Solving the mystery of the difference in sound speed in water through ion research

Although the speed of sound in a range of electrolytes has been intensively studied since the 1950s, only the overall effect of these ions in solution has been measured. Now, Theo van de Ven and Yiwei Jiang have quantified the speed of sound in aqueous solutions for specific ions and found that anions reduced the speed of sound as expected, while surprisingly, cations actually accelerated it.

What’s a bit surprising, van der Veen says, is that apparently no one has studied this problem before. He added that a better understanding of the impact of different components in a solution on the speed of sound could be a valuable tool in analyzing the composition of complex solutions.

The original motivation for the acoustic research was to try to replace plastics and textiles with more sustainable materials based on cellulose, the main component of wood. To this end, Van de Ven and Jiang created solutions containing nanocellulose and dissolved cellulose, which they could extrudate to create plastics and textiles. Previous studies have shown that the ratio of molecularly dissolved cellulose to nanocellulose determines the properties of extruded materials, but they had no reliable way to precisely identify the composition of their solutions.

difficult to separate [the components of the solution] Because everything is tangled and small, Vanderveen explains. So we wanted to find a technology that could send something and measure something, and sound seemed like an ideal way to research and interrogate these solutions.

Although there are numerous references in the literature to the speed of sound in different salt solutions, they all cite the net salt value and the combined effect of cations and anions. This makes sense, van de Ven points out, because you can’t have a solution with only positive ions or only negative ions. Still, they weren’t looking for a simple salt solution, but rather a complex mixture of different dissolved molecules. So they began looking for a way to quantify the contribution of single ions.

The team used an acoustic spectrometer to measure the speed of sound in 11 different salt solutions. They then turned to equations first devised by British physicist Albert Beaumont Wood to describe the inherent speed of sound in suspended particles. Wood’s equation extracts the sound velocity parameters based on the volume fraction of particles in the suspension. With the measurements in hand, van de Ven and Jiang were able to solve Wood’s equation to obtain the speed of sound in water containing these 11 salts.

Typically, ions reduce the speed of sound, and the same goes for anions. However, the speed of sound is faster for cations than in water. Van de Ven and Jiang attribute this to the hydration layer surrounding the cations, whose positive charge causes water molecules to arrange around the ions in a structure more similar to ice than to water. Since sound travels faster in ice than in water, this explains why sound travels faster when passing through cations.

The discovery helps van de Ven explain why sound speed measurements in cellulose solutions and suspensions differ despite having the same concentration. “We’re not experts on the speed of sound,” Van de Ven said. We just thought it was an interesting way to use it as an analysis tool to get information about your system.

Chen Ling of the School of Chemistry at Beijing Normal University in China, who studies phonons in crystalline solids, said the work is a major advance. She added that the introduction of intrinsic sound speed to quantify the contribution of ions in solution to sound speed clarifies a previously challenging aspect of solution analysis and is expected to have broad applications in different fields, including hydrogel thermoelectricity, ion transport engineering, thermal transport management and manipulation of crystal-liquid duality.