Selection of emulsifiers

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. 

The HLB 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 0/W emulsions. A summary of the HLB range required for various purposes is given in Chapter 6. For O/W emulsions HLB is in the range 8-18. 

HLB number of 3.8, i.e. it is suitable for a W/O emulsifier. However, in many cases, accurate estimation of the saponification number is difficult, e.g. ester of tall oil, resin, beeswax and linolin. For the simpler ethoxylate alcohol surfactants, HLB can be calculated simply from the weight per cent of oxyethylene E and poly-hydric alcohol P, i.e. 

If the surfactant contains poly(ethylene oxide) as the only hydrophilic group, e.g. in the primary alcohol ethoxylates R(CH2-CH2-0) -0H, the HLB number is simply (E/5) (the content from one OH group is simply neglected). 

The above equation cannot be used for nonionic surfactants containing propylene oxide or butylene oxide, nor can it be used for ionic surfactants. In the latter case, ionisation of the head groups tends to make them even more hydrophilic in character, so that the HLB number cannot be calculated from the weight per cent of the ionic groups. In that case, the laborious procedure suggested by Griffin [51] must be used. 

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. 

A guide to the selection of surfactants for particular apphcations is provided in Table 10.1. As the HLB number depends on the nature of the oil, the HLB numbers required to emulsify various oils are listed in Table 10.2, as an illustration. 

The relative importance of the hydrophihc and HpophiHc groups was first recognised when using mixtures of surfactants containing varying proportions of 

Griffin developed simple equations for calculating the HLB number of relatively simple nonionic surfactants. For example, for a polyhydroxy fatty acid ester  

For a simple alcohol ethoxylate, the HLB number can be calculated from the weight percentage of EO (E) and polyhydric alcohol (P)  

The selection of different surfactants in the preparation of either 0/W or W/0 emulsions is often still made on an empirical basis. A semi-empirical scale for selecting surfactants is the hydrophilic-lipophilic balance (HLB number) developed by Griffin 

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 fi om the weight percent of ethylene oxide (E) and polyhydric alcohol (P), 

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

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Hasenhuetti, G.L. Design and Selection of emulsifiers in the Food Industry in Food Emulsion and Foams Theory and Practice,... 

At this point, we have assembled a sufficient mathematical model for the selection of emulsifiers to be used in agricultural emulsifiable concentrates so that we may write a computer program which will take into consideration all the variables mentioned thus far. The mathematical model will take into consideration these variables, analyze the data in a logical systematic method, yield results which should give us a "near-neighborhood" to a "best" type of surfactant system. 

The selection of emulsifiers to prepare food emulsions is mainly based on their HLB number. This index is based on the relative percentage of hydrophilic to lipophilic groups within the emulsifier molecule. Lower HLB numbers indicate a more lipophilic emulsifier, while higher numbers indicate a more hydrophilic emulsifier. Emulsifiers showing HLB numbers between 3 and 6 are best for water-in-oil (W/O) emulsions, and emulsifiers with numbers between 8 and 18 are best for O/W emulsions. 

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. 

Several ideas have been put forward to explain the driving force for formation of the different liquid crystalline phases. One of the simplest methods for predicting the shape of an aggregated structure is based on the critical packing parameter concept (P) introduced by Israelachvili and his co-workers [22, 23]. This concept will be discussed in detail in the chapter on emulsions (selection of emulsifiers). Basically, P is the ratio between the cross sectional area of the alkyl chain (that is given by v/l, where v is the volume of the hydrocarbon chain and is the maximum length to which the alkyl chain can extend) and the optimum head group area ao, i.e.,... 

Alternative approaches to the selection of emulsifiers have been investigated. Prediction of optimum emulsifier mixtures has been made by way of solubilization measurements [44]. Lin et ai [44] found a correlation between the maximum amount of aqueous phase that could be solubilized in the oil phase containing the surfactant and the average droplet size of the emulsion subsequently formed (Fig. 8.9). The relationship held even when ionic-non-ionic mixtures of surfactants were used and thus displays an advantage over the PIT method as ionic surfactants do not produce PIT values. It is telling that the method works in the presence of additives such as lauryl alcohol. Fig. 8.9 shows the shift in optimum surfactant ratio when lauryl alcohol is added to the oil phase, in this case mineral oil. The addition of a polar oil to a non-polar oil will result in a predictable shift in required HLB. However, a shift of no more than 1 HLB unit would be expected from the linear additivity rule, while Fig. 8.9 shows a shift of some 2.4 HLB units. In some systems that Lin and his colleagues [44] investigated, the position of maximum solubilization did not coincide with the optimal O/W emulsion, an effect believed to be due to phase inversion at the point of maximum solubilization. 

Garti N, Aserin A, Tiunova I, Binyamin H. 1999. Double emulsions of water-in-oil-inwater stabilized by alpha-form fat microcrystals. Part 1 Selection of emulsifiers and fat microcrystalline particles. 7 Am Oil Chemists Soc 76 383-389. 

Even today, empirical laboratory experiments are still used for the selection of emulsifiers, although a few fundamental rules exist. Thus, for example, Griffin classified the nonionic emulsifiers numerically between 0 and 20 [40]. A rising num-... 

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