Surfactant structure and the thermodynamics of micelle formation

Surfactants are molecules which possess a dualistic character. They combine a water-preferring (hydrophilic) and a water-rejecting (hydrophobic)part in one molecule. This dualistic character is the basis of their surface activity. ... Zie: Summary

Effect of Counterion Structure on Micellar Growth of Alkylpyridinium Surfactants in Aqueous Solution

This paper describes the influence of counterions on the unidirectional growth of micelles formed by alkylpyridinium surfactants in aqueous solution. It is shown that the growth of spherical micelles to form wormlike micelles is strongly dependent on counterion structure. More hydrophobic counterions induce the formation of wormlike micelles at lower surfactant concentrations. Next to hydrophobicity and the type of substituent, the substitution pattern of the aromatic ring plays the most important role in micellar growth. The formation of a network of entangled, elongated wormlike micelles by alkylpyridinium surfactants with o-hydroxybenzoate and p-chlorobenzoate counterions is discussed in terms of surfactant structure. It is concluded that, next to counterion structure, the microenvironment of the counterion (substituent) in the Stern region and the structure of the surfactant monomer (i.e., the surfactant cation) play the most important role in the formation of these elongated wormlike micelles. Headgroup effects are proposed to be the main driving force for this phenomenon.

Modeling dinuclear copper sites of biological relevance: synthesis, molecular-structure, magnetic-properties, and h-1- nmr spectroscopy of a nonsymmetric dinuclear copper(ii) complex - microcalorimetric determination of stepwise complexation of copper(ii) by a nonsymmetric dinucleating ligand

The new nonsymmetric dinuclear copper(II) complex [Cu(2)L(1)(OAcW(ClO4) (7) was synthesized by complexation of Cu(OAc). H2O with a new nonsymmetric dinucleating ligand (5) which' is formed in situ by condensation of 2-formyl-6-((4-methylpiperazin-1-yl)methyl)phenol (3a) with 2-(aminoethyl)pyridine. Complex 7 crystallizes in the monocli,nic space group P2(1)/n with cell constants a = 7.486(1) Angstrom, b 22.472(2) Angstrom, c = 16.892(2) Angstrom, beta = 102.7(1)degrees, V = 2771.6(6) Angstrom(3), and Z = 4. The structure was determined at 130 K from 4890 out of a total of 7034 reflections with R(F) = 0.060 and R(wF) = 0.060. The crystal structure establishes the presence of a dinuclear copper core with distinct copper sites. One of the copper ions is coordinated by two sp(3) nitrogens whereas the other copper is coordinated by two sp(2) nitrogens. The two copper ions are bridged by a phenoxo group, by a syn-syn bidentate acetato bridge, and by a monodentate acetato bridge resulting in a Cu-Cu distance of 3.0293(10) Angstrom. The copper centers are weakly antiferromagnetically coupled with 2J = -15 cm(-1). This weak antiferromagnetic coupling is reflected in the large isotropic shifts for the ligand protons in the H-1 NMR spectrum of 7. 2D-COSY experiments and selective substitutions, leading to p-methyl-substituted complex 8, were performed to allow assignments of the proton resonances. The complexation behavior of nonsymmetric ligand 5 is studied by microcalorimetric and UV-vis spectroscopic measurements. These techniques unambiguously establish the stepwise complexation behavior of the ligand; nonsymmetric dinuclear complex 7 is formed via the initial formation of a mononuclear copper complex. The enthalpy of formation of the first complexation step is -8.4 (+/-0.2) kcal/mol (K = 1.8 (+/-0.1) x 10(8)); for the second step a value of Delta H = -1.9 (10.1) kcal/mol (K = 1.1 (+/-0.1) 10(5)) was obtained using microcalorimetry

Isobaric Heat Capacities of Micelle Formation by 1-Methyl-4-n-dodecylpyridinium Iodide in Aqueous Solution; Effects of Added Urea

Over the temperature range from 303 to 333 K, the enthalpy of micelle formation by 1-methyl-4-n-dodecylpyridinium iodide in aqueous solution is exothermic, characterised by an isobaric heat capacity of micelle formation equal to -439 ± 10 J K-1 mol-1. At 303 K, the critical micellar concentration (2.8 · 10-3 mol dm-3) increases when urea, 4 mol kg-1 and 8 mol kg-1 is added. Surprisingly, the isobaric heat capacity of micelle formation is unaffected although micelle formation is slightly less exothermic compared to that recorded for aqueous solutions.

Study of Self-Associating Amphiphilic Copolymers and Their Interaction with Surfactants

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.