Isolation of amphoterics

These four classes behave differently in ion-exchange systems, and the best method of isolation depends on which amphoteric and which other classes of surfactant are present. The behaviour of WW and WS amphoterics in ion-exchange systems is well characterised, at least for those with only one acid and one basic group, but that of SW amphoterics is not. They are believed to behave as described here, but there is little direct experimental evidence. SS amphoterics (sulphobetaines) do not interact with ion-exchange resins. 

One might speculate about the anionic, cationic or zwitterionic character of amphoterics with more than one acid or basic group or with both strong and weak acid or basic functions (some of which are commercially available), but there appears to be no published information about their behaviour in ion-exchange columns. If confronted by one of these, it is a simple matter to pass samples through the columns of acidic and basic resins in the acid, base or salt form and see what happens. It should then be straightforward to work out a system for the 

1 WW and SW amphoterics. Make up a solution in 80% methanol, if possible. If carboxylates are present, remove them by extraction with petroleum ether from acidified 50% methanol. Use the minimum possible amount of acid. Add IV2 volumes of methanol to give a final concentration of 80%. The procedure then depends on what other surfactants are present. 

Anionics only present. Remove the anionics on a column of either a strongly basic anion exchanger chloride or a weakly basic anion exchanger hydrochloride. The effluent contains the amphoteric and the displaced chloride ions. For SW amphoterics ensure that the solution entering the column is acid, so that the amphoteric is in the zwitterionic form and will therefore not be retained. 

Anionics and any other surfactant present. Combine the two columns already described, in the order given. The first column retains sulphates and sulphonates, the second retains the amphoterics, and all other species pass through. Elute as before. 

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