Sulfosuccinate head groups

A great variety of chemical reactions can be advantageously carried out in microemulsions [860-862]. In one of the first papers in this field, Menger et al. described the imidazole-catalyzed hydrolysis of 4-nitrophenyl acetate in water/octane microemulsions with AOT as an anionic surfactant [=sodium bis(2-ethyl-l-hexyl)-sulfosuccinate] [864]. The solubilized water, containing the imidazole eatalyst, is confined in spherical pools encased by surfactant molecules, which have only their anionic head groups (-SOb ) immersed in the aqueous droplets. When the ester, dissolved in water-insoluble organic solvents, is added to this water/octane/AOT/imidazole system, it readily undergoes the catalysed hydrolysis under mild reaction conditions (25 °C). 

Micelles form when a suitable amphiphile [e.g., sodium bis(2-ethyl-hexyl)sulfosuccinate (AOT)], is introduced into a hydrocarbon solvent (e.g isooctane). Reverse micelles containing water form when water is taken up by an isooctane—AOT solution. At water contents exceeding what is needed to saturate the polar head groups forming the micelle wall, the system can properly be termed a water-in-oil microemulsion, in which water droplets stabilized by a monolayer of surfactant are dispersed in an organic solvent. For convenience, the terms reverse micelle and microemulsion are sometimes considered equivalent. There is a considerable literature on the properties of proteins, particularly enzyme activity, in reverse micelles (see Luisi and Steinmann-Hofmann, 1987, and references cited therein). 

Finally, in the discussion of reverse microemulsion systems, mention should be made of one of the most widely studied systems. The surfactant, sodium bis(2-ethylhexyl) sulfosuccinate or Aerosol-OT (AOT), is one of the most thoroughly studied reverse micelleforming surfactants since it readily forms reverse micelle and microemulsion phases in a multitude of different solvents without the addition of cosurfactants or other solvent modifiers. The phase behavior of AOT in liquid alkane/water systems is already well documented. Indeed, the first report of the existence of the formation of microemulsions in a supercritical fluid involved an AOT/alkane/ water system. A The spherical structure of an AOT/nonpolar-fluid/ water microemulsion droplet is shown in Fig. 1. In the now well-known structure, it can be seen that the two hydrocarbon tails of each AOT molecule point outward into the nonpolar phase (e g., supercritical fluid). These tails are lipophilic and are solvated by the nonpolar continuous phase solvent whereas the hydrophilic head groups are always positioned in the aqueous core. 

Holmes et al. reported the first enzyme catalyzed reactions in water-in-CO2 microemulsions (67). Two reactions, a lipase-catalyzed hydrolysis and a lipoxygenase-catalyzed peroxidation, were demonstrated in water-in-C02 microemulsions using the surfactant di(l/7,l/7,5/7-octafluoro- -pentyl) sodium sulfosuccinate (di-HCF4). A major concern of enzymatic reactions in CO2 is the pH of the aqueous phase, which is approximately 3 when there is contact with CO2 at elevated pressures. Holmes et al. examined the ability of various buffers to maintain the pH of the aqueous solution in contact with CO2. The biological buffer 2-(A-morpholino)ethanesulfonic acid sodium salt (MES) was the most effective, able to maintain a pH of 5, depending on the pressure, temperature, and buffer concentration. The activity of the enzymes in the water-in-C02 microemulsions was comparable to that in a water-in-heptane microemulsion stabilized by the surfactant AOT, which contains the same head group as di-HCF4. 

The reverse micelles refer to the aggregates of surfactants formed in nonpolar solvents, in which the polar head groups of the surfactants point inward while the hydrocarbon chains project outward into the nonpolar solvent (Fig. 7) [101-126], Their cmc depends on the nonpolar solvent used. The cmc of aerosol-OT (sodium dioctyl sulfosuccinate, AOT) in a hydrocarbon solvent is about 0.1 mM [102]. The AOT reverse micelle is fairly monodisperse with aggregation number around 20 and is spherical with a hydrodynamic radius of 1.5 nm. No salt effect is observed for NaCl concentration up to 0.4 M. Apart from liquid hydrocarbons, recently several microemulsions are reported in supercritical fluids such as ethane, propane, and carbon dioxide [111-113]. 

