Glassy water, or amorphous ice, is a non-crystalline solid form of water. Unlike familiar ice cubes, its molecules are not arranged in an ordered, repeating pattern. This unique state challenges our everyday understanding of water, behaving more like a frozen liquid than a typical solid.
The Unusual States of Water
Water is an exceptional substance, known for its various solid phases, far beyond the common hexagonal ice (Ice Ih) found on Earth. Amorphous ice lacks the long-range, ordered structure seen in crystalline ice, where molecules are precisely aligned. Glassy water is a specific type of amorphous ice, characterized by its glass-like properties. While crystalline ice is predominant on Earth, amorphous ice is thought to be the most common form of water in the universe, particularly in colder environments like space.
How Glassy Water Forms
Scientists primarily create glassy water in laboratory settings using methods that prevent water molecules from forming a crystalline structure. One common approach is rapid cooling, also known as vitrification. This involves cooling liquid water extremely quickly (greater than 10^5 K per second), preventing molecules from arranging into a crystalline lattice. Examples include spraying fine water droplets into very cold liquefied gas (e.g., propane) or onto a supercooled solid surface below 80 Kelvin. Another method cools capillary tubes of water with liquid helium at 4.2 Kelvin.
Another way to create amorphous ice is through immense pressure. Applying high pressure to ordinary ice can transform it into different amorphous ice phases. For instance, compressing hexagonal ice at 77 Kelvin to its extrapolated melting point of 10 kilobars can produce an amorphous phase. This method essentially “melts” a solid by pressure, resulting in a new type of amorphous ice.
Distinct Properties of Glassy Water
Glassy water exhibits unique physical and chemical characteristics that set it apart from liquid water or crystalline ice. Its defining feature is its amorphous structure, lacking the long-range molecular order found in crystalline ice. This disordered arrangement is more akin to liquid water, but with the molecules frozen in place.
Glassy water can also display density anomalies, with its density varying significantly depending on its formation method. For example, low-density amorphous ice (LDA) has a density of about 0.94 g per cubic centimeter, while high-density amorphous ice (HDA) can reach densities of 1.17 g per cubic centimeter at 0.1 MPa. When heated, glassy water undergoes specific phase transitions; hyperquenched glassy water tends to crystallize around 150 Kelvin before melting. Glassy water is metastable, meaning it is kinetically stable but not the most thermodynamically favored state.
Why Glassy Water Matters
Glassy water holds significant scientific importance and various practical applications. In cryopreservation, it plays a role in preserving biological materials like cells, tissues, and organs by preventing the formation of damaging ice crystals. Vitrification, the process of forming glassy water, helps achieve this crystal-free state, maintaining the viability of preserved biological samples.
Glassy water is also relevant to astrobiology and space science. It is considered the most common form of water in the universe, prevalent on interstellar dust grains and icy moons. Understanding its behavior in extreme extraterrestrial environments helps scientists comprehend water’s role in the cosmos and its potential for supporting life beyond Earth. Studying glassy water also contributes to fundamental research, deepening our understanding of water’s complex properties and anomalies. This research could also lead to advancements in fields such as materials science.