The National Institute for Computational Sciences

The Surfactants of the Matter

XSEDE Resources Help Syracuse University Researchers Investigate the Relationship Between Molecular Arrangement and Fluid Flow

By Scott Gibson

Although the word is fancy and scientific sounding, surfactants (short for surface-active agents), consisting of molecules derived from fat, are products we use every day. They are surface-tension-reducing compounds with many applications—from consumer products such as soap, toothpaste, cosmetics, diapers, and shampoo, to industrial settings, where they are put to work as hydraulic fracturing fluids for mining natural gas and petroleum, drag-reducing agents in heating and cooling systems, and additives in asphalt, a black, sticky form of petroleum. The amount of surfactant in a substance determines how thick or thin it is, and, consequently, its rate of flow.

Surfactants are even present in our body. For example, they ensure the function of vital organs such as the lungs by reducing the interfacial tension of the alveoli, balloon-like structures in which oxygen exchange takes place. Also in the context of human well being, surfactants are included in many therapeutic applications.

Surfactants are composed of molecules that have hydrophilic (water-loving) polar heads and hydrophobic (non-water-loving, or "water-fearing") tails, and when surfactant becomes concentrated in water, like parts come together—heads with heads and tails with tails—to form aggregate particles called micelles.

When we wash our hands, the hydrophobic tails of micelles in the soap's surfactant surround and collect, oil, dirt, and germs on our skin for those unwanted elements to be washed away.

The "Interplay" Between Shapes and Flow

As commonplace as surfactants are, they nonetheless have some mysterious properties that remain scientifically slippery despite hundreds of years of investigations, says researcher Subas Dhakal.

Dhakal has been working in Professor Radhakrishna Sureshkumar's group in the Department of Biomedical and Chemical Engineering at Syracuse University. The group's research interest encompasses understanding the structure, dynamics, and rheology (flow) of complex fluids, both analytically and computationally via numerical simulations.

Dhakal and coworkers have been applying the advanced digital resources provided by XSEDE to probe the relationship between the particles in surfactants and the property of viscosity, a measure of a fluid's resistance to flow. What's learned could enable the fine-tuning of surfactants to make them even more beneficial to society, Dhakal says.

Specifically, the project in which the researchers are engaged involves relating the mathematical study of the shapes (topology) of micelles in surfactants with the rheology of the solutions containing them.

Dhakal explains that flow properties of micellar solutions change dramatically when surfactant chemistry, ionic environment, temperature, and chemical concentrations are altered, and what he and the other group members want to do is relate the topology of the emerging assembled micelle structures with the resultant rheological properties. "This interplay between topology and rheology can be understood via detailed large-scale molecular dynamics simulations," he says.

Dhakal and his coworkers performed such simulations of Cetrimonium bromide surfactant in an aqueous environment with added salt.

A Unique Look at Micellar Solutions

"We were able to simulate a [micellar solution] system at large scale for the first time," he says. "Our simulations were done in a box of 40 nanometers in each direction that requires simulating a half-million particles to represent surfactant, salt, and water." He adds that they have typically employed 200,000 compute hours on Kraken for each simulation to probe the trajectory of the system over a few hundred nanoseconds, and have also relied on two other XSEDE resources to perform the simulations—Blacklight at the Pittsburgh Supercomputing Center and Lonestar at the Texas Advanced Computing Center.

Dhakal says he and his coworkers have investigated different emerging shapes of micelles as a function of surfactant and salt concentration, deduced various scaling laws to different length scales in the micellar solutions, and found that micelle length distribution is not universal and strongly depends on the electrostatic interactions within the system. Electrostatic pertains to electric charges at rest. The research has also revealed that salt concentrations have an effect on the thickening and thinning rates in surfactant solutions.

Simulations Could Enhance the Understanding of Experiments

Dhakal explains that determining the shapes of micelles is difficult because they maintain a chemical equilibrium and are continuously combining and breaking with the neighboring micelles, making them pesky moving targets for the scientists trying to characterize them. "In this regard, simulations might help experiments," he adds.

"Micelles of different shapes are possible—sphere, cylinder, torus, and others," Dhakal explains. "It depends upon the geometrical and chemical properties of the surfactant molecules as well as the salinity of the solutions. In other words, micelles follow certain rules of geometry. For theorists like me, it is also of interest from the fundamental point of view to investigate optimal micelle shapes using the knowledge of geometry; theory and simulations could tell us more in that regard."

Dhakal notes that the simulation techniques developed so far have created a foundation for developing a comprehensive understanding of structure–dynamics–rheology relationships in micellar solutions.

"These simulation methods can also be considered as in silico (computer-performed) rheological experiments," he says. "In another collaborative project that uses computational resources from XSEDE, Abhinanden Sambasivam, a fourth-year graduate student in the group, has discovered that the addition of suitable nanoparticles might enable more precise tuning of the viscous properties of micellar solutions. In particular, the addition of nanoparticles substantially increases the viscosity of the solutions. This is the first time that has been illustrated in theory and simulations, and is quite remarkable. The simulation model and the technique developed so far could help to further unfold many mysteries that remain in the micellar solutions."

Sureshkumar's research group has discovered that metal nanoparticle solutions stabilized by surfactants can be spin-coated onto thin-film silicon solar cells to create light-harvesting nanoparticle interfaces. "Such interfaces are used to enhance the efficiencies of thin-film silicon solar cells," Dhakal explains. "Computer simulations provide a means to understand the structure of nanoparticle-surfactant solutions and thereby aid knowledge-based process design."

Dhakal and his coworkers have presented on the details of their research at various professional conferences in recent months, and he notes that they also have plans to submit their results for publication in scientific journals.

Presentations Made on This Research

  • "Structure and flow properties of micelle-nanoparticle solutions from Molecular Dynamics simulations," APS March Meeting, Denver, Colo., March 2014
  • "Structure and Rheology of Micelle-Nanoparticle and Polymer-Nanoparticle Networks from Molecular Dynamics Simulations," AIChE Annual Meeting, San Francisco, Calif., November 2013
  • "Structure and rheology of micelle and micelle-nanoparticle solutions from molecular dynamics simulations," 247th ACS National Meeting, Dallas, Texas, March 2014
  • "Dynamics of nanoparticles in an entangled polymer matrix," APS March Meeting, Denver, Colo., March 2014

Article posting date: 9 April 2014

About NICS: The National Institute for Computational Sciences operates the University of Tennessee supercomputing center, funded in part by the National Science Foundation. NICS is a major partner in NSF's Extreme Science and Engineering Discovery Environment, known as XSEDE.