Structure, Diffusivity and Linear Rheology of Sodium Ether Dodecylsulfate in Aqueous Solutions – (I)

Static and dynamic light scattering measurements have been performed on dilute and semidilute solutions of sodium ether dodecylsulfate (sodium laurylethersulfate, RO(CH₂CH₂O)₂SO₃^- with R=C₁₂) in the presence of NaCl at 25°C and were combined with rheology measurements. At dilute concentrations (0.92–1.82C*) with C*=[3M_r/4π({R_g²}_z)³ N_A; in the presence of 0.05M NaCl sodium ether dodecylsulfate (SLES) show a first normal mode of the chain entanglement which agrees reasonable well with that of a free draining Zimm-model. The slow component which is Q² independent has a larger amplitude than the fast component which is Q² dependent, and reflects the disruption and/or coalescence kinetics of SLES-aggregates. Increasing the salt concentrations or the SLES-concentrations resulted in the formation of isotropic rod-like micelles. The light scattering results were supplemented with rheology experiments revealing the different transition states of SLES-micelles, too. In addition, above C>C*, with C*=0.15 (w/w) of SLES, the viscoelastic properties are described by an almost ideal Maxwellian behaviour. The rheological data obtained are consistent with scaling laws of the SLES concentration where the exponents are close to those predicted from recent stress relaxation models applied to living polymers. At high angular frequency, the complex modulus is explained in terms of a cross over between the regimes of reversible scission and of breathing of the polymer-like chains. The total average chain lengths of the SLES-aggregates was found to increase with SLES concentration, e.g. L(ø)∼ø_SLES^0.37-0.44 with ø_SLES expressed as weight percent of surfactant, measured between 0.05%–15%, encompassing both dilute and semidilute regimes. The observed findings are different from reports on cationic surfactants of the cetylpyridinium or cetyltrimethyammonium ion types in the presence of strongly bound counterions, e.g. salicylate or high salt, respectively. Moreover, the experiments conducted in the presence of n-dodecanol as a contamination of the SLES reveal distinct differences to previous reported experiments, missing all the transitions of this surfactant. In addition no indication has been found for intramolecularly crosslinking or connections between entangled chains in the presence of 0.1M NaCl. However, at high salt concentrations a transition to a rod-like structure of SLES has been observed with hydrodynamic parameters of R_G=260Å, N_agg∼3000, L∼900Å and d=45.0Å.