The initial freezing point Tf of a solution is directly
related to the water activity aw as stated in the Clausius–
Clapeyron relation:
lnðawÞ ¼ ð1=Tf 1=Tf 0Þk=R ð1Þ
Here R is the gas constant, k is the latent heat of fusion,
and Tf 0 is the freezing point of pure water. For ideal
solutions the water activity follows from Raoults law.
As stated in the introduction one has to account for the
effect of bound water, and hence the water activity is
equal to the mole fraction of free (unbounded) water xwf
(Schwartzberg, 1976):
aw ¼ xwf ¼ ðyw ybÞ=½ðyw ybÞ þ MwRsgsys=Ms ð2Þ
Here yw is the mass fraction of water, yb is the mass
fraction of bound water, and Mw is the molar weight of
water. The index ‘s’ indicates solutes, ys is the solutes
mass fraction, Ms is the (effective) molar weight of the
solute, and gs is the dissociation number of salts.
Following previous studies on initial freezing point
predictions (Miles et al., 1983; Murikami & Okos, 1996;
Pham, 1987), we assume that the amount of bound
water is linear dependent on the mass fractions of soluble
biopolymers: