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In most analyses, it may be assumed that the ambient fluid pressure in the external environment is zero, thus $p_{\ast}=0$ , where the subscripted asterisk is used to denote environmental conditions. The ambient solute concentration may be represented by $c_{\ast}$ . It follows that the effective fluid pressure in the external environment is $\tilde{p}_{\ast}=-R\theta\Phi_{\ast}c_{\ast}$ and the effective concentration is $\tilde{c}_{\ast}=c_{\ast}/\tilde{\kappa}_{\ast}$ . Therefore, in biphasic-solute analyses, whenever the external environment contains a solute at a concentration of $c_{\ast}$ , the user must remember to prescribe non-zero boundary conditions for the effective solute concentration and the effective fluid pressure.