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This study represents an attempt to achieve a better understanding of the stomatocyte-echinocyte transition in the shape of red blood cells. We determined experimentally the index of cell shape at various ionic strengths and osmolarities for native and trypsin-treated human erythrocytes. For every given composition of the outer phase, we calculated the ionic strength in the cells and the transmembrane electric potential using a known theoretical model. Next, we described theoretically the electric double layers formed on both sides of the cell membrane, and derived expressions for the tensions of the two membrane leaflets. Taking into account that the cell-shape index depends on the tension difference between the two leaflets, we fitted the experimental data with the constructed physicochemical model. The model, which agrees well with the experiment, indicates that the tension difference between the two leaflets is governed by the different adsorptions of counterions at the two membrane surfaces, rather than by the direct contribution of the electric double layers to the membrane tension. Thus, with the rise of the ionic strength, the counterion adsorption increases stronger at the outer leaflet, whose stretching surface pressure becomes greater, and whose area expands relative to that of the inner leaflet. Hence, there is no contradiction between the bilayer-couple hypothesis and the electric double layer theory, if the latter is upgraded to account for the effect of counterion-adsorption on the membrane tension. The developed quantitative model can be applied to predict the shape index of cells upon a stomatocyte-discocyte-echinocyte transformation at varying composition of the outer medium.

The surface tension isotherms of dodecyl acid diethanole amide (DADA), sodium dodecyl sulfate (SDS) and their mixtures at different molar ratios are measured by the Wilhelmy-plate method. The isotherms are analyzed by the theory of nonideal interactions in binary surfactant mixtures (NIBM) and by the nonideal multicomponent mixed micelle model (NMMM). The molecular interaction parameters (MIP) on the interface, β S , and in the micelles, β M , are determined by the NIBM method. By using the Ingram's modification of this method, we calculate the compositions of the mixed adsorption layers and of the micelles, as functions of DADA and SDS concentrations both below and above the critical micelle concentration (CMC). It is shown that the DADA molecules prevail on the surface and in the micelles (>50%) at concentrations around CMC even when DADA presents a small fraction in the surfactant mixture (5 mol%). This result is explained by the much higher surface activity of DADA in comparison with SDS. At concentrations, which are about 1 order of magnitude above the CMC, the composition of the micelles approaches the composition of the total surfactant mixture. The composition of the surfactant adsorption layer also becomes much closer to the composition of the total surfactant mixture at concentrations well above the CMC. The results for the micelles composition at moderate and high molar fractions of DADA in the surfactant mixture (above 10 mol%), obtained by NMMM and by NIBM methods, agree well with each other. A discrepancy between the results obtained by these two models is found at low DADA fraction in the mixture, and this discrepancy is explained by the presence of traces of dodecanol (DOH) in the used SDS sample.

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