Micelle Characterization by DLS: Bringing Viscosity into the Equation

Dynamic viscosity is an integral part of the Stokes-Einstein equation, which enables the calculation of particle size from DLS measurements. While performing calculations using the dynamic viscosity of the solvent might be an acceptable strategy for some samples, it introduces a sizeable bias in DLS results when particles strongly affect the suspension’s viscosity. Micelles are composed of amphiphilic surfactant molecules which spontaneously assemble into spherical structures in solution. Here we investigated particle size in a micellar solution submitted to variations in ionic strength, pH and surfactant concentration with the Litesizer™ 500, while viscosity was measured using a Lovis 2000 ME viscometer coupled to a DMA™ 5000 M densitometer. Especially for undiluted sample we observed that the particle size calculated using the measured dynamic viscosity differed significantly from that calculated using solvent viscosity. Diluting the sample in order to diminish the particles’ influence on overall viscosity is frequently used as an alternative to measuring dynamic viscosity. However this strategy was not applicable here, as we observed that dilution had a dramatic impact on the size of micelles. Together these data stress the importance of measuring the dynamic viscosity of samples to obtain accurate particle size results with DLS.

Micelles are spherical aggregates of surfactants with surface-active and amphiphilic properties. If the concentration of surfactant is below the critical micelle concentration (CMC), surfactant molecules are free to move in solution and position themselves on surfaces. If the surfactant concentration is high enough to reach the CMC, surfactants spontaneously start to form micelles in solution.

When surfactant concentration increases, the number of micelles increases as well. Micelle properties are also influenced by pH, temperature and the ionic strength of the solvent. Disodium cocoamphoacetate is a mild amphiphilic surfactant which is derived from coconut oil fatty acids of different chain lengths. It is frequently used in skin and hair conditioning products because it conditions as well as cleanses. Moreover, it increases the foaming capacity of cosmetic formulations when used as a co-surfactant. Unlike other surfactants like sodium lauryl sulfate, which is very widely used in cosmetics, disodium cocoamphoacetate has a relatively low detergent capacity and is thus mild to the skin and non-irritating.

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