Hydrophobic Interactions of Methylureas in Aqueous Solutions Estimated with Density, Molal Volume, Viscosity and Surface Tension from 293.15 to 303.15 K

Density (ρ), viscosity (η), and surface tension (γ) for 0.005–0.25 mol ⋅ kg−1 solutions of urea, 1-methylurea, and 1,3-dimethylurea solutions have been measured at intervals of 0.005 mol ⋅ kg−1. Apparent molal volume (Vo, cm3 ⋅ mol−1) and intrinsic viscosity coefficients (B and D) are calculated from the ρ and η values, respectively. Primary data were regressed and extrapolated to zero concentration for the limiting density (ρ0), apparent molal volume (Vφ0), viscosity (η0), and surface tension (γ0) values for solute–solvent interactions. The –CH3 (methyl) groups of N-methylureas weaken hydrophilic interactions and enhance hydrophobic interactions, and the values of the ρ0 and Vφo reflect the intermolecular forces due to electrostatic charge, whereas the η0 and γ0 values reflect the frictional and surface forces. The B values depict the size of hydrodynamic sphere due to heteromolecular forces whereas D shows the effect of concentration. The molar surface energy (ΔEm/sur) for dropwise flow was calculated from the γ values and decreases with concentration and temperature, but increases with –CH3 weakening of the hydrophilic interactions and strengthening the hydrophobic interactions.