Abstract
The incorporation of the sodium lauryl ether sulfate (SLES) on phosphatidylcholine (PC) liposomes has been studied with time using the fluorescent probe 2-(p-toluidinyl)-naphthalene-6-sodium sulfonate (TNS). This probe reports changes on the surface potential of PC liposomes by effect of the anionic surfactant adsorption. The addition of small SLES amounts promoted an abrupt fall in the fluorescence intensity (F.I.) and this fall was already detected 10 secs after mixing. Only slight changes with time were observed in the F.I. of SLES—liposome-probe systems. These results indicate a fast and almost complete incorporation of SLES on the liposome surface. The surfactant/lipid molar ratios (Re) and the surfactant partition coefficients bilayer/aqueous phase (K) were determined from the linear dependence between lipid and surfactant concentrations obtained for a fixed number of surfactant molecules on the liposome surface. TheRevalues indicated that the highest SLES ability to adsorb on the surface occurs at the initial adsorption moments. TheKvalues indicated that the affinity of the surfactant by the liposomes decreased after about 7500 surfactant molecules were adsorbed on the bilayer. This fact is probably caused by the increase of electrostatic repulsion between surfactant monomers in the bulk solution and the bound surfactant. The aforementioned linear dependence obtained from the data for a given number of SLES molecules on the bilayer and the range of SLES concentrations used (below/above its critical micelle concentration, CMC) suggest an adsorption mechanism regardless of the surfactant concentration: a monomeric adsorption of SLES is always assumed in both above/below surfactant CMC. In comparison with the higher adsorption reported for the analogous sodium dodecyl sulfate, the ethylene oxide moles in SLES could hinder its incorporation on PC liposomes. Thus, this study of sublytic interactions allows to follow the incorporation of surfactants on the lipid bilayers and to compare the effect of different surfactants on the membranes before the lysis.
Highlights
The anionic surfactants sodium dodecyl sulphate (SDS) and sodium lauryl ether sulphate (SLES) are widely used surfactants in industry, household and personal care products because of its good foaming properties
The higher critical micelle concentration (CMC) for the SDS shows that the ethylene oxide (EO) units in the sodium lauryl ether sulfate (SLES) structure decrease its CMC, in line with the results reported by Vollhardt et al [19]
From the data concerning to the SLES solutions, we see that the EO units in SLES decrease its CMC and its surface activity
Summary
The anionic surfactants sodium dodecyl sulphate (SDS) and sodium lauryl ether sulphate (SLES) are widely used surfactants in industry, household and personal care products because of its good foaming properties. The structural difference between these molecules is the ethylene oxide (EO) units spacing the hydrophilic and the hydrophobic part in the SLES. This surfactant has demonstrated to be milder than SDS [1] on the skin. The presence of EO moles appears to be the cause of the milder behaviour of the surfactants on the skin. In this sense, the use of phosphatidylcholine (PC) liposomes could help to understand the role of EO in the surfactant molecule and its relation with the irritation. We are aware that the PC liposomes are a simple membrane model, they can provide valuable information that would be difficult to attain in vivo
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