Study of the size and morphology of aggregates formed by pentaethylene glycol monooctyl ether (C8EO5) in n-heptane

The size and morphology adopted by aggregates formed by a non-ionic surfactant, pentaethylene glycol monooctyl ether (C8EO5) in deuterated heptane were studied in detail. Self-diffusion coefficients were estimated by means of 1H-NMR DOSY measurements. Considerable differences in the diffusion coefficients were found for samples with concentrations below and above the critical aggregate concentration – CAC – of the surfactant. The difference is attributed to the presence of aggregates. No significant change in the diffusion coefficient was observed by increasing surfactant concentration; therefore the aggregates exhibit a homogeneous size distribution. Through the diffusion coefficient, a study of the possible size and shape of these aggregates was made. Four different structures were considered: prolate and oblate ellipse, sphere and cylinder, for which structural parameters were calculated using the general Stokes–Einstein equation. The results allow delimiting the size for the possible configurations that C8EO5 aggregates exhibit in heptane.

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► We established the presence of aggregates in C8EO5-heptane solution.
► The DOSY 1H-NMR technique enables aggregate characterization.
► We determined a compact aggregate which completely excludes solvent.
► Our analysis established that aggregates can adopt a prolate ellipse or cylindrical structure with an aggregation number higher than two dozens.