(423h) Controlled Self-Assembly of Cationic Polyelectrolytes and Anionic Surfactants in Microfluidics Channels

The Nobel Prize-winning American theoretical physicist, Richard Feynman, pondered the potential of miniaturization in science and technology in his celebrated lecture at the 1959 meeting of the American Physical Society. His famous phrase “There is plenty of room at the bottom” beaconed the development of microelectronics and nanotechnology in the next 50 years. Computer chips with millions of conducting ‘channels’ for electrons are the cornerstone of our civilization today. It is fascinating that the same approach is now used in chemistry: polymer chips with microchannels inside can handle dozens of liquid flows simultaneously thus heralding the era of new chemistry for processing various reactants and nanoparticles. Similar to ‘microelectronics’ in physics, this approach is called ‘microfluidics’.

Microfluidics is an interdisciplinary area combining chemistry, biology, fluid mechanics, nanotechnology and engineering. It deals with liquid flows in channels with the diameter between several micrometers and a millimeter, reduces dramatically the amount of required reagents; only minutes are needed to carry out complex reactions and analytic procedures which usually require weeks for preparation. In addition, microfluidics provides unique conditions for synthesizing and fine-tuning nanoparticles, as well as making drug delivery complexes which cannot be obtained in bulk solutions.

Polymers and surfactants are indispensable components of living matter and chemical industry processes: nucleic acids and proteins, detergents, enhanced oil recovery reagents, and drug delivery systems. They can easily form nanoparticles by self-organization. They are ‘smart systems’ as they can be easily controlled by many factors, such as pH, temperature, solvent, and etc. Thus, it is not a surprise that they are common objects of soft matter nanotechnology.

It is thrilling that practical application of such systems often involves microchannels (!): blood capillaries, micropores of catalyst carriers, 3D-printing systems, and etc. If we want to know how to apply them properly, we have to study them in microchannels. Microfluidics, however, mostly deals with flow hydrodynamics, fabrication of chips and chemical analysis. Polymer-surfactant systems are also mostly studied in bulk solutions, and not in microchannels. Several publications discussing polymers and surfactants in microfluidic devices are rather individual papers than a focused approach. Thus, a systematic and focused study of polymer-surfactant association in microfluidic environment will be an excitingly new interdisciplinary area which will contribute both to fundamental knowledge and practical applications of such systems.

This research is dedicated to synthesis and characterization of polyelectrolyte-surfactant nanocomplexes and to tuning their properties with the use of microfluidic approach.

The research is the joint project of Carnegir Mellon University and Kazan National Research Technological University (Russia).

This research project implementation was focused on the following objectives:

1. Designing and fabricating microfluidics devices for polymer-surfactant associations;
2. Synthesizing polymer-surfactant associates in microfluidic confinement through self-organization; characterization of complexes in bulk solutions;
3. Tuning of size and surface properties of polymer-surfactant complexes in microfluidic chips.

Research Methodology:

Microfluidic devices: PDMS-based chips made by microlithography method.

Polymers to be used for the research: chitosan and carboxymethyl cellulose (natural-occurring and eco-friendly polyelectrolytes of different charges).

Surfactants: sodium dodecyl sulfate and cetyltrimethylammonium bromide (model synthetic surfactants of different charges).

Project results:

Microfluidic chips for synthesis of polymer-surfactant nanocomplexes have been designed. The size of microchannels was selected in the 50-200 μm range.

Microfluidic chips were prepared using soft lithography method: PDMS samples with microchannels were bonded to glass plates using plasma processing to make the contacting surfaces hydrophylic.

Research methods: Microtensiometry; Optical Microscopy; Microviscometry; Soft Lithography (for Fabrication of microfluidics chips), Dynamic light scattering.

Individual fows of polymers and surfactants were studied in microchannels. The size of polymer nanoparticles was controlled by the solvent composition: alcohol solutions of various concentrations were added as co-solvents. Dynamic light scattering studies demonstrated that the size of polymer nanoparticles changed in the range of 50 - 200 nm, while microfluidic confimenement provided excellent environment to control the flows of polymer solutions.

Polymer-surfactant complexes have been synthesized in microfluidic channels. The following methods were used to tune the properties of these nanocomplexes:

- polymer-to-surfactant ratio provided control of sizes in the 20-100 nm range

- change of solvent composition by ading low-molecular salt and alcohol (20-200 nm range of nanoparticles)

Phase behavior of polymer-surfactant complexes was also studied. The study revealed that soluble and insoluble complexes form in microfluidic channels and formation of phases can be teracked by optical microscopy.

Polymer-surfactant nanocomplexes synthesized in microchannels demonstrate similar associative and phase behavior as the same complexes synthesized by conventional "in vitro" mixing. Microfluidics, however, is an excellent tool to accelerate synthesis of polyelectrolyte-surfactant nanoparticles and apply various automated methods to control their properties.