S. Dai et al., Binding characteristics of hydrophobic ethoxylated urethane (HEUR) and an anionic surfactant: Microcalorimetry and laser light scattering studies, J PHYS CH B, 105(42), 2001, pp. 10189-10196
The binding behaviors of an anionic surfactant, sodium dodecyl sulfate (SDS
), to an associative polymer, hydrophobic ethoxylated urethane (HEUR) were
studied by isothermal titration calorimetric (ITC) and laser light scatteri
ng (LLS) techniques. Static and dynamic light scattering results suggest th
at HEUR polymers form flowerlike micelles with an average aggregation numbe
r of about 20. Isothermal titration calorimetric studies of SDS and HEUR so
lutions were conducted by varying both the concentrations of SDS and HEUR.
For the titration of SDS into HEUR solutions, the thermograms display an en
dothermic peak after the critical aggregation concentration (cac), followed
by an exothermic peak that eventually merges with the SDS dilution curve.
The endothermic and exothermic peaks are interpreted by different binding m
echanisms at different surfactant concentrations. Although the values of ca
c are independent of HEUR concentrations, the saturation concentration C-2
is strongly dependent on HEUR concentrations. With increasing HEUR concentr
ations, C-2 increases proportionately. Dynamic light scattering results cor
roborate with the findings from ITC studies. The apparent hydrodynamic radi
i increase with increasing SDS concentrations for cac < CDSDS< C-2, and rem
ained constant at C-SDS > C-2. However, the thermograms for titration of HE
UR into SDS micellar solutions differ from those of titrating SDS into HEUR
, suggesting that different binding mechanism must be in operation. The bin
ding enthalpy changes increase With SDS concentration and decrease with HEU
R concentration when titrating HEUR into SDS solutions.