Practical Examples of Nanoemulsions

The HLB temperature was determined using conductivity measurements, whereby 10 moldm NaCl was added to the aqueous phase (to increase the sensitivity of the measurements). The concentration of NaCl was low and hence had Uttle effect on the phase behaviour. 

Nanoemulsions were prepared by rapid cooling of the system to 25 °C, and the droplet diameter was determined using PCS. The results are summarised in Table 14.1, which shows the exact composition of the emulsions, HLB temperature, z-average radius, and polydispersity index. 

O/W nanoemulsions with droplet radii in the range 26-66 nm could be obtained at surfactant concentrations between 4% and 8%. The nanoemulsion droplet size and polydispersity index was shown to decrease with increases in surfactant concentration this effect was considered due to the to the increase in surfactant interfadal area and the decrease in interfacial tension, y. 

As mentioned above, y reaches a minimum at the HLB temperature, and therefore the minimum in interfacial tension would occur at a lower temperature as the surfactant concentration increased. This temperature would become closer to the cooling temperature as the surfactant concentration increased, and this would result in smaller droplet sizes. 

The results with isohexadecane are summarised in Table 14.2. As with the hexadecane system, the droplet size and polydispersity index were decreased with increases in surfactant concentration. Nanoemulsions with droplet radii of 25-80run were obtained at 3-8% surfactant concentration. It should be noted, however, that nanoemulsions could be produced at lower surfactant concentration when using isohexadecane, when compared to results obtained with hexadecane. This could be attributed to the higher solubility of isohexadecane (a branched hydrocarbon), the lower HLB temperature, and the lower interfacial tension. 

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