Salt and surfing

In Western Australia we have some of the best beaches in the world, where you can relax and forget about chemistry. Nothing could be further from science than the sight of seagulls scattering over sand and surf. Or could it?

Imagine a beach scene where the waves never bristle with foam and don’t end in a sizzle. Just a few gurgles, a swish, and a thump might accompany each wave - like water in your bathtub, or coming out of your kitchen tap. It just wouldn’t be the same, would it? If our oceans weren’t so salty, this is exactly what would greet us as we lay down our towel on the yellow sand.

Chemists know that the surface chemistry of aqueous solutions is altered by the addition of surfactants such as detergents. These surfactants reduce surface tension and stabilise bubbles, even when the surfactants are present in quite small amounts. However, most chemists have tended to ignore the effect of inorganic ions on the stability of bubbles, and on the foam in the oceans. No doubt this is because most chemists are very poor at surfing.

In low concentrations - less than 0.1 M - most salts do not stabilise a froth. At higher concentrations, such as (surprise!) the salinity of seawater, i.e. about 0.5 M, a lot of salts stabilise small bubbles. However, to date there has not been a comprehensive explanation of why this happens. Try asking someone!

Since surfboard riding is so popular in Western Australia, we are compelled to provide an explanation. After all, we don’t want waves suddenly losing their white crests simply because there’s no scientific reason for their existence, do we?

First off - the effervescent foam in our oceans IS NOT simply the result of organic substances acting like detergents. Scientists - and you - can easily show this by bubbling air through pure water containing sufficient pure sodium chloride. Even when the organic substances have been carefully removed, solutions of salt in water give the same effervescence.

So what on earth, or rather what in the oceans, is happening? The best explanation seems to be that the inorganic ions crowd at the surface of the air bubbles, and in doing so stabilise the bubbles momentarily.

At the air-water interface, ions with a small ionic radius are likely to be adsorbed in preference to larger ions. The smallest ions are usually the positive ions (like sodium in sodium chloride), and these give the surface a positive charge. Immediately below this atomic layer of sodium ions, a layer of negative (chloride) ions is formed. Below the layer of chloride ions, a less well ordered layer of sodium ions can be found. After a few ionic layers formed in this fashion, the ions are no longer well organised and occur randomly.

So, a possible explanation for the nice surf at West Australian beaches is that the forces inherent in the structured layers of ions near the surface are enough to prevent bubbles coalescing as fast as they would in pure water. Unfortunately, you won’t read textbooks mentioning our great surf - or the reason for it.

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