An investigation of the aggregation and water-air interfacial behaviour of a cationic tentacle molecule

Abstract

Abstract:The aggregation and air-water interface properties of a three-legged tentacle molecule tris (11-pyridinium undec-l-yl)benzene-1, 3, 5-tricarboxylate tribromide have been investigated by measurement of the molar conductance, 4, and equilibrium surface tensin, y, over a wide range of bulk concentration (8 x 10 -6 -0.3 M). In contrast to the single chain analogue, dodecyl pyridinium bromide, DDPB, the tentacle species shows no evidence for a conventional micellar transition in solutions up to 100 fold more concentrated than the critical micelle concentration of DDPB, though aggregation of small numbers of molecules cannot be excluded. The conductance behaviour suggests the occurrence of ion-pair (or higher) equilibria, which are likely to complicate the interpretation of the data. Also, in contrast to a previous tentacle molecule described in the literature, the present species is significantly surface active, and in the high concentration limit reduces y to < 30 mNm -1, significantly lower than the values obtained for simple ionic amphiphiles. In this region the tentacle species seems to stand like a 'tripod' at the waterair interface with both its hydrocarbon legs and its aromatic body lifted clear of the water surface. At much lower concentrations two small plateau regions in y are apparent (-60 and 52 mNm-1). Here, the tentacle molecule occupies a much larger surface area, and seems to adopt a crab-like crouch with its aromatic body and polar ester substituents lowered into the water surface.

Molecular concepts, self-organisation and properties of polysoaps

“…Defined "oligomeric" surfactant structures obviously have stimulated the imagination of many colloid chemists, as a number of imaginative names were suggested for them: bola-surfactants [410][411][412], gemini-surfactants [413][414][415], tentacle molecules [416][417], octopus molecules [418], hexapus [419], multiarmed amphiphiles [420][421][422]. .…”

“…This is the more true, as the majority of the "tail end" structures represents "stars" [416,417,420,422,425,449] rather than linear or cyclic oligomers [418,419,423]. Most attempts to produce "head-type" trimers suffer from the poor water solubility of the products [450], but recently some water-soluble examples have been reported [427].…”

“…CMC-like transitions become less clear, in particular of studied by surface tension and conductivity measurements [416][417][418][419][420][421][422]425]. The reduction of the surface tension by such oligomers is often quite low [416-420, 422, 423, 425], and the shape of the plots of surface tension vs the logarithm of the concentration can be bizarre [417,425] though not necessarily for all systems studied [423]. For "hexamers" the surface activity becomes very weak [419].…”

“…The reason for such discrepancies is obscure. There are indications that solubilization and surface tension or conductimetric studies respectively do not give consistent CMC values for such uncoventional surfactants [425] which may be due to the formation of very small aggregates at low concentrations [417]. The surprising behaviour of trimeric surfactants of the "head" type was explained analogously [427].…”