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

Holder, Simon J., Sriskantha, Bruntha C., Bagshaw, Stephen A., Bruce, Ian J. (2012) Headgroup effects on the krafft temperatures and self-assembly of ?-hydroxy and ?-carboxy hexadecyl quaternary ammonium bromide bolaform amphiphiles: Micelles versus molecular clusters? Journal of Colloid and Interface Science, 367 (1). pp. 293-304. ISSN 0021-9797. (doi:10.1016/j.jcis.2011.10.017) (KAR id:36817)

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Eight bolaform amphiphiles were synthesised and characterised; 4 ?,?-hydroxy-alkane trialkyl (and pyridyl) ammonium bromides and 4 ?,?-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 (TK) of the eight amphiphiles were high, with 6 of the eight possessing TKs 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 ?,?-hydroxy-hexadecyl triethylammonium bromide (2b) with a TK of 19.1 °C, another amphiphile, ?,?-carboxy-hexadecyl tripropylammonium bromide (2c) has been identified with a TK near ambient temperatures (TK 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 TK and the product of the enthalpies and Tms of the compounds was observed for 7 of the eight compounds. No correlation between the amphiphile critical micelle concentrations (cmc) and TKs was observed confirming previous reports that TK values are predominantly determined by crystalline stability rather than solubility. Considerable differences were observed between the various amphiphile TKs 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 (Nagg) obtained for all compounds were very low (Nagg < 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.

Item Type: Article
DOI/Identification number: 10.1016/j.jcis.2011.10.017
Uncontrolled keywords: Micelle; Aggregate; Pre-micellar; Ammonium; Bolaform; Amphiphile; Self-assembly; Dynamic light scattering; Krafft temperature; Aggregation number; Critical micelle concentration
Subjects: Q Science
Divisions: Divisions > Division of Natural Sciences > Biosciences
Divisions > Division of Natural Sciences > Physics and Astronomy
Depositing User: Simon Holder
Date Deposited: 22 Nov 2013 16:07 UTC
Last Modified: 16 Nov 2021 10:13 UTC
Resource URI: (The current URI for this page, for reference purposes)
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