Dynamic Light Scattering Detection for Accurately Sizing Small Surfactant Micelles

Sodium Dodecyl Sulfate (SDS) is a commonly used amphiphilic surfactant with a range of applications, including chemical disinfection to protein denaturation in SDS-PAGE gels, cell lysing for DNA extraction and nanoparticle solubilization. SDS has a well-known structure and properties as shown in Figure 1, including a critical micelle concentration of 8.3mM at 25°C. Critical micelle concentration (CMC) is the amphipilic surfactant concentration at which 50% of the surfactant exists in micelles while 50% is free surfactant.

An increase in the surfactant concentration results in a higher percentage being incorporated into micelles. The ultra-small SDS micelles are difficult to size accurately in solution but experimental evidence and theoretical predictions estimate the micelle radius at approximately 1.6-2.1nm.

A free energy model for micelle formation has been developed and rigorously tested against many surfactants, including SDS. Although the mathematical underpinnings are beyond the scope of this article, it is important to understand that the micelle radius depends to a degree on several variables, including temperature, pH, and ionic strength of the solution.

SDS preferentially forms into spherical micelles above the CMC as a single hydrophobic chain surfactant. The DelsaMax software optimization calculator is a very helpful feature for studying micelle formation.

In case the molecular weight and concentration of a sample are known, the calculator can determine the least number of acquisitions and time per acquisition needed to acquire accurate data. Time is saved by the calculator and uncertainty is eliminated for dynamic light scattering measurements with DelsaMax series instruments.

Structure of sodium dodecyl sulfate.

Figure 1. Structure of sodium dodecyl sulfate.

Experimental

The feature is most useful when working with smaller particles that scatter exponentially less light, such as surfactant molecules or protein. Within this study, the optimization calculator was used for determining the measurement parameters for all three SDS solutions that were studied as shown in Figure 2.

DelsaMax software optimization.

Figure 2. DelsaMax software optimization.

SDS (Sigma Aldrich) was weighed and dissolved in deionized water. Three solutions were prepared of SDS (molecular weight 288.4g/mol); 20 mM, 8 mM, and 2 mM. Solutions were filtered with a 0.02 µm filter (Anatop) before running in the DelsaMax CORE to remove any dust particles.

A volumetric pipette was used, 50 µl of SDS solution was placed into the disposable DelsaMax CORE cuvette. A magnifying glass was used to confirm that no bubbles were present in the 4µl sample reservoir.

The DelsaMax CORE was run at 25°C using the conditions listed in Figure 3. Eight trials were run at 20 mM, 4 trials were run at 8 mM, and 4 trials were run at 2 mM. The number of acquisitions/acquisition time was determined using the optimization calculator on the DelsaMax Software.

SDS sizing experiment run parameters.

Figure 3. SDS sizing experiment run parameters.

Auto-correlation functions. ACF functions were plotted for representative runs of SDS at all three concentrations. Representative runs were selected by having the lowest SOS (Sum of Squares) for Cumulant fitting. Cumulant fitting fits an exponential decay to the ACF in order to directly find the hydrodynamic diameter of the SDS.

Figure 4. Auto-correlation functions. ACF functions were plotted for representative runs of SDS at all three concentrations. Representative runs were selected by having the lowest SOS (Sum of Squares) for Cumulant fitting. Cumulant fitting fits an exponential decay to the ACF in order to directly find the hydrodynamic diameter of the SDS.

The formation of micelles was apparent at 20 mM SDS, while 2 and 8mM SDS had no indication of micelles. The fact that no micelles were present at 8mM, approximately at the CMC may be due to some loss of SDS during filtration of the solution, lowering the actual concentration.

It is apparent that the 2 mM and 8 mM SDS solutions contain mostly small molecules because of the rapid decay of the auto-correlation function (ACF) as shown in Figure 4. The ACF is the direct output from a dynamic light scattering measurement and can be constantly displayed on the DelsaMax CORE touch screen.

Results

The published values for the diameter of an SDS micelle are between 3.5 to 4nmf agreeing well with the average value found of 3.72 nm at 20 mM SDS as shown in Figure 5. At 2 mM, where predominantly free surfactant exists in solution, the average diameter was 0.5 nm with low polydispersity as shown in Figure 5, demonstrating the ultra-low limit of detection for the DelsaMax CORE.

This study helps confirm that the DelsaMax CORE is capable of handling tricky surfactant sizing experiments.

Hydrodynamic diameter and polydispersity vs. SDS concentration.

Figure 5. Hydrodynamic diameter and polydispersity vs. SDS concentration.

Beckman Coulter Life Sciences - Auto-Cellular and Proteomics

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.

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