エントロピーと溶媒和

Entropy of mixing, or ΔSmixing, is the change in entropy when a solute and solvent combine during solution formation. This entropy change is independent of intermolecular interactions. During mixing, the solute disperses throughout the solvent, and solvent molecules directly interacting with solute molecules form a solvent shell or solvent cage around each solute particle.

When hydrocarbons dissolve in water, water molecules at the hydrocarbon-water interface rearrange to maximize hydrogen bonding with each other. This creates an ordered solvent shell with reduced motional freedom around each hydrocarbon molecule. The water in this shell has lower entropy than bulk solvent water, resulting in an overall entropy decrease during dissolution.

The hydrophobic effect is the entropy-driven separation of hydrocarbon and water molecules. When hydrocarbons clump together, the low-entropy solvation water is released to become higher-entropy solvent water, increasing total system entropy. This entropy gain favors hydrocarbon aggregation and the formation of separate hydrocarbon and water layers over dissolution.

The dielectric constant measures a solvent's ability to shield ions from electrostatic interactions. Polar solvents have high dielectric constants (ϵ ≥ 15) and effectively separate oppositely charged ions, reducing their tendency to associate. Apolar solvents have low dielectric constants and provide poor ion separation. The rule of thumb is that like dissolves like: polar solvents dissolve polar solutes, and apolar solvents dissolve apolar solutes.

Gas hydrates are crystalline solid forms of water and gas that form when methane and water freeze under high pressure and low temperature. The ice crystal structure contains relatively large open spaces where hydrocarbon molecules fit within the holes. This arrangement allows prediction of the maximum size of hydrocarbon molecules capable of forming clathrates, making gas hydrates one of the largest natural gas reserves.

Since entropy is the driving factor of hydrocarbon insolubility in water, temperature significantly influences the process. Lower temperatures and high pressures favor gas hydrate formation, where hydrocarbon molecules become enclosed within stable ice cages. Temperature changes alter the entropy balance between solvation and phase separation, affecting whether hydrocarbons remain dissolved or aggregate.

A good solvent has molecular characteristics similar to those of the compound to be dissolved, following the principle that like dissolves like. Polar solvents with high dielectric constants effectively dissolve polar solutes by shielding ionic interactions, while apolar solvents with low dielectric constants dissolve apolar solutes. Matching solvent and solute polarity maximizes favorable interactions and solubility.