Properties of micellar solutions. Part 2. - N.m.r. chemical shift of water protons in solutions of sodium alkyl sulphates

The n.m.r. chemical shifts of water protons in solutions of sodium alkyl sulphates have been measured. The results show that hydrocarbon chains in water cause a shift to higher applied field of the water proton resonance. The first four CH₂ groups from the sulphate groups have little effect. This is attributed to the influence of the head group on water structure in this region. For CH₂ groups after the first four, the effect is proportional to the number of CH₂ groups outside the region of influence of the head group. When micelles are formed the alkyl chains are removed from contact with the water and the effect disappears. The micelles have much the same effect on the chemical shift of the water protons as an equivalent number of isolated -SO₄^- groups. These results could be interpreted as showing that when alkyl chains are dissolved in water, hydrogen bonds in the water are broken. However, an alternative explanation is possible. This attributes the effect of the hydrocarbon chains on water to an increase in the covalent character of the hydrogen bonds in water.