Phase Inversion Temperature PIT Concept

This concept, developed by Shinoda [24, 25], is closely related to the HLB balance concept described above. Shinoda and co-workers found that many 0/W emulsions stabilised with nonionic surfactants undergo a process of inversion at a critical temperature (PIT). The PIT can be determined by following the emulsion conductivity (a small amount of electrolyte is added to increase the sensitivity) as a function of temperature. The conductivity of the O/W emulsion increases with rising temperature till the PIT is reached, above which there will be a rapid reduction in conductivity W/O emulsion is formed). 

The optimum stability of the emulsion is relatively insensitive to changes in the HLB or the PIT of the emulsifier, but instability is very sensitive to the PIT of the system. It is essential, therefore, to measure the PIT of the emulsion as a whole (with all other ingredients). At a given HLB, the stability of the emulsions against coalescence increases markedly as the molar mass of both the hydrophilic and lipophilic components increases. 

The enhanced stability using high molecular weight surfactants (polymeric surfactants) can be understood by considering the steric repulsion, which produces more stable films. Films produced using macromolecular surfactants resist thinning and disruption, thus reducing the possibility of coalescence. 

The emulsions showed maximum stability when the distribution of the PEO chains was broad. The cloud point is lower but the PIT is higher than in the corresponding case for narrow size distributions. The PIT and HLB number are directly related parameters. 

The above results suggest a correlation between emulsion stability against coalescence and the PIT. Coalescence results in an increase in the droplet size, which is usually followed by a reduction in the viscosity of the emulsion. This trend is only observed if the coalescence is not accompanied by flocculation of the emulsion droplets (which results in an increase in the viscosity). Ostwald ripening can also complicate the analysis of the rheological data. 

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Lack of understanding of the interfacial chemistry involved in production of nano-emulsions. For example, few formulation chemists are aware of the use of the phase inversion temperature (PIT) concept and how this can be usefully applied for the production of small emulsion droplets. 

Two methods may be applied for the preparation of nano-emulsions (covering the droplet radius size range 50-200 nm). Use of high-pressure homogenisers (aided by appropriate choice of surfactants and cosurfactants) or application of the phase inversion temperature (PIT) concept. 

For nonionic surfactants, particularly those of the ethoxylate type, a selection can be made based on the hydrophUic-lipophilic balance (HLB) concept (Chapter 6). A closely related system developed by Shinoda and his collaborators is based on the phase inversion temperature (PIT) concept. This is also described in detail in Chapter 6. 

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. 

Later we discover another parameter, the phase inversion temperature(PIT), which helps us to predict the structure of emulsions stabilized by nonionic surfactants. The PIT concept is based on the idea that the type of an emulsion is determined by the preferred curvature of the surfactant film. For a modern introduction into the HLB and PIT concepts see Ref. [546],... 

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. 

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. 

One limitation of the HLB concept is its failure to account for variations in system conditions from that at which the HLB is measured (e.g., temperature, electrolyte concentration). For example, increasing temperature decreases the water solubility of a nonionic surfactant, ultimately causing phase separation above the cloud point, an effect not captured in a temperature-independent HLB value. When both water and oil are present, the temperature at which a surfactant transitions from being water soluble to oil soluble is known as the phase inversion temperature (PIT). Below the PIT, nonionic surfactants are water soluble, while above the PIT. they are oil soluble. Thus, from Bancroft s rule, a nonionic surfactant will form an 0/W emulsion below its PIT and a W/0 emulsion above its PIT. Likewise, increasing salt concentrations reduces the water solubility of ionic surfactant systems. At elevated salt concentrations, ionic surfactants will eventually partition into the oil phase. This is illustrated in Fig. 13. which shows aqueous micelles at lower salt concentrations and oil-phase inverse micelles at higher salt concentrations. Increasing the system temperature will likewise cause this same transition for nonionic surfactant systems. 

The. second intent to numerically characterize (he formulation concept was the so-called phase inversion temperature (PIT), originally introduced by K. Shitioda in 1964 as (he temperature at which a polyethoxylated nonionic surfactant switched its dominant afitnity from the aqueous phase to the oil phase to produce a change in emulsion type. This was both easy to determine experimentally and simple to undersund as far as the related phenomenology was concerned (48-50). 

Nevertheless, it is now understood that HLB essentially depends on the surfactant, while the phase behavior and emulsion properties are also related to the water and oil phase nature, as well as to the temperature (100). The temperature was the preferred variable in the case of nonionic surfactants which are very sensitive to it, and an experimentally based concept was first introduced by Shinoda to quantify the formulation, i.e., the phase inversion temperature (PIT) (105, 106). It is known that the hydrophilicity of a nonionic surfactant decreses when temperature decreases. In water solution there exists a temperature at which the surfactant is no longer soluble and thus produces a separate phase. This so-called cloud point occurrence is related to the Shinoda PIT, which is essentially the same phenomenon, but in the presence of an oil phase whose nature could facilitate this separation and make it happen at a lower temperature. Although the PIT is limited to the liquid water temperature range of nonionic surfactants, its introduction was an important milestone because it was related not only to the surfactant, but also to the whole physicochemical environment (107), a feature that was shown to be essential by Winsor. 

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 second intent to numerically characterize the formulation concept was the so-called phase-inversion temperature (PIT), originally introduced by Shinoda in 1964 as the temperature at which a nonionic surfactant switched its dominant affinity from the aqueous phase to the oil phase. This inversion occurred in a so-called phase transition process that was both easy to determine experimentally and simple to understand as far as the associated phenomenology was concerned [107,108]. Later, it was related to the temperature at which the emulsion inversion takes place [57,58,109], which is the same in most cases, with scarce exceptions and with no real significance [110]. It was finally fine-tuned again and renamed the HLB temperature [111], as a way of stating that it was the temperature at which the hydrophilic and lipophilic tendencies of the surfactant were balanced (i.e., whenever a Winsor Type III system is occurring). 

An alternative surfactant combination which is free of ethoxylated molecules is based on rapeseed sorbitol ester and sodium lauroyl glutamate [53]. Here, the phase inversion from a w/o- to an o/w-emulsion can be initiated by the addition of lauroyl glutamate, which is a hydrophilic surfactant, instead of using the temperature. Penetration studies for the release of Vitamin E from PIT emulsions in comparison with other formulation concepts have been performed [54]. It has been shown that the penetration of Vitamin E into the skin is better for a w/o-cream than a PIT emulsion. The free diffusion of Vitamin E might be hindered by the oil-water interface, which acts as a barrier around the oil droplets. 

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