关键词: 微乳, 抗凝聚稳定性, 耐盐稳定性, 显微成像分析, 双光学切片分析

Abstract:

The average diameter and size distribution of dispersed-phase droplets are important factors affecting the properties of emulsions, and the changes in these parameters with time and environment can be used to evaluate the emulsion stability. Traditional size characterization methods such as dynamic light scattering (DLS) are not applicable to highly concentrated emulsions. Herein, we report an imaging-based method to measure the droplet size in highly concentrated emulsions. This method comprises three steps: 1) emulsions are labeled with a fluorescent dye, 2) three optical slices with a certain distance between two adjacent focal plans are measured sequentially via confocal laser scanning microscopy, 3) the sizes of dispersed-phase droplets are determined from the apparent diameters of droplets in the optical slices. When the apparent diameter of a droplet in the three optical slices increases or decreases monotonically, droplet diameter is calculated according to the following equations: D_C1–2 = {D₂² +[(D₁² − D₂²)/4δ_z + δ_z]²}^1/2 or D_C2–3 = {D₃² + [(D₂² – D₃²)/4δ_z + δ_z]²}^1/2, where D₁, D₂, D₃ is the apparent diameter of the droplet measured from the consecutively-obtained optical slices 1−3, respectively; D_C1–2 represents the calculated diameter of the droplet from the slices 1 and 2, and D_C2–3 is that from the slices 2 and 3, and δ_z is the distance between two focal planes of the adjacent optical slices. To avoid an obvious interference from the droplet movement, we use the equation 2|D_C1–2− D_C2–3|/(D_C1–2 + D_C2–3) = X, where a smaller X value indicates a less extent of movement during measurement, and that the calculated average diameter (D_C1–2 + D_C2–3)/2 is closer to the measured size of the droplet. The experimental results showed that when X was 15%, the difference between the calculated and measured diameters was about 10%. When X was less than 15%, the calculated average droplet diameter was adopted as an effective diameter. However, when the condition D₁= D₂ ≥ D₃ (or D₃ = D₂≥ D₁) was met, D₂ was used as the effective droplet diameter. The present method combines the advantages of fluorescent labeling, double optical slices analysis, and a strategy for eliminating the error caused by droplet movement. The stability of highly concentrated emulsions (oil volume percentage: 60%−80%), prepared by mixing a crude oil model mixture containing n-decane, naphthaline, decalin, and tetraphenylporphyrin (92.3%, 4.1%, 3.6%, and 0.1‰ by mass, respectively) with aqueous solutions containing surfactants, was studied with the proposed method. The experimental results indicated that the present method allowed for the effective and accurate measurement of the anti-coalescence stability of emulsion dispersed-phase droplets. In contrast, the widely adopted "bottle test" method could not provide accurate information on the anti-coalescence stability of the dispersed phase droplets.

Key words: Emulsion, Anti-coalescence stability, Salt-tolerance, Micro-imaging analysis, Double optical slice analysis