A Formulator’s Guide to Ingredient Compatibility: Solubility, Charge, Polarity and pH Explained

Introduction

When you first begin formulating, it’s easy to focus on choosing a variety of exciting ingredients for your formulas. But behind every successful cosmetic product lies one essential principle: ingredient compatibility. If your ingredients don’t work together, your formula won’t perform and it won't stay stable over the course of a standard shelf life. Understanding how solubility, charge, polarity and pH interact gives you the confidence to create formulas that are safe, and effective. Let’s break down the four core pillars of compatibility and why each one matters.

Solubility

Solubility is the first compatibility checkpoint. Put simply, solubility determines when and where to add an ingredient into your formula.

Why solubility matters?

If an ingredient is placed in the wrong phase, or added without the correct solubiliser, one of three things can happen:

• It separates (floating oils, sinking powders)

• It crystallises or forms gritty particles

• It loses efficacy because it hasn’t properly dispersed

  
  

Solubility basics every formulator should know

• Oil soluble ingredients (e.g. vitamin E, essential oils, many emollients) are soluble in oils, not water.

• Water soluble ingredients (e.g. hyrosols, niacinamide, humectants) dissolve in water, not oils.

• Alcohol soluble ingredients (e.g. menthol, salicylic acid in some cases) dissolve best in alcohol.

• Partially soluble or insoluble powders may require solubilisers, dispersants, or liposomes.

  
  

Common solubility issues

• Fragrance or essential oils added directly to water → separation

• Powders added to oils → gritty texture

• Oil soluble actives added to the water phase → instability

  
  

Always check solubility first - it determines your formula’s structure from the very beginning.

Charge (Ionic Compatibility):

Many cosmetic ingredients carry a charge. Some are positive, some negative, and some neutral. When incompatible charges meet, you can get:

• Thickening failure

• Separation

• Curds or coagulation

• Loss of viscosity

• Weakened preservation

  
  

Types of charge

• Cationic (positive) – e.g. BTMS, cationic conditioning agents

• Anionic (negative) – e.g. SLS, SLES, and many types of emulsifiers and surfactants

• Non ionic (neutral) – e.g. most oils, esters, butters, silicones, many natural emulsifiers and surfactants

Charge compatibility rules

1. Cationic + Anionic = Often incompatible This is the most common area where beginners are caught out. Example: BTMS (cationic) + anionic surfactants → separation or clumping.

2. Non ionic + anything = Usually compatible Non ionic emulsifiers and oils tend to play well with others.

3. Cationic + cationic = Generally fine Ideal for conditioners and hair masks.

4. Anionic + anionic = Usually fine Common in cleansing products and gels.

   
   

Why this matters: Charge influences texture, stability, and overall behaviour. Even the most beautifully structured formula will fail if the ionic interactions clash.

pH Compatibility:

pH affects ingredient performance and preservation. Every ingredient has a pH range where it works best. Remember pH is only a consideration in formulas that contain water.

What can go wrong with pH incompatibility

• Actives degrade or lose efficacy

• Gels collapse or turn watery

• Emulsions destabilise

• Preservatives stop working

• Skin irritation increases

  
  

Examples of pH dependent ingredients

• Niacinamide prefers pH 5–7

• AHA acids (glycolic, lactic) need pH 3–4 to exfoliate effectively

• Vitamin C (ascorbic acid) needs pH < 3.5

• Carbomer can lose viscosity outside pH 5–6

• Preservatives like benzoic acid derivatives need a lower pH to function

  
  

pH tips for compatibility

• Always adjust pH at the end of formulation.

• Check the pH after adding preservatives, because they can shift it.

• Ensure active ingredients all fall within a compatible range.

• If your formula contains pH sensitive materials, test stability over time.

  
  

Polarity:

Polarity is often overlooked, but it’s incredibly important, especially when working with oils, esters, and oil soluble actives.

What is polarity?

Polarity refers to how molecules distribute electrical charge. In formulation, it determines whether oils blend well, stay uniform, and remain stable in an emulsion. Not all oils are alike. Some are high polarity, some are medium, and others are low polarity and each behaves differently.

Why polarity matters?

Some oils blend beautifully, others separate. Actives dissolve only in oils with compatible polarity

Choosing the wrong polarity oils can cause:

• Bleeding or sweating in balms

• Graininess

• Separation over time

• Reduced solubility of oil based actives

  
  

Emulsion stability can change depending on polarity mismatches.

Examples of polarity differences

• High polarity oils/esters: propanediol dicaprylate, castor oil

• Medium polarity: sunflower oil, jojoba oil

• Low polarity oils: mineral oil, isopropyl myristate, many silicones

  
  

Practical application

If an oil soluble active requires a high polarity carrier, you must choose an oil that matches. Likewise, blending oils with drastically different polarities can destabilise a balm or butter.

Polarity is the hidden reason why some oil phases feel greasy, some feel dry, and some simply refuse to stay mixed.

Bringing It All Together: How to Assess Ingredient Compatibility

Before finalising any formula, run through these steps:

1. Check solubility: Can each ingredient dissolve in its assigned phase?

2. Check charge: Are you combining cationic and anionic materials?

3. Check pH: Do all ingredients function well within the final pH range?

4. Confirm preservative compatibility: Preservatives have their own solubility, charge, and pH requirements.

5. Check polarity of lipids: Are all the lipids being used in the formula compatible with one another, will they blend well and stay stable over time?

6. Observe behaviour during stability testing, watch for:

• Separation

• Texture changes

• Colour changes

• pH drift

  
  

Conclusion

Ingredient compatibility is one of the foundational skills every cosmetic formulator must master. Once you understand how solubility, charge, polarity and pH work together, you’ll be able to:

• Create more stable formulations

• Troubleshoot problems more easily

• Choose ingredients with confidence

• Design products that perform beautifully on the skin

  
  

Whatever you’re formulating these four factors will guide every decision you make.