Lower Consolute Solution Temperature Research Articles

Dichotomous behaviors of stress and dielectric relaxations in dense suspensions of swollen thermoreversible hydrogel microparticles.

While the mechanical disruption of microscopic structures in complex fluids by large shear flows has been studied extensively, the effects of applied strains on the dielectric properties of macromolecular aggregates have received far less attention. Simultaneous rheology and dielectric experiments can be employed to study the dynamics of sheared colloidal suspensions over spatiotemporal scales spanning several decades. Using a precision impedance analyzer, we study the dielectric behavior of strongly sheared aqueous suspensions of thermoreversible hydrogel poly(N-isopropylacrylamide) (PNIPAM) particles at different temperatures. We also perform stress relaxation experiments to uncover the influence of large deformations on the bulk mechanical moduli of these suspensions. All the sheared PNIPAM suspensions exhibit distinct dielectric relaxation processes in the low and high frequency regimes. At a temperature below the lower consolute solution temperature (LCST), the complex permittivities of highly dense PNIPAM suspensions decrease with increase in applied oscillatory strain amplitudes. Simultaneously, we note a counter-intuitive slowdown of the dielectric relaxation dynamics. Contrary to our rheo-dielectric findings, our bulk rheology experiments, performed under identical conditions, reveal shear-thinning dynamics with increasing strain amplitudes. We propose the shear-induced rupture of fragile clusters of swollen PNIPAM particles to explain our observations. Our work illustrates that rheo-dielectric studies have enormous potential for providing deep insights into the length scale-dependent dynamical properties of complex systems such as dense suspensions and soft glasses.

Elaborating the phase behaviour of ethylene oxide oligomers and analogues in 2H, 3H-perfluoropentane

Pressure metered dose inhalers (pMDIs) are seen as an attractive option for the delivery of active molecules to the bloodstream via the lungs. Formulation issues are important as it is necessary to disperse the active substance into a volatile propellant. The phase behaviour of a range of common formulation agents – excipients – in 2H, 3H-perfluoropentane is reported here, focussing on the effects of simple elaboration of a central ethylene-oxide (EO) moiety. The base series of hydroxyl terminated EO fragments exhibited lower consolute solution temperature (LCST) behaviour that was strongly molecular weight-dependent. Substitution of methyl groups in place of the hydroxyl groups at the terminus of the EO fragment had a pronounced effect on the solubility of the oligomeric or polymeric material, the material becoming fully miscible with the HPFP. Substitution of methyl groups along the backbone of the hydroxyl terminated EO greatly enhanced the solubility of the materials, but the exhibited phase behaviour was still that of LCST. Longer alkyl groups at the terminus of the EO were found to promote the solubility compared to the hydroxyl terminated material, but to a relatively smaller degree compared with the methyl group. It is hoped that these simple “design rules” should facilitate the development of designer excipients tailored for a particular application.

Fits of Scaling and UNIQUAC Equations to Liquid−Liquid Equilibrium (LLE) Phase Compositions of the Binary Amphiphile 2-Butoxyethanol (1) + Water (2) Measured by the Phase Volume Method

The liquid−liquid coexistence curve of the binary nonionic amphiphile 2-butoxyethanol (1) + water (2) was determined around the lower consolute solution temperature, Tc, by phase volume measurements. The phase volume method uses the linear relationship between the phase volume and the component concentration according to the lever rule. The method is much more convenient to use for low-pressure phase studies than other methods. Moreover, the results show that this method yields equilibrium compositions as accurate and precise as other methods. The coexistence curve of the binary system is fitted to (critical-)scaling equations. The scaling equations have a valid region that is limited near the critical point. This region is extended from ΔT = |T − Tc| = 8 K to ΔT = 20 K by defining a new order parameter and incorporating this parameter into the equations. These new equations yield excellent fits to the liquid−liquid equilibrium data of the binary system. When compared to UNIQUAC, the scaling equations with a new order parameter are slightly better than UNIQUAC. In addition, they give correct information on the critical points, which are usually predicted erroneously by UNIQUAC.

Liquid-Liquid Phase Equilibrium in Binary Mixtures of the Nonionic Amphiphile CH3(CH2)5(OCH2CH2)2OH and Water

The coexistence curves of binary mixtures of the nonionic amphiphile CH3(CH2)5(OCH2CH2)2OH with water or with aqueous 10 mM NaCl electrolyte were determined around the lower consolute solution temperature, Tc, by phase volume measurements. From these data Tc = 284 K and the critical composition (weight fraction of the amphiphile = 0.130) were determined by critical-scaling equations.

Contact angles of water-2-butoxyethanol system against glass: Exploration of the phase diagram

We have measured the contact angles of water/2-butoxyethanol binary liquid mixture against glass and vapor in the whole phase diagramm. The water/2-butoxyethanol system is characterized by a Lower Consolute Solution Temperature (T c = 48.8°c). At 25°c, far from the critical temperature, we observe a wetting isotherm, that can be considered as standard , starting from litterature results for other binary systems, including surface active agents. Starting from a equation relating the contact angle variation to liquid-vapor surface tension fluctuations, it is possible to calculate the contact angle values. The agreement with our experimental values is quite good in the case of our binary system. Near to the critical point, we have measured the contact angle of the critical interface against glass. This value is also obtained by surface tension and contact angles measurements at the triple line water/vapor/glass. All these results are in agreement, showing a contact angle of 90° at the critical point. This result is in agreement with the experimental litterature.

Characterization of Monodisperse Aqueous Latex Dispersions Prepared with N-Isopropylacrylamide

AbstractPoly-N-isopropylacrylamide (poly-NIPAM) is a water soluble nonionic polymer that exhibits a lower consolute solution temperature (LCST) of about 31°C. This paper describes the preparation and characterization of a monodisperse, hydrophilic polymer latex of poly-NIPAM, crosslinked by N,N'-methylenebisacrylamide. The presence of these crosslinking moieties will prevent dissolution of the particles at low temperatures; rather the particles will be swollen with solvent such that a microgel is formed. The swelling and rheological behaviour of the latex has been studied. The results show that at high temperatures the particles are ˜ 30 nm whilst at 25°C, the particles were ˜ 130 nm in diameter. This four fold increase in the particle diameter correspond to a ˜ sixty fold increase in volume. The rheological properties, both continuous and oscillatory shear, of these systems have also been studied and is strongly temperature dependant, reflecting the changes in phase volume of the NIPAM latex.

An experimental test of catastrophe and critical-scaling theories of emulsion inversion

Electrical conductivities of the conjugate phases and emulsions of the binary system 2-butoxyethanol/water were measured over the temperature range T-T{sub c} = 4-21 K (T{sub c} is the lower consolute solution temperature). From these data, T{sub c}, the emulsion morphologies, and the emulsion inversion and its hysteresis were determined. Equations from critical-scaling theory were fit to the phase conductivities, and the emulsion data were used to test catastrophe and critical-scaling theory equations for emulsion inversion.

An ellipsometric study of the liquid-vapor interface of a binary system near its lower consolute solution temperature: water-2-butoxyethanol

An ellipsometric study of the liquid-vapor interface of a binary system near its lower consolute solution temperature: water-2-butoxyethanol