Surface Tension and Cohesive Forces fewer interactions at surface:
higher energy Surface tension measures the energy required to increase the surface area of a liquid • maximize IM interactions by minimizing exposed surface, keeping maximum molecules in low energy surroundings • higher IM forces = higher surface tension But what about forces between molecules at the surface? more interactions in interior: lower energy ☺
Cohesive forces: IM forces within a (liquid) substance Adhesive forces: IM forces between two substances at a surface If cohesive >> adhesive: beads and drops • liquid more attracted to itself than surface • e.g. water (H‐bonds) on Teflon or wax (weak dispersion) If cohesive ≈ adhesive: pools • e.g. water (H‐bonds) on glass (strong dipoles) If cohesive << adhesive: film • liquid more attracted to surface than itself • e.g. oil (dispersion) on glass or water (strong dipoles) Surfactants decrease surface tension and cause even materials with large cohesive forces to form films: polar ends and non‐polar tails act as a bridge between a polar solvent and a non‐polar surface. They can also create water‐soluble micelles with a non‐polar interior to render oily material soluble.