The Design Logic of "pH Buffering": Why Can't Makeup Removers Rely Solely on "Adjusting Acidity/Alkalinity"?

I. "Just Add Some Citric Acid to Adjust to 6.5" — Where is This Statement Wrong?

In the formulation sampling process of many small and medium-sized brands, "pH adjustment" is often the very last step: measure the pH, if it's too high add citric acid, if it's too low add sodium hydroxide, adjust to the target value, and confirm the sample.

There is nothing inherently wrong with this process, but it only completes half the job of pH management—to be precise, it completes the initial pH adjustment while ignoring a far more critical question: Can this pH value remain stable throughout the product's entire shelf life?

From the moment a makeup remover is filled until it reaches the consumer, it undergoes warehousing, transportation, temperature fluctuations, and repeated opening and exposure to air and skin. During this process, if the formulation lacks a true buffering system, pH drift is almost inevitable. And this drift will simultaneously impact the product's skin safety, preservative efficacy, and even the performance expression of active ingredients.

II. The Skin's pH Ecology: Why the pH of Makeup Removers is Not an Isolated Value

To understand the necessity of pH buffer design, one must first understand the skin's sensitivity to pH.

Skin pH is a critical factor in regulating normal skin function. Current research supports that the acidic pH range of 4.0–5.8 under healthy stratum corneum conditions (slightly higher in covered areas) is crucial for maintaining stratum corneum integrity and optimal barrier function.

The high pH of soap solutions disrupts the skin's physiological pH (approx. 4.5–5.5), promoting the degradation of lipids and structural proteins. Research data shows that two natural soaps caused a significant increase in skin pH upon application, and the elevated values persisted for 30 minutes post-use, potentially interfering with the skin's Acid Mantle and its buffering capacity.

In cleansing care scenarios, skin pH gradually recovers to the normal range of 4.5–5.5 after cleansing, but the time required depends on the alkalinity of the cleanser. This means: the pH design of a makeup remover not only affects the product's own stability but is also directly related to the skin's pH recovery rate and barrier repair efficiency post-use.

For makeup removers, a target pH range of 6.5–7.0 is currently the most widely recognized design direction in the industry—it is close to the physiological pH of tear fluid (ensuring eye safety), not overly acidic to cause unnecessary irritation to keratin, and simultaneously provides an effective working environment for most preservation systems.

III. Why is "Just Adjusting pH" Not Enough? The Essence of Buffer Capacity

The fundamental difference between a Buffer System and "adjusting pH" is: the former is the ability to resist external disturbances and maintain pH within a specific range, while the latter is merely a transient numerical setting.

Taking actual disturbance sources in makeup remover formulations as examples:

• CO₂ Dissolution: CO₂ in the air dissolves in the aqueous phase to form carbonic acid, gradually acidifying the formula.

• Acid Production via Hydrolysis: Certain ester raw materials (such as PEG derivatives) hydrolyze during storage, producing free fatty acids and lowering the pH.

• Preservative Degradation: Benzoate-based preservatives degrade under specific conditions, producing small amounts of acidic byproducts.

• pH Equilibrium After Skin Contact: When consumers dip into the makeup remover, skin secretions (pH approx. 4.5–5.5) remaining on the bottle opening introduce trace acidic sources with every use.

Formulations without a buffering system will experience unidirectional pH drift when faced with any of the above disturbances. The chain reactions caused by pH drift are often harder to detect than the pH change itself:

Direction of pH Drift	Main Chain Risks
Too Acidic (<5.5)	Increased risk of eye irritation; enhanced adsorption of anionic surfactants onto keratin, exacerbating irritation.
Too Alkaline (>7.5)	Risk of preservative failure; skin flora imbalance; increased risk of Staphylococcus aureus proliferation under alkaline conditions.

