Structure–property relationships in metallosurfactants

Abstract

The morphology of micelles formed by three sub-classes of metallosurfactants-those with macrocyclic, linear and gemini head groups-has been studied by small-angle neutron scattering (SANS) for a series of metal-and counter-ions. All the data may be described by a model that invokes a globular micelle morphology in which the dimensions of the micelle are consistent with the known chemical structure of the constituent groups within the metallosurfactant. For two macrocyclic head group metallosurfactants, viz. 1-(2-hydroxy-tetradecyl)-1,4,7-triazacyclonane that forms predominantly spherical micelles and 1-(2-hydroxy-tetradecyl)-1,4,7,10-tetraazacyclononane that forms disc-like micelles, the metal ion and its counter-ion have a negligible effect on the morphology of the micelle. Binary mixtures of surfactants with these two macrocyclic head groups (with homo-or hetero-metal ions/counter-ions) form mixed micelles whose morphology is an average of the two single component micelles. Further, as found for the single surfactant solutions, the metal and counter-ion had no effect on the morphology of the mixed surfactant micelle. Lastly, the micelle morphology of two gemini surfactants was also shown to be insensitive to the number and nature of the metal and counterions present, but sensitive to the structure of the head group. These observations considerably extend our understanding of the relationship between chemical structure and micelle morphology for these interesting molecules.

A contrast variation small-angle scattering study of the microstructure of 2,5-dimethyl-7-hydroxy-2,5-diazaheptadecane–toluene–butanol oil-in-water metallomicroemulsions

“…The neutron scattering data from the surfactant-only system (h-surfactant in D 2 O) were fitted using the FISH data analysis program. 13 The data were best described by a core-shell model in which the core radius was constrained to the all trans-equivalent hydrocarbon chain length (R core ¼ 15 Å ) yielding a shell thickness (d) of 10 Å and an ellipticity (X) of 2, in good agreement with previous data, [9][10][11] see Table 1.…”

“…For example, NiCl 2 and CuSO 4 led to significant amounts of insoluble material that could not be dissolved with reasonable variations in pH, temperature or ionic strength. However, Cu(NO 3 ) 2 and Zn(NO 3 ) 2 produced stable, clear micellar solutions (critical micelle concentrations, CMC ¼ 2-3 mM [9][10][11] ) with no solid precipitate, and these were selected for further study.…”

“…Recently we showed that catalytic centres present within the different regions of a conventional (sodium dodecyl sulfate-based) microemulsion system allowed control over the product distribution and extent of a hydrolysis/oxidation reaction. 11 Specifically, a manganese-Schiff base catalyst covalently linked to an organic framework that constrained its activity to the surfactant head group region of a microemulsion droplet, was found to provide a more efficient mechanism to degrade mustard gas that partitions between the oil and water phases, compared with the same catalytic centre engineered to reside predominantly in either the oil or water phase. The ability, therefore, to localise metal ions at fluid interfaces is an important facet of catalysis.…”

Structural evolution in metallomicroemulsions – the effect of increasing alcohol hydrophobicity

“…Although the S(Q) term is essential to fit the data, previous work 22 has demonstrated that the fitting process can be insensitive to the individual parameters in the Hayter-Penfold S(Q) structure factor used for micelle-micelle charge repulsions 23 Due to the presence of a metal ion in the headgroup, there is a possibility to use small-angle X-ray scattering (SAXS) to investigate the location of the surfactant headgroups in a micelle, or to identify the surfactant shell around a microemulsion droplet. We have used this approach previously in both types of system, 1,19 but SAXS measurements cannot provide the level of detail required to determine contributions of different components to microemulsion structure. To do this a multiple contrast small-angle neutron scattering (SANS) experiment is required, using selective deuteration to highlight individual components.…”

“…Metallosurfactants may be defined as containing a polar transition metal ligand complex in conjunction with a hydrophobic moiety 1,2 , forming a construct in which the physical behaviour of conventional surfactants is combined with the properties of the bound transition metal ion, such as redox 3,4,5 , catalytic 6,7 , photophysical 8,9 and paramagnetic 10,11 properties. This has led to their uses in imaging 12 , DNA binding 13 The classical adsorption and self assembly behaviour of metallosurfactants observed in aqueous solution 14,15,16 affords a high local concentration of the reactive metal species and allows one to constrain a particular characteristic of the metal at an interface in a compartmentalised system, such as a microemulsion.…”