Effects of temperature on the association behavior in aqueous solutions of a series of charged thermoresponsive poly(N-isopropylacrylamide)-block-poly((3-acrylamidopropyl) trimethylammonium chloride) (abbreviated as PNIPAAM(n)-b-PAMPTMA(+)(20)) with different lengths of the PNIPAAM block (n = 24, 48, and 65) have been studied with the aid of turbidimetry, zeta sizer, and dynamic light scattering (DLS). The turbidity results show that the transition to high turbidity values is shifted to lower temperatures when the length of the PNIPAAM block increases. It was observed that the value of the cloud point (CP) dropped with increasing polymer concentration, enlarged length of the PNIPAAM block, and augmented salinity. It was found that the decay of the correlation function from DLS is bimodal at temperatures well below CP, where the fast mode represents the motion of the unimers and the slow mode the dynamics of micelles/intermicellar complexes. At higher temperatures, larger particles of the system grow at the expense of the smaller ones in the spirit of Ostwald ripening, and clusters with a narrow size distribution evolve at high temperatures. By adding salt (NaCl), enhanced aggregation occurs at elevated temperatures because of screening of Coulomb repulsions.

Nanoparticles with an internal structure have been prepared by dispersing under dilute conditions poly(acrylic acid) with a polymerization degree n = 6000 (PAA6000) together with a cationic surfactant hexadecyltrimethylammonium hydroxide (C16TAOH) and the non-ionic surfactant penta(ethylene glycol) monododecyl ether (C12E5) in water. The nanoparticles are formed at different mixing ratios in the corresponding two-phase regions (liquid crystalline phase/dilute isotropic phase) of the C16TAPA6000 complex salt/C12E5/water ternary phase diagram. The particles consist of polyacrylate PA6000– polyions, C16TA+ surfactant ions, and C12E5. Their internal ordering was identified by small-angle X-ray scattering (SAXS) to be either bicontinuous cubic with the Ia3d crystallographic space group or normal hexagonal depending upon the amount of C12E5. The bicontinuous cubic phase, to our knowledge never observed before in polyelectrolyte–surfactant particle systems, was inferred by SAXS experiments. The data also showed that this structure is thermoresponsive in a reversible manner. The bicontinuous cubic space group transforms from Ia3d to Im3m as the temperature decreases from 25 to 15 °C. According to dynamic light scattering and electrophoretic mobility measurements, the particles have a well-defined size (apparent hydrodynamic radii RH in the range of 88–140 nm) and carry a positive net charge. The size of the nanoparticles is stable up to 1 month. The faceted nanoparticles are visualized by cryogenic transmission electron microscopy that also reveals their coexistence with thread-like C12E5 micelles.

A comprehensive experimental study on the interaction between the PEO-PPO-PEO block copolymer P123 (EO20PO68EO20) and the anionic bile salt sodium glycodeoxycholate (NaGDC) in water has been performed. The work was aimed at investigating the suitability of using P123 as bile salt sequestrant beside the fundamental aspects of PEO-PPO-PEO block copolymer-bile salt interactions. Various experimental techniques including dynamic and static light scattering, small-angle X-ray scattering, and differential scanning calorimetry (DSC) were employed in combination with electrophoretic mobility measurements. The system was investigated at a constant P123 concentration of 1.74 mM and with varying bile salt concentrations up to approximately 250 mM NaGDC (or a molar ratio n(NaGDC)/n(P123) = 144). In the mixed P123-NaGDC solutions, the endothermic process related to the self-assembly of P123 was observed to gradually decrease in enthalpy and shift to higher temperatures upon progressive addition of NaGDC. To explain this effect, the formation of NaGDC micelles carrying partly dehydrated P123 unimers was proposed and translated into a stoichiometric model, which was able to fit the experimental DSC data. In the mixtures at low molar ratios, NaGDC monomers associated with the P123 micelle forming a charged "P123 micelle-NaGDC" complex with a dehydrated PPO core. These complexes disintegrated upon increasing NaGDC concentration to form small "NaGDC-P123" complexes visualized as bile salt micelles including one or a few P123 copolymer chains.

Self-assembly of peptides and bile acids has been widely investigated because of their biological role and their potential as a tool for the preparation of nanostructured biomaterials. We herein report both the synthesis and the self-association behavior of a compound that combines the aggregation properties of bile acid- and amino acid-based molecules. The derivative has been prepared by introducing a L-tryptophan residue into the C-3 position of the deoxycholic acid skeleton and resulted in an amphoteric fluorescent labeled bile acid that shows a pH-dependent self-assembly. Under alkaline conditions it assembles into 28 nm diameter tubules, thus showing a completely different behavior compared to the precursor bile acid, which forms micelles under similar conditions. Upon heating the tubules break and turn into micelles, leading to an increase in the exposure to water of the tryptophan residue. On the other hand, in acidic solutions it aggregates into elongated micelles that further self-assemble forming a gel network, when an electrolyte is added.

Mixed micelle formation between two oppositely charged diblock copolymers that have a common thermosensitive nonionic block of poly(N-isopropylacrylamide) (PNIPAAM) has been studied. The block copolymer mixed solutions were investigated under equimolar charge conditions as a function of both temperature and total polymer concentrations by turbidimetry, differential scanning calorimetry, two-dimensional proton nuclear magnetic nuclear Overhauser effect spectroscopy (2D 1 H NMR NOESY), dynamic light scattering, and small angle X-ray scattering measurements. Well-defined and electroneutral cylindrical micelles were formed with a radius and a length of about 3 nm and 35 nm, respectively. In the micelles, the charged blocks built up a core, which was surrounded by a corona of PNIPAAM chains. The 2D 1 H NMR NOESY experiments showed that a minor block mixing occurred between the core blocks and the PNIPAAM blocks. By approaching the lower critical solution temperature of PNIPAAM, the PNIPAAM chains collapsed, which induced aggregation of the micelles. a Transition enthalpy given kJ (mol polymer) 21 , values in parenthesis in kJ (mol NIPAAM unit) 21 . b PNIPAAM 41 -b-PAMPTMA(1) 24 . c PNIPAAM 17 -b-PAMPS(-) 11 .