Phase inversion temperature , emulsion stability

Izquierdo, P., Esquena, J., Tadros, T.F., Dederen, C., Garcia, M.J., Azemar, N. and Solans, C. (2002) Formation and stability of nano-emulsions prepared using the phase inversion temperature method. Langmuir, 18 (1), 26-30. 

The first kinetics measurements about coalescence were reported by Kabalnov and Weers in water-in-oil emulsions [40]. These authors measured the characteristic time at which the layer of free water formed at the bottom of the emulsions corresponded approximately to half of the volume of the dispersed phase. This time was assumed to be equal to t. By measuring r at different temperatures, the activation energy was deduced from an Arrhenius plot. Kabalnov and Weers were able to obtain the activation energy for a water-in-octane emulsion at 50%, stabilized by the nonionic surfactant C12E5 (pentaethylene glycol mono n-dodecyl ether), above the phase inversion temperature (PIT), and found a value of 47 kgTr, Tr being the room temperature. 

Shinoda K, Aral H (1967) The effect of phase voliune on the phase inversion temperature of emulsions stabilized with nonionic simfacatnts. J Colloid Interface Sci... 

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],... 

Sudden changes in the storage condition of an emulsion is usually detrimental to its physical stability. Whereas slight changes in temperature may be acceptable, large changes in temperature may cause phase inversion, that is, conversion from o/w to w/o and vice versa. The temperature at which this process occurs is called the phase inversion temperature. Usually, emulsions should be stored at least 2C below the phase inversion temperature (Attwood and Florence, 1985). 

Just as solubilities of emulsifying agents vary with temperature, so does the HLB, especially for the non-ionic surfactants. A surfactant may thus stabilize O/W emulsions at low temperature, but W/O emulsions at some higher temperature. The phase inversion temperature (PIT), at which the surfactant changes from stabilizing O/W to W/O emulsions, is discussed in Section 3.6.1. 

Tornberg and Ediriweera, 1987). Phase inversion temperature (Shinoda and Saito, 1969) and emulsifying capacity (Swift et al., 1961) have been used to evaluate the effects of low molecular weight and protein emulsifiers, respectively. Unfortunately, it is not possible to measure the size of the large droplets present in unhomogenized water-in-oil emulsions because the droplets coalesce very quickly. The phase inversion temperature is not a relevant test, as it may not be related directly to the stability to inversion at the emulsification temperature. Furthermore, it has been stated (Matsumoto and Sherman, 1970) that water-in-oil emulsions do not exhibit a true phase inversion temperature, unlike oil-in-water emulsions. 

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. 

Consequently, the following four components were selected to prepare the photoresponsive emulsion system equal volumes of n-dodecane and 0.3 M NaNOs aqueous solution, the C12E4 surfactant, and an azobenzene-modified poly(acrylate). C12E4 is known to stabilize direct and inverse emulsions below and above the so-called phase inversion temperature (PIT here 24°C), respectively. Emulsions are unstable in the vicinity of the PIT, a temperature domain corresponding to the CTR of the light-responsive system. Emulsions made of equal oil and water volumes are directly below the PIT (and display a high electric conductivity because of the water continuum), but inverse (and of low conductivity) above the PIT (Khoukh et al., 2005). 

Since the cloud point of a surfactant is a structure related phenomenon, it should also be related to HLB, solubility parameter, cmc, and other parameters, as is found to be the case. Clearly, temperature can play an important role in determining surfactant effectiveness where hydration (or hydrogen bonding) is the principal mechanism of solubilization. Because of the temperature sensitivity of such materials, their activity as emulsifiers and stabilizers also becomes temperature sensitive. In particular, their ability to form and stabilize o/w and w/o emulsions may change dramatically over a very narrow temperature range. In fact, an emulsion may invert to produce the opposite emulsion type as a result of temperature changes. Such a process is termed phase inversion, and the temperature at which it occurs for a given system is its phase inversion temperature (PIT). 

Formation and stability of nano-emulsions prepared using the phase inversion temperature method, Langmuir 18, 26-30 (2002). 

The phase inversion temperature (PIT) concept which has been developed by Shinoda [95,96] is closely rated to the HLB balance concept described above. Shinoda and coworkers found that many 0/W emulsions stabilized with nonionic surfactants undergo a process of inversion at a critical temperature (PIT). The PIT can be determined by following the emulsion conductivity (small amount of electrolyte is added to increase the sensitivity) as a function of temperature. The conductivity of the 0/W emulsion increases with increasing temperature until the PIT is reached, above which there will be a rapid reduction in conductivity (W/0 emulsion is formed). 

Enever [25] conelated the stability of liquid paraffin emulsions stabilized by E04 cetyl ether with phase-inversion temperature. PITs were determined by viscosimetry or conductivity. He also emphasized the stabilizing effect of the liquid crystals. 

O/W emulsions stabilized with non-ionic surfactants tend to form W/O emulsions at elevated temperatures as the surfactant molecules dehydrate and become more lipophilic. The phase inversion temperature (PIT) can thus be ascertained by experiment. Arai and Shinoda [39] have found that the PIT of emulsions in which the oil phase consists of oil mixtures can be expressed as... 

Figure 8.6 The effect of the mixture of n-heptane with various oils on the phase inversion temperatures of emulsions stabilized with 3 % w/w in water of polyoxyethylene (9.6) nonyl phenyl ether. From Arai and Shinoda [39] by permission of Academic Press.

It is now well established that the choice of emulsification conditions is an important consideration in determining the ultimate drop size (and hence stability) of an emulsion. Using nonionic surfactants, Shinoda and Saito demonstrated that emulsification at the phase inversion temperature (PIT) followed by cooling led to the formation of stable O/W emulsions of small drop size. Emulsification at temperatures higher than the PIT, initially producing W/O emulsions, resulted in very stable emulsions on subsequent cooling. The inversion process, forming a... 

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