SELF-ASSEMBLY AND COMPLEX PHASE BEHAVIOUR IN THE AMMONIUM PENTADECAFLUOROOCTANOATE / WATER / AMMONIUM CHLORIDE SYSTEM.

AUTOR(ES)
FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

199609

RESUMO

The ammonium pentadecafluorooctanoate in heavy water system (APFO/\ pot.2\ H\ ind.2\ O) exhibits exceptional phase behaviour stemming from strong tendency of the surfactant to form disc-like micelles which are stable over a wide concentration and temperature intervals. An understanding of this phase behaviour requires exposing the factors which govern the stability of these disc-like micelles. To this end the effect of salt on the phase behaviour and the structure of the micelles has been studied. The ternary phase diagram has been established in part. It reveals the occurrence of a cloud point \ \"curve\" with an upper consolute temperature, in the dilute regime of surfactant concentration. This represents coexistence of a dilute F and a concentrated I\ \ micellar solution. Small angle X-ray scattering experiments carried out in the ternary system above the cloud point curve reveal that the dilute solution is comprised mainly of long rod-like micelles (for weight fractions of APFO Wa ^ou= 0.05) whilst the concentrated micellar solution is formed by disc-like micelles (for Wa >0.20). It is found that with increasing surfactant concentration a gradual change from rod-to-disc micelles occurs at the concentration interval 0.05 0.30. At higher concentrations of APFO, the phase diagram shows a discotic nematic phase ND+, and significantly two distinct lamellar phases co-existing in equilibrium with each other. This is precisely the kind of behaviour which is predicted by two-dimensional lattice models for the in layer concentration of disc-like micelles. The lower concentration lamellar phase L\ being a smectic-A-phase of discs, and the higher concentration one L\" being a bilayer with holes in it. At low concentration of surfactant L\ is shown to co-exist in equilibrium with a dilute micellar solution F, in which it can be dispersed as large vesicles, polydisperse in size. The observed phase behaviour is rationalised in terms of the effect of salt on both the intramicellar and inter-micellar electrostatic forces.

ASSUNTO(S)

micelles fisico-quimica

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