Hydrophilic-lipophilic balance microemulsions

Different surfactants are usually characterised by the solubility behaviour of their hydrophilic and hydrophobic molecule fraction in polar solvents, expressed by the HLB-value (hydrophilic-lipophilic-balance) of the surfactant. The HLB-value of a specific surfactant is often listed by the producer or can be easily calculated from listed increments [67]. If the water in a microemulsion contains electrolytes, the solubility of the surfactant in the water changes. It can be increased or decreased, depending on the kind of electrolyte [68,69]. The effect of electrolytes is explained by the HSAB principle (hard-soft-acid-base). For example, salts of hard acids and hard bases reduce the solubility of the surfactant in water. The solubility is increased by salts of soft acids and hard bases or by salts of hard acids and soft bases. Correspondingly, the solubility of the surfactant in water is increased by sodium alkyl sulfonates and decreased by sodium chloride or sodium sulfate. In the meantime, the physical interactions of the surfactant molecules and other components in microemulsions is well understood and the HSAB-principle was verified. The salts in water mainly influence the curvature of the surfactant film in a microemulsion. The curvature of the surfactant film can be expressed, analogous to the HLB-value, by the packing parameter Sp. The packing parameter is the ratio between the hydrophilic and lipophilic surfactant molecule part [70] ... 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

Microemulsions are transparent or translucent, thermodynamically stable emulsion systems (Griffin 1949). Forming a middle phase microemulsion (MPM) requires matching the surfactant system s hydrophobicity with that of the oil. The HLB (hydrophilic-lipophilic balance) number reflects the surfactant s partitioning between water and oil phases higher HLB values indicate water soluble surfactants while lower values indicate oil soluble surfactants (Kunieda et. al. 1980, Abe et. al. 1986). While a balanced surfactant system produces middle phase microemulsions, an underoptimum surfactant system is too water soluble (high HLB) while an over-optimunTSystem is too oil soluble (low HLB). 

Four different emulsifier selection methods can be applied to the formulation of microemulsions (i) the hydrophilic-lipophilic-balance (HLB) system (ii) the phase-inversion temperature (PIT) method (iii) the cohesive energy ratio (CER) concept and (iv) partitioning of the cosurfactant between the oil and water phases. The first three methods are essentially the same as those used for the selection of emulsifiers for macroemulsions. However, with microemulsions attempts should be made to match the chemical type of the emulsifier with that of the oil. A summary of these various methods is given below. 

Kunieda, H. and Shinoda, K. (1980) Solution behaviour and hydrophile-lipophile balance temperature in the Aerosol OT-isooctane-brine system-correlation between microemulsions and ultralow interfacial tensions. /. Colloid Interface Sci., 75, 601-606. 

Khan, A., Lindstrom, B., Shinoda, K. and Lindman, B. (1986) Change ofthe microemulsion structure with the hydrophile-lipophile balance of the surfactant and the volume fractions of water and oil. /. Phys. Chem., 90, 5799-5801. 

In conclusion, the amphiphilicity factor is important for the understanding of microemulsion structuring. It was also suggested as an additional means of classifying surfactants, together with the packing parameter or the hydrophilic-lipophilic-balance (HLB) scale [84],... 

In W/O microemulsions the enzyme molecules are invariably located (entrapped) in the water droplet region, but the exact position may vary depending on the hydrophilic-lipophilic balance of the enzyme (and to some extent on the nature of the solvent). A truly hydrophilic enzyme will be located in the water core of the droplet, surrounded by a water layer. a-Chymotrypsin is an example of such a protein. A surface-active enzyme, such as lipase, has a strong driving force for the oil/water interface, where it competes with... 

In a 1994 work, the effect of the surfactant on lipase-catalyzed hydrolysis of palm oil in microemulsion was further investigated [62]. Three surfactants were used one anionic, one nonionic, and one cationic. As shown in Fig. 10, all three compounds were double-tailed, with similar hydrophilic-lipophilic balance, giving large regions of L2 microemulsions with isooctane and water at 37°C. 

As mentioned earlier, the formation of bicontinuons microemulsions is governed by the nature of the water-soluble monomer. It is present in large proportions (fti 25%) and affects interfacial properties as well as the hydrophile-lipophile balance of the system. For this reason, monomer/surfactant interactions cannot be ignored. 

Inverse (or water-in-oil) emulsions (315, 401) are emulsions in which an aqueous phase is dispersed within a continuous organic phase. This system is essentially the inverse of a conventional emulsion, hence the name inverse emulsion. The organic phase is typically an inert hydrocarbon (such as mixed xylenes or low-odour kerosenes), and the aqueous phase contains a water-soluble monomer such as acrylamide (268). The aqueous phase may be dispersed as discrete droplets or as a bicontinuous phase (335), depending upon the formulation and conditions of the inverse emulsion. The hydrophilic-lipophilic balance (HLB) value of the stabiliser determines the form and stability of an inverse emulsion, with HLB values of less than 7 being appropriate for inverse emulsions. Steric stabilisers such as the Span , Tween , and Plutonic series of nonionic surfactants are usually used in preparing inverse emulsions. Inverse emulsions, suspensions, miniemulsions (199), and microemulsions have been prepared, primarily as a function of the stabiliser concentration. Commercial products produced by inverse emulsion polymerisation include polyacrylamide, a water-soluble polymer used extensively as a thickener. 

Unfortunately, ionic surfactants do not form balanced middle-phase microemulsions without the addition of at least a fourth component, namely salt, and often a fifth component, i.e. an alcohol cosurfactant . Since the head-groups of ionic surfactants tend to be substantially more hydrophilic than lower-molecular-weight poly(ethylene oxide) moieties j = 4 to 8) (36), salts and alcohol cosurfactants must be added to move the overall hydrophilic-lipophilic balance of the mixture into the range required for formation of optimally balanced middle-phase microemulsions. 

Without addition of salt, Aerosol OT forms a much-studied region of oil-rich microemulsions (41). However, the even more hydrophilic single-tailed surfactants such as sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) are so far from the optimal hydrophilic-lipophilic balance that addition of salt alone is not enough for the formation... 

The above discussion of phase behaviour clearly shows that while a wide variety of mixtures form microemulsions, the conditions under which optimal microemulsion form are narrow and sharply defined. Above all, the proper hydrophilic-lipophilic balance must be set by an appropriate choice of surfactant and additives. 

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