The Freundlich Isotherm

The heat of adsorption is many instances decreases in magnitude with increasing extent of adsorption. This decline in heat of adsorption is logarithmic, implying that adsorption sites are distributed exponentially with respect to adsorption energy. This isotherm does not indicate an adsorption limit when coverage is sufficient to fill a monolayer ( theta = 1). The equation that describes such isotherm is the Freundlich Isotherm, given as:

where n > 1.

k is a temperature-dependent constant.

With n = 1, the equation reduces to the linear form: q = k p

On linearization, the equation becomes:

The plot of log (q) vs. log (p) gives a straight line with slope of (1/n) and y-intercept of log (k).