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 (V _o, 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 (ρ ⁰), apparent molal volume (V _φ ⁰), viscosity (η ⁰), and surface tension (γ ⁰) values for solute–solvent interactions. The –CH₃ (methyl) groups of N-methylureas weaken hydrophilic interactions and enhance hydrophobic interactions, and the values of the ρ ⁰ and V _φ ^o reflect the intermolecular forces due to electrostatic charge, whereas the η ⁰ and γ ⁰ 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 (ΔE _m/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.