O/W emulsion system performance: Difference in phase behavior between alkyl phosphate and PEG Emulsifier
The emulsification stability of O/W emulsion is not a single performance index, but the result of the combination of the molecular structure of the emulsifier, the interfacial adsorption method, and the macroscopic phase organization.At present, the mainstream O/W emulsifiers on the market are mainly divided into two categories, namely PEG emulsifiers that have been used for many years and alkyl phosphate emulsifiers with advanced performance.Both seem to be able to achieve the effect of oil-water emulsification, but there are essential differences in molecular adsorption, interfacial film formation, phase stability, and macroscopic performance.
I.The core logic of O/W emulsion stability
To distinguish the performance differences between the two types of emulsifiers, we must first clarify the core logic of O/W emulsion stability.A qualified stable emulsion needs to rely on the complete phase organization support of the molecular layer, the interfacial layer, and the macrostructure. Any imbalance in the structure of any layer will lead to the collapse of the overall system stability. This is also the key reason why the phase behavior determines the quality of the emulsion.
The first layer is the molecular adsorption layer, which is the basis for the stability of the emulsion.The emulsifier molecules need to be evenly adsorbed at the oil-water interface. Through the directional arrangement of hydrophilic and hydrophobic groups, the tension of the oil-water interface is reduced so that the grease can be evenly dispersed in the aqueous phase.The second layer is the interfacial film layer. The adsorbed emulsifier molecules will gather to form a continuous interfacial film, which wraps the oil droplets to resist the collision and aggregation between the oil droplets and avoid agglomeration and stratification.The third layer is a macroscopic droplet network structure, with countless stable wrapped oil droplets evenly dispersed to form a regular and uniform overall system, and ultimately achieve long-term stability.
The core gap between PEG emulsifiers and alkyl phosphate emulsifiers is reflected in the molding quality and stability of these three-layer structures.Different molecular structures have created very different interface orientations and phase organization forms, which also directly determine the stability, skin feeling, weather resistance and other core properties of the finished emulsion.In formula design, reasonable matching of oil in water emulsifiers is the key to controlling the basic performance of the system. The difference in the selection of the two types of emulsifiers will directly rewrite the comprehensive performance of the finished product.
II.Molecular structure differences
The phase behavior of an emulsifier is essentially determined by its own molecular structure. The molecular hydrophilic and hydrophobic group characteristics of PEG emulsifiers and alkyl phosphate emulsifiers are different. The adsorption method, binding strength, and migration ability at the oil-water interface are very different, and two completely different interfacial membrane structures are eventually formed.
2.1 PEG emulsifier
PEG emulsifiers such as Ceteareth-20 and Steareth-21, which are commonly used in daily formulas, are typical nonionic emulsifiers. The core stabilization mechanism relies on the hydration of the EO chain.The polyethylene glycol chain segment in its molecular structure can form a large number of hydrogen bonds with water molecules, build a dense hydrated layer on the surface of the oil droplets, and rely on solvation to achieve oil-water emulsification.
This adsorption method forms a flexible interfacial adsorption layer, which is characterized by high dynamic activity.The exchange speed of PEG emulsifier molecules on the interface is fast, and the interface can be quickly rearranged, which is suitable for the production needs of rapid emulsification, and is suitable for batch and efficient emulsion production.However, the shortcomings are also very obvious. The structural strength of the flexible interfacial membrane is limited, the binding force of molecules to the interface is weak, and the interfacial mobility is high, making it impossible to form a stable interfacial skeleton.
From the perspective of phase behavior, the O/W system built by PEG emulsifiers is a typical kinetic stable system, relying on continuous dynamic balance to maintain the emulsion state, rather than the rigid stability of the structure itself.This also explains why many PEG system emulsions are in good condition for short-term storage at room temperature, but after long-term placement, heating up or disturbance by external forces, they are prone to state fluctuations.
2.2 Alkyl phosphate emulsifier
The alkyl phosphate emulsifier represented by potassium potassium cetyl phosphate and c20-22 alkyl phosphate has a unique molecular structure and dual advantages.One end is a strongly polar phosphoric acid group, which can produce a strong ionic interaction with the aqueous phase and firmly bind the molecules of the aqueous phase. The other end is a long-chain hydrophobic alkyl structure, which can deeply anchor the oil phase and realize the strong binding of the oil and water phases.
Compared with the flexible membrane of PEG emulsifiers, alkyl phosphates can form a semi-rigid interfacial membrane, the interfacial binding force is greatly improved, the mobility of molecules at the oil-water interface is significantly reduced, and the regularity and stability of the interfacial structure are higher.The molecular bulk density on the surface of the oil droplets has been greatly increased, which effectively reduces the overall interfacial energy and builds a quasi-structured interfacial network.This structure is not rigid and cured, but has both stability and flexibility, which can resist interference from the external environment and ensure the smooth texture of the emulsion when applied.
Relying on the unique advantages of interfacial structure, alkyl phosphate emulsifiers are often used in the formulation of
Thickening suspension system. While achieving emulsification stability, they can also assist the system in thickening and suspending the powder, and solve the problems of emulsion stratification and powder sedimentation in one stop. Suitable for high-demand makeup and sunscreen products.
