title: Cloud point of aqueous solutions containing oxyethylated methyl dodecanoates: effects of surfactant hydrophilicity, nature of added electrolyte, and water activity.
abstract: 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).
authors.affiliation: Poznan University of Technology, Institute of Chemical Technology and Engineering, Pl. Sklodowskiej-Curie 2, 60-965, Poznan, Poland. katarzyna.materna@put.poznan.pl
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Materna
authors.forename: Gerard
authors.initials: G
authors.lastname: Cote
authors.forename: Jan
authors.initials: J
authors.lastname: Szymanowski