Surfactant-based Aqueous Biphasic Systems for Extraction Processes

more hydrophobic tails. They are classified regarding their head-group’s charge as anionic (negatively charged group), cationic (positively charged head-group), zwitterionic (a head-group carrying a positive and negative charge), or nonionic (uncharged but polar head-group) [26]. The investigations in this thesis focused on the nonionic surfactants due to their application in extraction processes.

The chemical structure of three nonionic surfactants of importance for this work is presented in Figure 2.6.

Figure 2.6: Chemical structure of Triton X-114 (fig. a): average number of EO in head group 8 [27];

Silwet L7230 (fig. b): poly(ethylene oxide) (m)/poly(propylene oxide) (n) = 40/60 wt% and polydimethylsiloxane (x)/ polymethylsiloxane (y) = 21/79 wt% (own representation according [28];

and ROKAnol NL-5 (fig. c): number of C-atoms in tail (n) = 7-11 and number of EO in head group (m) = 3-5 (own representation according chemical data sheet). Me: methyl group.

A typical nonionic surfactant consists of a hydrophobic alkyl chain group and hydrophilic ethylene oxide groups. The molecules of Triton X-114 (fig Figure 2.6a) and ROKAnol NL-5 (fig. Figure 2.6c) represent such chemical structure. Triton X-114 is an alkylphenol ethoxylate with a hydrophobic branched tail containing an aromatic ring. The molecule of ROKAnol NL-5 is represented by a fatty alcohol (C

= 7-11) which is ethoxylated with 3-6 ethylene oxide moieties [3]. Silwet L7230 (fig. Figure 2.6b) represents another class of nonionic surfactants. This amphiphile is a block copolymer of silicone, ethylene oxide, and propylene oxide. The hydrophobic part of the molecule is built of a siloxane backbone which is grafted to the ethylene oxide-propylene oxide head group [28].

Due to the diversity of their chemical structures, nonionic surfactants exhibit variations in their chemical and physical properties. For instance, the hydrophilic-lipophilic balance (HLB) can change by adjusting the weight ratio between the head group and the tail moiety. The HLB is specific for each surfactant and is expressed on a scale of 3.5 to 18 divided in water-in-oil (w/o) emulsifiers (HLB=3.5÷6), wetting agents (HLB=7÷9), oil-in-water (o/w) emulsifiers (HLB=8÷18), detergents (HLB=13÷15), and solubilizers (HLB=15÷18) [29,30]. In case of Silwet

L7230, the HLB-value is 6.3 which makes the amphiphile less soluble in water than ROKAnol NL-5 (HLB = 11.6).

Nonionic surfactants can form complex structures, such as bilayers, liquid crystals, and spherical or cylindrical vesicular structures (micelles). A typical phase diagram for an aqueous nonionic surfactant binary mixture (C5E12/water) is shown in Figure 2.7

Figure 2.7: Phase diagram for C5E12/water [31].

In aqueous surfactant solutions, a change in the surfactant concentration or the system temperature leads to different phase transitions, as illustrated in Figure 2.7. At low concentrations and temperatures, the surfactant dissolves entirely into a single isotropic liquid phase.

However, above the critical micelle concentration (cmc) regular micellar solutions (L1) and reverse micellar solutions (L2) are formed. The normal micelle is a dynamic aggregate composed of a hydrophobic core, formed by the lipophilic tails of the monomers, and a hydrophilic outer layer of the water-soluble monomer moieties.

The reversed micelle structure is the opposite. The hydrophobic core of the micelles can solubilize lipophilic substances (solutes). Hence, by solubilizing an oil-soluble substance in the micelle, one can homogeneously disperse it in an aqueous bulk. This property is the reason for the broad application of surfactants in cleansing agents, food and cosmetic products [32].

At higher temperatures, above the cloud point temperature (Tc), the micellar solution separates into one surfactant-lean phase (L1’) and one surfactant-rich phase (L1’’).

Additionally, a sponge-like phase (L3) can also exist in surfactant/water mixtures.

