Temperature Effects on the Interactions between EHEC and SDS in Dilute Aqueous Solutions. Steady-State Fluorescence Quenching and Equilibrium Dialysis Investigations

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Abstract

The interaction between the hydrophobic, nonionic cellulose derivative ethyl hydroxyethyl cellulose (EHEC; fraction CST-103) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied as a function of temperature from 20 to 50 °C in dilute aqueous solutions, i.e. a polymer concentration slightly below the critical overlap concentration (c*) and a surfactant concentration up to three times the normal critical micelle concentration (cmc). Methods utilized in this investigation include equilibrium dialysis and steady-state fluorescence quenching. The results show that the average aggregation numbers of the polymer-bound SDS clusters decrease with an increase in temperature although the magnitude of the effect is composition dependent and is most pronounced for compositions which give the largest cluster sizes. The adsorption of SDS to EHEC shows a break-point at an intermediate value of the adsorption isotherm above which the cooperativity increases. This break-point diminishes and disappears, i.e. the cooperativity decreases, as the temperature increases up to 50 °C. It is suggested that the mechanism responsible for these two steps in the adsorption process is at first adsorption of SDS to aggregated EHEC chains and then to a mainly deaggregated state of EHEC. The critical surfactant concentration where the adsorption to the polymer starts seems to be slightly shifted toward lower values as the temperature is raised from 20 to 50 °C. To summarize the results, the interaction between EHEC and SDS gets more intensive as the temperature is raised. Two fluorophore/quencher pairs, which previously have been used for determination of average aggregation numbers in aqueous surfactant and polymer−surfactant systems utilizing the steady-state fluorescence quenching technique, are compared. A good agreement between the two pairs is reported. Reference measurements of average aggregation numbers and adsorption isotherms for the PEO/SDS/water system are also given.

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