Cloud point of aqueous solutions containing oxyethylated methyl dodecanoates: effects of surfactant hydrophilicity, nature of added electrolyte, and water activity.

The aim of this work was to determine the cloud points of new oxyethylated methyl dodecanoates of various hydrophilicity (OMD-n, where n refers to the average degree of oxyethylation) and to correlate them with surfactant hydrophilicity and, for a given electrolyte, with water activity. Thus, it is shown that the cloud point in the absence of electrolyte (CP(0)) can be simulated by the following equation: CP(0)=165.5logn-112.0 (with R(2)=0.987). The effects of NaCl, NaHCO(3), and KSCN on the cloud point are also reported and discussed. The salting-out effect arising from the presence of NaCl or NaHCO(3) is explained by the existence of a hydration shell with enhanced water structure as well as a zone with decreased salt concentration around the -(OCH(2)CH(2))(n)- chain, as compared with the bulk. On the other hand, the salting-in effect is explained by depletion of water around the -(OCH(2)CH(2))(n)- chain. Thus, it is estimated that the number b of water molecules forced back into the bulk solution from the salt-deficient regions when the hydration shells of two -O-CH(2)-CH(2)- monomer units overlap ranges between 2 and 3 and 3 and 4 for NaCl and NaHCO(3), respectively, depending on the average degree of oxyethylation n of OMD-n. It is also shown that the water activity is a useful parameter to simulate the variation of cloud point in the presence of an electrolyte (CP) at low and moderate concentration (e.g., <1 M NaCl), CP(0)/CP approximately 1-(bR/alpha)lna(w), where R is the gas constant and alpha approximately 15 JK(-1)mol(-1). At high electrolyte concentration, the relationship between CP(0)/CP and lna(w) significantly deviates from linearity. In the particular case of KSCN, an inversion of the salt effect can be observed. The salting-in effect of KSCN increases up to about 2 M, but decreases at higher KSCN concentrations, so that KSCN can even act as a salting-out salt at high concentration (typically above 3.3 M for OMD-14).