Fluid Experts

What’s in Liquid Coolant? Understanding the Chemistry Behind Reliable Liquid Cooling

Brendan O’Byrne

Developed in collaboration with Dr. Curtis Breville

Liquid coolant is far more than a heat transfer fluid

As liquid cooling becomes standard for AI and high-performance computing deployments, most discussions still focus on processors, cold plates, coolant distribution units (CDUs), and heat rejection infrastructure. Comparatively little attention is given to one of the most important components in the entire system: the coolant itself.

This creates an operational blind spot.

Many operators assume coolant is simply a liquid that transports heat. In reality, modern coolants are engineered fluids whose chemistry is carefully formulated to perform two equally important functions: efficiently removing heat and protecting the cooling system from the chemical and material degradation that naturally occurs over time.

Modern liquid coolants are designed to do much more than transport heat. They must protect equipment constructed from multiple metals, maintain chemical stability over years of continuous operation, minimize biological activity, support efficient heat transfer, and help preserve the long-term reliability of mission-critical infrastructure.

Understanding what is inside a coolant, and more importantly why each component exists helps operators make better decisions about maintenance, testing, and lifecycle management.

Every ingredient has a purpose

Although formulations vary between manufacturers, most liquid cooling fluids contain several categories of components, each serving a specific engineering purpose.

The exact chemistry is proprietary. The functions, however, are well understood throughout the industry.

Base fluid

At its most fundamental level, the base fluid serves as the primary medium for transporting heat away from electronic components and ultimately rejecting that heat from the cooling system.

Heat transfer, however, is only one of the functions a coolant must perform.

Depending on the cooling architecture, the fluid may also be responsible for protecting system materials, maintaining chemical stability, supporting reliable long-term operation, providing electrical insulation, or enabling freeze protection and other environmental safeguards.

Different liquid cooling technologies therefore use different types of base fluids.

Direct Liquid Cooling (DLC) systems commonly use water-based formulations that may include propylene glycol and engineered additive packages.

Immersion cooling systems use dielectric fluids specifically designed to safely contact energized electronic components.

Other specialized applications may use alternative formulations based on operating temperatures, material compatibility, environmental conditions, or OEM requirements.

Regardless of the formulation, the base fluid largely determines the coolant’s physical and chemical characteristics, including:

Thermal conductivity

Specific heat capacity

Viscosity

Density

Electrical properties

Freezing behavior

Compatibility with system materials.

Selecting the appropriate base fluid always involves balancing thermal performance, reliability, operational requirements, environmental conditions, lifecycle maintenance, and equipment compatibility.

A coolant should not be viewed as a single chemical. It is an engineered fluid in which the base fluid and additive package work together as an integrated system. Altering one component whether by dilution, contamination, or improper top-offs can change the performance of the entire formulation.

Corrosion inhibitors

Few liquid cooling systems contain only one metal.

Copper, brass, stainless steel, aluminum, nickel plating, and other materials may all exist within the same cooling loop.

Whenever dissimilar metals share the same fluid, galvanic corrosion becomes a long-term engineering concern if coolant chemistry is not properly maintained.

Corrosion inhibitors are added to help create protective barriers on metal surfaces, reducing the electrochemical reactions that gradually remove material from components.

Without effective corrosion protection, operators may experience:

Metal loss

Reduced heat transfer efficiency

Increased particulate contamination

Fouling of cold plates

Premature equipment degradation

Corrosion rarely becomes a major problem overnight.

Instead, it develops gradually, making routine coolant monitoring essential for identifying early warning signs before reliability is affected.

pH stabilizers and buffering additives

Coolant chemistry is dynamic.

Over time, exposure to oxygen, dissolved contaminants, system materials, and operating conditions can slowly change the fluid’s chemistry.

One of the most important parameters affected is pH, a measure of a coolant’s acidity or alkalinity, and an important indicator of its overall chemical health.

