logarithmic normal distribution

of a macromolecular assembly
A continuous distribution with the differential mass-distribution function of the form:
f w x d x = 1 a π x exp ( − 1 a 2 ln 2 x b ) d x
where x is a parameter characterizing the chain length, such as relative molecular mass or degree of polymerization and a and b are positive adjustable parameters.
Source:
Purple Book, p. 57