Interfacial pH and polarity detection of amphiphilic self-assemblies using a single Schiff-base molecule

Abstract

The interfacial pH and polarity are two highly interrelated parameters in amphiphilic self-assembled systems. The hydronium ion (H₃O⁺) concentration and/or the pH value near the water/oil separating interface may change significantly due to large polarity gradients between water- and oil-exposed surfaces within the interface. Therefore, for precise detection of these two properties (pH and polarity) at a specific interfacial depth, a similar interfacial location of the same probe is a prerequisite. In this regard, we have synthesized a new interface-interacting Schiff-base (SBOH-Z-SBOH) molecule to detect both the interfacial pH and polarity of various amphiphilic self-assembled micelles and vesicles at a similar interfacial location. SBOH-Z-SBOH, existing mostly as a non-ionic species (SBOH⁰-Z-SBOH⁰) in nonpolar solvents, exhibits an exclusive solvent polarity-dependent linear interconversion equilibrium with its partially charge separated zwitterionic form (SBOH⁰-Z-SBOH^±) as the polarity of the medium increases, which makes it useful to detect the polarity. Additionally, the solvent pH-dependent conversion of both SBOH⁰-Z-SBOH⁰ and SBOH⁰-Z-SBOH^± into the deprotonated di-anionic species (SBO⁻-Z-SBO⁻) allows it to monitor the pH. We found that the interfacial dielectric constant (∼44.0–54.0) differs substantially from that of the bulk aqueous medium depending on the amphiphilic system. On the other hand, unlike the neutral interface of titron X-100 (TX-100) micelles or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) unilamellar vesicles (ULVs), a large positive pH-deviation of ∼1.8 and 2.2 units from the bulk to the interface was identified for cationic cetrimonium bromide (CTAB) micelles and dimethyldioctadecylammonium bromide (DDAB) ULVs, respectively. The present study provides a unique and simple Schiff-base molecule to monitor the pH and polarity at similar interfacial depths for amphiphilic self-assembled systems.