Presence of a Spatially Varying Electric Field at the Lipid–Water Interface with a Na:K Ratio in Water

We investigated ion-lipid interactions in dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers exposed to mixed K⁺/Na⁺ solutions at varying ratios (K⁺:Na⁺ = 0:100 to 100:0). Tensiometric studies of water, i.e., studies of surface pressure-specific molecular area (π-<i>A</i>) isotherms, revealed that monolayer rigidity decreased with incorporation of ions. The variation of rigidity with ionic ratio is clearly nonlinear. X-ray reflectivity measurements of these monolayers transferred onto Si(001) substrates could be explained by headgroups sitting on a buffer layer on the substrate and hydrocarbon tails in air. The presence of ions condensed the monolayer structure, with the mixtures of ions showing higher average electron densities in the headgroup region compared to single-ion environments, suggesting stronger ion-headgroup coordination. We observed nonlinear changes in both headgroup and tail tilt angles with varying K⁺:Na⁺ ratios, suggesting the spatial organization of ions rather than uniform dispersion. NEXAFS spectroscopy at the O K edge demonstrated that increasing K⁺:Na⁺ ratios led to nonlinear variations in P═O bond energy and progressive weakening of P-O bonds, consistent with the Fajans rule. Notably, at K⁺:Na⁺ = 50:50, a split in the C═O π-bond peak was observed, suggesting two distinct polarization states due to spatially varying electric fields. These findings indicate the presence of a structured interfacial ion layer causing spatially varying electric fields rather than a uniform screened electric field. This understanding of complex ion-membrane interactions under physiologically relevant conditions provides insights into cellular membrane behavior, where such mixed-ion environments are a norm.