On the Origin of Mesoscale Structures in Aqueous Solutions of Tertiary Butyl Alcohol: The Mystery Resolved

We have performed a detailed experimental study on aqueous solutions of tertiary butyl alcohol which were a subject of long-standing controversies regarding the puzzling presence of virtually infinitely stable large-scale structures in such solutions occurring at length scales exceeding appreciably dimensions of individual molecules, referred to also as mesoscale structures. A combination of static and dynamic light scattering yielding information on solution structure and dynamics and gas chromatography coupled with mass spectrometry yielding information on chemical composition was used. We show that tertiary butyl alcohol clearly exhibiting such structures upon mixing with water does not contain any propylene oxide, which was previously considered as a source of these structures (an impurity expected to be present in all commercial samples of TBA). More importantly, we show that no mesoscale structures are generated upon addition of propylene oxide to aqueous solutions of TBA. The ternary system TBA/water/propylene oxide exhibits homogeneous mixing of the components on mesoscales. We show that the source of the mesoscale structures is a mesophase separation of appreciably more hydrophobic compounds than propylene oxide. These substances are explicitly analytically identified as well as their disappearance upon filtering out the mesoscale structures by nanopore filtration. We clearly show which substances are disappearing upon filtration and which are not. This enables us to estimate with rather high probability the chemical composition of the mesoscale structures. Visualization of large-scale structures via nanoparticle tracking analysis is also presented. Video capturing the mesoscale particles as well as their Brownian motion can be found in the Supporting Information

Self-Assembly Thermodynamics of pH-Responsive Amino-Acid-Based Polymers with a Nonionic Surfactant

The behavior of pH-responsive polymers poly­(<i>N</i>-methacryloyl-l-valine) (P1), poly­(<i>N</i>-methacryloyl-l-phenylalanine) (P2), and poly­(<i>N</i>-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 pH_tr 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