The effect of nanoparticles on the phase separation of waxy corn starch + locust bean gum or guar gum

Phase separation of mixtures of a gelatinized waxy corn starch (S) and locust bean gum (LBG) has been studied in detail at pH 7 and room temperature in the presence and absence of up to 1 wt.% silica nanoparticles (primary particle size 20 nm diameter). A range of silica particles of varying surface hydrophobicities was used, surface modified so as to have 100% (i.e., unmodified) to 65% of their surface SiOH groups remaining. In the absence of particles the S + LBG system separated via spinodal decomposition. In the presence of particles the phase separation was significantly curtailed. Confocal microscopy showed that the particles had a strong preference for the starch microdomains that formed, rather than the gum phase, whilst there was an increasing tendency for particle aggregation to occur within the starch microdomains and possibly at the W/W interface between the two phases as the particle hydrophobicity and particle concentration was increased. Measurements on starch + guar gum system showed that this behaved similarly to the S + LBG system. Measurements of the bulk rheology of the gum and starch phases in the presence and absence of the particles suggested that the inhibition of the phase separation was not due to changes in the viscoelasticity of the starch or gum microdomains themselves, but flocculation of the particles in the gum phase, possibly via the depletion mechanism. It is suggested that the formation of large aggregates of particles aids their accumulation at the W/W interface and subsequently slows down phase separation.