You're observing a transition between a clear microemulsion and a turbid emulsion, which suggests phase separation beyond a certain dilution point. Here are some key points to consider for understanding and designing your system:

1. Microemulsion Formation Process

Microemulsions form when oil, water, and surfactant (with or without cosurfactants like propylene glycol) spontaneously organize into nanometer-sized droplets. The formation process follows these stages:

• Surfactant and oil mixing: Your oil phase and Tween 80/propylene glycol system form a clear solution because the surfactant stabilizes the oil.
• Water addition: Initially, adding small amounts of water may still result in a clear system due to microemulsion formation.
• Critical point of phase transition: When you exceed a certain water content, the system becomes unstable, leading to phase separation and a transition to an emulsion (milky white).

2. Order of Transparency and Turbidity Appearance

• Low water content → Clear microemulsion
• Increase water content gradually → System remains clear if microemulsion is stable
• Beyond critical water concentration → Turbidity appears as the system undergoes phase inversion or breaks down
• Further dilution → System may not return to clarity, indicating emulsion formation instead of microemulsion

3. Understanding the Phase Behavior

The behavior you describe suggests a Winsor IV (single-phase microemulsion) at lower water content, which transitions to a Winsor II or Winsor I (bicontinuous or water-in-oil / oil-in-water system) upon further dilution. The loss of clarity indicates that your system is moving out of the microemulsion region into an unstable emulsion phase.

4. Design Considerations

• Phase Diagram Construction: To optimize microemulsion formation, construct a pseudo-ternary phase diagram (Oil-Surfactant-Water) to map out microemulsion regions.
• Surfactant-to-Oil Ratio: Your ratios (1:9, 2:8, etc.) suggest a focus on oil dilution. If you want a stable microemulsion at higher water content, consider increasing surfactant concentration.
• Cosurfactant Role: Propylene glycol affects the hydrophilic-lipophilic balance (HLB). Adjusting this or incorporating additional short-chain alcohols (e.g., ethanol) may extend the microemulsion region.
• Titration Method: Adding water slowly while stirring thoroughly may help prevent abrupt phase transitions.

I hope this might be helpful for you.

Thanks.

For understanding you should take in mind that 'micro''emulsions' are actually micellar solutions. If the solubility limit is exceeded of 'normal' or reversed micelles, emulsions are formed, i.e., droplets of one phase in the other. Increasing the concentration of solubility enhancers of the system extents the 'micro''emulsion' range.