Self-assembly thermodynamics of pH-responsive amino-acid-based polymers with a nonionic surfactant

The behavior of pH-responsive polymers poly(N-methacryloyl-l-valine) (P1), poly(N-methacryloyl-l-phenylalanine) (P2), and poly(N-methacryloylglycyne-l-leucine) (P3) has been studied in the presence of the nonionic surfactant Brij98. The pure polymers phase-separate in an acidic medium with critical pHtr values of 3.7, 5.5, and 3.4, respectively. The addition of the surfactant prevents phase separation and promotes reorganization of polymer molecules. The nature of the interaction between polymer and surfactant depends on the amino acid structure in the side chain of the polymer. This effect was investigated by dynamic light scattering, isothermal titration calorimetry, electrophoretic measurements, small-angle neutron scattering, and infrared spectroscopy. Thermodynamic analysis revealed an endothermic association reaction in P1/Brij98 mixture, whereas a strong exothermic effect was observed for P2/Brij98 and P3/Brij98. Application of regular solution theory for the analysis of experimental enthalpograms indicated dominant hydrophobic interactions between P1 and Brij98 and specific interactions for the P2/Brij98 system. Electrophoretic and dynamic light scattering measurements support the applicability of the theory to these cases. The specific interactions can be ascribed to hydrogen bonds formed between the carboxylic groups of the polymer and the oligo(ethylene oxide) head groups of the surfactant. Thus, differences in polymer-surfactant interactions between P1 and P2 polymers result in different structures of polymer-surfactant complexes. Specifically, small-angle neutron scattering revealed pearl-necklace complexes and "core-shell" structures for P1/Brij98 and P2/Brij98 systems, respectively. These results may help in the design of new pH-responsive site-specific micellar drug delivery systems or pH-responsive membrane-disrupting agents.

Large-Scale Supramolecular Structure in Solutions of Low Molar Mass Compounds and Mixtures of Liquids: I. Light Scattering Characterization

Static and dynamic laser light scattering were used to bring evidence of large-scale supramolecular structure in solutions of low molar mass electrolytes, nonelectrolytes, and mixtures of liquids. It was shown that solutes are distributed inhomogeneously on large length scales. Regions of higher and lower solute concentration exist in solution and give sufficient scattering contrast for experimental observation. A detailed light scattering study showed that these regions can be characterized as close-to-spherical discrete domains of higher solute density in a less dense rest of solution. These domains do contain solvent inside and can be therefore characterized as loose associates (giant clusters, aggregates). Their size distributions are significantly broad, ranging up to several hundreds of nanometers. Characteristic sizes of these inhomogenities thus exceed angstrom dimensions of individual molecules by several orders of magnitude. The number of solute molecules per domain varies approximately in the range 10(3)-10(8). Phenomena described were observed in a very broad range of solutes and solvents. Among others, selected data on most common substances of great chemical and biological importance such as sodium chloride, citric acid, glucose, urea, acetic acid, and ethanol are presented.

Real-time monitoring of the origination of multimacroion domains in a polyelectrolyte solution

First real-time monitoring of the origination of multimacroion domains in an initially homogeneous polyelectrolyte solution was performed. Domains were generated by pH-induced increase of macroion charge in solution of poly(methacrylic acid). Monitoring was performed by static and dynamic light scatterings, in which scattering contributions from individual polyions and growing multimacroion domains were separated, such that amplitudes of both modes were expressed in absolute units. Kinetic results also yield new information regarding the nature of multimacroion domains.