Emulsification surfactant chemical structure

In the past [4-6] it was common to characterize amphiphiles according to their major performance in food systems (1) emulsification and stabilization, (2) protein interactions, (3) polysaccharide complexation, (4) aeration, and (5) crystal structure modification of fats. Such classifications correlate the surfactant chemical structure to its interaction (chemical or physical) with substrates such as fats, polysaccharides, and proteins. It was confirmed fhat certain surfactants interact molecularly with macromolecules, forming complexes and/or hybrids, and alter the macromolecular behavior at the interface. Such activity is an important new contribution of cosurfactants to the surface performance of other surfactants [7]. Such interactions are sometimes a very important contribution of amphiphiles to food systems. 

The physico-chemical theory of surface activity is a vast field and no more than broad principles can be touched on here major reference sources exist for those who require more detail of the relationship between chemical structure and the various surfactant properties such as wetting, detergency and emulsification-solubilisation [32-36]. 

The emulsifying properties of these polymeric surfactants demonstrate that the chemical structure influences the kinetic behaviour of interfacial tension reduction. An increase of sulfopropyl moieties reduces the interfacial tension slower while an increase in 2-hydroxy-3-phenoxy propyl moieties reduces the interfacial tension faster. The ionic strength of the emulsion appears to increase the rate of tension reduction. The average droplet size of oil-in-water emulsions in presence of previously dissolved 2-hydroxy-3-phenoxy propyl sulfopropyl dextran is around 180 nm immediately after preparation and increases with time. The presence of ionic moieties appeared to facilitate emulsification at low polymer concentrations due to electrostatic repulsions between the oil droplets [229]. 

It would be nice if the world of emulsion formulation were such that a simple correlation could be obtained between the chemical structure of a surfactant and its performance in practice. Unfortunately, the complicated nature of typical emulsion formulations (the nature of the oil phase, additives in the liquid phases, specific surfactant interactions, etc.) make correlations between surfactant structure and properties in emulsification processes very empirical. 

Therefore, in order to select which surfactants to use, one must first decide what performance is expected from the surfactant. To achieve the desired performance, the properties of the interface or the properties of the continuous phase must be altered by the adsorbed surfactant. Because performance is generally observable as a macroscopic property (such as emulsification or foaming) rather than as a chemical or molecular interaction (e.g., the formation of a monomolecu-lar film at an interface), it is important to understand the relationships between these macroscopic performance properties and the molecular level changes of the interfaces or the solution phase and to correlate them to the chemical structure of the surfactant. 

In contrast to the large achievements in investigations of kinetic stability, modest attention has been paid to the fundamentals of thermodynamic stability in emulsions, especially regarding the surfactant adsorption layer s influence on the coalescence time. There are several investigations devoted to the surface chemistry of adsorption related to emulsification and demulsification. However, the link between the chemical nature of an adsorption layer, its structure, and the coalescence time is not yet quantified. 

The HLB number can be calculated either from the structure of the surfactant molecule or from emulsification experiments. Several simple equations have been developed for certain types of non-ionic surfactants where HLB is calculated from the structural groups present (hydrophilic and lipophilic chemical contents). For example, for many non-ionic surfactants, HLB=E/5 where E is the wt% of hydrophilic content of the molecule. However, these equations often have no general suitability this is more so in case of ionic surfactants where the pH, the surrounding environment, salt content etc. also play a role. Hence, experimental determination is always a better approach. 

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