Molecular interactions in a solution of alcohol and water primarily involve hydrogen bonding between the molecules. Both alcohol and water molecules contain -OH groups capable of forming hydrogen bonds, which are relatively strong intermolecular forces compared to typical van der Waals forces. These hydrogen bonds occur between water molecules, between alcohol molecules, and importantly, between alcohol and water molecules, creating a network of interactions in the mixture. However, the hydrogen bonding between alcohol and water molecules is generally weaker and less extensive than the hydrogen bonding within pure water or pure alcohol. This weakening of intermolecular interactions upon mixing leads to several important effects:

• Increased vapor pressure: Because the intermolecular forces become weaker, molecules can escape more easily from the liquid phase, increasing the vapor pressure of the solution compared to pure components.
• Lowered boiling point: The increased vapor pressure results in a decrease in the boiling point of the alcohol-water mixture relative to pure water or pure alcohol.
• Partial molecular ordering and clustering: Studies show that alcohol and water form complex hydrogen-bonded networks, sometimes leading to micro-immiscibility or clustering, which affects entropy and enthalpy of mixing.
• Solubility dependence on alkyl group size: Smaller alcohols (like methanol and ethanol) mix well with water due to strong hydrogen bonding, while larger alkyl groups hinder hydrogen bonding and reduce solubility.

In summary, molecular interactions, especially hydrogen bonding, dictate the physical properties and behavior of alcohol-water solutions by influencing volatility, boiling point, solubility, and microscopic structure of the mixture