Effect of Co-surfactants on Bending Stiffness and Scission Energy of Wormlike Micelles: a Rheometry and Diffusing Wave Spectroscopy (DWS) Study
Final Report Abstract
In this study, we have investigated binary surfactant systems of C16TAB with different CnTAB (n=8, 10, 12, 14) in presence of potassium bromide (KBr). In the dilute regime, the surface tension measurements show a nonideal behavior. The negative values of the interaction parameter β for both C16TAB-C14TAB and C16TAB-C12TAB systems indicate an attractive interaction between the two surfactants that becomes stronger as the difference in chain length increases. In the entangled regime, the zero-shear viscosity η0 as well as the relaxation time TR decreases monotically upon increasing the concentration of cosurfactant with shorter chain length for all mixtures, at a fixed C16TAB concentration of 0.5 M. For mixtures C16TAB- C12TAB, C16TAB-C10TAB, and C16TAB-C8TAB, we observe a decrease of the plateau modulus G0 upon addition of short chain surfactant and for the C16TAB-C14TAB mixture G0 increases slightly, but this increase with total surfactant concentration is less pronounced than expected for the pure C16TAB system. These results are rationalized by a progressive diminution of the micellar length with addition of cosurfactant. The persistence length lp has been determined from high frequency rheological experiments and its variation with increasing short chain surfactant content show that the mixed micelles exhibit an increased flexibility even upon addition of a small amount of short chain surfactant with a saturation effect towards the pure short chain limit. We have also investigated the system formed from CTAB and NaSal. Results for this system are compared to data for the system CPyCl with NaSal. For the CTAB/NaSal system the micellar flexibility was found to depend on surfactant concentration: lp decreases slightly as the CTAB concentration increases. Additionally the latter system is more rigid than the CPyCl/NaSal system. The reason is probably due to the fact that pyridinium headgroups confer more flexibility than trimethylammonium headgroups on the micelles. The pyridinium has a planar head structure, whereas the geometry at the nitrogen element in the CTAB headgroup is tetrahedral. Finally we have used rotational rheometry and oscillatory squeeze flow to investigate the salt effect on the rheological properties of the system formed from CPyCl and NaClO3 in aqueous solution. The squeeze flow technique provides very precise G´ and G´´ data at frequencies between 10 and 104 rad/s and thus allows for an accurate direct determination of the plateau modulus of these surfactant solutions. Upon increasing salt concentration, at a fixed surfactant concentration, we observe a maximum for η0 as well as for TR whereas G0 increases continuously with ionic strength. This latter result is different from that obtained by Cappelaere and Cressely (2000) who reported a strong decrease of G0 with increasing ionic strength at salt concentrations higher than that corresponding to the viscosity maximum. We expect an inaccuracy of their oscillatory shear measurements in the high frequency regime to be responsible for a wrong G0 estimation and consequently a wrong interpretation of the viscosity decrease. The increase in G0 observed in our study is closely related to the formation of branched micelles providing an additional contribution to the crosslink density. This interpretation is supported by the power law exponent value of 2.42 found for the dependence of the zero-shear viscosity on surfactant concentration and by cryo-TEM images taken for the same salt/surfactant system but at lower surfactant concentration.
Publications
- “Rheological Properties of Aqueous Solutions of Cetylpyridinium Chloride in the Presence of Sodium Chlorate”, Rheological Acta (2011) 50, 655–660
Oelschlaeger, C., Willenbacher, N.
- “Mixed wormlike micelles of cationic surfactants: effect of the cosurfactant chain length on the bending elasticity and rheological properties”, Colloids and Surfaces A: physicochemical and engineering aspects (2012) 406, 31-37
Oelschlaeger, C., Willenbacher, N.
(See online at https://doi.org/10.1016/j.colsurfa.2012.04.045)