At low temperatures but higher concentration, the formation of lyotropic phases can be observed, viz. hexagonal (H1), cubic (V1) and lamellar (Lα) liquid crystalline phases, with the latter characteristically extending into the two-phase area [33,34]. The liquid crystalline lamellar, hexagonal, cubic phases, as well as the sponge-like phase, are highly structured. As a result, high viscosity may be exhibited in these phase regions [34–36].

The coexistence of the phases L1’ and L1’’is of higher importance for separation processes and is further discussed in this chapter. An aqueous solution containing a nonionic surfactant above the cmc can undergo a temperature-induced clouding.

The temperature, at which the solution turns turbid, is referred to as cloud point temperature (CPT) [37,38]. The thermodynamic mechanisms behind the clouding are complex. It is assumed, that elevated temperatures lead to dehydration of the micelles’ hydrated outer layers. Consequently, repulsive forces are decreased and attractive micellar interactions are more pronounced. Hence, the size of the aggregates increases and the solution becomes turbid [3,32].

For commercial water-soluble surfactants, the CPT is commonly defined at a surfactant concentration of 1 wt % in deionized water. Further, at a surfactant concentration, the minimum clouding temperature, also known as minimal lower critical solution temperature (LCST), is exhibited [39]. However, there are diverse methods to determine a CPT and the LCST at different solvents. Thus, one has to consider the manufacturer's data concerning the CPT and LCST carefully.

The molecular structure or the solution composition can affect the CPT of nonionic surfactants. These factors can cause steric hindrance, charge repulsion, or influence the solvent’s solubility [26,33]. The LCST values of an aqueous nonionic surfactant solution decrease with a lower number of ethylene oxide groups and increasing length of the alkyl chain [4]. However, electrolytes have the most significant influence on aqueous nonionic surfactant solutions. It is distinguished between electrolytes lowering the CPT, and such increasing the CPT. The first effect

is caused by a dehydration of the surfactant’s ethylene oxide chain and its consequently decreased solubility in water (salting-out effect). Vice versa, the second effect is caused by an increased solubility of the surfactant (salting-in effect) [38,40].

Moreover, if the aqueous surfactant solution (S) is further heated to a point above the coexistence curve (in the two-phase region, above the CPT), a macroscopic phase separation may occur (Figure 2.8).

Figure 2.8: Liquid-liquid equilibrium of water-surfactant solution. Phase separation mechanism [41].

Then the mixture splits into a surfactant-rich phase (R) and a surfactant-lean aqueous phase (P), as shown in Figure 2.8 [41]. The concentration of surfactant in the micellar phase (R) is a function of the temperature. The aqueous phase is poor in micelles whereas the concentration of surfactant is close to the cmc [42].

This type of phase behavior and the ability of the micelles to solubilize solutes, make surfactant-based aqueous two-phase systems (ATPS) attractive for separation processes. Hence, a technique for recovery of target substances from an aqueous bulk, referred to as cloud point extraction (CPE), is presented in Figure 2.9 [4]:

Figure 2.9: Scheme of the cloud point extraction with nonionic surfactants [43].

The feedstock for the CPE may be any solution that contains predominantly water, as well as dissolved organic compounds (Figure 2.9 A). Further, by addition of nonionic surfactant at a concentration above the cmc, micelles occur in the aqueous media. Thus, the dissolved hydrophobic solutes are solubilized in the micelle cores (Figure 2.9 B). By elevating the temperature of the solution, the macroscopic phase separation takes place as described previously (Figure 2.9 C).

Consequently, the micellar phase is rich in surfactant and hydrophobic solutes, leaving only low concentrations of both components in the aqueous phase [4]. The uneven distribution of the solute makes possible the separation of organic components from diluted bulks by CPE.

The general principles of the CPE are thereby analogous to a conventional liquid-liquid extraction, except that the solvent is completely miscible with the feed solution below CPT [39]. Additionally, the density difference between the micellar and the aqueous phase should be high enough to allow a proper phase separation [13].

Some nonionic surfactant solutions possess CPT similar to the ambient temperature [4]. In additions, some amphiphiles are known for their low toxicity [44]. Therefore, the cloud point system represents a suitable media for biotransformation. Moreover, it can be used to remove sensitive compounds due to the mild temperature. The basic knowledge regarding such in situ product removal techniques is presented in the next chapter.