Micropolarity and Order in the Reverse Micelles of L62 and L64 Pluronic Copolymers, As Studied by Molecular Probe Techniques

Abstract

The reverse micelles of triblock copolymers poly(ethylene oxide)-block-poly(propylene oxide)-block-poly-(ethylene oxide) EO 6 PO 36 EO 6 (Pluronic L62) and EO 13 PO 30 EO 13 (Pluronic L64) in ternary (copolymer/oxylene/water) and binary (copolymer/water) systems with different water contents were evidenced and investigated by fluorescence, absorption, and spin probe techniques. The spectral parameters of the polarity sensitive probes, 1-anilinonaphthalene-8-sulfonic acid (ANS), dansylhexadecylamine (Dansyl), pyrenesulfonic acid (PSA), 4-nitropyridine N-oxide (NP) and 4-(N,N′-dimethyl-N-alkyl)ammonium 2,2′,6,6′-tetramethylpiperidine-1-oxyl iodide (CAT n), were related to the local hydration and polarity by comparison with a series of poly(oxyethylene)/water (TEG/water) calibration mixtures. The data were concordant and complementary, resulting in polarity profiles of the core for all systems and the relative radial positioning of the probes. The order of decreasing hydrophobic character found was Dansyl > NP > CAT 16 > CAT 11 ∼ ANS > CAT 8 > PSA > CAT 4 > CAT 1. A linear relationship was found between the R A (the intensity ratio of two absorption vibronic bands) of PSA in ethanol/water mixtures and Kosower's Z parameter (Z ) -29.0R A + 177.83). Thus, by means of the calibration mixtures, converting different spectral parameters of the mentioned probes, the local polarity values were expressed in terms of Z values. The role of water as a prerequisite for micellization was evidenced and so was the minimum quantity of water required in the ternary systems (W ) 0.2 for L64 and W ) 0.4 for L62). Regarding the water distribution, a more advanced segregation of water is observed in the ternary systems as compared to the binary ones, pointing to the role played by the solvent in this process. Evidence was found for a considerable solvation of the poly(propylene oxide) block with xylene, leading to a looser packing in the corona and in the poly(ethylene oxide) core. In the case of ternary systems, the ordering of the polymer chains in the micelles, measured with a series of x-doxylstearic acid spin probes, indicates the progress of micellar organization with increasing water content, the order increasing and extending further from the polar core and the corona becoming less penetrable. In the corresponding binary systems, the polymer chain rotation is "frozen" at room temperature (295 K) and even at 320 K. The structural characteristics of L62 and L64 ternary systems are rather similar, a conclusion suggested especially by the information the polar probes supplied. There are differences in the microenvironment of more hydrophobic probes, which sense a higher hydrophobicity in the case of L62, the increase of microviscosity being also more marked in the L62 system.

Investigation of the micropolarity in reverse micelles of nonionic poly(ethylene oxide) surfactants using 4-nitropyridine-N-oxide as absorption probe

“…It should have its proper location within the aggregate, and it should be sensitive enough to relevant changes in the system to be able to provide meaningful information. The polarity profile of the nonionic RM has been evidenced by absorption, fluorescence, and spin probe techniques [15,[20][21][22][23].…”

Quenching of Pyrene Derivatives' Fluorescence by Nitroxide Radicals in Sodium Dodecyl Sulfate Micellar Solutions

“…Self-assembled molecular systems (micelles, vesicles, membrane) have been widely investigated in the past by spectroscopic methods (electron spin resonance (ESR), fluorescence, and ultraviolet-visible absorption), using the molecular probes incorporated into the microaggregates (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11).…”

“…The fluorescence measurements supply information about the micellization process and about the microenvironment of the fluorescent probe in terms of polarity (1,2), microviscosity (4,5), aggregation number (6)(7)(8)(9), and micellar size and shape (10,11). One important parameter that can be estimated using either steady-state fluorescence quenching (SSFQ) or time-resolved fluorescent quenching (TRFQ) is the mean aggregation number of the micellar surfactant aggregates, N .…”

Imidazole derivatives as absorption probes for pluronics core-shell aggregates micropolarity investigation

“…The micropolarity variation in reverse micelles (RM) of nonionic PEO surfactants of the alkyl ether type (Brij 30), alkyl-aryl ethers (Triton X-100), and of Pluronics L62, L64, P123, and F127 copolymers has been studied using a UV-vis absorption probe, 4-nitropyridine-Noxide (NP) [29,34,35]. In our previous article, the micropolarity and order in the RM of L62 and L64 in o-xylene…”

“…have been investigated using absorption, fluorescence, and spin probe methods [35]. The data revealed that the local polarity in the RM is dependent on the surfactant type, the block copolymer composition, the solvent nature, and the water content.…”