Hydrophilic-lipophilic balance cationic surfactants

Surfactants employed for w/o-ME formation, listed in Table 1, are more lipophilic than those employed in aqueous systems, e.g., for micelles or oil-in-water emulsions, having a hydrophilic-lipophilic balance (HLB) value of around 8-11 [4-40]. The most commonly employed surfactant for w/o-ME formation is Aerosol-OT, or AOT [sodium bis(2-ethylhexyl) sulfosuccinate], containing an anionic sulfonate headgroup and two hydrocarbon tails. Common cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB) and trioctylmethyl ammonium bromide (TOMAC), have also fulfilled this purpose however, cosurfactants (e.g., fatty alcohols, such as 1-butanol or 1-octanol) must be added for a monophasic w/o-ME (Winsor IV) system to occur. Nonionic and mixed ionic-nonionic surfactant systems have received a great deal of attention recently because they are more biocompatible and they promote less inactivation of biomolecules compared to ionic surfactants. Surfactants with two or more hydrophobic tail groups of different lengths frequently form w/o-MEs more readily than one-tailed surfactants without the requirement of cosurfactant, perhaps because of their wedge-shaped molecular structure [17,41]. 

Nonionic surfactants consist of a -(CH2CH20)n0H or -OH as the hydrophilic group and exhibit a variety of hydrophile-lipophile balances (HLB) which stabilize O/W or W/O emulsions. Unlike anionic and cationic surfactants, nonionic surfactants are useful for oral and parenteral formulations because of their low irritation and toxicity. Based on their neutral nature, they are much less sensitive to changes in the pH of the medium and the presence of electrolytes. The best use of nonionic surfactants is to produce an equally balanced HLB of two nonionic surfactants one... 

Surfactants are organic molecules that possess a nonpolar hydrocarbon tail and a polar head. The polar head can be anionic, cationic, or nonionic. Because of the existence of the two moieties in one molecule, surfactants have limited solubility in polar and nonpolar solvents. Their solubility is dependent on the hydrophile-lipophile balance of their molecular structure. At a critical concentration, they form aggregates in either type of solvent. This colloidal aggregation is referred to as micellization, and the concentration at which it occurs is known as the critical micelle concentration. The term micelle was coined by McBain (7) to designate the aggregated solute. In water or other polar solvents, the micellar structure is such that the hydrophobic tails of the surfactant molecules are clustered together and form the interior of a sphere. The surface of the sphere consists of the hydrophilic heads. In nonpolar solvents, the orientation of the molecules is reversed. 

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. 

With respect to the properties of polar groups, surfactants can be subdivided into ionic (cation- and anion-active, ampholytic, and zwitterionic) and nonionic surfactants. If the effect produced by the polar group of the surfactant molecule is more significant than that of the lipophilic group, this substance is soluble in water. It is less surface active as compared to any substance characterized by an optimum balance between the activities of hydrophilic and lipophilic groups. Similar conclusions can be drawn also with respect to the solubility in oil here, the role of the lipophilic group is determining. Clearly, the efficiency of a surfactant is not determined solely by the amphiphilicity, but depends on the hydrophilic/lipophilic balance (HLB) characteristic for this compound. Therefore, this balance is an important characteristic of both the surfactant and the interface. 

The cationic micellar eluents of ChTAB gave better linear correlations for k vs. log Pow than the anionic surfactant SDS [16]. This might indicate that the cationic micellar systems have a similar hydrophilic/lipophilic balance to octanol-water, and/or the polar interactions betw n the solutes and the cationic head group better resemble those in octanol-water. For the chromatographic correlations, one should also note that the cationic surfactants adsorbed on the stationary phase can better shield the residual silanol groups on the silica surface, than SDS. Consequently, the reduction (or elimination) of the silanophilic interactions would increase the correlations between RPLC retention and log P w ... 

Water-insoluble aryl iodides can be hydroxycarbonylated directly using solubilized media, such as canonical microemulsions or Shinoda s swollen micelles. Microemulsions formed by cationic and anionic surfactants can be used for both liquid and solid aryl iodides, giving high yields of benzoic acids in the presence of palladium salts in phosphine-less mode and inorganic bases. Though the microemulsions always contain aliphatic alcohols used to adjust the hydrophile-lipophile balance of the surfactant system, the formation of esters was never observed [109]. 

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