The potential for tail-to-tail interactions in the adsorbed surfactant was clearly demonstrated in an article by Kandori et al. [29]. They studied the adsorption of sodium dodecyl sulfate, cetyltrimethylammonium bromide (CTAB), and sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT) onto both an anionic and a zwitter-ionic latex. They found, as did Connor and Ottewill, that a one-to-one relationship existed between adsorbed amount and surface charge for the case of adsorption of an ionic surfactant onto a surface of opposite charge. They also determined, through zeta potential measurements, that for both CTAB and Aerosol OT there appeared to be two surfactant molecules adsorbed to the tail of each molecule adsorbed in the head-down conformation. This is interesting in view of the fact that Aerosol OT is a two-tailed surfactant, thus giving rise to twice as many tails for subsequent adsorption when it is adsorbed in the head-down conformation. The result by Kandori would thus indicate that the limiting factor for the second layer of adsorbed material is not the number of tails available but is perhaps the interactions between the head groups on this second adsorbed layer. 

The Na-AOT reverse micelle is a widely investigated reverse micelle system made up of the sodium salt of a two-tailed anionic surfactant, sodium di(2-ethylhexyl) sulfosuccinate. The interior of the aqueous reverse micelle is modeled as a rigid cavity, with a united atom representation for the sulfonate head group (Faeder and Ladanyi 2000 Pal et al. 2005). The head groups protrude from the cavity boundary and are tethered only in the radial direction by means of a harmonic potential. Interactions between reverse micelles are neglected in the model hence periodic boundary conditions and Ewald summations for the electrostatics are not required. Water is treated using the extended simple point charge, or SPC/E, model and the potential parameters for all the species are listed in Table 6.1. 

Falcone et al. used the anionic sodium l,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the cationic surfactant, benzyl-n-hexadecyldimethylammonium chloride (BHDC) to prepare IL microemulsions [48]. The ILs chosen were l-butyl-3-methylimidazolium trifluoromethanesulfonate ([C mim][CF3S03]) and l-butyl-3-methylimidazolium trifluoroacetate ([C mim][CF3COJ see Scheme 13.2). DLS experiments reveal the formation of microemulsions. Besides, the FTIR results suggest that the ionic interactions (with the surfactant polar head groups, surfactant counterions, or IL counterions) are substantially modified upon confinement. These interactions produce segregation of IL s ions, altering the composition of the microemulsion interfaces. 

The choice of surfactant is critical to the size, shape and stability of the particles. It should be chemically inert with respect to all other components of the microemulsion. This is particularly important when the system contains oxidizing or reducing agents. Surfactants are classified as cationic, anionic, non-ionic or zwitteri-onic, depending on the type of charge on their head group. Two of the most commonly used surfactants are the anionic surfactant sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and the cationic surfactant CTAB. 

While sulfosuccinates are currently not manufactured with fermentation-based carboxylic acids, it is reasonable that bio-based succinic, fumaric or maleic acid could be used as the four-carbon head group, replacing maleic anhydride. For these feedstocks to be successful, effective processes and catalysts for aqueous esterification [21] and sulfonation [22] of these carboxylic acids need to be better characterized. 

It is interesting to compare adsorption at the two interfaces imder investigation here, water-C02 interface and air-water interface (i.e., at the cmc). For example, with di-HCF4 the area per molecule is 60% larger in the CO2 system as compared with the normal air-water interface (105 A versus 65 A ). On the other hand, with branched-chain AOT at air-water (cmc) and water-in-oil microemulsion interfaces, the molecular area is essentially constant at about 76 (Table 2 and Refs. 45 and 54). The SANS results for various sulfosuccinates were consistent with an effective head group area of aroimd 110 A, which is more than 30 A greater than for AOT in equiv-... 

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