Emulsions hydrophilic-lipophilic balance

Emulsifiers are classified by the hydrophilic—lipophilic balance (HLB) system. This system indicates whether an emulsifier is more soluble in water or oil, and for which type of emulsion (water-in-oil or oil-in-water) it is best suited. Emulsifiers having alow HLB value are more oil soluble, and are better suited for water-in-oil appHcations such as margarine. Conversely, emulsifiers having a high HLB value are more water soluble, and function more effectively in oil-in-water emulsions such as ice cream (34). The use of this system is somewhat limited because the properties of emulsifiers are modified by the presence of other ingredients and different combinations of emulsifiers are needed to achieve a desired effect. The HLB values of some common emulsifiers are given (35). 

Emulsifiers. Removing the remover is just as important as removing the finish. For water rinse removers, a detergent that is compatible with the remover formula must be selected. Many organic solvents used in removers are not water soluble, so emulsifiers are often added (see Emulsions). Anionic types such as alkyl aryl sulfonates or tolyl fatty acid salts are used. In other appHcations, nonionic surfactants are preferred and hydrophilic—lipophilic balance is an important consideration. 

The performance of secondary alkanesulfonates in applications as emulsifiers in the widespread emulsion polymerization of vinyl monomers can be assessed by their hydrophilic-lipophilic balance (HLB) numbers. The HLB numbers can... 

Formation of emulsions of the oil-in-water or water-in-oil type depends mainly on the hydrophilic-lipophilic balance (HLB) of the emulsifier. Phosphate esters with their various molecular structures can be adjusted to nearly every HLB value desired. Therefore they are able to meet nearly all of demands in this field. 

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 are often characterized in terms of their hydrophile—lipophile balance (HLB) number (see Emulsions). For simple alcohol... 

At low temperature, nonionic surfactants are water-soluble but at high temperatures the surfactant s solubility in water is extremely small. At some intermediate temperature, the hydrophile—lipophile balance (HLB) temperature (24) or the phase inversion temperature (PIT) (22), a third isotropic liquid phase (25), appears between the oil and the water (Fig. 11). The emulsification is done at this temperature and the emulsifier is selected in the following manner. Equal amounts of the oil and the aqueous phases with all the components of the formulation pre-added are mixed with 4% of the emulsifiers to be tested in a series of samples. For the case of an o/w emulsion, the samples are left thermostated at 55°C to separate. The emulsifiers giving separation into three layers are then used for emulsification in order to find which one gives the most stable emulsion. 

Griffin suggested an empirical quantitative hydrophile-lipophile balance (HLB) scale which characterizes the tendency of a surfactant to form water-in-oil or oil-in-water emulsions [544], The HLB is a direct measure of the hydrophilic character of a surfactant the... 

The hydrophilic-lipophilic balance (HLB) classi Lcation system was L rst introduced by GrifL n (1949) to characterize the relative afLnity of a surfactant to the aqueous and oil phase. A HLB value is an empirical numerical value in the range of 1-30. The higher the HLB value, the more hydrophilic the surfactant is and in turn, the lower the HLB value, the more lipophilic the surfactant is. As a result, surfactants with higher HLB values (>8) are favorable for formation of o/w emulsions, while surfactants with lower HLB values (3-6) are more suitable for the formation of w/o emulsions. The HLB values of the surfactants used in parenteral emulsions are listed in Table 10.2. 

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. 

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). 

An empirical generalization that predicts that the continuous phase in an emulsion will be the phase in which the emulsifying agent is most soluble. An extension for solid particles acting as emulsifying agents predicts that the continuous phase will be the phase that preferentially wets the solid particles. See also Hydrophile-Lipophile Balance. 

An empirical generalization used to predict which phase in an emulsion will be continuous and which dispersed. It is based on a physical picture in which emulsifiers are considered to have a wedge shape and will favour adsorbing at an interface, such that most efficient packing is obtained that is, with the narrow ends pointed toward the centres of the droplets. A useful starting point, but there are many exceptions. See also Bancroft s Rule, Hydrophile-Lipophile Balance. 

