Effect of a third component on the interactions in a binary mixture determined from the fluctuation theory of solutions

The Kirkwood-Buff (KB) theory of solution is applied to a ternary mixture b y deriving explicit expressions for the various Kirkwood-Buff integrals (KB Is) and the corresponding excesses of the number of molecules around centra l ones. However, the ideal solution should be considered non-aggregated, an d the above expressions for the excesses provide non-zero values for such a case. For this reason, in order to obtain information about clustering one must subtract from the traditional excesses those which correspond to a re ference state, thus ensuring that for an ideal mixture the excesses are zer o. The expressions derived for the latter excesses have been applied to the investigation of the N,N-dimethylformamide-methanol-water mixture, to conc lude that: (i) in the vicinity of the water molecules there are excesses of water and N,N-dimethylformamide molecules and a deficit of methanol molecu les; (ii) in the vicinity of the methanol molecules there are excesses of m ethanol and N,N-dimethylformamide molecules and a deficit of water molecule s; (iii) in the vicinity of the N,N-dimethylformamide molecules there are e xcesses of methanol and water molecules and a deficit of N,N-dimethylformam ide molecules: (iiii) the excesses of N,N-dimethylformamide around water an d methanol molecules and those around N,N-dimethylformamide are weakly depe ndent on the concentration of the third component in a large range of conce ntrations of the latter, and these results are compatible with the existenc e of N,N-dimethylformamide-water and N,N-dimethylformamide-alcohol complexe s. (C) 2001 Elsevier Science B.V. All rights reserved.