HLB scale

Several empirical correlations between the HLB value and physicochemical parameters of the surfactants were proposed. Griffin suggested a correlation between the HLB value and the nonionic surfactant cloud point. Becher related the HLB value to the free energy of surfactant micellization, and to the Israelachvili-Mitchell-Ninham packing parameter i.e. spontaneous curvature). There was also an attempt to provide a mechanistic interpretation of the HLB scale by Davies et 60,61 authors examined the rates of coalescence of O/W and W/O emulsions. For high-HLB surfactants, the coalescence barrier of O/W emulsions is predicted to be high, while that of W/O emulsions is low. Eventually, O/W emulsions will dominate over the time, which explains the HLB. However, the way the coalescence barriers were estimated is rather primitive.  

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The hydrophile—hpophile balance (HLB) is an empirical system based on the fact that oil—water (o/w) emulsions are best stabilized by water-soluble-emulsifiers and water—oil (w/o) emulsions are best stabilized by oil-soluble ones (3). The HLB scale mns from 0—20 and is based on the ratio of the saponification number of ester, A, to the acid number of recovered acid, where HLB = 20 1-Sj A). The dispersibiUty of an emulsifier in water is related to HLB value. 

One of the most important characteristics of the emulsifier is its CMC, which is defined as the critical concentration value below which no micelle formation occurs. The critical micelle concentration of an emulsifier is determined by the structure and the number of hydrophilic and hydrophobic groups included in the emulsifier molecule. The hydrophile-lipophile balance (HLB) number is a good criterion for the selection of proper emulsifier. The HLB scale was developed by W. C. Griffin [46,47]. Based on his approach, the HLB number of an emulsifier can be calculated by dividing... 

Since the HLB scale greatly compresses the range of partition coefficients, the data of Fig. 6 are replotted as a function of Kp in Fig. 7 for the Makon surfactants. This analysis reveals a steady increase in extraction up to a cut-off level at which recovery drops precipitously. 

Whether the system formed on mixing oil, water, and surfactant will be an oil-in-water or a water-in-oil emulsion is a central problem in emulsion technology. It was realized very early that the volume fractions of oil and water are not that important and that the type of emulsion is primarily determined by the nature of the surfactant. Simply speaking surfactants with Ns < 1 tend to form oil-in-water emulsions, while surfactants with Ns > 1 are more likely to form water-in-oil emulsions. Two more detailed guiding principles which are used for practical emulsion formulation are Bancroft s rule of thumb and the more quantitative concept of the HLB scale ... 

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... 

HLB scale) An empirical scale categorizing surfactants in terms of their tendencies to be mostly oil-soluble or water-soluble, hence their tendencies to promote W/O or O/W emulsions, respectively. See also Phase Inversion Temperature. 

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. 

The HLB scale was introduced hy W. C. Griffin of Atlas now I.C.I, America) (1949, I9S4, 1980) as an aid particularly to the use of nonionic surfactants in the formulation of cosmetics. His scale (Table )) is related to the solubility of the materials in water for nonionic suifactants on polyfethylene oxide) the HLB number may be found simply by dividing the percentage of ethylene oxide in the product by five (Griffin, 1954). Davies (1957) has devised group constants that permit calculation of HLB values for other types of suifactani (Table II). Greth and Wilson (1961), also of... 

Figure 2 Removal rate as a function of surfactant HLB. The data are for the copper disk, polished on Struers (Table 2) at pH 2. The ionic surfactant SDS is included, although off the 0-20 nonionic HLB scale.

An empirical scale developed for categorizing single-component or mixed (usually nonionic) emulsifying agents, using this principle, is the hydrophile-lipophile balance or HLB scale. This dimensionless scale ranges from 0 to 20 a low HLB (<9) refers to a lipophilic surfactant (oil-soluble) and a high HLB (>11) to a hydrophilic (water-soluble) surfactant. In gen-... 

Figure 7.12 The HLB scale and the approximate ranges into which solubilising agents, detergents, emulsifiers and antifoaming agents fall.

Any pair of emulsifying agents that fall at opposite ends of the HLB scale - for example. Tween 80 (sorbitan monooleate with 20 mol EO, HLB = 15) and Span 80 (sorbitan monooleate, HLB = 5) - can be taken and used in various proportions to cover a wide range of HLB numbers. The emulsions should be prepared in the same fashion, with a few percent of the emulsifying blend. The stability of the emulsions can then be assessed at each HLB number, either from the rate of coalescence or qualitatively by measuring the rate of oil separation. In this way it should be possible to determine the optimum HLB number for a given oil. Subsequently, having found the most effective HLB value, various other surfactant pairs can be compared at this HLB value to identify the most effective pair. 

It was shown by Rusanov et al. that the empirical HLB scale is supported by thermodynamics the analysis of work of transfer of surfactant molecules from aqueous phase to hydrocarbon phase revealed that group numbers, B are proportional to the work of transfer of individual groups present in the molecule, and that the work of transfer of the entire molecule is given by summation of works of transfer of individual groups. 

Hydrophile—Lipophile Balance (HLB scale) An empirical scale categoriz-... 

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],... 

As an essential feature of the HLB scale, it was assumed that the HLB of a surfactant mixture could be calculated from a linear average mixing rule based on weight composition. Thus intermediate HLB could be attained simply by mixing two or more surfactants by using the relation.ship ... 

The physical gr physicochemical signiheafion of the HLB scale could be ascertained by associating the HLB value with some surfactant properties, e.g.. water dispersibility. Sur t its with HLB <8 ate not dispersible in waiei. wherea.s those with HLB >12 are fully soluble in water. Intermediate HLB surfactants are more or less dispersible depending on other factors. Scores of papers have related HLB to other surfactant properties, as reported in an exhaustive computer screening prepared by Becher (2). 

This experimental protocol proposed by Griffin is. however, tedious and lacks accuracy for several reasons. While the maximum at low HLB is essentially at the same position for most HLB scans, so that HLB, for W/O is about 5-6 for most oil phase.s, the maximum at high HLB is often difficult to pinpoint with accuracy because the 0/W emulsion stability does not change very much with HLB. On the other hand, the stability curve and thus minima position are not strictly dependent on the HLB scale but are found Co change when the surfactant type is changed. 

The adempts to rationalize GrifHn s HLB scale from a physicochemical point of view were made in a number of studies. Various correlations were shown to exist between the HLB numbers and the chemical structure or molecular composition of the siufactants. Correlations were also fotmd between the HLB number and physicochemical properties of surfactants and their solutions, for example, stffface and interfacial tension, solubility, and heat of solution, spreading and distribution coefficient, dielectric permittivity of the surfactant, cloud point and phase inversion point, critical micelle concenlration, foaminess, etc. These studies are reviewed in Ref. 262. However, the correlations found are not generally applicable moreover, the concept of the additivity of HLB numbers as such for mixtures of surfactants or oils cannot be proven expermentally when the surfactant characteristics are varied over a wider range (265). 

The HLB scale of Davies is based on the difference between the work of transfer of a surfactant molecule (or its constituents) from the vacuum into aqueous and oil phases. It can be expected that, similar to this difference, the ratio of these values can be used as a measure for the HLB (261). The work of surfactant transfer into a phase from the vacuum can be calculated as w = -i- wy, where the sub-... 

To summarize, among all the proposed characteristics of the HLB, Davies HLB scale is the most substantiated and most widely used, in spite of the number of deficiencies noted above. Here, the recent publication (279), which relates the electroacoustophoretic behavior of emulsions with Davies HLB numbers, can be referred to as an example. 

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