The influence of cationic and anionic surfactant solutions on the character of the fluorescence spectra of reagents of different charge and hydrophobicity in aqueous solutions of nonionic surfactant Triton X-100 has been studied. An increase in the fluorescence intensity and a shift in the position of the fluorescence maximum with increasing hydrophobicity of reagents and ionic surfactants have been shown. The analytical signal of the surfactant is further amplified in the proximity of the charge values of the reagent and the counterion of the surfactant. The non-monotonic nature of the hydrophobicity effect of cationic surfactants on their analytical signal in the system has been shown. The observed effects are explained by the realization of charge and hydrophobic matching in the interaction of surfactants with the fluorescent reagents. The obtained effects are significant in the design of fluorescent systems for the determination and study of surfactant micelles. Conditions for detecting the content of cetylpyridinium chloride by reaction with eosin Y and sodium tetradecyl sulfate by reaction with rhodamine 6G in the presence of Triton X-100 were proposed. The methods have been tested in detecting the content of the ionic surfactants in pharmaceuticals.
The statistical characteristics of the dependences of the fluorescence signal of analytical systems as a function of the integral parameters of the structure of fluorescent reagents and cationic surfactants on their association in aqueous solutions has been investigated. Molecular weight, surface area, and their first-order molecular connectivity index have been taken as parameters of the structure of the reagents and cationic surfactants. The influence of the hydrophobicity of the reagent and cationic surfactants, such as the octanol–water distribution constant and octanol–water partition coefficient, on the fluorescence signal of the reagent–cationic surfactant associates have also been investigated. It is shown that the associates of anionic reagents with cationic surfactant counter ions are characterised by high stability and a higher analytical signal compared with associates in which there is no electrostatic attraction between the reagent and the surfactant ion. The effect of hydrophobicity of the reagent and cationic surfactant in the absence of electrostatic attraction between the interacting particles is similar. The increase in the role of the influence of the structure of cationic reagents in their association with cationic surfactants, when the electrostatic attraction is absent and the stability of the associates is due mainly to hydrophobic interactions, is noticeable. The regularities of the influence of the colloid-chemical state on the analytical signal of associated cationic surfactants in solutions have been investigated. The study made it possible to formulate a rational basis for the search and design of analytical systems for the determination of large cations by the fluorescence method.
The
localization of guest molecules at the molecular scale in mesoporous
host materials is crucial for applications in heterogeneous catalysis,
chromatography, drug delivery, and in different biomedical applications.
Here, we present for the first time the precise localization of different
guest molecules inside the mesoporous organosilica material UKON2a
with a pore size of 6 nm. We exploited paramagnetic probe molecules
4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) and 4-carboxy-TEMPO
in combination with a deuteration strategy. Applying a complementary
set of different pulsed electron paramagnetic resonance methods, we
obtained information about the dimensionality of the spatial distribution
and local concentration via double electron–electron resonance
experiments, orientation of the guest molecules with respect to the
pore walls via electron spin echo envelope modulation spectroscopy,
and about the distance between guest molecules and pore walls via
electron nuclear double resonance spectroscopy. This allowed localizing
the guest molecules and shows that their spatial distribution in nanopores
strongly depends on their polarity.
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