This paper delineates the influence of counterions on the aggregation behavior of 1-methyl-4-ndodecylpyridinium surfactants, using conductometry and 1 H-NMR spectroscopy. Three types of counterions have been studied: (i) halides, (ii) alkanesulfonates, and (iii) aromatic counterions. The critical micelle concentration is found to decrease with increasing counterion size and increasing counterion hydrophobicity, whereas the degree of counterion binding increases. The aggregation behavior of 1-methyl-4-ndodecylpyridinium surfactants with aromatic counterions is shown to be markedly dependent on the substituent (hydrophobicity) and the substitution pattern in the aromatic ring of the counterion. Depending on the molecular architecture of the aromatic counterion, extremely long wormlike micelles can be formed instead of (nearly) spherical micelles. NMR experiments revealed that all aromatic counterions intercalate in between the pyridinium headgroups of the micelles, with more or less the same degree of penetration. All results can be explained on the basis of counterion-surfactant and counterion-water interactions, taking into account the specific microenvironment in the Stern layer.

Classification of calorimetric titration plots for alkyltrimethylammonium and alkylpyridinium cationic surfactants in aqueous solutions Bijma, K; Engberts, Jan; Blandamer, M.J; Cullis, P.M.; Last, P.M.; Irlam, K.D.; Soldi, L.G. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Calorimetric titration plots for deaggregation of micelles formed by alkylpyridinium and alkyltrimethylammonium surfactants are classiÐed into three types, A, B and C, depending on the shape of the plot of the enthalpy of dilution as a function of surfactant concentration. For Type A plots the recorded heat of injection q changes sharply between two parts of the titration curve over which the recorded heats are e †ectively independent of the composition of the solution in the sample cell. For Type B plots, the change is less sharp and both parts of the plot show dependences of heat q on solution composition, a pattern accounted for in terms of soluteÈsolute interactions. Type C plots are complicated, in that no sharp change in q is recorded, the complexity of the plots being accounted for in terms of micelleÈmonomer equilibria over a range of surfactant concentrations and related enthalpies of deaggregation.

Citation for published version (APA): Bijma, K., Blandamer, M. J., & Engberts, J. B. F. N. (1998). Effect of counterions and headgroup hydrophobicity on properties of micelles formed by alkylpyridinium surfactants. 2. Microcalorimetry. Langmuir, 14(1), 79 -83. DOI: 10.1021/la970216n Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. We report the influence of counterions and headgroup hydrophobicity on the enthalpies and heat capacities of micelle formation by 1-alkyl-4-n-dodecylpyridinium surfactants in aqueous solution. For micelle-forming 1-alkyl-4-n-dodecylpyridinium iodide surfactants (alkyl ) C1-C3), enthalpies of micelle formation show no trend with respect to 1-alkyl chain length. Changes in molecular architecture of the counterion, however, have a large influence on the thermodynamics of micelle formation. It is shown that next to solvation, London dispersion interactions, counterion (substituent)-water interactions, and the microenvironment of counterions adsorbed to micellar surfaces play an important role in determining the exothermicities of micelle formation. Isobaric heat capacities of micelle formation show that the formation of wormlike micelles for salicylate-containing cationic surfactants is due to a favorable microenvironment for salicylate counterions in the Stern layer of micelles.

The self-association of copolymers of N-isopropylacrylamide and N-n-octadecylacrylamide (Pnipam-C18) in aqueous solutions was studied by means of time-resolved fluorescence quenching. The discrete domains consist of several polymer chains interacting through their hydrophobic side chains, since the number of aliphatic side chains involved in the microdomain formation (aggregation number) is larger than the number of aliphatic side chains per polymer. By means of titration microcalorimetry, the interaction of the copolymer with surfactants was studied. Strong association between the copolymer and the cationic surfactants N-cetylpyridinium chloride (C16PyCl) and cetyltrimethylammonium bromide (CTAB) occurs by partitioning of the surfactants in a noncooperative mechanism. Prior to mixed-micelle formation, individual surfactant molecules adsorb to collapsed polymer coils as can be seen from the large exothermal contribution in the enthalpy curves which result from microcalorimetric titration of surfactant into aqueous Pnipam-C18 solutions.