Hydrophilic-lipophilic balance number

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). 

Hydrophile-Lipophile Balance) Numbers for Some Surfactants... 

The Span 80 with an HLB (hydrophilic-lipophilic balance number) of 4,3, which is an oil soluble liquid, was used as surfactant. The effect of the continuous medium was investigated by employing 1,1,2,2-tetrachloroethane, toluene, or decane, which have various degrees of hydrophobicity. The amounts of the components used are listed in Table 3. At room temperature (20 °C), concen-... 

The choice of an emulsifier is a step not completely random but it is driven by several empirical rules. These rules are based on the primary condition for the emulsifier to be efficient in a two phase, two-liquid emulsion. It has to be localised at the oil/water interface to a maximal extent. This criterion is exemplified in the HUB (Hydrophilic Lipophilic Balance) number. The HLB number for a certain molecule denotes the balance between its hydrophilic/lipophilic properties only the number itself does not give any information about the stabilising efficacy of the emulsitler. 

The amphiphilic nature of nonionic surfactants is often expressed in terms of the balance between the hydrophobic and hydrophilic portions of the molecule. An empirical scale of ffLB (hydrophile-lipophile balance) numbers has been devised (see Chapter 7, section 7.3.2). The lower the ffLB number, the more lipophilic is the compound and vice versa. ffLB values for a series of commercial nonionic surfactants are quoted in Tables 6.7 and 6.8. The choice of surfactant for medicinal use involves a consideration of the toxicity of the substance, which may be ingested in large amounts. The following surfactants are widely used in pharmaceutical formulations. 

Seven surfactants were selected with differing HLB (Hydrophile Lipophile Balance) numbers and used at concentrations of 0.5 - 2.5% w/v in aqueous solution. An intermediate concentrate was prepared by high shear mixing and subsequently diluted with 1% methylcellulose solution to give a final drug concentration of 10% w/v. Methods of analysis included laser diffraction particle size determination, optical microscopy and dissolution into an aqueous medium. The suspensions were analysed immediately after preparation and after 4 and 96 hours. 

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... 

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... 

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

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

The amphiphilic nature of many emulsifying agents (particularly non-ionic surfactants) can be expressed in terms of an empirical scale of so-called HLB (hydrophile-lipophile balance) numbers222 (see Table 10.1). The least hydrophilic surfactants are assigned the lowest HLB values. Several formulae have been established for calculating HLB numbers from composition data and they can also be determined experimentally - e.g. from cloud-point measurements123,125. For mixed emulsifiers, approximate algebraic additivity holds. 

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. 

The solubility of the surfactant of polyethyleneglycol type in different phases can be described by the HLB (hydrophilic-lipophilic-balance) concept [ 27]. This concept attributes to the molecule a HLB number that represents the geometric ratios of the hydrophilic and the hydrophobic moieties. It should, however, be emphasized that the HLB does not represent a fundamental property of the system but is based on experience. For fatty alcohol ethoxylates... 

Examination of the relevant theory indicates that the adjuvant effect of surface-active agents on herbicide action is maximized when the quantity FI = yL cos 0, or the film pressure at the liquid/solid interface, has a maximum value. Measurement of surface tension of 1.0% aqueous solutions and of contact angle on a number of substrates (Teflon, paraffin) and plant-leaf surfaces (soybean, com) as a function of hydrophile-lipophile balance show at least one maximum, and these values are in good agreement with earlier experimental data on herbicidal activity. 

One of the difficulties is that there is no convenient theory for selecting the appropriate surfactants a priori. The simplest procedure is to try to match the HLB (hydrophile-lipophile balance) character of the surfactants to the polarity of the particular polymers. Tabulated HLB values are available [23, 24] (the higher the HLB number the more hydrophilic is the soap), but the rating is essentially empirical and effective use requires some experience and intuition. 

Figure 47.2. Pluronic block copolymers with various numbers of hydrophilic EO (n) and hydrophobic PO (m) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions at concentrations above critical micelle concentration (CMC) these copolymers self-assemble into micelles.

Griffin devised the concept of hydrophile-lipophile balance (HLB) and its additivity many years ago for selection of non-ionic emulsifiers and this rather empirical method is still widely used. The enormous literature on the HLB of surfactants has been reviewed by Becher. Each surfactant is allocated an HLB number usually on a scale of 0-20, based on the relative proportions of the hydrophilic and hydrophobic part of a molecule. Water-in-oil emulsions are formed generally from oil-soluble surfactants of low HLB number and oil-in-water emulsions from more hydrophilic surfactants of high HLB number. The method of selection is based on the observation that each type of oil will require an emulsifying agent of a specific HLB number to produce a stable emulsion. Thus, oils are often designated two required HLB numbers, one low and one high, for their emulsification to form water-in-oil and oil-in-water emulsions respectively. A series of emulsifiers and their blends with HLB values close to the required HLB of the oil are then examined to see which one forms the most stable emulsion (c.f. Fig. lA). 

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. 

The hydrophile-lipophile balance (HLB) has been used to characterize surfactants. This number indicates relatively the tendency to solubilize in oil or water and thus the tendency to form water-in-oil or oil-in-water emulsions. Low HLB numbers are assigned to surfactants that tend to be more soluble in oil and to form water-in-oil emulsions. When the formation salinity is low, a low HLB surfactant should be selected. Such a surfactant can make middle-phase... 

HLB stands for hydrophile-lipophile balance. Surfactants with a low HLB are more lipid loving and thus tend to make a water-in-oil emulsion while those with a high HLB are more hydrophilic and tend to make an oil-in-water emulsion. The HLB number of a mixture composed of x% of surfactants of HLB A and y% of surfactants of HLB B is obtained by the following formula... 

The sorbitan esters are insoluble in water, but are soluble in most organic solvents (low hydrophilic-lipophilic-balance (HLB) number surfactants). The ethoxylated products are generally soluble in number, and have relatively high HLB numbers. 

The selection of different surfactants in the preparation of either O/W or W/O emulsions is often still made on an empirical basis. A semi-empirical scale for selecting surfactants, the hydrophilic-lipophilic balance (HLB number) was developed by Grifhn [18]. This scale is based on the relative percentage of hydrophihc to lipophilic (hydrophobic) groups in the surfactant molecule(s). For an O/W emulsion droplet the hydrophobic chain resides in the oil phase, whereas the hydrophilic head group resides in the aqueous phase. In contrast, for a W/O emulsion droplet the hydrophilic group(s) reside in the water droplet while the lipophilic groups reside in the hydrocarbon phase. 

---