Cell surfaces of virtually all taxa are negatively charged. This applies to both the Plasma Membranes (PMs) and the Cell Walls (CWs) or to other PM covering materials. These negative charges create negative surface potentials at the PMs (ψ_PM) and the CWs (ψ_CW), but it is the ψ_PM, rather than the ψ_CW, that plays the principal role in ionic interactions and biotic effects. These surface potentials are also controlled by the ionic composition of the bathing medium. ψ_PM controls the distribution of ions between the PM surface and the medium so that negative potentials increase the surface activity of cations and decrease the surface activity of anions. All cations reduce the negativity of ψ_PM because of ionic screening and binding. The common ions Al³⁺, H⁺, Ca²⁺, and Mg²⁺ are especially effective. These ions, especially the latter three, are known to reduce the uptake and biotic effectiveness of cations and to have the opposite effects upon anions. Toxicologists commonly interpret the interactions between toxic cations (commonly metals) and ameliorative cations (commonly H⁺, Ca²⁺, and Mg²⁺) as competitions for binding sites at a PM-surface ligand. ψ_PM is rarely considered. This biotic ligand model (BLM) is sometimes considered to be an extension of the free ion activity model (FIAM), and for either model the precise chemical composition of the bathing media must be measured or calculated (i.e., speciation must be performed). The thesis of this presentation is that ψ_PM effects are likely to be more important to bioavailability than site-specific competition. Models that do not consider ψ_PM, such as BLM and FIAM, as usually employed, are likely to lead to false conclusions about competition for binding at PM-surface ligands. The electrostatic approach can account for the bioavailability of anions whereas the BLM cannot, and it can account for interactions among cations when site-specific competition does not occur. ψ_PM is measured, approximately, as a ζ potential from electrophoretic mobility or is computed with a Gouy-Chapman-Stern model for which parameter values must be obtained. Although parameter values are available for plants, computation of ψ_PM requires dedicated computer programs. The first equation below presents a simpler method of computation of ψ_PM for plants. One may replace the activity of Na⁺ (a_Na+) with activities for K⁺, NH₄⁺, and most other monovalent cations, other than H⁺; the activity of Ca²⁺ (a_Ca2+) may be replaced with activities for Mg²⁺, Sr²⁺, and some other divalent cations. Anions, and cations at very low activities, may be ignored except for H⁺ and the trivalent cations. Ion activities at the PM surface may be computed with the second equation below. In these equations, a_i refers to the activity (in μM) of ion i in the bulk-phase medium, a_i,PM refers to the activity (in μM) of ion i at the PM surface, Z_i is the charge on ion i, and ψ_PM is in units mV. Ion uptake, intoxication, and the alleviation of intoxication are much better correlated with a_i,PM than with a_i, thus the ratio a_i,PM/a_i is an index of the bioavailability of an ion.

ψ_PM = 55.5 – 266/(3.08a_H+^1/2 + 0.100a_Na+^1/2 + 0.636a_Zn2+^1/2 + 1.00a_Ca2+^1/2 + 3.37a_Cu2+^1/2 + 14.0a_La3+^1/2 + 36.3a_Al3+^1/2 + 0.00323a_H+²)^0.306

a_i,PM = a_i exp(–Z_iFψ_PM/(RT)) = a_i exp(–Z_iψ_PM/25.7)