The Hydrophilic-Lipophilic Balance HLB

As will be seen in the chapter on emulsions (Chapter 12), the hydrophilic-lipophilic balance (HLB) is a very useful parameter which, in conjunction with several other semi-empitical tools (like the Bancroft mle), can be used in the design of emulsions. 

The HLB is a rather empirical parameter. Values below 8 indicate hydrophobic surfactants (emulsifiers), while values above 8 (or 10) indicate hydrophilic surfactants. 

The most weU-known method for estimating the HLB is the group contribution method of Davies and Rideal  

According to the values shown in Table 5.6, and as expected, ionized emulsifiers will have high HLB values. This can be attributed to the strong interaction with water molecules exerted by the charged head groups. The values were obtained by studying the behaviour (e.g. the type of emulsion formed) of selected surfactants in mixtures and referring the 

There are a few special estimation methods as well, e.g. for nonyl phenyl ethoxylates  

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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). 

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... 

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. 

Particularly useful is the physical classification of surfactants based on the hydrophile-lipophile balance (HLB) system [67,68] established by Griffin [69,70]. More than 50 years ago he introduced an empirical scale of HLB values for a variety of nonionic surfactants. Griffin s original concept defined HLB as the percentage (by weight) of the hydrophile divided by 5 to yield more manageable values ... 

The key-step of the synthesis of glycolipids, and more generally of amphiphilic carbohydrates, is the covalent coupling of a hydrophilic carbohydrate with a lipophilic compound. A hydrophilic or hydrophobic spacer may be inserted between them in order to control the hydrophilic-lipophilic balance (HLB). This modulation allows to obtain variously organized systems with the same polar head and apolar tail. 

Bourrel M, Chambu C (1983) The Rules for Achieving High Solubilization of Brine and OU by AmphiphUic Molecules. Soc Petrol Eng J 23 327-338 Kunieda H, Shinoda K (1985) Evaluation of the hydrophile-lipophile balance (HLB) of nonionic surfactants I. Multisurfactant systems. J Colloid Interface Sci 107 107-121 Kahlweit M, Strey R, Eirman P (1986) Search for tricritical points in ternary systems Water-oil-nonionic amphiphile. J Phys Chem 90 671... 

Molecules consisting of a long hydrophobic part and one or two hydrophilic headgroups. Able to form micelles and/or liposomes depending on the hydrophilic-lipophilic balance (HLB). 

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. 

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. 

In certain cases, cholesterol is required for vesicle formation. It is commonly accepted that the hydrophilic lipophilic balance (HLB) is a parameter that could indicate the vesicleforming potential of surfactants. For amphiphils such as sorbitan esters and alkyl ethers, low HLB values could predict vesicle formation [52,55]. However, niosomes were obtained from polysorbate 20 (HLB 16.7), a highly hydrophilic molecule, when cholesterol at an appropriate concentration was added to the amphiphil [44], In this case it could be assumed that a kind of amphiphilic complex with a lower HLB was responsible for the vesicle formation. An excellent review on the structure, characteristics, chemical composition, and mechanism of action of niosomes was published by Uchegbu and Vyas [41]. 

Polyoxyethylene (POE) (20) sorbitan monooleate (Tween 80, Lot 36218, ICI Surfactants, Inc.) was used as received with no further purification. The average molecular weight of Tween 80 is 1310 g/mole, the density is 1.07 g/cm3, the hydrophile-lipophile balance (HLB) is 15, the critical micelle... 

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. 

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]. 

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 correlation between the volume or stability of the foam and the hydrophile-lipophile balance (HLB) of surfactants or their mixtures is used to generalise the experimental results derived about the stabilising ability of complex foaming compositions or homologues surfactant series. HLB is an important surfactant parameter, characterising the relative... 

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. 

Generalizations from the aqueous-solution surface tensions in Table IX are risky, because values are as dependent on the hydrophilic-lipophilic balance (HLB) as on the intrinsic surface activity of the hydrophobe. The data in Table IX are consistent with earlier observations that longer per-fluorinated groups are most effective in producing low surface tensions (in this case CF3(CF2)5-) and that a terminal CF2H- is detrimental. 

UCON is a random copolymer of ethylene oxide (EO) and propylene oxide (PO) and can be considered as a synthetic oil. POE is poly(oxyethylene), which is a homopolymer of EO. Random EO-PO copolymers are typical of synthetic oils used as lubricants in textile processing (53, 54). PLURONICS (BASE) are block copolymers of EO and PO. Both of these copolymers can be produced with viscosities ranging from that of a thin oil to that of a thick paste. Oils of appropriate viscosity give hydrodynamic lubrication in the high-speed spinning of synthetic fibers. The hydrophile-lipophile balance (HLB) in these polymers can be varied, by changing the EO-PO composition, to suit the aqueous solubility requirements. 

It has been observed that as the hydrophilic lipophilic balance (HLB) value of the surfactant increases, the stabihty of the emulsion decreases [85]. The contribution of the surfactant to stabilization and swelling is discussed in Sections 25.4.2.1 and 25.4.2.2, respectively. 

The hydrophilic-lipophilic balance (HLB) and molecular dimensions of a preservative should also be considered in the design of contact lens products. For example, chlorobutanol (a non-ionic preservative used... 

In spite of many advances in the theory of stability of lyophobic colloids, resort has still to be made to an empirical approach to the choice of emulsifier, devised in 1949 by Griffin. In this system we calculate the hydrophile-lipophile balance (HLB) of surfactants, which is a measure of the relative contributions of the hydrophilic and lipophilic regions of the... 

It is necessary to use at least two surfactants, one for the primary emulsion and the other for the dispersion of this emulsion to form the multiple system. The optimum surfactant to emulsify a given oil can be determined by use of the hydrophile-lipophile balance (HLB) approach. The present authors have carried out an investigation into the optimal HLB required for both primary and secondary emulsification steps in the formulation of a water/ isopropyl myristate/water emulsion. W/o emulsions containing 47.5 o isopropyl myristate and 2.5% surfactant had an optimal HLB of 4.5. 

The most widely used system for classifying emulsifiers is the hydrophile-lipophile balance (HLB) concept (Griffin (1949)/77)- A low HLB number means that the emulsifier is lipophilic and a high value means that it is hydrophilic. 

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