RESUMEN
We use dynamic light scattering to investigate the effects of charge polydispersity and charge residence time on the dynamics of a micellar system. While in the corresponding uncharged system only one exponential relaxation is observed, two relaxation modes are seen when charging the micelles by adding charged co-surfactant molecules with a long residence time. We attribute the existence of these two relaxation modes to the combined effect of size polydispersity and charge polydispersity, i.e. frozen fluctuations of the number of charges per micelle. Further support to this scenario is provided by control experiments on a similar charged system, but where the charge residence time is short compared to the time scales probed by dynamic light scattering. Here, charge polydispersity is effectively suppressed due to the rapid exchange of charged molecules between micelles and only one single relaxation mode is seen, thereby demonstrating the key role of frozen charge fluctuations in the complex dynamics of our micellar system.
RESUMEN
The shape and interactions between microemulsion droplets (R = 8.2 nm, polydispersity 20%) either decorated with PEO modified with a single hydrophobic end function (PEO-m: C12H25 - (EO)n, M(PEO) = 5.2 kg/mol), or with telechelic polymers of twice the mass (PEO-2m: C12H25 - (EO)2n - C12H25, M(PEO) = 10.4 kg/mol) have been studied by small-angle neutron scattering (SANS). The results as a function of droplet and polymer concentration have been compared to the reference case of the bare microemulsion which was shown to be unchanged using Porod representations. The interactions between bare and decorated droplets have been analyzed using the structure factor S(q), at first in a model-free way based on its low-q limit S(q â 0). This analysis provides clear evidence on the concentration-dependent repulsive or attractive nature of the contributions to the pair droplet-droplet pair potential of the polymers. Model pair potentials describing the steric repulsions and attractions by copolymer bridging are used to describe the low-q behavior of the structure factor based on an integral equation approach, giving an estimate of the range and amplitude of the potentials. Moreover, they provide an explanation for the observed transient clustering in terms of a shallow minimum of the total potential, as they establish the respective repulsive and attractive contributions of the polymer molecules.
