Phase Transition Kinetics in Oil-to-Milk Cleansing Systems
The cleaning performance of the product is no longer only determined by the solubility of the oil phase, but more depends on a lower-level process-Oil-to-Milk Phase Transition.
From the perspective of formulation, this kind of system is closer to an unbalanced phase conversion system than the stable mixed structure of “oil + Emulsifier + water” in the traditional sense.What really determines the experience is the speed of structural reorganization of the system at the moment of contact with water, and how the interface quickly changes from the continuous phase of oil to a flushable dispersed and emulsified state.
Oil-to-Milk Transition
When makeup remover products are spread on the surface of the skin, the system first manifests as a typical oil-linked continuous structure.Ester oils, hydrocarbon oils, and some silicone oil systems will spread rapidly, and dissolve and wrap makeup. This stage is usually the smoothest period.
But when water begins to intervene, the system will enter a very critical stage of interface instability.The local interfacial tension fluctuated rapidly, the emulsifier molecules began to rearrange, and the original continuous oil film structure was gradually destroyed.This process is not the beginning of simple emulsification, but the pre-kinetic stage of phase inversion.
Then the system entered a real turning point: the oil phase broke from a continuous structure to dispersed oil droplets, and gradually transformed into oil-in-water emulsion, visually showing a state of “emulsifying into milk”.But from a microscopic point of view, this is actually a rapid transition from the continuous phase of oil to the dispersed phase.
From a thermodynamic point of view, the oil-water system is inherently incompatible, and the final stable state is still stratified.However, the perceived “good or bad use” of consumers does not depend on the final state, but on the dynamic path-that is, the speed at which the phase transformation occurs and the stability of the path.
In other words, the key is not whether it will be layered, but how it will change before it is layered.This is also the biggest difference between the oil-to-emulsion system and the traditional emulsification system.
Interface behavior
In system design, interface behavior is often more critical than oil phase selection.The emulsifier is not just a stable structure here, it is more like an interfacial dynamics dispatcher.
In the initial oil phase stage, the emulsifier usually exists in the form of a single-molecule membrane, which is arranged preferentially towards the oil phase to maintain the continuity of the system.However, when the aqueous phase enters, these molecules will quickly hydrate and re-orient, the interface curvature begins to change, and the system gradually shifts from W/O tendency to O/W structure.
In this process, different emulsifying systems will exhibit completely different kinetic paths.For example, some systems are designed to favor rapid interfacial rearrangement, causing the oil phase to quickly break into small oil droplets, while others retain a longer transition state, making structural changes more slow and gradual.
This difference will eventually directly affect the flushing speed and residual feeling.
In actual formulas, this kind of interface regulation is often achieved in combination with specific oil phase systems.For example, in some lightweight solvent systems, such as C13 14 isoproaffin, it will provide lower structural resistance and make the phase transition occur faster; while in high-moisture systems, such as capric capric triglyceride, it is more inclined to form a more stable oil film structure, thereby prolonging the milk-to-milk path.
This combination relationship is essentially controlling the time of existence of the oil film.
Three-stage evolution of microstructure
If viewed from the perspective of structural evolution, the entire oil-to-milk process can be understood as a continuous but unstable three-stage path.
The first stage is a continuous oil phase structure, the system has a high viscosity and a complete oil network, which is mainly responsible for dissolving makeup and spreading.
The second stage enters a double-continuous transition state. At this time, the oil phase and the water phase begin to penetrate each other, and the interface continues to break and rebuild. It is the stage where the energy of the entire system fluctuates most violently, and it is also the key window that determines the final skin feeling.
The third stage is transformed into a typical oil-in-water emulsion. The oil droplets are dispersed and wrapped in the aqueous phase, the fluidity of the system is improved, it presents a milky white appearance, and it has good flushing performance.
Differentiation of flushing sensation: quick flushing vs residual
From the point of view of user experience, the oil-to-milk system will eventually divide into two typical feelings.
One is a rapid flushing system: the milk transfer speed is fast, the interface rigidity is low, the particle size of the oil droplets is small, and the system can quickly enter a dispersed state, so it is rinsed clean and the skin feels light.
The other is a high residual system: the oil film breaks down slowly, the transition state exists for a long time, and part of the oil phase is not completely dispersed, resulting in a heavy skin feeling.
This difference often does not depend on the amount of oil, but on the design of the phase conversion dynamics.
In some systems, the initial aqueous phase intervention will be delayed through the pre-structural design of the Water in oil emulsifier, thereby improving the makeup melting efficiency.However, this structure needs to undergo a stronger reverse rotation when encountering water, so the interfacial regulation ability of the emulsifying system is more demanding.
On the contrary, in a more rapid flushing system, the tendency of Oil in water emulsifier will be introduced earlier, causing the system to enter a dispersed state faster, thereby shortening the milk transfer path.
These two paths are essentially different orientations of “efficiency” and “skin feeling”.
Kinetic control: the real role of Emulsifiers
In the modern formulation system, the emulsifier is no longer just a stabilizer, but a phase conversion control unit.
It affects the system through three key mechanisms：
The first is to adjust the interfacial tension so that water can enter the oil film structure faster or slower.
The second is curvature control, which determines whether the oil phase tends to maintain a continuous structure or break rapidly.
Finally, the oil droplet crushing control directly affects the final emulsified particle size and milky white fineness.
When these three factors work together, the entire oil-to-milk process changes from a natural occurrence to a designable process.
Conclusion
The performance of the oil-to-emulsion system is essentially not determined by the solubility of the oil phase alone, but by the phase transformation dynamics and the interface reconstruction process.
Truly excellent systems often have two characteristics: one is that the milk transfer process is fast enough, and the other is that the milk transfer path is controllable enough.
When these two conditions are met at the same time, the final experience is usually: rinsed, the skin feels light, and there is almost no residual burden.
In this sense, the Oil-to-Milk Cleaning System is more like a precision-designed interfacial phase change system than just a cleaning product system.
Hubei Mingya New Material Technology Co., Ltd.
