Rheology of Concentrated Isotropic and Anisotropic Xanthan Solutions. 1. A Rodlike Low Molecular Weight Sample

Abstract

The rheology of a rodlike xanthan sample (Fraction X13F3, Mw ) 1.54 × 10 5 g/mol) has been studied in aqueous NaCl (0.01, 0.1, 1.0 M) in oscillatory and steady shear experiments. Polymer concentrations W (in weight percent) spanning the isotropic, biphasic, and fully anisotropic regimes of the lyotropic liquid crystalline phase diagram have been investigated. The rheological parameters increase as power law functions of W in the isotropic regime, decrease with higher power law exponents in the biphasic regime, and change very slowly in the fully anisotropic regime. The peak in plots of steady shear viscosity η against W shifts toward lower W with increasing shear rate γ , because of the shearinduced orientation. In contrast, peaks in the plots of the complex viscosity η*, storage modulus G′, and loss modulus G′′ are independent of frequency ω, because shear-induced orientation is minimal under conditions of small strain. Power law curves fit to η*, G′, and G′′ in the isotropic, biphasic, and anisotropic regimes cross at W i and at Wa, the boundaries, respectively, between the isotropic and biphasic and between the biphasic and anisotropic regions as determined by phase separation measurements. No crossover of G′ and G′′ is observed for X13F3 at any salt concentration studied, and the loss tangent tan δ ) (G′′/G′) > 1 under all the conditions investigated. The parameters η* and G′′ conform to tight master curves when the appropriate reduced variables are plotted, but η and G′ do not. The Cox-Merz rule is obeyed at all three salt conditions for all polymer concentrations studied in the Newtonian plateau region of the reduced flow curves. The salt concentration plays an important role in determining W i and Wa for this ionic, rodlike polysaccharide, but it does not affect the Newtonian plateau viscosities in the isotropic regime. In contrast, the viscosities of anisotropic solutions at different salt concentrations increase as the salt concentration increases, because the order parameter in anisotropic solutions is greater at low salt concentration than it is at high salt concentration.

Formation of fibrillar whey protein aggregates: Influence of heat and shear treatment, and resulting rheology

“…9 were all between 0.71 and 0.74. These values are comparable to the reported power law indices of xanthan solutions around the transition from isotropic to biphasic regime (Lee & Brant, 2002a). This Shear rate (s -1 )…”

Newtonian Viscosity of Dilute, Semidilute, and Concentrated Polymer Solutions

“…With further increase in polymer concentration, the volume fraction of liquid-crystal phase increases until it becomes, first, the continuous phase and then completely liquid crystalline, the viscosity continues to decrease, reaches a minimum value, and then begins to increase again as the liquid-crystal phase becomes more concentrated in polymer molecules. In addition to PnHIC solutions, similar behavior has been documented for concentrated solutions of xanthan, a rigid ionic double-helical polysaccharide in aqueous NaCl [Lee and Brant, 2002]. the concentration-dependent viscosities of several stiff-chain polymer species, each of which have comparable contour lengths, L ∼ 200 nm, but which differ in their persistence lengths, p .…”

Effect of the addition of hydrocolloids and aloe vera gel (Aloe barbadensis Miller) on the rheological properties of a beverage of sweet corn (Zea mays var. saccharata)

“…2a). This is because the gum has a rigid isotropic structure of elongated rods with a random orientation relative, and its structure changes when the concentration is increased in the dispersion, resulting in formation of molecular interactions with high-molecularweight aggregates and causing an increase in apparent viscosity [31,[38][39][40]. Moreover, treatments 8, 11, 16, and 18, showed a higher coefficient of consistency at 10°C, which promotes suspension stability.…”