Isopropyl alcohol (IPA), also known as isopropanol or rubbing alcohol, is a common chemical found in many households and industrial settings. It is frequently used as a household disinfectant, a general cleaner for electronics, and a solvent in various products. This substance evaporates very quickly, which is why IPA is favored for cleaning surfaces that should not remain wet, such as electronic components.
The Physics of Rapid Evaporation
The speed at which isopropyl alcohol evaporates is governed by its molecular structure and physical properties compared to water. Evaporation, or vaporization, is the process where a liquid turns into a gas below its boiling point, and this tendency is known as volatility. IPA is highly volatile because the attractive forces holding its molecules together are relatively weak compared to those in water. Water molecules are held together by strong hydrogen bonds, but IPA molecules form these bonds less extensively and with less strength. This difference means alcohol molecules require less thermal energy to leave the liquid state.
A liquid’s propensity to evaporate is also measured by its vapor pressure, which is the pressure exerted by its vapor in a closed system. IPA has a significantly higher vapor pressure than water at the same temperature, indicating more molecules are escaping into the gaseous phase. This high vapor pressure is directly related to its lower boiling point, which is approximately 82.3°C for pure IPA, compared to 100°C for water. Even at room temperature, many IPA molecules possess the necessary energy to transition into a gas, making the disappearance of the liquid rapid. When IPA evaporates, it absorbs heat energy from the surface it contacts, creating a noticeable cooling sensation on the skin.
How Concentration Affects Evaporation Speed
Isopropyl alcohol is almost always sold as an aqueous solution, meaning it is mixed with water, which significantly alters its evaporation profile. Common concentrations are 70% IPA, which contains 30% water, and 91% IPA, which contains only 9% water. The presence of water slows down the overall evaporation process because of water’s own strong intermolecular forces and higher boiling point.
When a 70% solution is applied to a surface, the alcohol component vaporizes rapidly, leaving behind a liquid mixture increasingly concentrated with water. As the percentage of water increases, the evaporation rate of the remaining liquid slows considerably. The remaining water requires substantially more energy to change state.
Consequently, a 91% IPA solution will evaporate faster than a 70% solution simply because it contains less of the slow-evaporating component, water. This difference is important for applications where residue is a concern, such as cleaning sensitive electronics, where the faster drying 91% solution might be preferred. Conversely, the 70% solution is often considered more effective as a disinfectant because its slower drying time allows the alcohol to remain in contact with microbes for a longer period.
The slower evaporation of aqueous solutions allows the liquid to remain on the surface long enough to achieve its intended purpose. This extended contact time is beneficial for applications like dissolving substances or killing germs.
Safety Considerations Related to Vaporization
The rapid vaporization of isopropyl alcohol creates two primary safety concerns that users must consider: flammability and inhalation risk.
IPA vapor is heavier than air and highly flammable, meaning it can travel along a surface to an ignition source and flash back. The vapor can form an explosive mixture with air when its concentration is between 2% and 12.7% by volume. IPA must be kept away from heat, sparks, open flames, and any equipment that could cause a static discharge. The low flash point, the minimum temperature at which the liquid releases enough vapor to ignite, is approximately 12°C (53°F) for pure IPA.
The high concentration of vapor produced in a short time also presents a respiratory hazard. Inhaling high concentrations of IPA vapor can lead to irritation, dizziness, and drowsiness. Therefore, use isopropyl alcohol only in well-ventilated areas or outdoors to prevent the buildup of fumes.