HLB-temperature

At low temperature, nonionic surfactants are water-soluble but at high temperatures the surfactant s solubUity in water is extremely smaU. At some intermediate temperature, the hydrophile—Hpophile balance (HLB) temperature (24) or the phase inversion temperature (PIT) (22), a third isotropic Hquid 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 foUowing manner. Equal amounts of the oil and the aqueous phases with aU 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. 

A considerable amount of experimental work has been carried out on the so-called gel emulsions of water/nonionic surfactant/oil systems [9-14, 80, 106, 107]. These form in either the water-rich or oil-rich regions of the ternary phase diagrams, depending on the surfactant and system temperature. The latter parameter is important as a result of the property of nonionic surfactants known as the HLB temperature, or phase inversion temperature (PIT). Below the PIT, nonionic surfactants are water-soluble (hydrophilic form o/w emulsions) whereas above the PIT they are oil-soluble (hydrophobic form w/o emulsions). The systems studied were all of very high phase volume fraction, and were stabilised by nonionic polyether surfactants. 

The HLB temperature was found to be the most important factor in the formation of stable emulsions. In each case, w/o HIPEs [9,11,80] would only form at temperatures above the HLB temperature of the systems, while o/w HIPEs [14] formed below the PIT. The nature of the oil phase was also found to be of importance to the formation of stable w/o HIPEs [11] aromatic liquids, for example, did not produce highly concentrated emulsions. With aliphatic oils, the stability was observed to vary with chemical nature. This was due to the different HLB temperatures for each liquid. 

Phase diagrams of water, hydrocarbon, and nonionic surfactants (polyoxyethylene alkyl ethers) are presented, and their general features are related to the PIT value or HLB temperature. The pronounced solubilization changes in the isotropic liquid phases which have been observed in the HLB temperature range were limited to the association of the surfactant into micelles. The solubility of water in a liquid surfactant and the regions of liquid crystals obtained from water-surfactant interaction varied only slightly in the HLB temperature range. 

If the liquid crystalline phase is included in the diagram, the general features are those in Figure 7 (38). At this temperature (the PIT or HLB temperature) increasing amounts of emulsifier first give rise to an isotropic liquid (S) in a small concentration range (A-B), followed by a phase transition to a lamellar liquid crystal (N) in the concentration range C-D. 

PIT) The temperature at which the hydrophilic and oleophilic natures of a surfactant are in balance. As temperature is increased through the PIT, a surfactant will change from promoting one kind of emulsion, such as O/W, to another, such as W/O. Also termed the HLB temperature . 

Shinoda and Kuineda [8] highlighted the effect of temperature on the phase behavior of systems formulated with two surfactants and introduced the concept of the phase inversion temperature (PIT) or the so-called HLB temperature. They described the recommended formulation conditions to produce MEs with surfactant concentration of about 5-10% w/w being (a) the optimum HLB or PIT of a surfactant (b) the optimum mixing ratio of surfactants, that is, the HLB or PIT of the mixture and (c) the optimum temperature for a given nonionic surfactant. They concluded that (a) the closer the HLBs of the two surfactants, the larger the cosolubilization of the two immiscible phases (b) the larger the size of the solubilizer, the more efficient the solubilisation process and (c) mixtures of ionic and nonionic surfactants are more resistant to temperature changes than nonionic surfactants alone. 

Microemulsions should be formed near or at the phase inversion temperature (PIT) or HLB temperature for a given nonionic surfactant, since the solubilization of oil (or water) in an aqueous (or nonaqueous) solution of nonionic surfactant shows a maximum at this temperature. 

Near the HLB temperature, the interfacial tension reaches a minimum, as illustrated in Figure 14.4. Thus, by preparing the emulsion at a temperature 2-4 °C below the PIT (near the minimum in y), followed by rapid cooling of the system, nanoemulsions may be produced. The minimum in y can be explained in terms of the change in curvature H of the interfacial region, as the system changes from O/W to W/O. For an O/W system and normal micelles, the monolayer curves towards the oil such that H has a positive value. However, for a W/O emulsion and inverse micelles the monolayer will curve towards the water and H will be assigned... 

