Optimal HLB

Non-aqueous HIPEs have received even less attention indeed, to date, there have been only two publications dealing with this subject, to the authors knowledge [124,125]. These describe the preparation of highly concentrated emulsions of jet engine fuel in formamide, for use as safety fuels in military applications. The emulsifier system used was a blend of two nonionics, with an optimal HLB value of 12. 

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. 

Apart from the fact that the use of the HLB system is limited as it is based on the observation of creaming or separation of the emulsions, as an index of instability the HLB system also neglects the effects of surfactant concentration on stability (26) and of course it is irrelevant to the particular problems with multiple emulsion systems. Nevertheless, it provides a useful approach to the choice of optimal surfactant system. In general, in a w/o/w emulsion, the optimal HLB value of the primary surfactant will be in the range 2-7 and in the range 6-16 for the secondary surfactant. Equilibration of the systems after mixing will undoubtedly result in the transfer of surfactant between the aqueous and nonaqueous components. Saturation of the phases with the two surfactants used should prevent instability during this equilibration. 

Figure It. The change in the optimal HLB of secondary surfactants used to prepare multiple v/o/v emulsions in which the primary w/o emulsion has heen stabilized "by 10% Bri,1 92. Reproduced with permission from Ref. 2EL Copyright 1979> J. Colloid Interface Sci.

The choice of emulsifier is critical since it controls the stability of the emulsions prior to and after polymerization. Moreover, polymerization conditions typically represent destabilizing factors vigorous stirring, temperature rise and evolution of acrylamide content in the aqueous phase. In the case of inverse emulsions, the HLB values mostly used by the formulators range between 4 and 6. Some attempts were made to predict quantitatively the optimal HLB value corresponding to the most stable dispersions [18,19]. The treatment was based on the so-called cohesive energy ratio (CER) concept devekq)ed by Beer-bower and Hill for conventional emulsions [20]. Tins approach is based on a perfect chemical match between the partial solubility parameters of oil (ig)... 

Candau and Anquetil [30] investigated the effect of the type of surfactant on particle size for poly(AM-NaAMPS) microlatices obtained by polymerization in bicontinuous microemulsions. By using different nonionic surfactant blends at the optimal HLB conditions (see Sec. II.C) they showed a significant effect of this parameter on particle size (Fig. 12). The results were accounted for by more or less pronounced salting-out effects... 

There are marked minima in the curves for isopar M and cyclohexane. These correspond to the optimal HLB values HLBopt 9 for AM and 12 for MADQUAT. Such values are much greater than those used in forming water-in-oil emulsions, for which the HLB lies in the range 4-6. This indicates that the systems are located in a phase inversion zone and have sponge-type structure. 

Coimneicial production of emulsifiable concentrates often uses a matched pair emulsifier system. The matched pair uses two surfiu tant blends that are mixed at the iq>pnq>riate ratio to maximize the kinetic stability of die emulsion that is formed. One surfactant blend has a relatively low HLB, and the other surfactant blend has a relatively high HLB. These two blends ate mixed at various ratios until die optimal HLB for the desired s tem is found. 

For reasons not explained by the HLB system, mixtures of surfactants give more stable emulsions than single surfactants. In the experimental set up, creaming of the emulsion is observed and is taken as an index of stability. The system with the minimum creaming or separation of phases is deemed to have an optimal HLB. It is therefore possible to determine optimum HLB numbers required to produce stable emulsions of a variety of oils. 

The apparent inability to effect larger or demonstrable increases in emulsion stability by the addition of substances which raise the cloud point of the stabilizing molecules may be due to several factors. Increase in aqueous solubility may decrease the concentration of emulsifier at the interface, although interfacial tension measurements do not indicate that this is so as there is an optimal HLB for each oil to achieve stability, any change in HLB by addition of agents which salt-in or salt-out the stabilizing molecules will shift the stability index from the optimum position. 

