Impact of ethanol addition on the behaviour of xanthan gum in aqueous media

Abstract

The effect of ethanol (EtOH) addition on the behaviour of xanthan gum (XG) chains in water (W) was investigated at 25 °C by using turbidimetry, zeta potential, refractometry and viscometry. The presence of EtOH (nonsolvent) in aqueous solution of XG reduces the thermodynamic quality of solvent and increases the polymer-polymer contacts (i.e., electrostatic, H-bonding or hydrophobic interactions). Thus, the transition from an expanded chain conformation to a more compact one was evidenced by viscometry, zeta potential masurements and refractometry while the aggregation of macromolecules or phase separation were observed by turbidimetry. A phase diagram for XG in solvent/nonsolvent mixtures was provided.For volumetric fractions of ethanol (ϕEtOH) higher than 0.3, zeta potential increases above −30 mV and the aggregation of xanthan chains takes place. Measurements of refractive indices were used to determine the preferential adsorption coefficient (λ1) for different volumetric fractions of ethanol. The parameter λ1 quantifies the polymer chain affinity for one component of the solvent/nonsolvent mixtures.The polyelectrolyte effect of XG chains in solution and the variable polarity as a function of W/EtOH composition influence the overall thermodynamic behaviour of XG assessed through the viscometric parameters determined by using the Wolf approach.