Small phase inversion temperature

At low temperature, nonionic surfactants are water-soluble but at high temperatures the surfactant s solubility in water is extremely small. At some intermediate temperature, the hydrophile—lipophile balance (HLB) temperature (24) or the phase inversion temperature (PIT) (22), a third isotropic liquid 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 following manner. Equal amounts of the oil and the aqueous phases with all 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. 

There are other variations of this approach that involve the phase inversion temperature (PIT) (see Section 3.6.1). In one method an emulsion is formed at a temperature a few degrees lower than the PIT, where the interfacial tension is quite low and small droplets can be formed. The emulsion can then be quickly cooled. Another method uses a controlled temperature change to cause an emulsion to suddenly change from a coarse oil-in-water (O/W) emulsion, through a microemulsion phase, and into a fine water-in-oil (W/O) emulsion [432]. 

The interfacial tension y at the planar interface has a minimum near the temperature Indeed, at the latter temperature r is small, A/jt0 = 0 and because d ij w/d J and dfi /dT have opposite signs and s is also small (because T is small), dy/d I 0. The temperature T0 is provided by Eq. (25) and is independent of the concentration of surfactant. In other words, the two minima of Fig. 4 which correspond to the phase inversion temperatures of a macroemulsion (the curve with a positive minimum) and microemulsion (the curve with a negative minimum) are the same. When emulsions are generated from a microemulsion and its excess phase, the emulsion is of the same kind as the microemulsion, the phase inversion temperature is obviously located in the middle of the middle phase microemulsion range and the above conclusion remains valid. The above results explain the observation of Shinoda and Saito [6,7] that the phase inversion temperature (PIT) of emulsions can be provided by the ternary equilibrium phase diagram. 

A lack of understanding of the interfacial chemistry involved in production of nanoemulsions. For example, few formulations chemists are aware of the concepts of phase inversion composition (PIC) and phase inversion temperature (PIT), and how these can be usefuUy apphed to produce small emulsion droplets. 

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. 

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). 

It is believed that the formation of a microemulsion can enhance detergent action since the oil-water interfacial tension can be lowered considerably, which facilitates solubilization of oily dirt particles by surfactant. The microemulsion is of Winsor type III (Section 3.13.2), with small amounts of surfactant forming a middle phase microemulsion in equilibrium with excess oil and water. The oil-water interfacial tension is a minimum at the phase inversion temperature (PIT) of an oil-water-surfactant system, so it is desirable to optimize the properties of the detergent mixture so that the system is close to the PIT at the washing temperature. Microemulsions made from mixtures of nonionic surfactants are used in hard surface cleaning products. Usually they are sold in concentrated form and diluted prior to use. 

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... 

This is applied in the so-called PIT method of Shinoda et PIT stands for phase-inversion temperature. It is observed that many nonionic surfactants decrease in HLB number with increasing temperature. Below the PIT (which also depends on the composition of both phases), an 0/W emulsion tends to be formed, but above the PIT, a W/0 emulsion see further on for an explanation. At the PIT the interfacial tension is very small, and quite small droplets result. These are unstable to coalescence, but by rapidly cooling the emulsion after emulsification a stable 0/W emulsion having fine (h oplets can be obtained. The droplet break-iq) is presumably in regime TV and fairly small e values suffice. The method is widely applied in industrial practice. 

For this system the temperature of phase inversion (PIT) is between 45°C and 55°C. Variation of both the temperature and the surfactant concentration in a system with a fixed ratio of water and oil leads to a phase diagram that is called informally the Kahlweit fish due to the shape of the phase boundaries that resemble a fish. In Figure 3.24 (left), this diagram is given for the system water/tetradecane/CnEs. For small surfactant concentrations (<15%), the phases already discussed occur but, at higher emulsifier concentrations, the surfactant is able to solubilise all the water and the hydrocarbon which results in a one-phase microemulsion D or a lamellar phase La. 

As we will see in Chapter 11, it is usual to describe interactions in polymers using the Flory interaction parameter x, which varies as the inverse of the temperature. Thus if x is relatively small it is possible to form a single phase at temperatures below the degradation point of the polymer. Then, as you cool and X gets larger, the system phase-separates. The temperature or value of x at which this occurs, the order-disorder transition, varies with composition and it is possible to... 

Raman spectra as a function of temperature are shown in Fig. 21.6b for the C2B4S2 SL. Other superlattices exhibit similar temperature evolution of Raman spectra. These data were used to determine Tc using the same approach as described in the previous section, based on the fact that cubic centrosymmetric perovskite-type crystals have no first-order Raman active modes in the paraelectric phase. The temperature evolution of Raman spectra has indicated that all SLs remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below T. The Tc determination is illustrated in Fig. 21.7 for three of the SLs studied SIBICI, S2B4C2, and S1B3C1. Again, the normalized intensities of the TO2 and TO4 phonon peaks (marked by arrows in Fig. 21.6b) were used. In the three-component SLs studied, a structural asymmetry is introduced by the presence of the three different layers, BaTiOs, SrTiOs, and CaTiOs, in each period. Therefore, the phonon peaks should not disappear from the spectra completely upon transition to the paraelectric phase at T. Raman intensity should rather drop to some small but non-zero value. However, this inversion symmetry breakdown appears to have a small effect in terms of atomic displacement patterns associated with phonons, and this residual above-Tc Raman intensity appears too small to be detected. Therefore, the observed temperature evolution of Raman intensities shows a behavior similar to that of symmetric two-component superlattices. 

To apply the phase inversion principle, the transitional inversion method should be used, as demonstrated by Shinoda and coworkers [11, 12] when using nonionic surfactants of the ethoxylate type. These surfactants are highly dependent on temperature, becoming lipophilic with increasing temperature due to dehydration of the poly(ethylene oxide) (PEO) chain. When an O/W emulsion that has been prepared using a nonionic surfactant of the ethoxylate type is heated, at a critical temperature - the PIT - the emulsion will invert to a W/O emulsion. At the PIT, the droplet size reaches a minimum and the interfacial tension also reaches a minimum, but the small droplets are unstable and coalesce very rapidly. Rapid cooling of an emulsion that has been prepared close to the PIT results in very stable and small emulsion droplets. 

At the oil-rich side, the phase behaviour is inverted temperature-wise as can be seen in the T( wA)-section provided in Fig. 1.7(c). Thus, the near-critical phase boundary 2 —1 starts at low temperatures from the lower n-octane-QoEs miscibility gap (below <0°C) and ascends steeply upon the addition of water. With increasing wA, this boundary runs through a maximum and then decreases down to the upper critical endpoint temperature Tu. The emulsification failure boundary 1 —r 2 starts at high temperatures and low values of wA, which means that only small amounts of water can be solubilised in a water-in-oil (w/o) microemulsion at temperatures far above the phase inversion. Increasing amounts of water can be solubilised by decreasing the temperature, i.e. by approaching the phase inversion. At Tu the efb intersects the near-critical phase boundary and the funnel-shaped one-phase region closes. 

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