Salt-induced cloud point in anionic surfactant solutions: Role of the headgroup and additives

Anionic surfactants are not known to show a clouding phenomenon in aqueous solutions. On the other hand, this is a general feature for nonionic surfactants. Here the effect of addition of tetra-n-butylammonium bromide (Bu4NBr) on the clouding phenomenon in sodium dodecylbenzenesulfonate (SDBS) has been studied by measuring cloud points (CP) for each combination. Similar type of studies were also performed with poly(ethylene glycol) t-octylphenyl ether (TX-100). The CP varies in an opposite manner for the two classes of surfactants, which is explained in terms of charge variation in each type of micelles by the addition of Bu4NBr. A relationship between [SDBS] vs [Bu4NBr] has been worked out for getting the CP - phenomenon in SDBS solutions: nearly one Bu4NBr molecule is needed for each two SDBS monomers for getting the CP in the system. With 10-mM SDBS, the addition of Bu4NBr shows an interesting phase behavior, where a stable colloidal phase with bluish - white appearance (preclouded) appears preceded by conventional clouding. Effects of the addition of ureas (urea and tetramethylurea), thioureas (thiourea and tetramethylthiourea), amino acids (glycine, alanine, leucine and phenylalanine), and sugars (xylose, arabinose and dextrose) have also been seen on the 50-mM SDBS + 35-mM Bu4NBr system (this system was chosen because its CP has a wider window available for variations below and above the CP). Ureas and thioureas affect the CP in different manners, which are explained in the light of indirect and direct interactions with micelles. CP variation in the presence of amino acids depends on their polar and hydrophobic nature. On the other hand, sugars behave in a manner similar to their effect on solubility of hydrophobic compounds in aqueous solutions.