Rather than simply setting an initial pH value, modern coolant formulations often include buffering compounds that help resist sudden chemical changes.

This stability is important because rapid pH shifts can accelerate corrosion, reduce inhibitor effectiveness, and affect long-term material compatibility.

Operators should view pH as an indicator of overall chemical stability rather than an isolated measurement.

A changing pH often signals that other chemical processes are occurring within the cooling loop.

Biological control additives

Water-based cooling systems naturally create conditions where microorganisms may develop if left unmanaged.

Microbial growth can produce:

Biofilms

Flow restrictions

Reduced heat transfer

Sensor fouling

Changes in coolant chemistry

Many coolant formulations include additives intended to minimize biological activity under normal operating conditions.

These additives are not permanent.

Like other components within the coolant, they may degrade or become depleted over time, reinforcing the importance of periodic monitoring rather than assuming the fluid remains unchanged throughout its service life.

Additional performance additives

Depending on the formulation, manufacturers may also include additives designed to improve specific performance characteristics.

These may support:

Long-term chemical stability

Material compatibility

Oxidation resistance

Deposit prevention

Foam control

Operational longevity

Different manufacturers use different approaches, which is why operators should always follow OEM guidance regarding approved fluids.

The specific formulation matters less than understanding that every component contributes to maintaining reliable operation.

Direct Liquid Cooling and Immersion Cooling Use Very Different Fluids

Although both technologies fall under the umbrella of liquid cooling, they use fundamentally different types of fluids because they solve different engineering problems.

In Direct Liquid Cooling (DLC), coolant flows through cold plates and closed piping systems that remove heat directly from processors and other high-power components. These fluids are typically water-based formulations that may contain propylene glycol, corrosion inhibitors, buffering agents, and other additives designed to protect metallic components while maintaining efficient heat transfer.

In Immersion Cooling, the electronic equipment is submerged directly into a dielectric fluid. These fluids are electrically non-conductive and eliminate the need for water-based chemistry. Because they never come into direct contact with oxygen in the same way as aqueous systems, they present a completely different set of maintenance considerations, material compatibility requirements, and aging mechanisms.

Although both systems remove heat using liquids, they should never be treated as chemically equivalent.

Testing protocols, maintenance priorities, contamination risks, and lifecycle management strategies differ substantially between these two cooling architectures.

For operators managing mixed environments, understanding these differences helps ensure that maintenance programs remain aligned with the requirements of each technology rather than applying a one-size-fits-all approach.

For readers interested in broader liquid cooling architectures, the Open Compute Project Cooling Environments provides a useful overview of current deployment approaches.

Why OEM Recommendations Matter

Every OEM validates its equipment using specific coolant formulations that meet defined performance and compatibility requirements.

These recommendations are not simply procurement preferences.

They reflect extensive testing related to material compatibility, seal performance, corrosion resistance, thermal efficiency, and long-term reliability.

For operators, the objective should not be identifying a “better” coolant than the one recommended by the OEM.

Instead, the focus should be understanding three practical questions.

Is the coolant approved for this equipment?

Compatibility with pumps, seals, elastomers, cold plates, and heat exchangers should always be confirmed.

Has the coolant remained within its intended operating condition?

Even an approved coolant changes over time. Monitoring its condition is just as important as selecting the correct product.

Has anything changed inside the cooling loop?

Maintenance activities, component replacements, water additions, contamination events, or improper top-offs may all alter coolant condition without changing the product label.

Following OEM guidance while independently monitoring coolant health provides operators with two complementary layers of protection.

One validates that the correct fluid is being used.

The other verifies that the fluid continues performing as intended throughout its operational lifecycle.

For additional guidance, NVIDIA publishes public documentation describing liquid cooling requirements for many AI platforms.

There Is No Universal “Best” Coolant

One of the fastest ways to oversimplify liquid cooling is to ask which coolant is “best.”

