Clouding phenomena generally occur with nonionic surfactants when the temperature of the system is raised to a certain value. This critical temperature is termed the cloud point (CP). Here we report the CP phenomenon with anionic surfactant sodium dodecyl sulfate in combination with a few symmetrical quaternary bromides (tetra-n-butylammonium/phosphonium and tetra-n-amylammonium bromides). The CP has been found to depend upon the nature and concentration of the salt and the surfactant concentration. The CP appearance in these systems is discussed in terms of increased hydrophobic interactions due to the alkyl chains of the quaternary bromides. Measurements show that the viscosity either increases or decreases as the system approaches the CP. Most of the organic additives (aliphatic alcohols (C6-C8), amines (C6-C8), and hydrocarbons (n-hexane and n-heptane)) generally decrease the CP, while nonelectrolytes increase or decrease the CP depending upon their effect on water structure or interaction with the anionic micelle.

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Here we report the clouding phenomenon with sodium dodecyl sulfate (SDS) in combination with tetra-n-butylammonium bromide (Bu4NBr). Cloud point (CP) vs [Bu4NBr] curves are constructed for various fixed [SDS], and the minimum Bu4NBr concentrations required to produce CP are determined. A relationship between [SDS] and [Bu4NBr] is established that exists down to fairly low [SDS]. The presence of aromatic hydrocarbons (added up to their solubility limits) decreases the CP of a fixed SDS−Bu4NBr system. Furthermore, an increase in hydrophobicity of the aromatic hydrocarbon decreases the CP. An unusual CP-decreasing effect of thio compounds is observed, which is discussed in terms of direct interaction of these compounds with anionic SDS micelles.

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.

Viscosity measurements under Newtonian flow conditions have been performed at 40 °C to study the effect of aromatic hydrocarbons (benzene, toluene, or o-xylene) on aqueous micellar solutions of 0.1 M cetylpyridinium bromide (CPB) containing different salts. Two series of salts, viz. (i) inorganic (KX; X ) Cl, Br, or NO3) and (ii) symmetrical quaternary ammonium (R4NBr; R ) H, CH3, C2H5, n-C3H7, or n-C4H9), were used to explore the effect of their nature and concentration. The hydrocarbons had marginal effect on viscosity when added to CPB solutions having no salt. However, in the presence of salts, the viscosity behavior was quite different (synergistic effect). Relative viscosity (ηr) versus concentration of hydrocarbon plots were constructed for various fixed salt concentrations. Most of the time, after reaching a maximum value, ηr decreased on further addition of hydrocarbons, showing a peaked behavior. The peak position (maximum) as well as the viscosity at the maximum, ηr max , was found to be dependent on the nature/ concentration of salts, hydrocarbons, and counterions. However, the viscosity behavior was different with the R4N salts having a longer alkyl (R) part (the synergism progressively diminished). The effect of concentration of salt was reversed and peaked behavior was also lost. This reversal and change in behavior have been explained in terms of the salting-in nature of these salts as compared to the salting-out nature of the salts of series i.

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We have observed the cloud point (CP) in the ionic surfactant sodium dodecyl sulfate (SDS) in the presence of 0.2 M tetra-n-butylammonium bromide (Bu4NBr). To study the changes taking place in the system approaching the CP, we have performed SANS studies with 0.3 M SDS as a function of [Bu4NBr] and temperature. The spectra obtained with Bu4NBr addition show micellar growth. The temperature increase has different effects on 0.1 and 0.15 M Bu4NBr systems. Further, the ionic micelles show characteristics of nonionics for 0.2 M Bu4NBr as the temperature was raised. The overall phenomenon is conjectured in light of gradual displacement of water from the clefts which characterize the micellar surface.

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