A salty safety solution – Physics World (2022)

Taken from the December 2017 issue of Physics World

As the wintry months take their grip on the northern hemisphere, Johan Wåhlin and Alex Klein-Paste explain why salt keeps us safe on icy roads

You’ve spent the weekend skiing in the Norwegian mountains. The conditions were amazing: it had snowed all week and when you arrived on Saturday, the Sun was shining and there was a crisp chill in the air. But by Sunday afternoon the weather had warmed up and now, as you drive home through the dark forest-covered mountains, it’s raining. The road, whose covering of white packed snow had made driving up so easy, is now icy, slippery and dangerous.

Your car’s fitted with studded tyres, but you’re struggling to get a good grip. One false move and you could skid out of control, slide down the mountain, crash into a tree or hit another vehicle. Your heart’s in your mouth, your hands are sore from gripping the steering wheel and your eyes are focused wide.

Then, salvation: you reach the main highway. The glassy forest road merges into black, bare tarmac, and you soon see why this road is free of snow or ice – a winter maintenance truck is reassuringly trundling along in the opposite direction, spreading salt on the roadway. But what exactly happens when we spread salt on icy and snowy roads? How does it transform these dangerous highways into safe surfaces to drive on?

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Mounds of mineral

Salt – or sodium chloride (NaCl) – is the most common material used to increase grip on icy roads and it is an invaluable tool for winter maintenance services in many countries. In Norway, where we both live, some 250,000 tonnes of road salt are used on average every year, while the US gets through about 17 million tonnes and the UK uses two million tonnes. Salt is particularly efficient in those countries where winter temperatures fluctuate above and below 0°C, and for roads with high volumes of traffic (more than about 1500 vehicles per day).

In Norway, the government’s Public Roads Administration does not do maintenance itself but instead hires contractors for defined regions. Their staff follow the weather forecast closely so that they can respond both pre-emptively and reactively to weather events. This strategy leads to three different approaches to spreading road salt: anti-icing, anti-compaction and de-icing.

Anti-icing is a pre-emptive operation, in which salt is applied to stop ice forming on a road in the first place. This strategy is used, for example, when the road is wet and temperatures are expected to fall below 0°C, or when the air is humid and the roadway is cold, causing frost to build up.

Anti-compaction, meanwhile, refers to roads being salted before or during snowfall. In this case, the purpose is not to melt all the snow, but to prevent that snow compacting into a strong, hard crust that is difficult to remove. Contractors aim to get the salt down before too much snow has fallen, as salted snow is less cohesive so easily removed by traffic and snow ploughs.

As for de-icing, this involves using salt to get rid of ice that is already on the road and is strongly attached to the surface. Contractors try to avoid de-icing because it means their anti-icing or anti-compaction actions have failed and dangerous conditions have formed. De-icing, in other words, is a reactive strategy intended to regain control over a precarious situation.

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The de-icing power of chaos

But how does road salt work? The simple answer is that salt dissolves in water, increasing its entropy and thereby lowering its freezing point. Entropy – often loosely referred to as chaos – is a measure of disorder in a substance. Dissolving salt in water provides the solution with more possible geometric configurations than pure water and therefore creates a system that is more disordered. Since the universe tends to higher entropy, a more disordered phase (i.e. liquid) becomes more attractive relative to its other phases (i.e. vapour and ice). The salt therefore stabilizes the water relative to ice, meaning a lower temperature is needed to freeze the liquid. And the more salt we dissolve, the lower this freezing point becomes. For most de-icers the relationship between concentration and freezing point has been determined experimentally.

While this is a key mechanism behind salt’s powers, it does not explain everything. Take the anti-icing strategy, which is designed to stop water freezing. That should be simple, right? Presumably you just need to apply enough salt to give the water on the roadway a freezing point lower than the coldest road temperature expected from the weather forecast. It turns out, however, that to prevent freezing, contractors only need 40% of what they use to melt ice. The reason for this discrepancy is that salt also changes the way water freezes.

We are all taught at school that liquid water becomes solid when it reaches its freezing point (0°C). For the most part (minus the inherent complexities of any phase transition), this is true – you put a tray of water in the freezer and after a while there’s no liquid left, just solid ice. That’s simple enough. Salt water, on the other hand, doesn’t completely become a solid at its freezing point (below 0°C) – there will be both solid and liquid present, and the solid does not contain salt.

This is because ice does not accept salt ions into its lattice – it has strict geometric constraints on where and how water molecules can be placed. An ice lattice forms a pattern of layered hexagons and for a foreign ion to be included, it must fit either inside the hexagons or between the layers. However, salt ions (Na+ and Cl) are too large to do this. As a result, the freezing process only removes water molecules from the solution and leaves the salt unaffected in the remaining liquid water. The salt in the leftover solution therefore becomes more concentrated, the freezing point falls further and freezing stops – you end up with some ice and some salty liquid. If the temperature drops further, more ice will form but there will be liquid present all the way down to the “eutectic temperature” of the salt-water mixture – the lowest possible temperature for a solution to be stable. Below this temperature, the salt crystallizes and the remaining water freezes. For sodium chloride this occurs at –21°C.