A useful index for choosing surfactants for various applications is the hydrophilic-lipophilic balance (HLB), which is based on the relative percentage of hydrophilic-to-lipophilic groups in the surfactant molecule(s). Surfactants with a low HLB number normally form W/O emulsions, whereas those with a high HLB number form a O/W emulsion. A summary of the HLB range required for various purposes is given in Table I. 

Polysaccharides may exercise a protective action in an emulsion and foam as a thin film at liquid-liquid (emulsion) and liquid-air (foam) interfaces. The hydrophile-lipophile balance in the macromolecules as well as <(>, determines whether or not the emulsion is an oil-in-water or water-in-oil dispersion (Void and Void, 1983 Dickinson, 1992). 

For a constant amount of nonionic surfactant, the interfacial tension at the planar oil-water interface, for the same amounts of oil and water, passes through a minimum when plotted against the hydrophilic-lipophilic balance (HLB). The emulsion stability passes through maxima in the W/O and O/W ranges and through a minimum between the two at the phase inversion point. The minima in the two cases coincide. These observations are explained on the basis of thermodynamics. The stability of macroemulsions can be correlated with the surface excess of surfactant, which also passes through two maxima and a minimum between them [2.11]. 

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... 

The hydrophile-lipophile balance (HLB) system is the measure of the surfactant s polarity as well as other physical properties of surfactants and the emulsifying materials. The more lipophilic the surfactant is, the lower the HLB values will be. Table 4.5 empirically classifies and compares surfactants according to their optimum use. Table 4.6 shows the HLB values for a selected group of surfactants. The HLB value of the surfactant or surfactant mixture should be matched with that of the oil or the mixture of oils to ensure a stable emulsion. The required HLB values of a... 

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. 

Table 1 summarizes the relation between the hydrophilic-lipophilic balance (HLB) of surfactants and their ability to form concentrated emulsions. Because the continuous phase is that phase in which the surfactant is soluble, it is expected from the definition of HLB [17,18] that surfactants with low HLB values are oil-soluble and can therefore generate w/o concentrated emulsions, while those with high HLB values are water-soluble and can lead to o/w concentrated emulsions. Span 20, whose HLB is 8.6, can generate both w/o and o/w concentrated emulsions. 

The type of emulsion formed (normally water-in-oil or oil-in-water, commonly expressed as wlo or olw, w denoting the aqueous phase and o the organic phase) is determined by the volume ratio of the two liquids and also by the phase addition sequence and the nature of any additives used to promote emulsification [29] the affinity of emulsifiers for oil and water is measured on the hydrophile-lipophile balance (HLB) scale [30]. Oil-in-water emulsions are most common in all application fields. 

For nonionic surfactants, an optimization of the process was achieved by using a similar approach to the so-called Cohesive Energy Ratio (CER) concept developed by Beerbower and Hill for the stability of classical emulsions (H). Its basic assumption is that the partial solubility parameters of oil and emulsifier lipophilic tail and of water and hydrophilic head are perfectly matched. Thus, the Vinsor cohesive energy ratio Ro, which determines the nature and the stability of an emulsion, is directly related to the emulsifier HliB (hydrophile-lipophile balance) by... 

The manner in which lecithin is modified to achieve increased hydrophilicity will greatly affect its emulsification properties. Different modifications will create lecithin products with different apparent HLB (hydrophile-lipophile balance) values, a term used to convey the approximate degree of water dispersibility (hydrophilicity) of lecithin products (31). The higher its HLB value, the more water dispersible the lecithin product. In o/w emulsions, the type of fat to be emulsified may require a specific type of hydrophilic lecithin for optimum emulsion stability. Dashiell (31) provides a short listing of fat types, and the corresponding class of lecithin found to give the most stable emulsion in model systems of water/fat/ emulsifier. 

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