Solubilization of Oil Droplets by Micellar Surfactant Solutions

Abstract

Traditionally, small molecule surfactants have been used in the food industry for three major purposes: (i) to enhance the formation and stability of emulsions; (ii) to modify food structure and rheology by interaction with biopolymers; and (iii) to alter the morphology of fat crystals by interaction with triacylglycerols (Krog 1990, Dickinson 1992). Recently, a number of novel applications of surfactant molecules have been identified which are likely to lead to the development of new processing techniques in the near future (Dickinson and McClements 1995, Wolcott 1995). These applications are based on the ability of small molecule surfactants to form molecular aggregates called micelles (Figure 1). Typically, a micelle consists of between 30 and 100 surfactant molecules, orientated so that their hydrophobic tails are located in the interior of the micelle (away from the water), and their hydrophilic head groups are located at the exterior (in contact with the water). This type of structure is formed because it minimizes the unfavourable contact area between polar and non-polar regions. In contrast to emulsions, micelles are thermodynamically stable systems. They also have highly dynamic and flexible structures, with surfactant molecules rapidly entering and leaving the micelle. Even so, they do have a clearly defined average size (distribution) and shape over measurable time scales. Typically, micelles are between 5 and 20 nm in diameter, and are therefore about a thousand times smaller than emulsion droplets.