Self-assembly in surfactant-based mixtures driven byudacid–base reactions: bis(2-ethylhexyl) phosphoric acid–udn-octylamine systems

摘要

Structural and dynamic features of bis(2-ethylhexyl) phosphoric acid (HDEHP)–n-octylamine (NOA)udmixtures as a function of the NOA mole fraction (XNOA) have been investigated by SAXS, WAXS, IR,uddielectric spectroscopy and polarized optical microscopy. In the 0 ¡ XNOA , 0.5 range, mixtures areudtransparent liquids, while the abrupt formation of a waxy solid characterized by an hexagonaludbidimensional structure occurs at XNOA = 0.5. Such a composition-induced phase transition results fromudthe synergetic effect of the progressive increase in number density of ordered HDEHP–NOA nanodomainsudwith XNOA. Mainly driven by an HDEHP to NOA proton transfer, the increase of structural order with XNOAudarises from the progressive substitution of loosely hydrogen bonded HDEHP–HDEHP aggregates withudstrongly bonded NOA–HDEHP ones. Analysis of SAXS patterns at temperatures in the 10–70 uC rangeudemphasized that these local structures are scarcely impacted by an increase of thermal fluctuations. Effectsuddue to the steric compatibility between HDEHP and NOA apolar moieties have been highlighted. Overall,udthe results allow us to emphasize the role of specific polar and apolar interactions joined to steric effects inudregulating the molecular organization in surfactant mixtures and can be used to design novel materialsudwith planned physico-chemical properties.