Liquid crystalline phases and emulsion stability

The equilibrium surfeice pressure of the a-crystal form of monomyristin was also followed as a function of temperature, and it was found to have the same value as the plateau pressure at the transition from form 1 to form 11. One can then assume that the monolayer formed when excess of the a-crystal form is present has the structure illustrated as form 11 in Fig. 5.9. The agreement between molecular areas and transition temperatures indicates that the hydrocarbon chain structure of monolayer forms 1 and 11 is identical to that of the liquid crystalline phases and the a-crystalline gel phase respectively. 

The liquid crystalline state of the monolayer, which is always formed at a temperature above the existence of the crystalline monolayer, possesses ideal rheological properties. The monolayer with liquid hydrocarbon chains (form 1) can thus vary its curvature and cross-sectional area per molecule over wide ranges. Under certain conditions, however, it is possible to crystallize the monolayer after the emulsion has been formed. 

A maximum in emulsion stability is obtained when three phases exist in equilibrium, and it was therefore proposed that the lamellar liquid crystalline phase stabilizes the emulsion by forming a film at the 0/W interlace. This film provides a barrier against coalescence. This is illustrated in Fig. 5.12 which shows that the lamellar liquid crystalline phase exhibits a hydrophobic surface towards the oil and a hydrophilic surface towards the water. These multilayers cause a significant reduction in the attraction potential cuid they also produce a viscoelastic film with much higher viscosity than that of the oil droplet. In other words, the multilayers produce a form of mechanical barrier against coalescence. 

The rheological properties of monolayers of bineuy surfactant mixtures have been related to emulsion stability (see below) and to the structural properties of the lamellar liquid crystalline phases formed by the surfactant in water. It was suggested that the emulsifier molecules adsorbed at the 0/W interlace will adopt the same hydrocarbon 

Another important repulsive force that occurs when the surfactant him contains charged molecules, e.g. on addition of sodium stearate to lecithin, is the double layer repulsion arising from these ionogenic groups. An obvious consequence of the presence of charged chains is the increased distance between the surfactant bilayers. 

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