) and should be taken into account in the development of the extraction system.

A third relevant factor is the recyclability of the system, in terms of feasibility, contaminations of the algal oil, and losses. The efficiency of a non-ionic/ionic cycle with the SPS DBU/octanol in recovering pure hydrocarbons is about 81% of the total amount of hydrocarbons extracted, with 8.1% of hydrocarbons retained in the ionic SPS phase in the second half of the cycle, and about a 10% mechanical loss probably due to small samples size (in our work the extraction procedures were
accomplished on 30 mg of freeze-dried algal samples [25]). These results clearly indicate that the recovery of the hydrocarbons is very good, since the ionic form of SPS retains a rather small amount of the hydrocarbons in the extraction phase; moreover, this amount can be still reduced by scaling up the process and increasing the size of the samples.

The GC-MS evaluation of oil quality after the first cycle of extraction indicates that the oil still contains small amounts of octanol (0.3%) and DBU (0.4%), but bubbling extra CO2 for 1 h at 40°C decreases the levels of contamination to undetectable values because of the precipitation of all the ionic liquid from the oil [25].