Nanoemulsions Prepared by a Two-Step Low-Energy Process

A simple low-energy two-step dilution process has been applied in oil/surfactant/water systems with pentaoxyethylene lauryl ether (C<sub>12</sub>E<sub>5</sub>), dodecyldimethylammonium bromide, sodium bis(2-ethylhexyl)sulfosuccinate, sodium <i>n</i>-dodecyl sulfate−pentanol, and hexadecyltrimethylammonium bromide−pentanol. Appropriate formulations were chosen for the concentrate to be diluted with water to generate oil-in-water (O/W) emulsions or nanoemulsions. For the system of decane/C<sub>12</sub>E<sub>5</sub>/water, bluish, transparent nanoemulsions having droplet radii of the order of 15 nm were formed, only when the initial concentrate was a bicontinuous microemulsion, whereas opaque emulsions were generated if the concentrate began in an emulsion-phase region. Nanoemulsions generated in the system decane/C<sub>12</sub>E<sub>5</sub>/water have been investigated both by dynamic light scattering (DLS) and contrast-variation small-angle neutron scattering (SANS). The SANS profiles show that nanodroplets exist as spherical core−shell (decane−C<sub>12</sub>E<sub>5</sub>) particles, which suffer essentially no structural change on dilution with water, at least for volume fractions ϕ down to 0.060. These results suggest that the nanoemulsion droplet structure is mainly controlled by the phase behavior of the initial concentrate and is largely independent of dilution. A discrepancy between apparent nanoemulsion droplet sizes was observed by comparing DLS and SANS data, which is consistent with long-range droplet interactions occurring outside of the SANS sensitivity range. These combined phase behavior, SANS, and DLS results suggest a different reason for the stability/instability of nanoemulsions compared with earlier studies, and here it is proposed that a general mechanism for nanoemulsion formation is homogeneous nucleation of oil droplets during the emulsification.