Solubility-temperature relationship, for surfactants

The solubility-temperature relationship for nonionic surfactants shows a different behavior from ionic surfactants. Figure 20.6 shows the phase diagram of Ci2E06-The nonionic surfactant forms a clear solution (micellar phase) up to a certain temperature (that depends on concentration) above which the solution becomes cloudy. This critical temperature, denoted as the cloud point (CP) of the solution, decrease with increase in surfactant concentration reaching a minimum at a given concentration (denoted as the lower consolute temperature) above which the CP increases with further increase in surfactant concentration. Above the CP curve the system separates into two layers (water -I- solution). Below the CP curve, several liquid crystalline phases can be identified as the surfactant concentration exceeds a certain limit. Three different liquid crystalline phases can be identified, namely, the hexagonal, the cubic, and lamellar phases. A schematic picture of the structure of these three phases is shown in Fig. 20.7. 

Solubility-temperature relationship(s) sodium with other elements, 22 763 for surfactants, 24 125-126... 

One of the characteristic features of solutions of surfactants is their solubility-temperature relationship, which is illustrated in Fig. 3 for an anionic surfactant, namely, sodium decyl sulfonate. It can be seen that the solubility of the surfactant increases gradually with an increase in temperature, but above 22°C there is a rapid increase in... 

The foaming properties of the nonionic surfactants depend upon the temperature because of their inverse solubility temperature relationship. Above the cloud point they are nonfoamers and some nonionic surfactants may even function as defoamers above their cloud point temperature. Therefore, the nonionic surfactant selected for rinse aid formulations must have a cloud point below the temperature of the rinse water. 

The Kraft point (T ) is the temperature at which the erne of a surfactant equals the solubility. This is an important point in a temperature-solubility phase diagram. Below the surfactant cannot fonn micelles. Above the solubility increases with increasing temperature due to micelle fonnation. has been shown to follow linear empirical relationships for ionic and nonionic surfactants. One found [25] to apply for various ionic surfactants is ... 

FIGURE 15.2. The temperature-solubility relationship for typical ionic surfactants illustrating the important characteristics such as the Krafft temperature, the monomer solubility curve, and the limiting monomer concentration at the critical micelle concentration. 

Figure 43. Temperature-solubility relationship for typical ionic surfactants.

The PIT system of surfactant evaluation theoretically applies only to nonionic materials. However, it is often found that for a given oil-water system, a combination of two or more surfactants (e.g., a nonionic and an ionic) will produce better results than either surfactant alone, at the same (or less) total surfactant concentration. Ionic surfactants usually have the normal temperature-solubility relationship—higher temperature means greater solubility—and in mixtures can often swamp out the phase inversion effect of a nonionic material. However, if the ionic/nonionic mixture is used with an aqueous phase of relatively high ionic strength, the HLB/S/F value of the molecule will be reduced and the phase inversion effect may reappear and become a useful tool again. 

The temperature dependence of the cmc of polyoxyethylene nonionic surfactants is especially important since the head group interaction is essentially totally hydrogen bonding in nature. Materials relying solely on hydrogen bonding for solubilization in aqueous solution are commonly found to exhibit an inverse temperature-solubility relationship. As already mentioned, major manifestation of such a relationship is the presence of the cloud point for many nonionic surfactants. 

The Krafft temperatures of a number of common ionic surfactants are given in Table 4.1. It can be seen from the data that Tk can vary as a function of both the nature of the hydrophobic group and the character of the ionic interactions between the surfactant and its counterion. It should be noticed that no data are listed for nonionic surfactants. Nonionic surfactants, because of their different mechanism of solubilization, do not exhibit a Krafft temperature. They do, however, have a characteristic temperature-solubility relationship in water that causes them to become... 

---