Composition dependent non-ideality in aqueous binary mixtures as a signature of avoided spinodal decomposition
Sarmistha Sarkar Saikat Banerjee Susmita Roy Rikhia Ghosh Partha Pratim Ray Biman Bagchi
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We explore the potential energy landscape of structure breaking binary mixtures (SBBM) where two constituents dislike each other, yet remain macroscopically homogeneous at intermediate to high temperatures. Interestingly, we find that the origin of strong composition dependent non-ideal behaviour lies in its phase separated inherent structure. The inherent structure (IS) of SBBM exhibits bi-continuous phase as is usually formed during spinodal decomposition.We draw analogy of this correlation between non-ideality and phase separation in IS to explain observation of non-ideality in real aqueous mixtures of small amphiphilic solutes, containing both hydrophilic and hydrophobic groups. Although we have not been able to obtain IS of these liquids, we find that even at room temperature these liquids sustain formation of fluctuating, transient bicontinuous phase, with limited lifetime ($\tau \lesssim$ 20 ps). While in the model (A, B) binary mixture, the non-ideal composition dependence can be considered as a fluctuation from a phase separated state, a similar scenario is expected to be responsible for the unusually strong non-ideality in these aqueous binary mixtures.
Sarmistha Sarkar1 2 Saikat Banerjee1 Susmita Roy1 Rikhia Ghosh1 Partha Pratim Ray2 Biman Bagchi1
Volume 135, 2023
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