Abstract
Aggregation numbers (NAg) of micelles of the commercial anionic detergent sodium lauryl ether sulfate (SLES), with an average of two ethylene oxide subunits, were determined at 30 and 40o C by the time-resolved fluorescence quenching method with pyrene as the fluorescent probe and the N-hexadecylpyridinium ion as the quencher. The added-salt dependent growth of SLES micelles (γ = 0.11-0.15, where γ is the slope of a plot of log aggregation number vs. log [Yaq] and [Yaq] is the sodium counterion concentration free in the intermicellar aqueous phase) is found to be significantly lower than that of sodium alkyl sulfate micelles (γ ca. 0.25), a difference attributed to the larger headgroup size of SLES. The I1/I3 vibronic intensity ratio and the rate constant for intramicellar quenching of pyrene show that the pyrene solubilization microenvironment and the intramicellar microviscosity are insensitive to micelle size or the presence of added salt.
Highlights
The worldwide consumption of detergents is dominated by anionic surfactants 1 and, among these, alkylbenzenesulfonates (LABS) predominate, followed by alkyl ether sulfates (LESS, Scheme 1) and linear alkyl sufates (LAS, Scheme 1) such as sodium dodecylsulfate (SDS)
The purified sodium lauryl ether sulfate (SLES) was characterized by 1H nuclear magnetic resonance (NMR), which confirmed the presence of an average of 2 oxyethylene units
critical micelle concentration (CMC) were determined from measurements of the surface tension as a function of log [SLES], performed on a du Noüy tensiometer equipped with a platinum ring
Summary
The worldwide consumption of detergents is dominated by anionic surfactants 1 and, among these, alkylbenzenesulfonates (LABS) predominate, followed by alkyl ether sulfates (LESS, Scheme 1) and linear alkyl sufates (LAS, Scheme 1) such as sodium dodecylsulfate (SDS). They are more expensive to produce than either LABS or LAS, LESS have several unique properties that enhance their utility in higher added-value personal care products and dishwashing formulations. The present work examines the [detergent]- and [salt]-induced growth of an important commercial LESS surfactant sodium dodecyl ether sulfate (SLES), which has an average of two ethylene oxide subunits in the ether bridge. The purified SLES was characterized by 1H nuclear magnetic resonance (NMR), which confirmed the presence of an average of 2 oxyethylene units
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