Hydrophilic-Lipophilic Balance HLB Concept

The relative importance of the hydrophilic and lipophilic groups was first recognised when using mixtures of surfactants containing varying proportions of a low and high HLB numbers [20, 21]. The efficiency of any combination (as judged by 

Griffin [20, 21] developed simple equations to calculate the HLB number of relatively simple nonionic surfactants. For a polyhydroxy fatty acid ester 

For a glyceryl monostearate, S = 161 and A = 198 - The HLB is 3.8 (suitable for W/O emulsion). For a simple alcohol ethoxylate, the HLB number can be calculated from the weight percent of ethylene oxide (E) and Polyhydric alcohol (P), 

If the surfactant contains PEO as the only hydrophilic group, the contribution from one OH group can be neglected. 

Davies [23] has shown that the agreement between HLB numbers calculated from the above equation and those determined experimentally is quite satisfactory. 

---

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. 

A useful concept for choosing wetting agents of the ethoxylated surfactants is the hydrophilic-lipophilic balance (HLB) concept. 

There are a large number of emulsifiers available and making a choice for a new formulation or product is difficult. The use of the hydrophilic-lipophilic balance (HLB) concept originated by Griffin in 1949 can be a useful empirical method of preliminary selection. The HLB number quantifies the balance of hydrophilic-lipophilic characteristics of the surfactant molecule on an arbitrary numerical scale. The least hydrophilic surfactants are assigned the lowest HLB (see Table 14.2). The use of the HLB balance is also well described by Davies and Rideal. ... 

The hydrophile-lipophile balance (HLB) concept is probably the most useful approach to predict the type of emulsion that will be stabilized by a given surfactant or surfactant formulation. 

Full descriptions of the hydrophile-lipophile balance (HLB) concept are given by Becher and Becher and Schick. Griffin first defined the affinity of a nonionic surfactant in terms of an empirical quantity, the HLB. Surfactants are assigned an HLB number at 25 °C on a scale of 1 to 20, where low HLB numbers represent lipophilic surfactants and high HLB numbers represent hydrophilic surfactants. Generally, the application of a surfactant can be derived from its HLB number in accordance with Table 6.1. ... 

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 ... 

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). 

Four different emulsifier selection methods can be applied to the formulation of microemulsions (i) the hydrophilic-lipophilic-balance (HLB) system (ii) the phase-inversion temperature (PIT) method (iii) the cohesive energy ratio (CER) concept and (iv) partitioning of the cosurfactant between the oil and water phases. The first three methods are essentially the same as those used for the selection of emulsifiers for macroemulsions. However, with microemulsions attempts should be made to match the chemical type of the emulsifier with that of the oil. A summary of these various methods is given below. 

The formulation has been related with the type and properties of emulsions since Bancroft s rule of thumb (1913) and Langmuirs wedge theory (1917). The hydrophilic-lipophilic balance (HLB) was introduced by Griffin 60 years ago, probably as a selling argument for the (by the time) new non-ionic surfactants. It accounts for the relative importance of the hydrophilic and lipophilic parts of an amphiphilic molecule on a weight basis [19]. For decades there was no other numerical yardstick. The simplicity of the HLB concept was its main advantage in spite of very serious limitations, such as an inaccuracy sometimes over two units, and the fact that it does not take into account several variables which are known to alter the phase behaviour, independently of the surfactant. 

Hydrophilic-lipophilic balance (HLB) For a surfactant, the balance of tail hydrophobicity to headgroup hydrophilic-ity that determines the cmc, aggregation number, Kraft temperature, and other properties of micelles. The HLB is a quahtatively useful concept that is hard to apply quantitatively. 

In total, a corrosive is not exclusively an acid or a base. Other molecular reactive functions can develop, as we have just seen, an aggressive character toward the skin. A corrosive is not a substance only soluble in water as is the case of fatty amines or with some fatty acids, for example. This type of molecules can also present an amphiphilic character. One pole is hydrophilic and the other lipophilic. These properties are useful to classify the behavior and the effectiveness of emulsifiers. This is the concept of hydrophilicity/lipophilicity balance (HLB). The more the value is high (scale from 1 to 40), the more the emulsifier is hydrophilic. These concepts are also used to justify suitable and efficient conditions of washing skin splashes. 

A concept equivalent to spontaneous curvature is used in the petroleum industry, i.e. the / -ratio quantifying the hydrophilic-lipophilic balance (HLB), as introduced by Winsor (9). The R-ratio is arbitrarily set to 1 when the spontaneous curvature is zero. Then, by using differential values, the R-ratio for a given water-oil-surfactant combination is given by the following ... 

Because of the drastic energy increase involved in the emulsification process, the resulting emulsion is not thermodynamically stable and the deemulsification process must be either slowed down to get stable goods or accelerated in separation operations. This chapter will focus on stabilizing emulsions. This stabilization can be explained using two different concepts the interfadai film using the hydrophilic-lipophilic balance (HLB) method and the Deijaguin, Landau, Verwey, and Overbeek (DLVO) theory. 

