Functional Application Areas

Surfactant Interactions

Surfactants are amphipathic molecules, i.e. they have a hydrophilic part and a hydrophobic part. These can be cationic, anionic, zwitterionic or non-ionic. Surfactants form micelles when the concentration goes above its CMC (critical micelle concentration).  Surfactants are additives to solubilize membrane preparations and are used to make consumer products like shampoo and detergent.  Surfactants are also used as part of drug delivery systems.

The active components of many detergents, cosmetic and pharmaceutical products are a blend of surfactants with long-chain synthetic polymers. These mixtures impact phase transition and interfacial properties.  The behavior of surfactant-polymer mixtures are quite different from individual polymer or surfactant solutions due to surfactant micelles binding to polymers below their CMC.

Knowledge of interactions involving surfactants is important to understand how they function in biological systems. The complete characterization of any binding interaction requires a quantification of the affinity, number of binding sites, and the thermodynamics. 

Thermodynamic data, specifically enthalpy (ΔH) and entropy (ΔS), reveal the forces that drive complex formation and mechanism of action. Thermodynamics provide information on conformational changes, hydrogen bonding, hydrophobic interactions, and charge-charge interactions.  This information is used to describe the function and mechanism at a molecular level. 

Isothermal Titration Calorimetry (ITC) is a powerful analytical tool which measures the binding affinity and thermodynamics between any two biomolecules. ITC is considered the “gold standard” assay for binding.  ITC is used to study the CMC of surfactant solutions, as well as the interaction of surfactants with membranes, proteins, and other biomolecules.  ITC provides a direct measurement of surfactant-polymer interactions, and can characterize the processes which occur during such an interaction.

The binding interactions will depend on the charge of the surfactant, the ratio of binding components, and other conditions. For a full characterization of a biomolecular interaction, it is important to observe how salt, pH, temperature, etc affects binding affinity and thermodynamics.

Surfactants and surfactant-polymer mixtures are being incorporated into pharmaceuticals as part of a drug formulation and drug delivery, making this an important area of research.

References

Enthalpies of Micellization of Double Chain and Gemini Cationic Surfactants.
Bai G., Wang Y., Yan H., and Thomas R. K.  
J Colloid Interface Sci 240, 375-377 (2001)

The demicellization of alkyltrimethylammonium bromides in 0.1 M sodium chloride solution studied by isothermal titration calorimetry.
Beyer K., Leine D., and Blume A.
Colloids Surf B Biointerfaces 49, 31-39 (2006)

Titration calorimetry of surfactant-membrane partitioning and membrane solubilization.
Heerklotz H. and Seelig J.
Biochim Biophys Acta 1508, 69-85 (2000)

Insights on polymer-surfactant complex structures during the binding of surfactants to polymers as measured by equilibrium and structural techniques.
Tam K.C., and Wyn-Jones E.
Chem Soc Rev 35, 693-709 (2006)

Isothermal Titration and Temperature Scanning Calorimetric Studies of Polymer-Surfactant Systems
Olofsson G., and Wang G.  
in Polymer-Surfactant Systems.  Kwak J.C.T. ed., Marcel Dekker Inc. New York, pp. 317-356 (1998)

Aggregation behavior of p-n-alkylbenzamidinium chloride surfactants.
Talhout R., Stafforst T., and Engberts J. B.  
J Colloid Interface Sci 276, 212-220 (2004)

ITC – Surfactant Interactions, Critical Micelle Concentration Reference List

ITC – Surfactant-Polymer Interactions Reference List

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