Hydrophile-lipophile balance of surfactants

Vol. 9 Hydrophile-Lipophile Balance of Surfactants and Solid Particles. Physicochemical Aspects and Applications. By P.M. Kruglyakov Vol. lO Particles at Fluid Interfaces and Membranes. Attachment of Colloid Particles and Proteins to Interfaces and Formation of Two-Dimensional Arrays. 

Stabilising Ability and Hydrophile-Lipophile Balance of Surfactants... 

Shinoda, K., Hanrin, M., Kunieda, H., and Saito, H. (1981) Principles of attaining ultra-low interfadal tension the role of hydrophile-lipophile-balance of surfactant at oil/water interface. Colloids Surf., 1, 301-314. 

In most cases, these active defoaming components are insoluble in the defoamer formulation as weU as in the foaming media, but there are cases which function by the inverted cloud-point mechanism (3). These products are soluble at low temperature and precipitate when the temperature is raised. When precipitated, these defoamer—surfactants function as defoamers when dissolved, they may act as foam stabilizers. Examples of this type are the block polymers of poly(ethylene oxide) and poly(propylene oxide) and other low HLB (hydrophilic—lipophilic balance) nonionic surfactants. 

H Schott. Solubility parameter and hydrophilic lipophilic balance of nonionic surfactants. J Pharm Sci... 

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] ... 

K. Shinoda and H. Takeda, The effect of added salts in water on the hydrophile-lipophile balance of nonionic surfactants the effect of added salts on the phase inversion temperature of emulsions, J. Colloid Interface Sci. 32 (1970) 642-646. 

The concentration of black spot formation in microscopic films Cm characterises not only the threshold concentration of the surfactant at which stable foams and emulsions can be obtained but it can also be used as an indirect measure of film stability. The relations between film stability and Cm of the emulsifier depend on the polarity of the organic phase of the emulsion films (aqueous and hydrocarbon) [58], on the hydrophilic-lipophilic balance of the surfactant mixture [59] as well as on other properties. 

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. 

Tween 80 (also known as At-Plus 109 and Polysorbate 80) is a polyoxyethylene sorbitan monooleate and was obtained from Atlas Chemical Industries, Inc. This nonionic surfactant has an HLB (hydrophile-lipophile balance) of 15.0 and is used as an emulsifier, solubilizer, and dispersant. It was used without further purification at the 0.1% (w/v) level in partitioning and greenhouse herbicidal evaluations as described below. 

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. 

I.J. Lin, The hydrophilic-lipophilic balance (hlb) of fluorocarbon surfactants and its relation to the critical micelle concentration (cmc), J. Phys. Chem., 1972, 76, 2019 I.J. Lin, J.P. Friend and Y. Zimmels, The effect of structural modifications on the hydrophilic-lipophilic balance of ionic surfactants, J. Colloid Interface Sci., 1973,45,378. 

I summarize briefly below the basic concepts of this approach, which is derived from that developed by Beerbower and Hill [31] for the stability of classical nonionic emulsions, which is referred to as the cohesive energy ratio (CER) concept. The treatment lies in a perfect chemical match between the partial solubility parameters of oil ( ) and surfactant lipophilic tail 6]) and of water and hydrophilic head. Under these conditions, one obtains for the optimum HLB (hydrophile-lipophile balance) of the surfactant the relation... 

Figure 6.11 illustrates the difference in surface composition with respect to the surfactant for a polymerizable amide ethoxylate and a conventional nonionic surfactant of similar hydrophilic-lipophilic balance (HLB). Surfactant concentrations at the film-air interface were... 

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. 

Determination of hydrophilic-lipophilic balances of some surfactants using tensio-active compoimds as stationary phases and injection of substances of suitable structure (normal hydrocarbons and alcohols) has also been recorded [126—127]. 

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. 

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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