Soil adsorption behavior is crucial for hydrogeological and geotechnical engineering applications. However, existing experimental and theoretical methods fail to accurately quantify adsorbed water film thickness ( \({x}_{c}\) ) under varying ion conditions. To address this, a theoretical model was developed to quantify \({x}_{c}\) by combining ion equilibrium conditions and ion exchange work based on electric double-layer theory, incorporating ion concentration ratios and their equilibrium states. The model also derives soil pore distribution characteristics and enables adsorbed water content determination. The accuracy of the theoretical adsorbed water content was validated through nuclear magnetic resonance (NMR) tests on soils with varying salinities, demonstrating excellent predictive capability ( \({R}^{2}\approx0.96\) ) and confirming the model’s reliability in analyzing soil adsorption behavior. The relationship between charge density and zero potential point can simplify calculation process for adsorbed water films. Solutes significantly influence \({x}_{c}\) . Ionic concentration ( \({n}_{0}\) ) exhibits diminishing effects on \({x}_{c}\) exceeding 65 \(\text{m}\text{o}\text{l}/{\text{m}}^{3}\) , while a low ionic valence (z) enhances \({x}_{c}\) by 1.71–3.14 times under similar conditions. Higher \({n}_{0}\) and z restrict the charge adsorption on soil particle surface. This work provides a precise method for saline soil adsorption analysis, offering theoretical support for industrial, agricultural, and ecological applications in saline soil regions.
