Question

One of the attractive features of ionic liquids is their low vapor pressure, which in turn tends to make them nonflammable. Why do you think ionic liquids have lower vapor pressures than most room-temperature molecular liquids?

Short answer

Ionic liquids have lower vapor pressures because their strong ionic bonds limit evaporation compared to the weaker forces in molecular liquids.

Step-by-step solution

Define Ionic Liquids

Ionic liquids are salts in the liquid state, typically composed of ions held together by ionic bonds. Unlike molecular liquids, which consist of neutral molecules, ionic liquids are made up entirely of charged particles: cations and anions.

Understand Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. It reflects the tendency of particles to escape from a liquid to the gaseous phase. Higher vapor pressure means a liquid easily evaporates.

Analyze Ionic vs. Molecular Liquids

In molecular liquids, molecules are held together by weaker intermolecular forces such as Van der Waals forces, which allow them to escape into the vapor phase easily, contributing to a higher vapor pressure.

Examine Ionic Bonds in Ionic Liquids

Ionic liquids have strong ionic bonds between cations and anions. These bonds significantly restrict the movement of individual ions into the vapor phase, leading to a very low tendency for vaporization.

Conclusion on Low Vapor Pressure

Thus, due to the strong ionic interactions that require much more energy to overcome compared to the weaker intermolecular forces in molecular liquids, ionic liquids have much lower vapor pressures.

Key concepts

Vapor Pressure

Vapor pressure is a fundamental concept when discussing the physical properties of liquids, including ionic and molecular liquids. It describes the pressure exerted by the vapor of a liquid when it is in equilibrium with its liquid or solid phase at a given temperature. Essentially, vapor pressure shows how readily molecules escape from the liquid to become gas.
When you heat a liquid, the molecules gain energy and start to move more vigorously. Some succeed in breaking free from the liquid's surface and transitioning into gas. If a container is closed, these gas molecules will collect above the liquid, creating vapor pressure. The stronger the tendency for the molecules to become vapor, the higher the vapor pressure of a liquid.
In general, a liquid with high vapor pressure evaporates easily, meaning molecules require less energy to break free from intermolecular forces and escape into the air. This quality makes them volatile and often flammable. Conversely, a low vapor pressure indicates that the liquid does not evaporate easily, as molecules stay bound by their existing forces, like in ionic liquids.

Ionic Bonds

Ionic bonds are the forces that hold together the cations and anions in ionic liquids. These bonds are incredibly strong due to the electrostatic attraction between oppositely charged ions. This attraction essentially holds the structure of ionic liquids quite firmly.
In terms of chemistry, an ionic bond is formed when one atom donates an electron to one of its neighboring atoms, converting them into ions. The positive and negative charges that result—cations and anions—are attracted to each other. So, when we talk about ionic liquids, the liquid's nature is defined by these charged particles and the robust ionic bonds that exist between them.
The strength of ionic bonds in ionic liquids makes them excel in stability and low vapor pressure. Because the energy required to break these strong ionic bonds and allow ions to enter the gaseous phase is so high, ionic liquids don't evaporate easily. Unlike molecular liquids, where weaker bonds like Van der Waals become easier to overcome, ionic liquids remain quite stable, manifesting in their low volatility and making them an excellent choice for various industrial applications.

Intermolecular Forces

Intermolecular forces provide insights into how molecules within a liquid interact. These are generally weaker forces found between molecules, such as Van der Waals forces, hydrogen bonding, and dipole interactions that hold molecular liquids together. In contrast, ionic liquids are held together mainly by ionic bonds, which differ fundamentally from intermolecular forces.
In molecular liquids, these weaker interactions allow molecules to escape more readily into the gas phase, which contributes to a high vapor pressure. They require relatively less energy to overcome, causing the liquid to evaporate easily. This is why water, for example, has a noticeable vapor pressure even at room temperature, facilitated by hydrogen bonds—a type of dipole-dipole interaction.
For ionic liquids, the key lies in understanding that the charged ions perform a significantly different balancing act. With strong ionic bonds taking the place of typical intermolecular forces, the escape of ions into the vapor phase demands much higher energy. This fact is why ionic liquids, used increasingly in green chemistry, possess very low vapor pressures, highlighting their sustainability and reduced environmental impact.