Complex Coacervation and Overcharging during Interaction between Hydrophobic Zein and Hydrophilic Laponite in Aqueous Ethanol Solution

In this paper, for the first time, we have reported the formation of complex coacervate during interaction between hydrophobic protein, zein, and hydrophilic nanoclay, Laponite, in a 60% v/v ethanol solution at pH 4. Dynamic light scattering and viscosity measurements revealed the formation of zein-Laponite complexes during the interaction between zein at fixed concentration, C Z = 1 mg/mL, and varying concentrations of Laponite, C L (7.8 × 10-4 - 0.25% w/v). Further investigation of the zein-Laponite complexes using turbidity and zeta potential data showed that these complexes could be demarcated in three different regions: Region I, below the charge neutralization region (C Z = 1 mg/mL, C L ≤ 0.00625% w/v) where soluble complexes was formed during interaction between oppositely charged zein and Laponite; Region II, the charge neutralization region (C Z = 1 mg/mL, 0.00625 < C L ≤ 0.05% w/v) where zein-Laponite complexes form neutral coacervates; and Region III, the interesting overcharged coacervates region (C Z = 1 mg/mL, C L > 0.05% w/v). Investigation of coacervates using a fluorescence imaging technique showed that the size of neutral coacervates in region II was large (mean size = 1223.7 nm) owing to aggregation as compared to the small size of coacervates (mean size = 464.7 nm) in region III owing to repulsion between overcharged coacervates. Differential scanning calorimeter, DSC, revealed the presence of an ample amount of bound water in region III. The presence of bound water was evident from the presence of an additional peak at 107 °C in region III apart from normal enthalpy of evaporation of water from coacervates.