The Velocity and Absorption of Sound in Aqueous Sugar Solutions

Abstract

Measurements of the absorption of sound have been made by a pulse method due to Pinkerton, and measurements of the velocity by a pulse-interference method, in the frequency range 6 to 25 Mc/s. Solutions of sucrose, d-glucose and lactose have been investigated at various concentrations and temperatures. The velocity results confirm the conclusion, originally derived from viscosity measurements, that the sugar molecules have `solvation envelopes' (attached layers of water molecules) which decrease in thickness as the temperature of the solution rises. The ratio of the actual absorption to the `classical' absorption is found to be fairly close to the value of this ratio for water (3.1) at all concentrations, while elementary considerations suggest that it should decrease towards unity as the concentration increases. A theoretical treatment reveals, however, an additional source of energy dissipation at high concentrations which approximately accounts for the constancy of this ratio. This dissipation arises from the high local viscous stresses which result from the squeezing-out of water molecules from between the sugar molecules when the solution is compressed. Similar considerations must apply to pure associated liquids if these contain two different states of molecular arrangement, one crystalline and one disordered. On this account a value of about 2.25 for the ratio of actual to classical absorption is to be expected for such liquids, while the observed values for alcohols and water range from 1.64 to 3.10. Some doubt is thus cast upon the theory that the major part of the absorption in water is due to a structural relaxation process.