The interface –log[H+] defined as pH′ of a mimic inner mitochondrial membrane is ∼3.9 at bulk pH ∼ 6.8, which affects cytochrome c activity.

It is possible that a defined curvature at the membrane interface controls its pH/polarity to exhibit specific bioactivity. By utilizing an interface-interacting spiro-rhodamine pH probe and the Schiff base polarity probe, we have shown that the pH deviation from the bulk phase to the interface (ΔpH)/interfacial dielectric constant (κ(i)) for amphiphilic self-assemblies can be regulated by the curvature geometry (positive/negative) and its radius. According to H NMR and fluorescence anisotropy investigations, the probes selectively interact with an anionic interfacial Stern layer. The ΔpH/κ(i) values for the Stern layer are estimated by UV-vis absorption and fluorescence studies. For the anionic sodium bis-2-ethylhexyl-sulfosuccinate (AOT) inverted micellar (IM) negative interface, the highly restricted water and proton penetration into the Stern layer owing to tight surfactant packing or a reduced water-exposed headgroup area may be responsible for the much lower ΔpH ≈ -0.45 and κ(i) ≈ 28 in comparison to ∼-2.35 and ∼44, respectively, for the anionic sodium dodecyl sulfate (SDS) micellar positive interface with a close similar Stern layer. With increasing AOT IM water-pool radius (1.7-9.5 nm) or [water]/[AOT] ratio ( w) (8.0-43.0), the ΔpH and κ(i) increase maximally up to ∼-1.22 and ∼45, respectively, due to a greater water-exposed headgroup area. However, the unchanged ΔpH ≈ -0.65 and κ(i) ≈ 53.0 within radii ∼3.5-8.0 nm for the positive interface of a mixed Triton X-100 (TX-100)/SDS (4:1) micelle justify its packing flexibility. Interestingly, the continuously increasing ΔpH trend for IM up to its largest possible water-pool radius of ∼9.5 nm may rationalize the increase in ΔpH (∼-1.4 to -1.6) with the change in the curvature radii (∼15 to 50 nm) for sodium 1,2-dimyristoyl- sn-glycero-3-phosphorylglycerol (DMPG)/1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) (2:1) large unilamellar vesicles (LUV) owing to its negative interface. Whereas, similar to the micellar positive interface, the unchanged ΔpH at the positive LUV interface was confirmed by fluorescence microscopic studies with giant unilamellar vesicles of identical lipids composition. The present study offers a unique and simple method of monitoring the curvature-radius-dependent interfacial pH/polarity for biologically related membranes.

A newly synthesised Schiff base molecule (PMP) existing in equilibrium between non-ionic and zwitterionic forms displays solvent polarity induced ratiometric interconversion from one form to another, such novelty being useful to detect the medium polarity. The specific interface localisation of PMP in versatile amphiphilic self-assembled systems has been exploited to monitor their interfacial polarity by evaluating such interconversion equilibrium with simple UV-Vis spectroscopy. In spite of the large differences in pH and/or viscosity between the bulk and interface, the unchanged equilibrium between the two molecular forms on varying the medium pH or viscosity provides a huge advantage for the exclusive detection of interfacial polarity.

The interfacial pH and polarity for different amphiphilic self-assemblies are estimated at a similar interfacial depth utilizing a unique Schiff-base molecule containing two identical phenol-conjugated-imine moieties.

An increase of bulk-to-interface pH-deviation due to partially protonated interface of tri-block polymer micelle makes an acidic membrane interface transiently permeable.