In view of the presence of Na+ and K+ ions as metal complexes of proteins and nucleic acids in our body and their physiological importance, the ultrasonic velocity and its derived parameters were employed to investigate the solute-solvent and ion-solvent interactions in leucine-aqueous urea, NaCl-aqueous urea, and KCl-aqueous urea systems. The presence of short-lived clusters of water flickering among them was employed to understand the solute-solvent interactions. The successive increases in solute concentration resulted in increasing the ultrasonic velocity, specific acoustic impedance, internal pressure, and solubility parameter. Similarly, the corresponding decreases exhibited by the compressibilities and pseudo-Gruneisen parameters suggest an increase in the solute-solvent interactions. The change and the relative change in adiabatic compressibility with solute concentration suggest an almost ideal behaviour. The structure of leucine-aqueous urea may consist of a mixture of several probable zwitterionic/native leucine entities, which are associated with flickering clusters of water. The water clusters of the bulk experiencing weak influence of the latter surround these, in turn, followed by successive sheaths of a large number of hydrogen-bonded water molecules/clusters. The field of influence caused by specific interaction dies out in the successive sheaths with distance. Such a field of influence affecting the extent of interaction may increase with successive increases in solute concentration. This seems to be due to the close proximity of the interacting entities. Successive increases in temperature brings the structural entities close to each other as a result of increased thermal/kinetic motion. The structures of sodium and potassium chlorides in their solutions differ in being surrounded in their first spheres by relatively fewer or more of flickering water clusters due to their ionic size differences. Urea and KCl contribute more in disrupting the H-bonds compared to NaCl-aqueous urea in which only urea disrupts the hydrogen bonds. On the other hand, the chloride ions may be present in their probable clusters, Cl--(H2O)(n=1-10) and swim in the bulk along with the said cationic structures. (C) 2004 Elsevier B.V. All rights reserved.