Equation explains forces that repel water, oil

UC SANTA BARBARA (US) — A new equation is the first to predict the hydrophobic interactions of molecules. Such interactions explain why oil and water don’t mix, how proteins are structured, and what holds biological membranes together.

“This discovery represents a breakthrough that is a culmination of decades of research,” says Jacob Israelachvili, professor of chemical engineering at the University of California, Santa Barbara. “The equation is intended to be a tool for scientists to begin quantifying and predicting molecular and surface forces between organic substances in water.”

Using a light-responsive surfactant—a soap-like molecule related to fats and lipids—the researchers developed an innovative technique to measure or change the forces between layers of the molecule in water by using beams of UV or visible light. The result is a general equation that applies to even more complicated systems, such as cellular membranes or proteins.

Jacob Israelachvili’s equation solves the mystery of forces between water-repelling and water-attracting molecules, critical to a variety of industrial and medical applications.  (Credit: UC Santa Barbara)

The research is reported in Proceedings of the National Academy of Sciences.

“We were fortunate to find the right combination of experimental methods and theory,” says Brad Chmelka, professor of chemical engineering and co-author of the study. “The keys to our research were using a light-responsive surfactant molecule, a means of measuring these delicate surface forces, and applying knowledge of what to look for.”

The highly-sensitive instrument they used to sense these molecular-level hydrophobic forces, called a surface forces apparatus, is a now-standard technique that was originally pioneered by Israelachili and colleagues in the 1970s.

“In basic chemistry, students learn about van der Waal forces—the weak forces that act between all molecules. That theory was developed more than 100 years ago,” explains Israelachvili.

“According to the van der Waals theory, however, oil and water shouldn’t separate and surfactants shouldn’t form membranes, but they do.  There has been no proven theory to account for these special hydrophobic interactions. Such behaviors are crucial for life as we know it to exist.”

Hydrophobic and hydrophilic interactions are central to the disciplines of chemistry, physics, and biology that have fueled modern developments in industries from detergents to pharmaceuticals and new biotechnologies. The new equation is expected to affect applications in water filtration, membrane separations, biomedical research, gene therapy methods, biofuel production, and food chemistry.

Virus and disease propagation in the human body are directly linked to hydrophobic properties on a cellular level. One of the problems related to chemotherapy treatments for cancer is being able to direct a drug specifically to cancer cells, instead of the entire body. Israelachvili and colleagues foresee their discovery having an impact in biomedical research that attempts to understand and treat diseases.

“Cell membranes are complex and discriminating structures, allowing the transmission of various signals into cells and mediating specific interactions with bacteria and viruses,” says Jean Chin, PhD, who oversees membrane structure grants at the National Institute of General Medical Sciences of the National Institutes of Health.

“This study, by enhancing our understanding of the role played by hydrophobic forces in membrane dynamics, will expand what we know about membrane structure and function, as well as microbial infection pathways.”

“Understanding how water and oil-like substances interact is enormously important for explaining the properties and functions of many biological and engineering materials,” says Robert Wellek, program director in the Directorate for Engineering at the National Science Foundation.

“We’ve known for a long time what we were aiming for. It’s a bit like climbing a mountain,” says Israelachvili. “The whole thing started at the very bottom. I’ve been searching for the keys to this interaction for 30 years. We are thrilled with the findings, but it took a lot of steps over carefully chosen paths to get there.”

The research was supported by the National Science Foundation, the National Institutes of Health, and the Procter & Gamble Company.

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