Surface tension and refractive index of guar and tragacanth gums aqueous dispersions at different polymer concentrations, polymer ratios and temperatures

Surface tension and refractive index of guar gum (GG) (up to 0.97% w/w) and tragacanth gum (TG) (up to 1.41% w/w) aqueous dispersions were determined for systems with only one and both gums (mixtures) (up to 1.25% w/w). Experimental determinations were carried out from 5 to 30 °C for surface tension and from 10 °C to 40 °C for refractive index. For mixtures, the effect of polymer ratio (1:3, 1:1 and 3:1) was analyzed at different total gum concentration and temperature. GG showed lower surface activity and refractive index than TG at the same experimental condition. Models for the estimation of surface tension and refractive index as function of gum concentration and temperature were proposed and satisfactorily tested. Both physical and optical properties, in the tested gum concentration range, allowed the determination of different behaviour for both gums because of a critical aggregation concentration for TG dispersions while this phenomenon was not observed for GG.

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► Tragacanth (TG) shows higher refractive index (IR) than guar (GG).
► Surface tension (ST) of TG dispersions are lower than GG.
► ST and RI allow the determination of a critical aggregation concentration for TG.
► ST and RI modelling with gum concentration and temperature is proposed.