The interaction between the nonionic surfactant C12E5 and a high molar mass (M ) 5.94 × 10 5 ) poly(ethylene oxide) (PEO) in aqueous solution has been examined as a function of temperature by dynamic light scattering and fluorescence methods over a broad concentration range. Clusters of small surfactant micelles form within the PEO coil, leading to its extension. The hydrodynamic radius of the complex increases strongly with temperature as well as with the concentrations of surfactant and polymer. At high concentrations of the surfactant, the coil/micellar cluster complex coexists with free C12E5 micelles in the solution. Fluorescence quenching measurements show a moderate micellar growth from 155 to 203 monomers in PEO-free solutions of C12E5 over a wide concentration range (0.02-2.5%) at 8°C. Below 0.25% C12E5, the average aggregation number (N) of the micelles is smaller in the presence of PEO than in its absence. However, N increases with increasing surfactant concentration up to a plateau value of about 270 at about 1.2% (ca. 30 mM) C12E5. At high surfactant concentrations, N is larger in the presence of polymer than in its absence, a finding which is connected to a significant lowering of the clouding temperature due to the PEO at these compositions. Similar results of increasing aggregation number followed by a plateau were also found at a fixed concentration of surfactant (2.5%) and varied PEO.

The changes in size and microstructure of alkyltrimethylammonium halide (CnTAX) micelles as a function of alkyl chain length (even number of carbons between 12 and 18) and NaCl concentration (0-2 M) in aqueous mixtures with and without trihydroxy or dihydroxy bile salts, sodium cholate or sodium desoxycholate (NaC or NaDOC), have been investigated by cryo-TEM, time-resolved fluorescence quenching, and relative viscosity measurements. Without additions, dilute solutions (25 mM) of all the surfactants form globular micelles, with aggregation numbers increasing with the chain length (to 230 ( 3 in C 18TAB at 40 °C). Addition of NaCl results in a growth of the micelles. For C16TA + a transition to long threadlike micelles occurred in 2 M NaCl, whereas the micelles remain globular at shorter chain lengths. For C18TA + a mixture of globular micelles and large structures was observed at 0.5 and 1. 0M NaCl. The pseudoternary CnTAB/NaC/NaCl systems showed results dependent on the alkyl chain length. On addition of NaC to C12TA + and C14TA + , in the presence of salt, a monotonous decrease in aggregation numbers with increasing concentration of NaC is found. For the longer alkyl chains, a micellar growth resulting in a transition to threadlike cylindrical micelles occurs first, giving a peak in the viscosity. The transition is most pronounced in the C 18TA + case. Further addition of NaC give smaller micelles. The various stages in the transition are seen from cryo-TEM results. In the pseudoternary C16TAB/NaDOC/NaCl system, the viscosity was orders of magnitude higher than in the corresponding compositions of the NaC system. The structure of the cylindrical micelles depend on the composition and mole fraction of NaDOC. The change in size and structure on a molecular level is discussed in terms of the structure of bile salt anions and its preferred modes of orientation in the mixed micelle under different conditions.

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Surface tension measurements have been performed on solutions containing cetyltrimethylammonium bromide and a bile salt, either sodium cholate or desoxycholate, in 50 mM NaCl. For the individual ionic surfactants the interfacial area per surfactant molecule (A s) and critical micelle concentration (cmc) were determined. For bile salt solutions a break in surface tension before the cmc is explained as an indication of a change in packing of the anions at the interface from a flat (A s > 150 Å2) to an upright orientation (A s ≈ 41−45 Å2) and is compared with results from the monolayer studies of Ekwall and Small. For systems containing binary mixtures of the oppositely charged surfactants, the experimentally determined mixed cmc (C*) was used to estimate the mixed micelle composition and the molecular interaction parameter, β, using the treatment of Rubingh for nonideal mixtures. The mixtures showed significant deviation from ideal mixing, giving an average β = −4 and −2.7 for the CTAB−NaC and CTAB−NaDOC systems, respectively. For both bile salts, the mixed cmc has a minimum when the bile salt fraction in the mixed micelles is close to 0.3, suggesting that a particularly favorable packing of the micelles is obtained in this composition range. However, using the average value of the interaction parameter in the theory of Rubingh, the cmc values of the systems are predicted within the precision of the experimental findings.

We have used surface tension measurements, differential scanning calorimetry (DSC), dynamic light scattering (DLS), and cryo-transmission electron microscopy (cryo-TEM) to investigate the dynamic and structural behavior of octadecyltrimethylammonium bromide (C18TAB) micelles in water and NaBr solution. The surface tension data for fixed C18TAB concentrations of 25 mM and varied NaBr additions (0−50 mM) shows that the critical micelle concentration (cmc) increases after an initial decrease at 0.5 mM NaBr. This unusual effect has been explained using results from DSC and DLS. At low salt concentrations (below ca. 25 mM) the relaxation time distribution is bimodal with a dominant fast mode due to spherical micelles. Above ca. 35 mM NaBr disklike structures are favored and the relaxation time distribution is more closely unimodal. The postulated sphere-to-disk transition is supported by cryo-TEM micrographs. A pronounced increase in the micellar effective hydrodynamic radius (R H) is observed as the NaBr concentration is increased above about 35 mM; below 35 mM the R H of the spherical micelles changes little with ionic strength.