Breakthrough in micelle technology for effective dye and drug dispersion

Micelles are spherical molecular structures usually formed by amphiphilic molecules with block structure, which contain both hydrophilic and hydrophobic parts. The hydrophobic tails of these molecules cluster together to form a core, while the hydrophilic heads face outward, creating a protective shell. This structure allows micelles to encapsulate hydrophobic substances within their core and disperse them in a water-based environment.

An example of micelles in action is in soap, which traps dirt and oil, making them easy to rinse off with water. Micelles can be created using block copolymers, which have distinct hydrophilic and hydrophobic segments, or random copolymers with a mixed distribution of hydrophilic and hydrophobic segments. The former, used in the pharmaceutical industry, offers precise control over the micelle's properties but is more complex and expensive to produce, while the latter, used in the dye industry, is simpler and cheaper to produce.

Researchers led by Mr. Masahiko Asada and Professor Hidenori Otsuka from Tokyo University of Science (TUS) and DIC Corporation, are investigating how to make micelles more effective at dissolving dyes. In a study featured on the cover of Volume 20, Issue 26 of the journal Soft Matter published on July 14, 2024, they compared the performance of block copolymers and random copolymers to determine the most optimal micelle for dye dispersion.

“There is a trade-off between utilizing random copolymers as dispersants for ink production and their inadequate dispersion performance. We investigated block copolymer micelles and compared their dispersion performance with those of random copolymers to determine the micelle structure required for adequate dye solubilization,” says Prof. Otsuka, the lead author of the study.

The researchers synthesized various block copolymers (BL01 to BL05) using different ratios of styrene (St), n-butylmethacrylate (BMA), and methacrylic acid (MA) as monomers. They compared the performance of these block copolymers with random copolymers (RD01, RD02, RD03, and RD04), which were made from styrene and either methacrylic acid or acrylic acid. The copolymers and random copolymers were dispersed in water at a 0.5% concentration, and the micelle structures were examined using Small Angle X-ray Scattering (SAXS) analysis.

The SAXS results showed that micelles formed from block copolymers had a well-defined spherical structure with a clear core-shell boundary. Micelles from random copolymers were found to have a more diffuse and continuous structure, resembling a random-coil pattern with no distinct core-shell boundary. The absence of a clear core-shell structure in the micelles formed from random copolymers reduced their ability to hold hydrophobic dyes. In Critical Micelle Concentration (CMC) tests, the researchers measured the concentration at which micelles form by detecting changes in the polarity around a fluorescent pyrene probe. The results showed that block copolymer micelles had a much lower polarity, meaning the pyrene molecules were better protected inside the hydrophobic core of these micelles.

The researchers made similar observations on measuring the extent of solubilization of hydrophobic orange oil SS dye in the micelles. The micelles made using random copolymers were found to let the dye in easily. However, BL01, BL03, and BL05 prevented the dye from penetrating the core, resulting in a longer time to reach saturation (2 days compared to 10 hours for the random copolymers). Micelles (BL01, 03, and 05) with larger core volumes and more polymer molecules (higher aggregation numbers) were found to hold or solubilize more dye (0.2 to 2 dye molecules per micelle) than the smaller micelles (BL02, BL04).

While the larger micelles with well-defined core-shell structures took longer to become saturated with dye, they could hold a significantly higher amount of dye. The micelle with the highest dye solubilization was BL02. Its shell contained a random mixture of methacrylic acid (MA) and butyl methacrylate (BMA), resulting in a highly polydisperse or varied interface between the core-shell and shell-solvent boundaries, which allowed the dye to enter and be expelled from it quickly.

Prof. Otsuka explains, “The block copolymer micelles exhibited a higher dye solubilization capacity, which correlated with their core volume, clear core-shell contrast, and slow solubilization rate.” This finding could lead to more efficient and cost-effective micelles for the ink and dye industries as well as pharmaceutical industries. 

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Reference                     

DOI: https://doi.org/10.1039/D4SM00009A

About The Tokyo University of Science
Tokyo University of Science (TUS) is a well-known and respected university, and the largest science-specialized private research university in Japan, with four campuses in central Tokyo and its suburbs and in Hokkaido. Established in 1881, the university has continually contributed to Japan's development in science through inculcating the love for science in researchers, technicians, and educators.

With a mission of “Creating science and technology for the harmonious development of nature, human beings, and society," TUS has undertaken a wide range of research from basic to applied science. TUS has embraced a multidisciplinary approach to research and undertaken intensive study in some of today's most vital fields. TUS is a meritocracy where the best in science is recognized and nurtured. It is the only private university in Japan that has produced a Nobel Prize winner and the only private university in Asia to produce Nobel Prize winners within the natural sciences field.

Website: https://www.tus.ac.jp/en/mediarelations/

About Professor Hidenori Otsuka from Tokyo University of Science
Prof. Hidenori Otsuka completed his Ph.D. from the Division of Natural Science Chemistry, Tokyo University of Science (TUS) Graduate School, and currently heads his own laboratory at TUS. With more than 100 research publications to his credit, his research focuses mainly on the basics and applications of physical chemistry, especially colloid and surface chemistry. His profile can be accessed here: https://www.tus.ac.jp/en/fac/p/index.php?3f8f

Funding information
This study was supported by the Iketani Science and Technology Foundation of Japan. This research was financially supported by the Japan Agency for Medical Research and Development (AMED) under the Grant Number 22ym0126812j0001.