Researchers have developed strategies for solubilization of cellulose

Studies of intermolecular interactions and dissolution mechanisms in cellulose conducted by scientists at Sweden’s Lund University may contribute to the expansion of chemical applications.

By Elton Alisson  |  Agência FAPESP – Cellulose is the most abundant natural material on Earth and is used as a raw material (pulp) in paper manufacturing but has been underused as an input for chemical processes. One of the reasons for this lack of use is that many chemical applications would require the dissolution of cellulose, and it is insoluble in water and most organic solvents (solubilizing substances).

Björn Lindman, Professor of Physical Chemistry at Lund University in Sweden, has devoted many years to endeavors to break through this barrier by gleaning more knowledge of cellulose and developing strategies for its solubilization.

He outlined some of his latest research findings on October 31, 2018, in a presentation to the São Paulo School of Advanced Science on Colloids (SPSAS Colloids).

Funded by FAPESP through its São Paulo School of Advanced Science Program (SPSAS), the event took place between October 28 and November 7 at the University of Campinas (UNICAMP) and the National Energy and Materials Research Center (CNPEM, also in Campinas), with activities at the University of São Paulo’s Chemistry Institute (IQ-USP) in São Paulo City. It was attended by 94 graduate students and young researchers, 52 of whom were Brazilian and 42 from other countries.

“The advance in knowledge and the development of cellulose dissolution strategies have led to enhanced use of the material in such applications as cellulose fiber regeneration, the promotion of homogeneous chemical reactions for the development of ‘green’ materials and chemicals, and the development of composites such as nanocellulose and coatings,” Lindman said.

One of the explanations found in the literature for the insolubility of cellulose in water, he added, is its tendency to form many strong intramolecular and intermolecular bonds with hydrogen.

This explanation suggests that the key to solving the problem may be identifying a solvent that can effectively destroy the hydrogen bonds between chains in cellulose.

Breaking these intramolecular and intermolecular hydrogen bonds, however, gives rise to the formation of hydrogen bonds between cellulose and water, which are just as strong as the former, according to Lindman.

“This leads to the conclusion that the hydrogen bonds between chains in cellulose aren’t sufficient to explain the polymer’s low water solubility. Other factors may be involved,” he said.

In a study of the intermolecular interactions of cellulose and the mechanisms of its dissolution in collaboration with colleagues at Algarve University in Portugal, Lindman found that cellulose is significantly amphiphilic, with a polar or hydrophilic region (which dissolves in water) and a nonpolar or hydrophobic region (which does not dissolve in water).

“We discovered that hydrophobic interactions play a key role in the crystal structure of cellulose and are important to explain its low water solubility,” Lindman said.

From their analysis of the structure of cellulose crystals, he and his team found that owing to intra- and intermolecular hydrogen bonding, flat ribbons are formed with sides that differ markedly in their polarity. The hydrophilic sides tend to stick to each other in an aqueous environment, contributing to the low solubility of the material.

Based on these findings, the researchers predicted that the solubility of cellulose would be facilitated in solvents that are also amphiphilic (with polar and nonpolar parts), such as electrolytes with a highly polarizable ion and N-methylmorpholine-N-oxide (NMNO).

Cosolutes (substances dissolved in the same solution), such as urea and its derivatives or polyethylene glycol, tend to weaken hydrophilic interactions, and other surfactants (amphiphilic substances) can also promote the water solubility of cellulose, according to the researchers.

“These discoveries have opened the doors to the development of cellulose dissolution strategies different from those based on disrupting hydrogen bridges to induce or increase the material’s solubility,” Lindman said.

Colloidal systems

The sessions at SPSAS Colloids included the discussion of several types of colloids, mixtures with properties between those of a solution and a fine suspension. The particle size in a colloidal system or dispersion ranges from 1 to 1,000 nanometers (a nanometer is a billionth of a meter). One substance is very finely dispersed in another without a new true solution forming. The two components are called the dispersed and continuous phases. Either phase can be a solid, liquid or gas, so a great many colloidal systems are possible.

Many colloids exist in nature, but there are also numerous man-made colloids. Common examples include milk (liquid fat dispersed as fine droplets in an aqueous continuous phase), smoke (solid articles dispersed in air), bone (small particles of calcium phosphate dispersed in a solid collagen matrix), paint (small solid particles dispersed in a solvent), and shaving foam (minute air bubbles dispersed in liquid soap).

“Colloids are the foundation of nanoscience and nanotechnology. SPSAS Colloids covered the different techniques used to characterize and investigate these systems, which have many applications in cosmetics, formulations and medications, for example,” said Watson Loh, a professor at the University of Campinas’s Chemistry Institute (IQ-UNICAMP) and coordinator of SPSAS Colloids.

The program included presentations on key aspects of colloidal systems and hands-on activities involving the main techniques used to investigate them, such as light, X-ray and neutron scattering, cryogenic and electron microscopy, fluorescence, surface characterization, and simulation of molecular dynamics.

“The program took place at three centers: UNICAMP, CNPEM and USP. The students learned about cryoelectronic microscopy, the technique that won last year’s Nobel Prize for Chemistry. The National Nanotechnology Laboratory is acquiring this equipment. We also used a line at the National Synchrotron Light Laboratory, conducting experiments on the storage ring itself, and visited Sirius, Brazil’s new synchrotron light source. The students were able to experience developments at the knowledge frontier,” said Loh, who is also a member of FAPESP’s multiuser equipment steering committee.

Following SPSAS Colloids, the 6th Meeting on Self Assembly Structures in Solution and at Interfaces took place at São Pedro, São Paulo State, on November 7-9. According to Loh, the two events shared speakers and attendees.

“The study of colloids is a transdisciplinary field, so we were able to provide twin opportunities to assemble all these experts,” he said.