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9 January 2008 Positron annihilation lifetime spectroscopy (PALS) and small angle x-ray scattering (SAXS) of self-assembled amphiphiles
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Self-assembled amphiphile systems are utilized in a wide variety of applications including drug delivery and energy storage. Nano-scale physical and chemical interactions govern the packing of self-assembled amphiphilic molecules, resulting in thermodynamically stable phases of defined geometries. Possible phases include micellar, hexagonal, cubic, lamellar and sponge phases. The internal nano-structure of the amphiphile self assembly materials play an important role in the properties of these systems and their application. To date small angle x-ray scattering (SAXS) has been the most common technique used to characterise their structure. Positron annihilation lifetime spectroscopy (PALS) offers a possible alternative technique as it is sensitive to both the internal cavities and the intermolecular forces and in combination with SAXS, may provide more detailed structural information such as trends with composition and temperature variations. The phase behaviour of a bulk phytantriol sample, consisting of 33 % w/w water was explored using PALS, and it was found that PALS was sensitive to phase transitions from bicontinuous cubic (Pn3m) to reversed hexagonal (H2) to reversed micellar (L2) phases. These boundaries agreed well with SAXS data. Trends observed for the PALS parameters ι3 and I3 as a function of temperature largely supports the concept that the ortho-positronium is annihilating in the organic regions of the self-assembled structure. However, further investigation is required. We have also developed an innovative data analysis technique to analyse PALS spectra for pore information, with the aim of minimising operator bias and error, which leads to better quantitative comparison of PALS results between laboratories.
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Aurelia W. Dong, Carlos Pascual-Izarra, Yao-Da Dong, Steven J. Pas, Anita J. Hill, Ben J. Boyd, and Calum Drummond "Positron annihilation lifetime spectroscopy (PALS) and small angle x-ray scattering (SAXS) of self-assembled amphiphiles", Proc. SPIE 6800, Device and Process Technologies for Microelectronics, MEMS, Photonics, and Nanotechnology IV, 68001C (9 January 2008);

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