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The structure of water near a synthetic clay surface, Laponite, is probed experimentally by total neutron scattering, in both liquid and arrested states. It is shown that the crystalline structure of the Laponite platelets and their charge distribution determine polarization of the closest water layers, with a local density higher than that of bulk water. Interestingly these layers show long range order in the plane parallel to the Laponite surface, while no indication of strong hydrogen bonding between water and Laponite is found. © 2014 Elsevier B.V. All rights reserved.

Microgel particles composed of Interpenetrated Polymer Network (IPN) of poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAAc) dispersed in water have been investigated through dynamic light scattering. The study of the temperature, concentration and pH dependence of the relaxation time has highlighted the existence of a thermoreversible transition corresponding to the swollen-shrunken volume phase transition. The presence of PAAc introduces an additional pH-sensitivity with respect to the temperature-sensitivity due to PNIPAM and leads to interesting differences in the transition process at acid and neutral pH. © 2014 Elsevier B.V. All rights reserved.

Microgels composed of stimuli responsive polymers have attracted worthwhile interest as model colloids for theorethical and experimental studies and for nanotechnological applications. A deep knowledge of their behaviour is fundamental for the design of new materials. Here we report the current understanding of a dual responsive microgel composed of poly(N-isopropylacrylamide) (PNIPAM), a temperature sensitive polymer, and poly(acrylic acid) (PAAc), a pH sensitive polymer, at different temperatures, PAAc contents, concentrations, solvents and pH. The combination of multiple techniques as Dynamic Light Scattering (DLS), Raman spectroscopy, Small Angle Neutron Scattering (SANS), rheology and electrophoretic measurements allow to investigate the hydrodynamic radius behaviour across the typical Volume Phase Transition (VPT), the involved molecular mechanism and the internal particle structure together with the viscoelastic properties and the role of ionic charge in the aggregation phenomena.

The phase behaviour of soft colloids has attracted great attention due to the large variety of new phenomenologies emerging from their ability to pack at very high volume fractions. Here we report rheological measurements on interpenetrated polymer network microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylic acid (PAAc) at fixed PAAc content as a function of weight concentration. We found three different rheological regimes characteristic of three different states: a Newtonian shear-thinning fluid, an attractive glass characterized by a yield stress, and a jamming state. We discuss the possible molecular mechanisms driving the formation of these states.

Small-Angle Neutron Scattering (SANS) and Dynamic Light Scattering (DLS) are two crucial techniques to probe, respectively, the static and the dynamic structure factor of soft matter systems. SANS provides the time-averaged structure of soft materials from a few nm to hundreds of nm [1, 2] while DLS [3] supplies their microscopic dynamics on time scales of nanoseconds and above and allows one to derive their hydrodynamic size [4]. However, usually SANS and DLS measurements are performed separately, in different laboratories, on different samples and at different times, missing the clear benefit that simultaneous DLS-SANS measurements on soft materials would bring from the synergy between the two techniques. Here we report on a new compact and portable DLS-SANS apparatus that allows one to simultaneously measure both the microscopic dynamics (through DLS) and the static structure factor (through SANS) on the same sample.

The relevance of anisotropic interactions in colloidal systems has recently emerged in the context of the rational design of new soft materials1. Patchy colloids of different shapes, patterns and functionalities2 are considered the new building blocks of a bottom-up approach toward the realization of selfassembled bulk materials with predefined properties3-7. The ability to tune the interaction anisotropy will make it possible to recreate molecular structures at the nano- and microscales (a case with tremendous technological applications), as well as to generate new unconventional phases, both ordered and disordered. Recent theoretical studies8 suggest that the phase diagram of patchy colloids can be significantly altered by limiting the particle coordination number (that is, valence). New concepts such as empty liquids8--liquid states with vanishing density--and equilibrium gels8-10-- arrested networks of bonded particles, which do not require an underlying phase separation to form11--have been formulated. Yet no experimental evidence of these predictions has been provided. Here we report the first observation of empty liquids and equilibrium gels in a complex colloidal clay, and support the experimental findings with numerical simulations.

Clay-polymer compounds have recently attracted increasing attention due to their intriguing physical properties in colloidal science and their rheological non-trivial behaviour in technological applications. Aqueous solutions of Laponite clay spontaneously age from a liquid up to an arrested state of different nature (gel or glass) depending on the colloidal volume fraction and ionic strength. We have investigated, through dynamic light scattering, how the aging dynamics of Laponite dispersions at fixed clay concentration (Cw ¼ 2.0%) is modified by the addition of various amounts of poly(ethylene oxide) (PEO) (CPEO ¼ (0.05 O 0.50) %) at two different molecular weights (Mw ¼ 100 kg mol1 and Mw ¼ 200 kg mol1 ). A surprising and intriguing phenomenon has been observed: the existence of a critical polymer concentration C* PEO that discriminates between two different aging dynamics. With respect to pure Laponite systems the aging will be assisted (faster) or hindered (slower) for PEO concentrations respectively lower (CPEO < C* PEO) or higher (CPEO > C* PEO) than the critical concentration. In this way a control on the aging dynamics of PEO-Laponite systems is obtained. A possible explanation based on the balance of competitive mechanisms related to the progressive saturation of the clay surface by polymers is proposed. This study shows how a real control on the aging speed of the PEO-Laponite system is at hand and renders possible a real control of the complex interparticle interaction potential.

The process of dynamical arrest, leading to formation of different arrested states such as glasses and gels, along with the closely related process of aging, is central for both basic research and technology. Here we report on a study of the time-dependent structural evolution of two aqueous Laponite clay suspensions at different weight concentrations. Neutron diffraction experiments have been performed with the near and intermediate range order diffractometer (NIMROD) that allows studies of the structure of liquids and disordered materials over a continuous length scale ranging from 1 to 300 angstrom, i.e., from the atomistic to the mesoscopic scales. NIMROD is presently a unique diffractometer, bridging the length scales traditionally investigated by small angle neutron scattering or small angle x-ray scattering with that accessible by traditional diffractometers for liquids. Interestingly, we have unveiled a signature of aging of both suspensions in the length scale region of NIMROD. This phenomenon, ascribed to sporadic contacts between Laponite platelets at long times, has been observed with the sample arrested as gel or as repulsive glass. Moreover, water molecules within the layers closest to Laponite platelets surface show orientational and translational order, which maps into the crystalline structure of Laponite.

An unexpected dichotomic long time aging behaviour is observed in a glassy colloidal clay suspension investigated by X-ray photon correlation spectroscopy and dynamic light scattering. In the long time aging regime the intensity autocorrelations are non-exponential, following the Kohlrausch–Williams–Watts functional form with an exponent bQ. We show that for spontaneously aged samples a stretched behaviour (bQ < 1) is always found. Surprisingly a compressed exponent (bQ > 1) appears only when the system is rejuvenated by application of a shear field. In both cases the relaxation times scale as Q 1. These observations shed light on the origin of compressed exponential behaviour and help in classifying previous results in the literature on anomalous dynamics.