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Abstract
The clouding of sodium dodecyl sulfate (SDS) in strongly acidic solutions has seen analytical use, but its mechanism has generally been misinterpreted. In the present work it was found that as SDS slowly hydrolyzes to form dodecanol, the solution passes through a series of compositions at which the aggregation of surfactant is promoted by nucleation onto traces of insoluble dodecanol. This occurred at concentrations well below the critical micelle concentration of SDS and resulted in mixed aggregates that grew to macroscopic size, giving the solution a cloudy appearance. The increasing dodecanol content eventually caused coalescence into a coacervate phase which evolved into a solid layer of dehydrated dodecanol. The process, which continued over an extended period, depended on the temperature and the concentration and type of acid used. The early stages of SDS aggregation were monitored through the I^sub 1^/I^sub 3^ ratio of pyrene fluorescence, which confirmed the existence of micelle-like aggregates at low surfactant concentration. The mixed SDS/dodecanol systems formed in acid hydrolysis were mimicked in neutral solution by combining the appropriate amounts of SDS, dodecanol, and NaCl. Clouding and the formation of a coacervate phase generally proceeded in a similar manner in these solutions.
Keywords Surfactant clouding * Acid hydrolysis * Sodium dodecyl sulfate * Aggregation * Dodecanol
Introduction
Temperature induced clouding of nonionic surfactants in aqueous solution has been studied extensively [1-5]. The occurrence of cloud points in these solutions is thought to be due primarily to the decrease of the dielectric constant of water with temperature, resulting in the coalescence of micelles into opaque particles [3, 6]. The clouding of anionic surfactants, in contrast, is only observed in acid media, or with the use of large hydrophobic counterions [7], and has been studied relatively little. A number of key aspects of this type of clouding remain unexplored. The work presented here deals with several important features of the clouding process in acidic sodium dodecyl sulfate (SDS) solutions.
When highly acidic solutions of SDS (e.g., in 2^1 M HCl) are heated, they first turn cloudy and then separate into two layers. The entire process typically extends over a period of several hours. Its analytical utility has been described along the lines of the more familiar clouding of nonionic surfactant solutions,...