The concept of hydrophile-lipophile balance (HLB) was first developed by Griffin [13] to correlate the structure of surfactant molecules with their surface activity. The HLB number (0-20) reflects the hydrophilicity of surfactant, and it increases with increasing hydrophilicity. A general trendi often observed in a family of surfactants is the increased CMC with HLB. Table 2.1 serves as a general guide for the formulator to choose surfactants that are most suited to meet the requirements of end-users. This semiempirical approach has been proved to be quite useful. Davies and Rideal [14] proposed that the HLB value of a particular surfactant could be calculated according to the group contribution approach. 

The property of interest to characterize a surfactant or a mixture of surfactants is its hydrophilic-lipophilic tendency, which has been expressed in many different ways through a variety of concepts such as the hydrophiUc-lipophilic balance (HLB), the phase inversion temperature (PIT), the cohesive energy ratio (CER), the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [1], which were found to be more or less satisfactory depending on the case. In the next section, the quantification of the effects of the different compounds involved in the formulation of surfactant-oil-water systems will be discussed in details to extract the concept of characteristic parameter of the surfactant, as a way to quantify its hydrophilic-lipophilic property independently of the nature of the physicochemical environment. 

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... 

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... 

Intensive work has been carried out in order to estab lish a relationship between emulsion properties and the properties of surfactant systems. The classical HLB (hydrophile-lipophile balance) concept is widely used in emulsion science to describe the balance of the hydrophilic and lipophilic properties of a surfae tant at oil/water interfaees. The HLB value deter mines flie emulsion inversion point (EIP) at which an emulsion ehanges from W/O to 0/W type. This was of particular importance for nonionic surfactants that change their properties with ehanges in tempera ture (59). Various NMR techniques have provided significant contributions to this basic understanding of surfactant systems and some of those were reviewed in Ref. 7. The usefulness of NMR techni ques in studying surfactant solutions lies in the direct information they provide about the microstracture ofmicroheterogeneous systems (8,60— 64). It is beyond the scope of this chapter to summarize the use of NMR techniques in the study of surfactant systems, but we will present some representative examples related to emulsions. 

The first question one may ask is if an oil-in-water emulsion or a water-in-oil emulsion is formed then the two solvents are dispersed into each other with the use of a given emulsifier. There are several empirical roles addressing this problem. The first is due to Bancroft (1) who stated that if the emulsifier is most soluble in the water phase, then an oil-in-water emulsion will be formed. A water-in-oil emulsion will be obtained when the reverse is true. The HLB (hydrophilic-lipophilic balance) concept is used for describing the nature of the surfactant. It was first introduced by... 

Research into optimal formulations is based on the idea of cohesive energy ratio (CER). This was originally developed to stabilise classic non-ionic surfactant emulsions [6.13]. Despite its limitations, the CER concept unifies the ideas of solubility parameters and HUB. Recall that the HLB is a measure of the emulsifying power of surfactants and is based on their hydrophile-lipophile balance [6.3]. It can be calculated from a simple formula involving only relative weights of sequences HLB = 20 x Mh/Mt, where Mh is the molecular weight... 

The selection of different surfactants in the preparation of EWs emulsion is still made on an empirical basis. This is discussed in detail in Chapter 6, and only a summary is given here. One of the earliest semi-empirical scales for selecting an appropriate surfactant or blend of surfactants was proposed by Griffin [49, 50] and is usually referred to as the hydrophilic-lipophilic balance or HLB number. Another closely related concept, introduced by Shinoda and co-workers [51-53, 58], is the phase inversion temperature (PIT) volume. Both the HLB and PIT concepts are fairly empirical and one should be careful in applying them in emulsifier selection. A more quantitative index that has received little attention is that of the cohesive energy ratio (CER) concept introduced by Beerbower and Hill [54] (see Chapter 6). The HLB system that is commonly used in selecting surfactants in agrochemical emulsions is described briefly below. 

There are other approaches for characterizing the hydrophobic-hydrophilic tendencies of surfactants (and other molecules). One useful approach, often employed in the study of emulsions, is the so-caUed HLB (hydrophilic-lipophilic balance) which is described in Appendix 5.2. All of these ways of assessing the hydrophilicity/hydrophobicity of molecules are different expressions of more or less the same concept and because of this they are correlated to each other (see, for example, Cheng et al, 2003, 2005). Some correlations are shown in Appendix 5.2. 

The properties (e.g. cleaning and stabilizing capabilities) of surfactants depend on both solution properties (temperature, time, presence of salts and cosurfactants) and their own characteristics, especially CMC, the Krafft point and their chemistry. The surfactant chemistry and especially the balance between hydrophobic and hydrophilic parts is quantified using tools like the CPP or HLB (critical packing parameter, hydrophilic-lipophilic balance, respectively). For example, it is often observed that detergency increases with concentration especially up to CMC and is often best at CPP values around 1. We will meet the important concept of CPP again in Chapter 7 where we will see that surfactant adsorption on solid surfaces is connected to CPP. 

The first quantitative measure of the balance between the hydrophilic and hydrophobic moieties within a particular surfactant came in 1949 when Griffin introduced the concept of the HLB, or hydrophile-lipophile balance, as a way of predicting emulsion type from surfactant molecular composition. A major... 

---