Originally Posted by

**vlai9180** Sorry, yes, it would be

$\displaystyle \sum\limits_{i=1}^2{\sum\limits_{j=1}^2{y_{i}y_{j} a_{ij}}}$

I don't know where you would get a matrix from, though, or how to fill it.

It's sort of weird to explain - my thermodynamics book explains van der waals parameters for a 2-component mixture as $\displaystyle a_{mix} = \sum\limits_{i=1}^2{\sum\limits_{j=1}^2{y_{i}y_{j} a_{ij}}}$, and then suddenly somehow comes up with

$\displaystyle a_{1}y_{1}^{2} + a_{12}a_{21}y_{1}y_{2} + a_{2}y_{2}^{2}$

without any real explanation.

Does it make any sense to you?