Effect of Cation Type and Concentration on the Changes in the Forces between Two Charged Surfaces Modulated by Audible Sound

We aimed to determine how audible sound can modify the electric double layer (EDL) and forces between a negatively charged silica particle and silicon wafer in aqueous electrolyte solutions when the electrolyte type (LiCl, NaCl, and KCl) and concentration (0.01, 0.1, 1, and 10 mM) were varied. Atomic force microscopy (AFM) was used to measure force-distance curves in the absence and presence of sound with a frequency of 15 000 Hz by attaching an earphone to our AFM liquid cell. Information about the EDL was obtained from the force curves. Decreased surface potential (Ψ₀) and Debye length (1/κ) values were clearly observed for concentrations of ≤0.1 mM when sound was applied. This decrease was explained by an increased electrostatic screening due to a more compressed EDL, which could result from a sound-wave-induced pressure in the aqueous solution. Sound may also induce a streaming current, which could increase the number of cations in the EDL and therefore reduce both Ψ₀ and 1/κ. The difference between the Debye lengths (Δ[1/κ]) and surface potentials (ΔΨ₀) in the presence of sound compared with those in the absence of applied sound decreased with an increased concentration. The ability to compress the EDL would decrease with an increased EDL ion density, which would increase with concentration. The values of Δ[1/κ] and ΔΨ₀ increased in the order of K⁺ < Na⁺ < Li⁺; this is the same order as the cation hydration number. The EDL of highly hydrated cations is therefore thought to be compressed more by sound than the EDL of poorly hydrated cations. Sound is therefore concluded to be capable of changing the EDL and forces acting between charged surfaces in electrolyte solutions, where the degree of this change can be controlled via the concentration and cation type.