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where x is the concentration of emulsifier in the aqueous phase, R is the gas constant, and T is the absolute temperature. These equations are used to determine the surface excess concentration of emulsifiers from experimental measurements of the surface tension versus emulsi-fier concentration, with r as the slope of the initial part of the curve (Figure 5.7). The factor 2 appears in the denominator of Equation 5.3 because the counterions associated with the head groups of ionic surfactants also accumulate at the interface (Hunter 1986). Equation 5.3 is only applicable at low ionic strengths where the interaction between the head groups and counterions is strong. As the ionic strength increases, the electrostatic interactions between the head groups and counterions are screened, and so Equation 5.2 becomes more applicable. Knowledge of the surface excess concentration is important for formulating food emulsions because it determines the minimum amount of emulsifier which can be used to create an emulsion with a given size distribution. The smaller the value of r, the greater the area of oil-water interface which can be covered per gram of emulsifier, and therefore the smaller

Surface Tension y/mN

Surface Tension y/mN

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