Henry's function

Henry's function (related to electrophoretic mobility) is given by (Eq. 2 in ):

$f(\kappa a)=1+{\frac {1}{16}}(\kappa a)^{2}-{\frac {5}{48}}(\kappa a)^{3}-{\frac {1}{8}}(\kappa a)^{4}\left[{\frac {1}{12}}(1-\kappa a)-\left(1-{\frac {1}{12}}(\kappa a)^{2}\right)e^{\kappa a}E_{1}(\kappa a)\right]$

where $a$ is the radius of the particle, $\kappa$ is the Debye length and $E_{1}(\kappa a)$ is the first order exponential integral.