Summary
Surfactant micelles are self-assembled colloidal structures used extensively in pharmaceutical development for solubilization of hydrophobic drugs, membrane protein studies, and as components of drug delivery formulations.
This application note demonstrates SAXS structural characterization of sodium dodecyl sulfate (SDS) micelles in aqueous solution. The study determines micelle size, shape, aggregation number, and internal structure using dedicated core-shell modeling — providing quantitative parameters essential for formulation development and quality control.
Background & Pharmaceutical Relevance
Micelles in Pharmaceutical Development
Surfactant micelles are used for:
- Drug solubilization: Increase aqueous solubility of hydrophobic APIs
- Oral formulations: Self-emulsifying drug delivery systems (SEDDS)
- Injectable formulations: Micelle-based IV formulations
- Protein extraction: Membrane protein solubilization and purification
- Model systems: Membrane mimetics for drug-membrane interaction studies
Critical Quality Attributes
- Micelle size: Affects drug loading capacity and bioavailability
- Aggregation number: Determines stability and CMC
- Shape: Influences viscosity and injectability
- Core-shell structure: Defines drug localization and release
Methods & Experimental Design
Sample Preparation
Surfactant: Sodium dodecyl sulfate (SDS), pharmaceutical grade
Concentration: 1% wt aqueous dispersion (well above CMC of ~0.24% wt)
Preparation: Mixed with stirrer for 24 hours at 40°C, then filtered
Measurement: Room temperature
Analysis: Differential SAXS pattern (SDS dispersion vs. buffer)
SAXS Measurement
Instrument Configuration
- X-ray sourceCu Kα (λ = 1.54 Å)
- Q-range0.01 – 0.50 Å⁻¹
- Exposure time45 minutes
- Sample holderFlow-through capillary
- BackgroundPure water subtracted
Core-Shell Modeling
- Model: Ellipsoidal core-shell structure
- Core: Hydrophobic alkyl chains (low electron density)
- Shell: Hydrophilic headgroups + bound water (high electron density)
- Fit parameters: Core radius, shell thickness, ellipticity, aggregation number
Results
Figure 1. SAXS differential pattern of SDS dispersion in buffer.
Figure 2. Reconstructed PDDF function. Minimums and maximums indicate the characteristic distances within the particle. Point C at ~41 Å estimates the average distance between opposite hydrophilic groups in the micelle. Point D at ~55 Å indicates the maximum distance within the particle.
Figure 3. Core-shell model of micelle showing hydrophobic core and hydrophilic shells.
Micelle Structural Parameters
Core Radius
Rcore = 1.55 nm
Hydrophobic alkyl chains (σ=13%)
1st Shell Thickness
0.38 nm
Inner hydrophilic heads (σ=9%)
2nd Shell Thickness
0.19 nm
Outer hydration layer (σ=22%)
PDDF Characteristic Distances
Dmax ~5.5 nm
Maximum dimension in micelle
PDDF Analysis
From PDDF function reconstruction:
- ~4.1 nm: Average distance between opposite hydrophilic groups
- ~5.5 nm: Maximum distance within the particle
- Hydrophilic-hydrophobic contrast: Clearly resolved in PDDF showing core-shell structure
Conclusion
This study demonstrates the reconstruction of the internal structure of SDS micelles in aqueous solution using SAXS. Both methods - PDDF analysis and core-shell modeling - result in comparable values for the particle size. The advantage of PDDF is flexibility and independence from the model, while core-shell modeling provides actual numerical values for the refined parameters.
* PDDF is a self-correlation function of relative scattering density within the particle. Maximums of PDDF function show the most populated vectors inside the particles connecting the areas with the largest scattering density.