III.Phase organization and phase zone characteristics
Based on the different interfacial film-forming characteristics, the phase organization morphology, phase zone range, and phase drift resistance of the two types of emulsifiers in the O/W system are significantly differentiated. This is also the core source of the stability difference in the formula. The critical difference between the two can be clearly distinguished from the perspective of the phase diagram.
3.1 PEG emulsifier system
The range of O/W phase regions of PEG emulsifiers is relatively wide, and the HLB value can be adjusted to adapt to a variety of grease systems, and the formula is highly adaptable, which is also the main reason for its high popularity.However, the shortcoming of this system is that the phase stability is poor and it is sensitive to temperature, electrolyte, and storage time.
3.2 Alkyl phosphate system
The O/W phase region of alkyl phosphate emulsifiers is relatively concentrated, which requires higher accuracy of formula ratio, but in exchange for extremely strong phase stability.The quasi-structured interface network it builds has a stable structure, is not susceptible to temperature, electrolyte, and external force disturbances, and there is almost no phase drift problem.
When using an appropriate amount of fat-soluble active ingredients such as
Ascorbyl palmitate in the formula system, the alkyl phosphate system can be stably compatible with various active substances, and there will be no problems of system demulsification and component inactivation, and it is suitable for the research and development and production of high-value-added emulsions.
IV.Macro application performance comparison
The microscopic phase behavior differences will eventually be intuitively reflected in the macroscopic properties of the finished emulsion, including viscosity stability, skin texture, weather resistance stability and other dimensions, and the applicable scenarios of the two types of emulsifiers are directly divided.
4.1 Viscosity stability
PEG emulsifiers themselves have almost no thickening ability, and the viscosity of the system depends on the adjustment of additional polymers and thickeners, and the overall viscosity system is passive and fragile.At the same time, the PEG system is very sensitive to electrolytes. After adding minerals and active electrolyte components to the formula, it is prone to sudden drops in viscosity, thinning of the system, and poor storage stability.
The alkyl phosphate system has the ability to actively thicken, and the regular interfacial accumulation structure can directly provide viscosity support for the system. There is no need to add a lot of exogenous thickeners, and the viscosity of the system is more natural and stable.At the same time, its excellent electrolyte tolerance can be adapted to various additives, long-term storage viscosity fluctuations are minimal, and the finished product is in a more stable state.
4.2 Skin texture performance
PEG system lotion is smooth to apply and light to use.
The skin feeling of the alkyl phosphate system is more advantageous. Relying on the semi-rigid interface structure, it presents a structured and smooth texture during application, and the texture is uniform and delicate.
V.Exclusive product landing
In view of the current formulation pain points of beauty and skin care lotions, two green-grade alkyl phosphate emulsifiers PHSPTER™16PK and PHSPTER™922P, relying on the advantages of differentiated phase behavior, accurately adapt to different scenarios of O/W emulsion research and development, taking into account stability, skin feeling and functionality.
5.1 PHSPTER™16PK：
INCI is called potassium cetyl phosphate.
Its core advantage is excellent water resistance. After the film is formed, a uniform and breathable waterproof film can be formed on the surface of the skin, which can effectively enhance the waterproof ability of sunscreen and makeup-free products to hold makeup, and avoid the problem of sweating, water-stained makeup and sunscreen.At the same time, the skin feels extremely light, without a heavy film, and is suitable for lightweight makeup creams, moisturizing sunscreens, refreshing skin care lotions and other products.The product has a wide range of adaptation systems, and the recommended addition amount of the formula is 1-5%.
5.2 PHSPTER™922P
INCI is named c20-22 alkyl phosphate & Stearyl phosphate & Behenyl alcohol.
The appearance is white particles. As a high-stability skeleton-type emulsifying system, this product focuses on optimizing the thickening capacity of the system on the basis of retaining excellent water resistance.
Compared with single phosphate emulsifiers and PEG emulsifiers, 922P adapts to scenes covering high-end products such as high-stability makeup creams, high-magnification sunscreens, and makeup lotions with powder. The recommended dosage of the formula is also 1-5%. It has wide adaptability and simple operation. It is the preferred raw material for emulsifier makeup scenes.
VI.Formula selection summary
In the formula selection, if you pursue rapid emulsification, extremely refreshing and light texture, and control the cost of raw materials, ordinary skin care lotions and basic moisturizing lotions can choose PEG emulsifiers to meet the basic emulsification needs.However, if the product needs long-lasting stability, waterproof makeup, uniform powder dispersion, high temperature resistance freeze-thaw, or high-demand categories such as sunscreen, makeup cream, and high-end makeup lotion, alkyl phosphate emulsifiers are the best choice.
Among them, PHSPTER™16PK is mainly light and water-resistant, suitable for lightweight and moisturizing high-end emulsions, and PHSPTER™922P is mainly high-stability thickening, suitable for powder, high-stability, and high-holding makeup products. The two products accurately cover the core formula needs of high-end O/W emulsions.
Hubei Mingya New Material Technology Co., Ltd.
