摘要:

IntroductionMicroemulsions (MEs) are systems in which nanoscale domains of one liquid are stably dispersed in another immiscible liquid, with the aid of one or more surfactants, and occasionally co-surfactants.1Their unique properties make them important vehicles for drug delivery,2synthesis of high-value nanoparticles3and other applications. However, the same components can also form a variety of other phases (micellar, vesicular and liquid crystalline, such as cubic and lamellar, Lα, phases) given appropriate concentration, temperature and pressure conditions. Studies of transformations and transitions between these phases are of great interest; such phase transitions can be induced by a changes in composition,4,5temperature (T-jump),6pressure (P-jump),7–10or in the case of some specialised systems, other external stimuli such as light.11Previous work has shown that the transition from an ME to an Lαphase can be effected by either temperature or pressure,6,8and that for a given set of conditions, the preferred curvature of the surfactant film controls the nature of the final phase.12It has also been noted that at certain pressures and temperatures, both lamellar and microemulsion phases may coexist.4,8Ghoshet al., using small-angle X-ray scattering (SAXS) with C12E5–octane–water systems, observed ME–Lα–ME phase transitions with increasing temperature, bound by regions of coexistence.12Nagao and Seto employed both small-angle neutron scattering (SANS) and SAXS with AOT–water–decane systems, showing that increasing pressure induced phase changes from an ME to Lα+ bicontinuous ME coexistence, which was explained by an increase in inter-droplet interactions with pressure.7,8Colloidal and soft matter phase transition kinetics are experimentally challenging to study as they can occur on fast timescales (on the order of ms to s), although timescales tend to be highly dependent on the nature of the system.13Small-angle scattering represents the ideal tool for the equilibrium investigation of these soft matter systems,13as it can provide direct structural information. Neutron scattering is particularly suited to studying MEs and related systems, as substitution of H2O by D2O generates convenient contrast variation. One potential disadvantage of neutron scattering for time-resolved studies is the limitation of signal-to-noise, requiring long data acquisition times to achieve good statistics (typically 5–30 min depending on the system of interest). However, the new generation of high-flux spectrometers like D22 at the ILL opens the possibility of real-time measurement with acquisitions of the order of 100 ms, coupled with histogramming memory data acquisition.Ternary systems comprising the common non-ionic surfactant polyethylene glycolp-(1,1,3,3-tetramethylbutyl)-phenyl ether (Triton X-100, TX-100) toluene and water have previously been studied, gaining information on the equilibrium phase behaviour and microstructural properties,14,15In this work, a lamellar phase comprising TX-100 and water (2.7 : 1 w/w) was subject to a rapid jump in composition, taking <100 ms, by stopped-flow mixing with an equal volume of toluene. The inset toFig. 1indicates the compositions of these initial and final states. The initial phase contains only two components lying on the D2O–TX-100 axis of the phase diagram, and importantly not inside the body of the three-phase triangle. Rapid addition of toluene (<100 ms) by stopped-flow results in a concentration step change; now the composition is located inside the three phase body, in a region that will eventually form a microemulsion after equilibrium is attained. The time-resolved SANS experiments reported here monitor the structural relaxations on the path to equilibrium, up to 280 s after this rapid step change in composition.Main: static SANS data for the undiluted Lαphase and the final ME. Solid lines represent fits to the data using the models described in the text; fit parameters for (a) ME:dm= 63.1 ± 3 Å,ξm= 53.2 ± 3 Å, (b) Lα:ξl= 46.0 ± 5 Å,dl= 43.3 ± 0.5 Å. Inset: partial phase diagram for TX-100–toluene–water. Symbols indicate initial (&z.squt;) and final (○) phase compositions.

ISSN:1744-683X    

DOI:10.1039/b819196d

出版商:RSC    

年代:2008

数据来源: RSC