IV. The Design Logic of Buffer Systems: Two Core Parameters

Parameter 1: Selection of the Conjugate Acid-Base Pair

An effective buffer system must contain a conjugate acid-base pair—a weak acid and its corresponding salt. For makeup removers with a target pH in the 6.5–7.0 range, here are the mainstream buffer pair choices:

Citric Acid is rarely used alone as a stable system. Formulators usually pair it with Sodium Citrate or Potassium Citrate to constitute a buffer system. Buffer systems can resist pH changes caused by the introduction of external acids or bases, thereby maintaining formulation stability during storage and use. Citric acid has three dissociation constants, allowing it to act as an effective buffer in the pH 3–6 range, and citrate buffer systems are also commonly used to stabilize preservative systems requiring specific pH windows.

However, the effective buffering range of the Citric Acid / Sodium Citrate pair is concentrated at pH 3.0–6.0. For makeup removers targeting pH 6.5–7.0, the buffering capacity has significantly decreased.

Practical Advice: For makeup removers targeting pH 6.5–7.0, it is recommended to adopt a compound buffer scheme of Citric Acid / Sodium Citrate (approx. 0.1–0.3%) + Disodium Phosphate (approx. 0.05–0.1%). This balances buffering capacity while keeping the total introduction of inorganic salts at a low level, avoiding interference with the stability of the surfactant system.

Parameter 2: Quantitative Design of Buffer Capacity

Buffer Capacity (β) describes the amount of strong acid/strong base required to cause a unit change in pH; it is the quantitative indicator for evaluating the "robustness" of a buffer system.

For general skincare products, the practical design goal for buffer capacity is: to be able to resist a shift of 0.1–0.2 pH units without introducing excessive electrolytes (which would affect surfactant micelle stability). If the buffer pair concentration is too low, the buffering capacity is insufficient; if too high, it introduces a massive amount of ions, which could cause the viscosity of carbomer-based thickening systems to collapse (carbomer is highly sensitive to electrolytes).

In practice, many preservative failure cases actually stem from pH drift rather than insufficient bacteriostatic ability of the preservatives themselves. Therefore, correct buffer design has a direct supportive effect on microbiological safety.

V. The Linkage Between pH Buffering and the Preservation System

This is the most underestimated linkage effect in formulation design.

Many common preservatives (such as Parabens, formaldehyde-releasers, phenoxyethanol, etc.) work best when the pH is below 5.5. High-pH products require preservatives that are effective under alkaline conditions, such as certain multifunctional acids or hydantoin derivatives. Using them outside their optimal pH range can lead to insufficient microbial control and product spoilage. Formulators must match the pH compatibility of preservatives with the finished product's pH to ensure sufficient bacteriostatic activity is maintained.

Taking the Benzoic Acid system, most commonly used in makeup removers, as an example, its efficacy is highly sensitive to pH: Organic acid preservatives like benzoic acid significantly lose their bacteriostatic activity when the pH exceeds 6 due to an increased degree of ionization; Parabens undergo hydrolysis under alkaline conditions, also leading to preservative failure.

This means: if a formulator adjusts the pH to 6.8 during sampling, but after 3 months of storage the pH drifts to 7.2 due to a lack of buffering, the effective preservative concentration of the benzoic acid system has already approached the failure threshold. What the consumer receives is not a product with "diminished efficacy," but a product with "preservative failure"—this is a microbiological safety risk, not merely a formulation quality issue.

The efficacy of almost all natural-origin preservatives is constrained by pH dependency. A deviation of just half a pH unit can completely inactivate certain preservatives in the formula.

Conclusion: pH Buffering is the "Foundation" of the Formulation, Not the "Last Step"

The essence of pH buffer design is to establish a dynamic stability mechanism at the formulation level for the product's entire lifecycle. It should not be a "finishing" processing action after the formula is adjusted, but should be incorporated into the overall consideration at the very beginning of the formulation architecture design—forming the foundational framework of the formula alongside surfactant selection, preservation systems, and thickener types.

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