Comparison of HLB, MCX and MAX, under Optimized Sorbent Conditions for Recovery of /3-Agonists and Antagonists... 

The difficulty with HLB as an index of physicochemical properties is that it is not a unique value, as the data of Zaslavsky et al. (1) on the haemolytic activity of three alkyl mercaptan polyoxyethylene derivatives clearly show in Table 1. Nevertheless data on promotion of the absorption of drugs by series of nonionic surfactants, when plotted as a function of HLB do show patterns of behaviour which can assist in pin-pointing the necessary lipophilicity required for optimal biological activity. It is evident however, that structural specificity plays a part in interactions of nonionic surfactants with biomembranes as shown in Table 1. It is reasonable to assume that membranes with different lipophilicities will"require" surfactants of different HLB to achieve penetration and fluidization one of the difficulties in discerning this optimal value of HLB resides in the problems of analysis of data in the literature. For example, Hirai et al. (8 ) examined the effect of a large series of alkyl polyoxyethylene ethers (C4,C0, Cj2 and C 2 series) on the absorption of insulin through the nasal mucosa of rats. Some results are shown in Table II. 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

The use of the HLB number to select a surfactant (or mixture of surfactants) is achieved by matching the surfactant HLB number to that of the material being dispersed. Unfortunately, little information is available on the HLB munber for ceramic powder surfaces. What data there exists is given in Table 9.16. For ceramic systems, the HLB of the surfactant is usually optimized by experiments with various surfactants. 

The oil structure influence on the formulation is illustrated in Figure 1. It represents the minimum percentage of emulsifiers required to induce the transition aacro-raicroeaulsion versus their HLB values for monomer-water mixtures dispersed in different oils. It can be seen that in the case of acrylamide (AH) or acrylamide-sodium acrylate (Aa) mixtures, the amount of surfactant needed to form a microemulsion is much larger for toluene or cyclohexane than for Isopar K (11,12[). When methacrylcxyethyltrimethylammonium chloride (HA0OU.AT) is the monomer, the optimal conditions are obtained in cyclohexane. These results closely follow the differences calculated for the solubility parameters between oils and lipophiles as shown in Table I. 

The main features of inverse microemulsion polymerization process have been reviewed with emphasis given to a search for an optimal formulation of the systems prior to polymerization. By using cohesive energy ratio and HLB concepts, simples rules of selection for a good chemical match between oils and surfactants have been established this allows one to predict the factors which control the stability of the resultant latices. The method leads to stable uniform inverse microlatices of water-soluble polymers with high molecular weights. These materials can be useful in many applications. 

The HLB number method is based on the fact that the emulsifier is optimal in a water-oil system in which the properties of the oil matches the surfactant. Hence, each water-oil combination is characterised by an HLB number. More practically, emulsifiers characterised by HLB number ... 

Davis summarized the concepts about HLB, PIT, and Windsor s ternary phase diagrams for the case of microemulsions and reported topologically ordered models connected with the Helfrich membrane bending energy. Because the curvature of surfactant lamellas plays a major role in determining the patterns of phase behavior in microemulsions, it is important to reveal how the optimal microemulsion state is affected by the surface forces determining the curvature... 

HLB values of the surfactants 6a-c, f, g and llg have also been evaluated experimentally by using the required HLB concept of the oil/water system [40]. The HLB system predicts the optimum emulsion stability when the HLB value of the surfactant systems matches the required HLB of the oil/water system. The required HLB is the value at which enhanced emulsion stability will be attained. Optimization of the performance can be achieved by only including surfactant systems with similar HLB values. Mixtures composed of a mannuronate-type surfactant and a commercial cosurfactant with a known HLB value (Span 85, Brij 72, Span 40, Span 20) were formulated with various surfactant/cosurfactant ratios (20, 40, 60, and 80 wt%) to create different HLB values of the system. Then, the performance was determined and plotted vs the HLB. A maximum appears in the plot and the... 

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