Clouding oxyethylene surfactants

For the case of nonionic surfactants, such as poly-oxyethylenated surfactants, their aqueous solutions become turbid if they are heated to a temperature known as cloud point [25]. 

Surfactant blends of interest will exhibit clouding phenomena in aqueous solutions undergoing a phase transition from a one phase system to a two phase system at a discrete and characteristic temperature, referred to as the Cloud Point (CP). This value indicates the temperature at which sufficient dehydration of the oxyethylene portion of the surfactant molecule has occurred and this results in its "displacement" from solution. The addition of lyotropic salts will depress the CP, presumably due to the promotion of localised ordering of water molecules near the hydrophilic sheath of the surfactant molecule (8). Furthermore, the addition of different oils to surfactant solutions can induce either an elevation or a depression of the recorded CP and can be used to qualitatively predict the PIT (8x9). 

The physical chemical properties of the surfactants that contain an ester bond between the hydrophobic tail and the polar head group are very similar to those of alcohol ethoxylates of the same alkyl chain length and the same number of oxyethylene units. The CMC and the cloud point values of the linear ester surfactant 1 of Fig. 4 are approximately the same as those of the straight chained alcohol ethoxylate tetra(ethylene glycol)monooctyl ether (C8E4), i.e., around 10 mM and 40 °C, respectively. Thus it appears that the... 

Nonionic surfactants dissolve in aqueous solutions through hydrogen bonding between the water molecules and the oxyethylenic portion of the surfactant. These interactions are weak but enough in number to maintain the molecule in solution up to the cloud point temperature, at which the surfactant separates as a different phase (4). Figure 3 shows that electrolytes like calcium chloride, potassium chloride, or sodium chloride reduce the cloud point of Triton X-100. Hydrochloric acid instead promoted a salting-in effect similar to that observed for ethanol. 

PAGS-8 and PAGS-12 are also water insoluble. The tendency toward side-chain crystallization and the hydrophobic backbone of the polymer contributes to their poor water solubility. Gloud points could be measured in 3% aqueous solutions of PM MS-8 in the presence of various salts (Figure 1). Gonsistent with data reported for nonionic surfactants and polymers with oxyethylene moieties, the cloud points are sensitive to the type of salts added, especially that of the anion. Fluorides and sulfates are effective salting-out agents, whereas thiocyanates raise the cloud points. 

KIM Kim, H.C. and Kim, J.-D., The polydispersity effect of distributed oxyethylene chains on the cloud points of nonionic surfactants, J. Colloid Interface Sci., 352,444, 2010. 

In a study by Kahn et al. [9] on a polydisperse octadecyl amide with nine oxyethylene units, the pure surfactant is found to be a viscous liquid at room temperature. Upon solubilization in water, a clear isotropic solution phase is observed for all concentrations between the cloud point and the solidification temperature. No liquid crystal formation is observed. 

If, indeed, a complex containing oxyethylene groups is formed, then it should exhibit cloud point phenomena like ethoxylated nonionic surfactants. This is found to be so as shown in Fig. 8. Note the shift with pH. At different pH s, the number of dissociated APEs is different thus, a different amount of TTAB is needed to neutralize it. The decrease in pH of an APE solution when TTAB is added to it is an indication that a complex is formed. The exhibition of cloud point phenomena for mixtures of APE-TTAB is an indication that the resulting complexes have a pseudononionic character. 

It is important to note that complexes, other than those containing oxyethylene moieties can also exhibit cloud point phenomena. Figure 12 shows the phase diagram of the anionic-cationic surfactant system sodium- -lauroyl-iV-methyl-p-alanine-stearyltrimeth-ylammonium chloride [38]. 

The cloud point temperature of the anionic and cationic mixtures decreased with an increase in the hydrophobicity of the surfactant portion. The cloud point temperature of mixtures of AEOS and alkyltrimethylammonium bromides of different chain lengths is shown in Fig. 13. The cloud point temperature decreased by approximately 10°C for every increase of the methyl group. The cloud point temperature of the anionic and cationic mixtures increased with an increase in the hydrophilicity of the surfactant components. The cloud point temperature of mixtures of AEOS and two ethoxylated cationic surfactants with three and five ethylene groups is shown in Fig. 14. The cloud point temperature increased by at least 60°C for an increase of two oxyethylene groups. 

Increasing temperature for ionic surfactants usually opposes micellization due to enhanced molecular motion that reduces p. For oxyethylene-containing nonionics, temperature causes the association number to increase up to the cloud point, when phase separation occurs because polyoxyethylene becomes insoluble in water. The cloud point decreases with decreasing oxyethylene (E) chain length indeed, surfactants of the type CmE (Section 4.2) with n < 4 are insoluble in water and so the system is always phase separated. As the association number increases with temperature, the solvent quality for oxyethylene becomes worse, causing the corona to shrink. A compensation between the increase in p and a decrease in corona size results in an approximately constant overall micellar radius. 

---