Resiliently Spherical Micelles of Alkyltrimethylammonium Surfactants with Multivalent, Hydrolyzable Counterions

Abstract

A series of C(12)-C(16) alkyltrimethylammonium surfactants with hydrolyzable phosphate (PO(4)(3-), HPO(4)(2-), and H(2)PO(4)(-)), oxalate (HC(2)O(4)(-) and C(2)O(4)(2-)), and carbonate (HCO(3)(-) and CO(3)(2-)) counterions have been prepared, and their micellar solution behavior has been characterized. Critical micelle concentrations were measured using electrical conductivity and were found to depend on both the counterion and its hydrolysis state. All monovalent counterions bind less strongly to the micelle surface than does bromide or chloride, whereas multivalent species bind more strongly. Small-angle neutron scattering reveals that, unlike alkyltrimethylammonium bromides and chlorides, micelles are small and spherical in the presence of hydrolyzable counterions of all valences and remain spherical even in the presence of added electrolyte. This is consistent with the strong solvation of even strongly bound hydrolyzable counterions, which prevents the screening of repulsions between adjacent headgroups necessary for sphere-cylinder transformations. Salts of multivalent hydrolyzable counterions could thus be used to control the micelle structure in novel ways.

Hexagonal closest-packed spheres liquid crystalline phases stabilised by strongly hydrated counterions

“…The formation of HCPS phases in all DTA + phosphates, oxalates and bicarbonate, but not in DTAC, underscores that these counterions at all hydrolysis states behave like a more weakly binding ion than chloride. This is further highlighted by the persistence of the HCPS phase as the alkyl chain length is increased from C 12 to C 16 22 Para and Warszynski have shown in a related system that HSO 4 À binds more strongly to a surface than SO 4 2À , as the divalent ions are more strongly hydrated. 40 The almost ubiquitous observation of small, spherical micelles in both the L 1 and HCPS phases of these cationic surfactants, together with the recurring phase progression on to I 1 (Pm3n) and H 1 suggests to us that curvature changes drive changes in self-assembly structure.…”

“…Alkyltrimethylammonium surfactants with hydrolysable carbonate, phosphate and oxalate counterions were prepared and characterized as described previously. 22 These solutions will undergo some degree of anion speciation by hydrolysis, but are identied throughout according to the parent salt e.g. (DTA) 3 PO 4 , DTAHC 2 O 4 , etc.…”

“…12,17 Its widespread occurrence in these surfactants is consistent with our recent small-angle neutron scattering investigation of the L 1 phase of these systems, which showed the spherical micelles to be extremely stable and resistant to a sphere-to-rod transformation. 22 Specic counterion effects are one of the best known ways of controlling surfactant self-assembly structure. Changing counterions changes the effective head group area a 0 and therefore the surfactant packing parameter, v/a 0 l c .…”

“…conductivity. 22 Both phosphate and oxalate ions are, however, known to remain strongly hydrated and to have large radii of hydration. [36][37][38][39] Thus, despite binding strongly to micelles, these ions are unable to effectively screen the repulsions between neighbouring headgroups within a micelle.…”

“…This suggested to us that ionic surfactants with weakly binding counterions which would resist sphere-to-cylinder transformation by added electrolyte should stabilize the HCPS phase. We have recently shown 22…”