In reality, coolant selection is an engineering decision influenced by multiple variables, including system architecture, operating temperature, materials of construction, environmental conditions, maintenance strategy, and OEM validation.

A formulation optimized for one application may not be appropriate for another.

For example, a coolant designed for a closed Direct Liquid Cooling loop has different design priorities than a dielectric immersion fluid.

Likewise, two water-based coolants may share similar ingredients while using different inhibitor packages or buffering systems to meet the requirements of specific equipment manufacturers.

From an operational perspective, the question should not be

“Which coolant is the best?”

Instead, operators should ask:

Is this coolant approved for my equipment?

Is it still performing as intended?

Am I monitoring its condition throughout its service life?

These questions shift the discussion away from product comparisons and toward operational reliability, which is ultimately the outcome that matters most.

Why operators should care

Most operators will never need to know the exact chemistry of their coolant.

They do, however, need to understand one important reality.

Every component inside the coolant has a job to perform.

Over time, those components can change.

Corrosion inhibitors may become depleted.

Contaminants may enter the system.

Trace metals may begin appearing in solution.

Biological activity may increase.

Buffering capacity may decrease.

None of these changes are necessarily visible during normal operation.

A system can continue cooling equipment effectively while subtle chemical changes are already developing inside the fluid.

By the time cooling performance is noticeably affected, corrective actions may become significantly more expensive and disruptive.

This is why coolant health should be viewed as an operational reliability issue rather than simply a chemistry issue.

Common misconceptions

“Coolant lasts forever.”

No engineered fluid remains unchanged indefinitely.

Temperature cycles, oxygen exposure, material interactions, and operational events gradually alter coolant chemistry.

The objective is not necessarily replacing coolant on a fixed schedule.

The objective is understanding its condition over time.

“If there are no leaks, the coolant must be healthy.”

Leaks represent only one potential failure mode.

Chemical degradation, contamination, corrosion, and biological activity may all occur without visible evidence outside the cooling loop.

“More glycol means better protection.”

Not necessarily.

Glycol concentration affects several fluid properties, including freeze protection and viscosity.

However, coolant performance depends on the complete formulation working as an integrated system.

Increasing glycol concentration without engineering justification can reduce heat transfer efficiency and alter overall fluid performance.

Field Note

One of the most common misconceptions encountered in liquid cooled environments is treating coolant as a permanent utility rather than a managed asset.

Experienced operators monitor UPS batteries, generators, filters, pumps, and water treatment because they understand these systems change over time.

Coolant should be viewed through the same operational lens.

Its condition evolves throughout the lifecycle of the cooling system, and understanding those changes allows maintenance decisions to become proactive instead of reactive.

Coolant is part of the infrastructure

In many respects, coolant behaves much like any other critical infrastructure asset.

It requires commissioning.

It requires monitoring.

It requires periodic assessment.

It occasionally requires corrective action.

Ignoring coolant chemistry simply because the system appears to be operating normally increases the likelihood that small issues will become larger operational problems.

Organizations investing millions of dollars in AI infrastructure are increasingly recognizing that fluid lifecycle management deserves the same discipline applied to electrical systems, mechanical equipment, and environmental controls.

Shield Perspective

At Shield by Guardian, we believe operators do not need to become chemists.

They do need to understand that coolant is an engineered fluid whose condition changes over time.

Knowing what functions the coolant performs provides the foundation for understanding why routine sampling, laboratory analysis, trending, and lifecycle management contribute directly to long-term system reliability.

The objective is not simply maintaining coolant.

The objective is maintaining confidence in the infrastructure that depends on it.

Key Takeaways

Liquid coolant is an engineered fluid, not simply a heat transfer medium.

Every additive performs a specific engineering function that supports reliability.

Coolant chemistry changes throughout the operational life of the system.

Many forms of coolant degradation develop long before visible operational symptoms appear.

Monitoring coolant health supports better maintenance decisions, reduces operational uncertainty, and helps protect long-term infrastructure performance.