Micelle Characterization by Dynamic Light Scattering: Bringing Viscosity into the Equation
Investigating particle size in a micellar solution using Litesizer DLS 500 while measuring viscosity using a Lovis microviscometer coupled to a DMA density meter proves the significance of correct dynamic viscosity to obtain accurate particle size 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. The influence of pH, surfactant concentration and ionic strength on the particle size of micelles was investigated by DLS using the Litesizer DLS 500. A disodium cocoamphoacetate-based commercial micellar solution was used. As viscosity is an integral part of the Stokes-Einstein equation (Equation 1), it is essential to know the viscosity of the solution in order to calculate particle size accurately.
To this end, we coupled DLS measurements with dynamic viscosity measurements of the micellar solution using the Lovis, a rolling ball viscometer for viscosity measurements of dilute solutions.
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