The HLB temperature was determined using conductivity measurements, whereby 10 moldm NaCl was added to the aqueous phase (to increase the sensitivity of the measurements). The concentration of NaCl was low and hence had Uttle effect on the phase behaviour. 

Figure 14.10 shows the variation of conductivity versus temperature for 20% O/W emulsions at different surfactant concentrations. It can be seen that there is a sharp decrease in conductivity at the PIT or HLB temperature of the system. The HLB temperature then decreases with increases in surfactant concentration. 

Nanoemulsions were prepared by rapid cooling of the system to 25 °C, and the droplet diameter was determined using PCS. The results are summarised in Table 14.1, which shows the exact composition of the emulsions, HLB temperature, z-average radius, and polydispersity index. 

Table 14.1 Composition, HLB temperature (Phlb) droplet radius r and polydispersity index (PI) for the system water-C,jE04-hexadecane at 25 C.

As mentioned above, y reaches a minimum at the HLB temperature, and therefore the minimum in interfacial tension would occur at a lower temperature as the surfactant concentration increased. This temperature would become closer to the cooling temperature as the surfactant concentration increased, and this would result in smaller droplet sizes. 

The results with isohexadecane are summarised in Table 14.2. As with the hexadecane system, the droplet size and polydispersity index were decreased with increases in surfactant concentration. Nanoemulsions with droplet radii of 25-80run were obtained at 3-8% surfactant concentration. It should be noted, however, that nanoemulsions could be produced at lower surfactant concentration when using isohexadecane, when compared to results obtained with hexadecane. This could be attributed to the higher solubility of isohexadecane (a branched hydrocarbon), the lower HLB temperature, and the lower interfacial tension. 

Table 14.3 HLB temperature droplet radius r, Ostwald ripening rate co and oil solu-...

Kunieda, H. and Shinoda, K. (1982) Phase behavior in systems of nonionic surfactant-water-oil around the hydrophile-lipopile balance temperature (HLB-temperature)./. Dispersion Sci. Technol., 3, 233-244. 

In determining the emulsification temperature for emulsions stabilised with EO containing nonionics, the consideration of the phase inversion temperature (PIT or HLB-temperature) suggested by Shinoda and co-workers [193] can be also important in order to select the surfactant of optimum HLB. The PIT of an emulsion depends not only on the structure of surfactant(s), but also on many other parameters, such as the surfactant concentration, nature of the oil, phase ratio, or the presence of salts. The lowest interfacial tension at the PIT is the important factor for obtaining emulsions with small average droplet size and hence good stability. 

In this relation. T is the temperature at optimum formulation where R = t, i.e.. [he PIT according to Shinoda s prcmi.se, an expression that deserves the latter label HLB-temperature. This relationship is very close to the one deduced b some re.searchers (78) who used the surfactant HLB instead EON-a to arrive to u similar result as far as the combined effects of temperature, salinity, and oil ACN are concerned. The above formula indicates how the PIT increases with the number of ethylene oxide groups in the surfactant molecule, increases with oil chain length, and decreases with electrolyte concentraiion and surfactant tail length (proportionally to a). It also predicts a variation with the alcohol type and concentration, a decrease with lipophilic. species. 

Research on microemulsions was a major topic in his scientific activity, since the earlier work under Prof. Shinoda s supervision [1, 2], through his entire scientific career. First the attention was focused to find the conditions to produce three-phase equilibria (balanced conditions) in both ionic [9-12] and nonionic [13-17] surfactant systems. In this context it was shown that the effect of temperature in ionic surfactant systems is opposite to that in polyoxyethylene-type nonionic surfactants [10] and that both types of surfactant systems display similarities in phase behavior [18]. The most detailed phase equilibria of a water/ nonionic surfactant/ahphatic hydrocarbon system around the HLB temperature (Figure 2) was reported in 1982 [16]. 

Figure 6 Schematic phase diagram of a water (W)/nonionic surfactant (S)/oil (O) system at the HLB temperature and self-organizing structures (a) normal" lamellar liquid crystal (b) normal vesicles (c) bicontinuous surfactant phase (middle-phase...

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