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In most analyses, it may be assumed that the ambient fluid pressure and electric potential in the external environment are zero, thus $p_{\ast}=0$ and $\psi_{\ast}=0$ , where the subscripted asterisk is used to denote environmental conditions. Since the external environment does not include a solid matrix, the fixed charge density there is zero. For example, for a triphasic analysis, $c_{\ast}^{+}=c_{\ast}^{-}\equiv c_{\ast}$ . It follows that the effective fluid pressure in the external environment is $\tilde{p}_{\ast}=-R\theta\Phi_{\ast}\sum\nolimits _{\alpha}c_{\ast}^{\alpha}$ and the effective concentrations are $\tilde{c}_{\ast}^{\alpha}=c_{\ast}^{\alpha}/\hat{\kappa}_{\ast}^{\alpha}\tilde{c}_{\ast}^{\alpha}$ . Therefore, in multiphasic analyses, whenever the external environment contains solutes with concentrations $c_{\ast}^{\alpha}$ , the user must remember to prescribe non-zero boundary conditions for the effective solute concentrations and the effective fluid pressure.