Henry's function

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

$f(\kappa a)=1+\frac{1}{16}(\kappa a{)}^2-\frac{5}{48}(\kappa a{)}^3-\frac{1}{8}(\kappa a{)}^4[\frac{1}{12}(1-\kappa a)-(1-\frac{1}{12}(\kappa a{)}^2)e^{\kappa a}{E}_1(\kappa a)]$

where $a$ is the radius of the particle, $\kappa$ is the Debye length and $E_1(\kappa a)$ is the first order exponential integral.