The enthalpically favoured hydration of hydrophobic entities, termed hydrophobic hydration, impacts the phase behaviour of numerous amphiphiles in water. Here, we show experimental evidence that hydrophobic hydration is strongly determined by the mean energetics of the aqueous medium. We investigate the aggregation and collapse of an amphiphilic polymer, poly-N-isopropyl acrylamide (PNiPAM), in aqueous solutions containing small amounts of alcohol and find that the thermodynamic characteristics defining the phase transitions of PNiPAM evolve relative to the solvent composition at which the excess mixing enthalpy of the water/alcohol mixtures becomes minimal. Such correlation between solvent energetics and solution thermodynamics extends to other mixtures containing neutral organic solutes that are considered as kosmotropes to induce a strengthening of the hydrogen bonded water network. This denotes the energetics of water as a key parameter controlling the phase behaviour of PNiPAM and identifies the excess mixing enthalpy of water/kosmotrope mixtures as a gauge of the kosmotropic effect on hydrophobic assemblies.

We investigate the co-nonsolvency of poly-N-isopropyl acrylamide (PNiPAM) in different water-alcohol mixtures and show that this phenomenon is due to two distinct solvation contributions governing the phase behavior of PNiPAM in the water-rich and alcohol-rich regime respectively. While hydrophobic hydration is the predominant contribution governing the phase behavior of PNiPAM in the water-rich regime, the mixing contributions governing the phase behavior of classical polymer solutions determine the phase behavior of PNiPAM in the alcohol-rich regime. This is evidenced by distinct scaling relations denoting the energetic state of the aqueous medium as a key parameter for the phase behavior of PNiPAM in the water-rich regime, while the volume fractions of respectively water, alcohol and PNiPAM become relevant parameters in the alcohol-rich regime. Adding alcohol to water decreases the energetics of the aqueous medium, which gradually suppresses hydrophobic hydration, while adding water to alcohol decreases the solvent quality. Consequently, PNiPAM is insoluble in the intermediate range of solvent composition, where neither hydrophobic hydration nor the mixing contributions prevail. This accounts for the co-nonsolvency phenomenon observed for PNiPAM in water-alcohol mixtures.

Self-assembly of core-shell nanoparticles (NPs) at liquid-liquid interfaces is rapidly emerging as a strategy for the production of novel nano-materials bearing vast potential for applications, including membrane fabrication, drug delivery and emulsion stabilization. The development of such nanoparticle-based materials is facilitated by structural characterization techniques that are able to monitor in situ the selfassembly process during its evolution. Here, we present an in situ high-energy X-ray reflectivity study of the evolution of the vertical position (contact angle) and inter-particle spacing of core-shell iron oxidepoly(ethylene glycol) (PEG) nanoparticles adsorbing at flat, horizontal buried water-n-decane interfaces. The results are compared with time-resolved interfacial tension data acquired with the conventional pendant drop method. We investigate in particular the effect of varying polymer molecular weights (2-5 kDa) and architectures (linear vs. dendritic) on the self-assembly process and the final structure of the interfacially adsorbed NP monolayers. Linear PEG particles adsorb more rapidly than dendritic PEG ones and reach full interface coverage and stable NP monolayer structure, while dendritic PEG particles undergo a slower adsorption process, which is not completed within the experimental time window of $6 hours. All NPs are highly hydrophilic with effective contact angles that depend weakly on PEG molecular weight and architecture. Conversely, the in-plane NP separation depends strongly on PEG molecular weight. The measured inter-particle separation at full interface coverage yields low iron oxide core content, indicating a strong deformation and flattening of the linear PEG shell at the interface. This finding is supported by modeling and has direct implications for materials fabrication, e.g. for the realization of core-shell NP membranes by in situ cross-linking of the polymer shells.

Water-dispersed silica nanoparticles (NPs) do not adsorb to the interface between immiscible bulks of water and hexane. Adding, however, a surfactant (cetyltrimethylammonium bromide, CTAB) to water induces the formation of a NP monolayer (ML) at this model liquid-liquid interface. We determined the ML's structure in situ at deeply buried planar water-hexane interfaces with sub-nanometer resolution by high energy X-ray reflectivity. Detailed modeling of the data yields the NPs' interfacial concentration and water immersion depth, allowing calculation of the NP average contact angle Q. At a CTAB concentration f c ¼ 0.75 mM, comparable to the critical micelle concentration, a dilute NP monolayer is found with Q $ 128 . At lower surfactant concentration (f c ¼ 0.05 mM) we find a dense ML of close-packed NPs with an unexpectedly high Q $ 146 . The structure and adsorption scenarios of the NPs at the interface are discussed, highlighting the relevance of the present method for quantitative studies of a broad range of systems and applications.

We explore the dynamical and mechanical characteristics of an evolving gel in diffusing wave spectroscopy (DWS) and rheometry, aiming to assess how the gel evolution impacts the creep response of the system. Our gel is formed by inducing the aggregation of thermosensitive colloids by a variation in temperature. We find experimental evidence that the long time evolution of this gel is due to two distinct processes: A coarsening process that involves the incorporation of mobile particles into the network structure and an aging process that triggers intermittent rearrangement events. While coarsening is the main process governing the evolution of the elastic properties of the gel, aging is the process determining structural relaxation. The combination of both processes in addition to stress hardening governs the creep behavior of the gel, a creep behavior that is determined by three distinct contributions: an instantaneous elastic, a delayed elastic, and a loss contribution. The systematic investigation of these contributions in recovery experiments provides evidence that losses and delayed elastic storage have a common origin, both being due to intermittent local structural relaxation events. V C 2017 The Society of Rheology.