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

摘要

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 thea bsence of electrolyte (CP~0) can be simulated by the following equation:CP~0=165.6 log n-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 expalined by the existence of a hydration shell with enhanced water structure as well as a zone with decreased salt cocnentration around the -(OCH_2CH_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_2CH_2)_n- chains.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 simualte the variation of cloud point in the presence of an electrolyte (CP) at low and moderate cocnentration (e.g.<1 M NaCl),CP~0/CP approx = 1-(bR/alpha)lna_w,where R is the gas constant and alpha approx =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 cocnentrations,so that KSCN can even act as a salting-out salt at high concentration (typically above 3.3 M for OMD-14).