Biomedical subjects
Publications and source records attributed to David C Steytler.
For some 15 years the attainment of efficient, nonfluorinated CO2-active surfactants has been a Holy Grail for researchers spanning pure and applied chemical sciences. This article tells the story of small-molecule CO2-active surfactants, from the first tentative observations with fluorinated compounds in 1991 up to recently discovered fluorine-free oxygenated amphiphiles.
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In this paper we investigate the use of supercritical carbon dioxide (sc-CO(2)) for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thickness, and d spacings. Small angle neutron scattering was used to probe the controlled swelling of the triblock copolymer surfactant templating agents, P123 (PEO(20)PPO(69)PEO(20)), P85 (PEO(26)PPO(39)PEO(26)), and F127 (PEO(106)PPO(70)PEO(106)), as a function of CO(2) pressure. The transition from the liquid crystal phase to the calcined mesoporous silicas, formed upon condensation and drying, was also studied in detail. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. Using a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with a high level of precision. The swelling process was observed to have no detrimental effects on the quality of silica formed, a distinct advantage over conventional swelling techniques, and all of the silicas synthesized in this study were highly ordered over distances of at least 2000 A.
Hybrid fluorocarbon-hydrocarbon (F-H) sulfate surfactants are shown to be efficient stabilizers in water-in-CO2 (w/c) microemulsions. The chain structure and F-H ratio affect the regions of P-T phase stability and aggregation structure in these w/c phases. High-pressure near-infrared spectroscopy and small-angle neutron scattering measurements of microemulsified water provide evidence for the stabilization of w/c microemulsion droplets. The relative lengths of the two chains were found to influence the favored aggregation structure: for symmetric chain surfactants (F8H8, F7H7) spherical reverse micelles are present, but for asymmetric chain surfactants (F7H4, F8H4) extended cylinder aggregates form. These changes in aggregation are consistent with different surfactant packing parameters owing to the controlled variations in molecular structure. Furthermore, the general order of w/c phase transition pressures (F8H8 < F7H7 and F8H4 < F7H4) is in line with estimations of surfactant fractional free volume, as proposed by Johnston et al. (J. Phys. Chem. B 2004, 108, 1962-1966). Studies of adsorption at the poly(dimethylsiloxane)-water interface are shown to be valuable for assessing the CO2-philicity of new surfactants. All in all, the symmetric F8H8 and F7H7 analogues are seen to be the most efficient compounds from this class for applications in CO2.
Polymerization of styrene-in-water microemulsions by photoinitiation using the initiator 2,2'-dimethoxy-2-phenyl acetophenone (DMPA) produces small nanolatices of the same size as the parent microemulsion (radius = 24-31 angstroms). This behavior is distinct from previously applied methods where significant particle growth accompanies the polymerization reaction. NMR measurements confirmed that polymerization is complete in under an hour and small-angle neutron scattering established the microemulsion structure before and after polymerization. The approach may be of more general application where retention of structure is sought in microemulsion-based polymer templating procedures.
Dilute aqueous phase behavior of a novel tris(hydroxymethyl)acrylamidomethane (THAM)-derived telomer bearing a perfluorohexyl hydrophobic chain, F6THAM6, has been investigated. Fluorinated polyhydroxy surfactants of this kind find use in emerging biomedical applications. Neutron reflection (NR) and drop volume surface tension (DVT) methods have been used to determine the critical micelle concentration (cmc=4.7 x 10(-4) mol x dm(-3)) and surface adsorption parameters (at the cmc NR gives a molecular area a(cmc)=67.4 and 62 A(2) and surface excess gamma(cmc)=2.46 x 10(-6) mol x m(-2)). The aggregation structures were determined by small-angle neutron scattering (SANS), indicating globular (polydisperse spheres) micelles of radius approximately 30 A are present. These findings are compared with literature on surfactants with related structures, to identify how the unusual molecular structure of F6THAM6 affects surfactant properties.
Stability and aggregation structures of various economically viable surfactants for CO(2) are reported. The compounds are either commercially available octylphenol nonionics (Triton X-100, X-100 reduced, and X-45) or custom-made analogues of aerosol-OT (J. Am. Chem. Soc. 123 (2001) 988). These were selected to reveal the influence of chain terminal group structure, namely highly methylated t-butyl units, on solubility and aggregation in CO(2). In addition the mean ethylene oxide block length is varied for the Triton surfactants (X-100 approximately EO(10), X-45 approximately EO(8)). High-pressure small-angle neutron scattering (SANS) experiments revealed the presence of aggregates, consistent with spheroidal reverse micelles. The nonionics show a temperature and pressure dependence on solubility. These results confirm the special affinity of highly methyl-branched tails for CO(2). However, none of these systems were able to disperse significant amounts of water or brine; therefore hydrated reversed micelles or microemulsion droplets were not stabilized. Hence the utility of these cheap methyl-branched surfactants in CO(2) is limited, and so groups of greater CO(2)-philicity are needed to achieve the goal of water-hydrocarbon surfactant-CO(2) dispersions.