The isocratic retention factor data were fit to either the original or re-parameterized NK The gradient retention time data were fit to either the original or re-parameterized NK model for gradient measurements using the Matlab lsqnonlin function found in the Optimization Toolbox. The default options were used except for the following: MaxFunctionEvaluations = 10 6 ; FunctionTolerance = 1 × 10 -12 ; OptimalityTolerance = 1 × 10 -12 ; StepTolerance = 1 × 10 -12 . The selection of the initial guess for the fit procedure is described in section 4.3.