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Issue 16, 2017
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Interfacial pH and polarity detection of amphiphilic self-assemblies using a single Schiff-base molecule

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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.

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

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Publication details

The article was received on 12 Apr 2017, accepted on 01 Jul 2017 and first published on 04 Jul 2017


Article type: Paper
DOI: 10.1039/C7NJ01222E
Citation: New J. Chem., 2017,41, 8536-8545
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    Interfacial pH and polarity detection of amphiphilic self-assemblies using a single Schiff-base molecule

    R. Majumder, Y. Sarkar, S. Das, A. Ray and P. P. Parui, New J. Chem., 2017, 41, 8536
    DOI: 10.1039/C7NJ01222E

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