I had talked about my specific heat measurements also with ionic solutions, and I had talked also about magnesium sulphates there, and magnesium is a divalent ion, has two charges, sulphate has two charges, so it has very strong hydration, and therefore a quite big effect on the specific heat. So after I finished my talk, and after Tamm and Kurtze had finished their talk, the theoretical physicist Becker said, 'Well, isn't there a relation between the two talks? Eigen has talked about the hydration of magnesium and sulphate ions, and you find a high sound absorption, and you find two maxima. Couldn't it be that one maximum is due to the magnesium ion and the other maximum due to the sulphate ion?'
Now we must first see what does it mean, how does sound absorption come about. The first who did a theory on that was Albert Einstein in about 1916, 1917, he did a theory on sound absorption in gases. At that time, by the way, Nernst tried to measure it... the methods were not good enough to see the effect. Now here... what is the cause of the sound absorption is what we call a relaxation effect. It means that, to give you an example, you have... let's say you have a chemical equilibrium. Now a sound wave is a pressure wave, and the equilibrium is pressure dependent, so in the sound wave the equilibrium will always adjust to the corresponding pressure. That means it will oscillate also, but it can only follow it if the reactions are fast enough. In other words, if you have a megacycle sound wave the reaction must be complete in a microsecond, in a millionth of a second, in order to get to megacycle, which is 10-6 cycles per second, or if it is at 100 megacycles reaction, must be as fast as being complete in 10-8 seconds and so. And now the question is: if the reaction does not go fast enough, then it would lag behind the pressure wave. There will be a face shift between the shifting equilibrium and the pressure wave, and always in physics where you get face shift, you get absorption of energy, some energy is lost in that process, and that causes the sound absorption. So when it goes through a maximum, you know, at this frequency, if you take the reciprocal of this frequency, must be the relaxation time of a physical process which gets out of phase with the sound wave, and Becker thought maybe that the hydration, the water, interaction with the magnesium ion, gets out of phase in that range and the same happening with the sulphate.
I wasn't so impressed by that, so I immediately told Tamm and Kurtze, you can easily show this within one day. You know already that sodium chloride does not cause any sound absorption. So take sodium sulphate. There you have only the sulphate ion which could make the effect, and on the other hand take magnesium chloride, you know all of the chloride ion doesn't do anything, there you have only the magnesium ion. They immediately did the measurement and neither magnesium chloride nor sodium sulphate gave any sound absorption, so it couldn't be true that it was solvation or hydration of the two ions. It must be that both ions, magnesium plus sulphate, interact, and we soon had a complete theory for that. It is the penetration of the sulphate ion into the co-ordination shell of water molecules around the magnesium, and that goes in two steps. The outer shell goes fast, in 10-8 seconds, and the inner shell takes a microsecond, 10-6. At that moment, I remembered Eucken's words of the immeasurably fast reaction. I said, 'This is the method to measure chemical reactions, because this is microsecond'. Hartridge-Roughton only went down to a millisecond, a thousandth, this is a millionth of a second, or 10-8, is 10 nanoseconds, that's billionths of a second.
[Q] So the method was, just to start in the solution?
Yes.
[Q] I understand.
... to prevent the mixing. In equilibrium mix, the compounds are mixed, you disturb the equilibrium, in this case it was the pressure which disturbed the equilibrium, temperature wave is almost negligible because of the density maximum of water, so it's purely the pressure wave... So put in a pressure change, and watch it. And that can be done fast enough, so you prevent the mixing. And I say, this is the idea. I went to Bonhoeffer and said, 'When I come can I have a laboratory?' Instead of using sound pressure I use electric field density. Electric field you know propagates with light velocity so you can be even faster than with your sound waves. And so, that was the birth of the relaxation methods which brought me the Nobel Prize a few years later.
