An appropriate response surface model for mixture responses is the Scheffé polynomial (Scheffé 1958). See the discussion of Cox Mixtures and the Scheffé Cubic macro in Cox Mixtures in the Fitting Linear Models book. The Scheffé polynomial model does the following:

For a mixture model with third-degree polynomial terms, the Scheffé Cubic model can be used. The Scheffé Cubic model includes terms of the form X1*X2*(X1-X2).