The non-miscibility of oil and water

is attributed to differences in chemical polarity, hydrogen bonding, and thermodynamic principles involving free energy and entropy.
Water (H_2O) is a polar covalent molecule; electrons are drawn more strongly toward the oxygen atom due to its high electronegativity compared to hydrogen, resulting in a permanent dipole moment (partial negative charge on oxygen and partial positive charge on hydrogen). This polarity enables water molecules to form a cohesive network via strong intermolecular hydrogen bonds.
In contrast, oils consist primarily of triglycerides—long, non-polar hydrocarbon chains held together by weak Van der Waals forces (London dispersion forces). Because electron density is uniformly distributed across these chains, oil molecules lack a dipole moment and cannot interact effectively with polar species.
According to the chemical principle "like dissolves like," mixing two liquids requires breaking existing intermolecular bonds and establishing new ones between dissimilar molecules:
1. Polarity Mismatch: Non-polar oil molecules cannot break or replace the strong hydrogen bonds between water molecules, making mixing energetically unfavorable.
2. Hydrophobic Effect & Thermodynamics: Introducing a non-polar molecule into water forces surrounding water molecules into a highly ordered, cage-like clathrate structure. This restriction causes a significant decrease in system entropy (\Delta S), violating the natural tendency toward disorder (Second Law of Thermodynamics). To minimize this entropic penalty, oil molecules aggregate to reduce their total surface area exposed to water, thereby releasing constrained water molecules back into a higher-entropy state.
Additionally, density differences dictate the final spatial separation; most oils have a lower density (\approx 0.91\text{--}0.93 \text{ g/cm}^3) than water (1.00 \text{ g/cm}^3 at 4^\circ\text{C}), causing the oil layer to float atop the water due to buoyant forces.

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