We describe a method to determine tielines for a mixture with a given
volume V-t and a given mole fraction x(t) at fixed temperature, which
is based on the orientation of the tangent plane to the Helmholtz free
energy surface. The method works equally well for gas-liquid separati
on as for liquid-liquid separation. The method is based on a rotation
in coordinate space followed by a rotation in function space which est
ablishes the correct direction and the end points of the tieline, The
mathematics is described and the general outline of a computer program
to implement the method is given, This is followed by an application
to a special form of the van der Waals equation, which describes the i
nfluence of chain-molecules as proposed by Tompa. The example given sh
ows a three phase separation and the influence of the chain-length the
reupon.