The effects of tetraalkylammonium bromides (TAABs) on the micellization of sodium dodecylsulfate (SDS) are studied using pyrene solubilization and several nuclear magnetic resonance (NMR) techniques. Two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) experiments confirm that tetraalkylammonium (TAA(+)) ions associate with SDS to form mixed micelles. TAA(+) ions attach to the surface of the mixed micelles and become inserted into the hydrophobic core of the mixed micelles. Because TAA(+) ions appear in the hydrophobic interior of the TAA-SDS mixed micelles, the micropolarity inside the mixed micelles sensed by pyrene might not reflect the true hydrophobicity of the micellar core. Using proton chemical shift analysis, the degree of hydration on the surface of the mixed micelles is determined from the chemical shift change of SDS α-CH2 protons. The self-diffusion coefficients of SDS and TAA(+) ions in the TAAB/SDS/D2O solutions are measured by using pulse-field gradient NMR, and the fraction of TAA(+) ions associated with the SDS to form the mixed micelles is calculated from the self-diffusion data. Moreover, secondary micelle formation for SDS and TAA(+) ions is observed on the basis of (1)H chemical shift analysis and the self-diffusion data. The 2D NOESY experiments also reveal unusual tumbling behavior of SDS alkyl protons. For Pr4NBr/SDS and Bu4NBr/SDS solutions, positive and negative nuclear Overhauser effects are simultaneously observed among the SDS alkyl protons.

We report alkyl-poly­(l-threonine)/cyclodextrin (alkyl-PLT/CD) supramolecular hydrogels with different molecular assemblies. Their properties are determined by the interplay between host–guest chemistry and hydrogen-bonding interactions. The gelation process was mainly dictated by the formation of alkyl chain/CD inclusion complex and PLT chain conformation. The dodecyl-PLT20/α-CD hydrogel exhibited laminar packing due to the sheet-to-coil conformational change upon forming inclusion complex. The hexadecyl-PLT20/β-CD hydrogel exhibited ribbon-like assemblies instead, because the peptide adopted mainly sheet conformation. The gel-to-sol transition occurred upon increasing temperature because of the decrease in hydrogen-bonding interactions and partly conformational change.

Pyrene solubulization and NMR spectroscopy were employed to study the competition between two surfactant aggregation processes in aqueous polymer/organic salt/surfactant systems. Two organic salts used in the study are tetrabutylammonium bromide (Bu 4 NBr) and tetrapropylammonium bromide (Pr 4 NBr). The anionic surfactant sodium dodecyl sulfate (SDS) is known to have moderate interactions with poly(N-vinylpyrrolidone) (PVP), which results in the formation of a PVP−SDS aggregation complex. On the other hand, SDS also can associate with tetraalkylammonium bromides (TAABs) to form mixed micelles by a strong electrostatic attraction. The pyrene solubilization experiments reveal that the dominant aggregation in PVP/TAAB/SDS systems is determined by the [SDS]/[TAAB] ratio, and complexation of PVP with SDS does not occur until the [SDS]/[TAAB] ratio is larger than a specific ratio. For example, when [SDS]/[Bu 4 NBr] is less than 1.1, only Bu 4 N + −SDS mixed micelles are formed, and the mixed micelles do not associate with PVP. When [SDS]/ [Bu 4NBr] is higher than 1.1, polymer-induced SDS aggregation is detected along with the mixed micellization between SDS and Bu 4 NBr. More interestingly, the Bu 4 N + ions do incorporate with the PVP-bound SDS aggregate to form a PVP−(Bu 4 N + −SDS) complex. By means of 2D NOESY and PFG NMR experiments, both structural and dynamic aspects of the aggregate species formed in the PVP/TAAB/SDS solution were studied. An interaction model was proposed to manifest the role of the two TAAB salts for PVP−SDS complexation in the PVP/TAAB/SDS systems. ■ INTRODUCTIONCombinations of polymers and surfactants presents peculiar synergistic behaviors and are of importance in many colloidal applications. 1−3 Many fundamental studies have been directed at understanding the basic mechanisms of polymer−surfactant interactions, especially those focusing on the details of polymer−surfactant interactions at the molecular level. 4−20 One of the features of polymer−surfactant complexation is that the surfactant aggregation appears to begin at a definite surfactant concentration, the critical aggregation concentration (cac), which is smaller than the normal critical micelle concentration (cmc) of the surfactant in the absence of the polymer. In most cases, when the surfactant concentration is higher than the cac, a complex aggregate structure is formed in association with the surfactant molecules and the polymer chain. 21,22 Subsequently, surfactant molecules continue to bind onto the polymer chain until a specific surfactant concentration (P sat ) is reached, at which point the polymer chain is saturated with micelle-like surfactant aggregates. The binding ratio of the surfactant to the polymer can thus be calculated from the difference between cac and P sat .In some situations, salts, including both inorganic and organic ones, are purposefully used together with polymer/ surfactant mixtures in technological formulations. 23−25 On the other hand, polymer/surfactant mixtures are usually used in salt environmen...

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