Syntheses of 2‐(N‐alkyl‐N,N‐dimethylammonio)ethyl hydrogen phosphates and their physicochemical properties

A series of new amphoteric surfactants, having a quaternary ammonium group and a phosphoric acid group, 2‐(N‐alkyl‐N,N‐dimethylammonio)ethyl hydrogen phosphates (alkyl: dodecyl, tetradecyl, hexadecyl), were prepared by introducing a phosphoric acid group into N‐alkyl‐N,N‐dimethyl‐(N‐2‐hydroxyethyl) ammoniumiodide, followed by neutralization with sodium hydroxide and removal of inorganic salts. By the evaluation of the physicochemical properties, it was found that the phosphobetaines behave like “nonionic” surfactant in the zwitterionic region, having very small CMC values in comparison with sodium 2‐(N‐alkyl‐N‐methylamino)ethyl hydrogen phosphates. Additionally, similar trends were also observed in the experiments from the point of the effect of electrolyte and temperature on CMC, respectively. It should be considered that the hydrophobicity of the surfactant molecule is increased by the electronic interaction between the quaternary ammonium group and a phosphoric acid group. On the other hand, the phosphobetaines gave the smaller values of the occupied area per molecule at the air/water interface than sodium 2‐(N‐alkyl‐N‐methylamino)ethyl hydrogen phosphates. Therefore, it is clear that the phosphobetaines have higher surface‐active properties than the amphoteric surfactants.