Purification of nonionic alkyl polyglycolether (CiEj) surfactants:: The "Inverse" 3PHEX technique

A new variant of the three-phase extraction technique (denoted as 3PHEX) for purifying nonionic alkyl polyglycolether (CiEj) surfactants is presented. Both the 3PHEX and its extension take advantage of the phase behavior of the ternary systems water-oil-CiEj. The conventional 3PHEX technique is used to extract simultaneously water- and oil-soluble impurities by removing the excess phases, keeping the microemulsion phase. The new variant, in contrast, aims at extracting surface-active ionic impurities into the microemulsion phase. Knowledge of the phase behavior permits choosing conditions where the microemulsion phase is small while the oil excess phase is large, minimizing the loss of surfactant. The oil excess phase contains most of the nonionic surfactant in monomerically dissolved form which is gained from it by distillation. The otherwise difficult to separate surface-active ionic impurities partition preferentially into the microemulsion phase, which is discarded. Thus, the new procedure is called "inverse" 3PHEX. The efficiency of the new method is demonstrated by measuring the lower miscibility gap (including the lower critical temperature T-c) and surface tension isotherms, which are both sensitive and suitable indicators for the presence of impurities. (C) 2001 Elsevier Science.