Adding to salt’s power, ice that forms in a salt solution consists of branch-like crystals (figure 1) that aren’t seen in fresh-water ice. These “dendritic needles” are not as strong as solid ice made from pure salt-free water and will be destroyed by traffic if they form on a road. Only when more than 60% of the solution has been transformed into ice will it be strong enough to become a problem for road users.

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No salty snowballs

So how does salt work when spread on snow to stop it from compacting? Again, the decreased freezing point is central but it doesn’t explain everything. Salt melts snow because, as mentioned earlier, salt solution has a higher entropy than ice – it is more attractive for the water molecules to be in the solution than be solid crystals. But to melt all the snow that falls would require extreme amounts of salt; much more than is actually used.

Instead, the salt melts a bit of snow and the resulting salt solution changes the mechanical properties of the remaining solid. It’s a bit like what happens when you make a snowball. “Wet” snow – snow close to its melting point (0°C) – sticks together really fast to make good, strong snowballs that can withstand being lobbed through the air. This happens through a process called sintering, whereby the small ice crystals constituting the snow bind together. Snow has a very large surface area – after all, a flat snowflake with all its arms has very little interior and a lot of surface. This structure is, however, unfavourable, from a thermodynamic point of view, giving the material a high surface energy. But because ice exists very close to its melting point, its molecules are very mobile. The molecules will then move from energetically unfavourable positions (snowflake arms) to more favourable positions at the contact point between crystals. The snow crystals therefore bond together, or “sinter”, providing strength to the snow.

Salt water, however, is very different to fresh water. If the snow is wetted by salt water, you can’t make a perfect, proper snowball – the crystals don’t stick to each other and the snow behaves more like a collection of very small pearls. The reason for this is not entirely understood, but scientists believe that the dissolved salt might attach to the water molecules on the surface of the ice, preventing them from forming bonds to other water molecules in other ice crystals. The result on a road is that we need to melt only 20% of the falling or fallen snow, and the remaining snow can be easily removed.

This brings us to de-icing – where water on the road has already frozen. Once again, de-icing is all about salt melting ice. But this only happens until meltwater dilutes the solution so much so that a new equilibrium is reached – the freezing point of the diluted salt solution becomes equal to the road temperature.

In the winter maintenance industry, we use the term “melting capacity” to state how many grams of ice one gram of de-icer can melt, with the lower the temperature, the lower the melting capacity of a de-icer. But remember that contractors perform de-icing when the road is already too slippery. It is therefore important that a de-icer not only melts a lot of ice, but also does it quickly – the melting rate is equally important as the melting capacity. Specifically, at low temperatures, there are salts other than sodium chloride that are more efficient de-icers, such as magnesium chloride or calcium chloride. Indeed, in some parts of the world, such as the US, these compounds are used when it becomes particularly cold.

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Good salt, bad salt

Salt can also have negative affects. Although it helps keep roads safe and predictable during winter, salt corrodes exposed metal, damaging vehicles as well as roadside installations, such as signs and barriers. Road salt can also damage vegetation near the salted roads and contaminate freshwater sources.

It is therefore important not to use more salt than necessary and systematically work on minimizing the salt exposure. Indeed, in Norway, salt-spreading is a finely honed affair, with the amount used depending on a road’s importance, usage and local climate. On 13,000 km of roads (about a seventh of the entire network), the aim is to keep them completely free of snow and ice throughout winter. These are the most important roads, seeing between 1500 and 80,000 vehicles per day, and are routinely salted and ploughed. On smaller roads, salt is used only when the temperature is close to 0°C and not when it’s colder – the roadway is kept bare when the weather is mild but a snow/ice crust is allowed to form during colder periods. On the rest of the network, the aim is to maintain a snow/ice crust throughout winter, and only use salt at the beginning and end of the season. This strategy requires snow ploughing to keep the layer level and spreading abrasive particles such as sand when the crust becomes too slippery. While this is still practised in many snow-rich countries, in more temperate climates virtually all roads are salted in winter.

At the Norwegian University of Science and Technology (NTNU) in Trondheim, our group is trying to find out how little salt we need to keep the roads safe. To do this, we need to understand the fundamental physics through which salt acts for its different purposes. For example, we are trying to work out the minimum amount of salt that needs to be added to snow to get sufficient anti-compaction and also what effect salt has on the formation of “hoar frost” (the deposition from humidity onto road surfaces). But knowing how salt works is only one piece of the puzzle. The mineral only helps as long as it is spread on the road and not in the roadside ditch. Unfortunately, traffic can easily blow or spray salt off the road, so another research topic in the field of “winter maintenance” is to develop methods to keep salt or other de-icers longer on the road and improve models to predict their operational longevity after they have been applied.

The purpose of all this work is to come closer to an optimal salting practice whereby contractors use as much salt or other de-icers as needed, but as little as possible without compromising safety. If we can reach this salting nirvana, it could mean fewer of those treacherous journeys on slippery, icy roads and a more relaxing end to your weekend skiing trip.

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