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Salt is commonly used to adjust the viscosity of liquid detergents.

At first, adding sodium chloride may make the product thicker. But after more salt is added, something unexpected can happen:

The detergent becomes thin again.

Why?

This does not necessarily mean that the thickener has failed. In many surfactant systems, viscosity does not increase continuously with salt concentration.

Many liquid detergents rely partly on surfactant micelles to build viscosity.

As salt is gradually added, the structure of these micelles can change. Viscosity may rise, reach a maximum, and then fall again when the salt concentration becomes too high.

This is often described as salt-thickening behavior.

Studies of surfactant-based household-cleaning systems have shown that viscosity can increase and then decrease as salt concentration changes because the micellar structure also changes.

Therefore, adding salt without measuring each step can easily move the formulation past its optimum point.

Different surfactant combinations respond differently to salt.

The final viscosity can depend on:

• anionic surfactant concentration;
• nonionic or amphoteric surfactants;
• surfactant ratio;
• active-matter content;
• salt concentration;
• water content.

This means the same amount of sodium chloride may work well in one detergent but perform poorly in another.

Hydroxyethyl cellulose (HEC) is a nonionic cellulose ether commonly used in detergents and personal-care formulations as a thickener, stabilizer, and rheology modifier.

Unlike relying only on salt thickening, HEC can provide polymer-based viscosity and improve formulation consistency.

This can be useful when:

• salt thickening is too sensitive;
• viscosity changes significantly between batches;
• suspension stability is insufficient;
• the product needs a smoother flow profile;
• the surfactant system alone cannot reach the required viscosity.

However, HEC dosage should still be optimized. Too much polymer can make the detergent overly thick or difficult to pour.

Viscosity should always be measured at a controlled temperature.

A detergent that looks thick at 20°C may become noticeably thinner at a higher temperature.

Research on detergent surfactant systems also shows that the viscosity response to salt can change significantly with temperature.

Therefore, if viscosity problems appear mainly in summer or during warm storage, check temperature before changing the cellulose ether dosage.

HEC must also be properly dispersed and hydrated.

If the polymer forms lumps or has not fully hydrated before viscosity is measured, the initial result may be misleading.

Keep these conditions consistent:

• addition sequence;
• mixing speed;
• hydration time;
• water temperature;
• final test temperature.

When comparing two HEC grades, do not judge them immediately after powder addition.

If liquid detergent becomes thinner after adding more salt, the problem may simply be that the formulation has passed its optimum salt concentration.

The final viscosity depends on:

• surfactant composition;
• salt level;
• HEC selection;
• water content;
• temperature;
• mixing and hydration.

The best approach is to determine the salt curve first and then use cellulose ether to fine-tune viscosity and stability.

For detergent formulators, the target is not the highest possible viscosity—it is stable, reproducible viscosity with good pouring and storage performance.