Headgroup effects on the Krafft temperatures and self-assembly of ω-hydroxy and ω-carboxy hexadecyl quaternary ammonium bromide bolaform amphiphiles: micelles versus molecular clusters?

Eight bolaform amphiphiles were synthesised and characterised; 4 α,ω-hydroxy-alkane trialkyl (and pyridyl) ammonium bromides and 4 α,ω-carboxy-alkane trialkyl (and pyridyl) ammonium bromides where the alkyl groups were methyl, ethyl and propyl. Four of these represented new compounds. Overall the Krafft temperatures (T(K)) of the eight amphiphiles were high, with 6 of the eight possessing T(K)s greater than 45 °C. Thus most of the amphiphiles could only expect to find applications at raised temperatures limiting their potential utility. However in addition to the previously reported α,ω-hydroxy-hexadecyl triethylammonium bromide (2b) with a T(K) of 19.1 °C, another amphiphile, α,ω-carboxy-hexadecyl tripropylammonium bromide (2c) has been identified with a T(K) near ambient temperatures (T(K) of 22.1 °C). This provides an acid functional ammonium bolaform amphiphile that micellises at ambient temperatures to complement the hydroxyl derivative. A correlation between T(K) and the product of the enthalpies and T(m)s of the compounds was observed for 7 of the eight compounds. No correlation between the amphiphile critical micelle concentrations (cmc) and T(K)s was observed confirming previous reports that T(K) values are predominantly determined by crystalline stability rather than solubility. Considerable differences were observed between the various amphiphile T(K)s at different pHs but no clear trend was apparent for the various compounds (despite the degree to which the compounds' carboxylic acid and hydroxyl functionalities were likely to be ionised). The cmcs for the amphiphiles were an order of magnitude larger than those for analogous mono-ammonium amphiphiles with little difference in between the hydroxyl- and carboxy-functionalised compounds. The aggregation numbers (N(agg)) obtained for all compounds were very low (N(agg)<7) and the apparent micellar diameters for the hydroxyl-bolaforms were in the range 1.0-1.4 nm whereas those for the carboxy-compounds were in the range 2.1-2.4 nm. These results strongly suggest a difference in the packing of the two sets of amphiphiles with loose low density aggregates or 'molecular clusters' for the carboxy compounds and denser classical micellar type aggregates for the hydroxyl-compounds. In both cases however the sizes and the low aggregation numbers point suggest that these aggregates are more characteristic of the pre-micellar aggregates observed for many amphiphiles but in particular gemini surfactants.