Interactions between Methylene Blue and Sodium Dodecyl Sulfate in Aqueous Solution Studied by Molecular Spectroscopy

Abstract

The interactions of methylene blue (MB, a cationic redox indicator and biological stain) and sodium dodecyl sulfate (SDS, a micelle-forming, anionic surfactant) in aqueous solution have been examined by using Rayleigh scattering, UV-visible absorption, and fluorescence spectroscopy. At SDS concentrations significantly below the critical micelle concentration (cmc), MB forms noncovalent dimers and aggregates with SDS that scatter light but do not fluoresce. For solutions containing 1 μM MB and < 3–5 mM SDS, shifts in the absorption spectrum characteristic of the formation of MB H-aggregates are noted. There appears to be little effect on the fluorescence emission spectrum, indicating that these MB aggregates do not fluoresce appreciably. At and above the known SDS cmc, MB is observed to interact with the micelles. The MB excited-state fluorescence lifetime (380 ps) remains constant until SDS micelles form, then increases to 615 ps. The MB rotational reorientation time similarly increases from 105 to 500 ps between 6 and 8 mM SDS. This finding suggests that the MB is encountering, on average, a microenvironment in the SDS micelles that is 5-fold more viscous than liquid water or the molar volume of the MB/SDS species that is reorienting is 5-fold larger than MB in water.

Photoacoustic lifetime contrast between methylene blue monomers and self-quenched dimers as a model for dual-labeled activatable probes

“…26 Carroll et al demonstrated that the scattering signal from a 1 μM MB solution exhibits a strong signal for ½SDS ¼ 1.5 to 4.5 mM, suggesting the presence of a large aggregate in that range. 36 Those results indicate the formation of MB-SDS clusters or premicellar aggregates before the CMC is reached. We suggest that these premicelles are formed by strong electrostatic interactions between anionic SDS and cationic MB with the surfactant possibly intercalating its hydrophobic tail between adjacent MB molecules [ Fig.…”

“…During this stage, premicelles may be disaggregating with MB molecules released as MB/SDS quenched ion pairs. 7,36 Finally, at SDS concentrations greater than 7 mM, the triplet transient absorption increases logarithmically until reaching a plateau corresponding to full monomerization. Concurrently, the absorption maximum at 665 nm is blue-shifted compared to the monomer spectrum signifying that the monomers are surrounded by a more polar environment than in water without SDS (most likely the negatively charged surface of the micelle) [ Fig.…”

“…SDS concentrations as low as 1 mM have been shown to significantly increase MB dimerization. 7,27,36 The following experiments were realized within 3 min after MB dilution to ensure that the solutions remained stable over time.…”

Photophysical and antibacterial properties of complex systems based on smectite, a cationic surfactant and methylene blue

“…As mentioned above, several studies have found MB to have a significantly lower photoactivity when in the form of molecular aggregates [14,[26][27][28]. This phenomenon was generally interpreted in terms of a very rapid deactivation of excited molecules via non-radiative relaxation pathways.…”

Phenotiazinium Dyes as Photosensitizers (PS) in Photodynamic Therapy (PDT): Spectroscopic Properties and Photochemical Mechanisms

“…It results in a decreasing of the fluorescence intensity and on the singlet oxygen formation [49]. These studies have shown that the interaction with micelles is responsible for the dimerization process and not the interaction with monomer of the surfactant, as it has been postulated in some works [ 169 ]. In this stage of the work it is important to study the nature of the aggregates formed in different negatives interfaces and in biological systems, more specifically in micelles, vesicles and mitochondria.…”