Surfactants and foam in toothpaste: what function do they really serve?

So why are surfactants used?

Because they serve real functions.

SLS has detergent and surfactant properties, can encourage the dispersion of the toothpaste and has activity against certain microorganisms.

It also provides abundant foam and contributes to a user experience that consumers immediately recognise as characteristic of a toothpaste.

The scientific literature also points to practical reasons for its use: foaming capacity, sensory acceptance and low cost compared with other surfactants. [2]

Therefore, the rigorous question is not:

"Does SLS serve no purpose?"

It does have functions.

The more interesting question is another one:

what does it really contribute to the result of brushing, and what effects does it simultaneously produce on the tissues with which it comes into contact?

The oral mucosa is not an inert surface

A surfactant does not distinguish between the surface we want to clean and the living tissues of the mouth.

And here a less well-known part of the scientific literature on SLS appears.

Phenomena of irritation and desquamation of the oral epithelium associated with toothpastes and hygiene products containing SLS have been described.

A systematic review on oral mucosa desquamation found cases associated with SLS and observed that the extent of the desquamation was related to its concentration. [3]

This does not mean that using an SLS-containing toothpaste necessarily causes visible lesions in every person.

It means something much more specific:

SLS is not biologically indifferent to the oral mucosa.

What happens to the mucosal barrier?

The oral mucosa constitutes a biological barrier.

Its function is not only to cover the inside of the mouth: it also regulates the interaction between our tissues and the substances present in the oral environment.

Various experimental works have studied how SLS interacts with this barrier. The scientific literature has linked its exposure, under certain conditions, with inflammation, desquamation and increased permeability of the mucosa to chemical substances. [3]

This requires an important qualification.

It does not mean that any everyday use of an SLS-containing toothpaste automatically produces a clinically relevant alteration of the barrier.

Nor does it demonstrate that other substances present in a toothpaste necessarily cross the mucosa.

But it does demonstrate that considering a surfactant simply as an ingredient that "makes foam" describes only one part of its interaction with the mouth.

We brush our teeth and expect foam to appear.

It is such a common association that it is easy to interpret foam as a sign of cleaning: the more foam a toothpaste produces, the more it seems to be working.

But foam and cleaning are not exactly the same thing.

Foam is, above all, a consequence of the presence of surfactants. And understanding why these substances are added to a toothpaste allows us to separate three concepts that we frequently confuse: cleaning, producing foam and generating a particular sensation during brushing.

What is a surfactant?

A surfactant is a substance capable of reducing the surface tension between different media.

In a toothpaste it facilitates the dispersion of its components, helps to loosen and keep certain residues in suspension, and encourages the product to spread throughout the mouth.

One of the most historically used surfactants in toothpastes is sodium lauryl sulphate, known by the abbreviation SLS (Sodium Lauryl Sulfate).

In addition to its detergent properties, SLS produces something immediately perceptible to the consumer:

foam.

And that foam has ended up becoming part of our idea of what "cleaning your teeth" means.

Does foam clean teeth?

We should not use foam and cleaning as synonyms.

The removal of dental plaque depends to a large extent on the mechanical action of brushing. Toothpastes also incorporate various ingredients with specific functions, including abrasives, fluorides, anti-sensitivity agents and, depending on the formulation, surfactants.

But a greater amount of foam does not by itself demonstrate a greater removal of plaque.

A randomised clinical trial carried out over eight weeks compared an SLS-containing toothpaste with an SLS-free one in 120 participants. The researchers found no statistically significant differences between the two groups in plaque indices or in gingival bleeding. [1]

They did find, however, a difference in the participants' perception:

the SLS-containing toothpaste was rated better for taste, sensation of freshness and foaming effect.

This raises an interesting distinction.

One thing is how much a product cleans. Another is how much it makes us feel that it is cleaning.

Saliva is not water: it is an active biological system

When a toothpaste enters the mouth, it does not come into contact only with the teeth.

Saliva contains proteins, glycoproteins, enzymes, immunoglobulins and numerous components that participate in lubrication, protection of the mucosae, microbial balance and the early stages of digestion.

Among them are mucins, which help to form a protective film over the oral tissues, and enzymes such as salivary α-amylase, which begins the digestion of starch already in the mouth.

Therefore, any substance used during brushing temporarily comes into contact with a biochemical system that was already functioning before the toothpaste was introduced.

The relevant question is not only what an ingredient does to the tooth.

We can also ask ourselves:

what does it do to the biological environment surrounding the tooth?

Surfactants, proteins and mucins

This question is especially interesting in the case of surfactants.

SLS has a high affinity for proteins and is known for its ability to modify their structure. In the oral environment, this property may also affect the glycoproteins that form part of the mucin layer that protects the surface of the mucosa. [4]

The scientific literature has precisely proposed the interaction of SLS with this protective layer as one of the mechanisms related to irritation and to the changes observed in the permeability of the oral mucosa. [4]

This introduces an important difference.

Mucin is not a residue that we need to eliminate.

It forms part of the natural mechanisms by which the mouth protects and lubricates its tissues.

Therefore, when we study a detergent used in oral hygiene, it is not enough to ask whether it removes residues or produces foam. We must also study which biological structures it interacts with while performing that function.

Can an ingredient in toothpaste interfere with the enzymes in saliva?

Yes, at least certain interactions are documented.

Interactions between surfactants and other active components of toothpaste have also been studied.

In in vitro experimental studies, the presence of SLS has reduced the uptake of fluoride by enamel in combinations with both sodium fluoride (NaF) and sodium monofluorophosphate (MFP). [5]

This result does not by itself establish the mechanism responsible, nor does it allow it to be automatically extrapolated to all commercial formulations.

This does not mean that SLS "cancels out the fluoride".

It shows something different and possibly more interesting:

the ingredients of a toothpaste do not act in isolation. They can interact with each other and also with the natural biochemical processes of the mouth.

What happens to the salivary protein film?

Teeth and the surfaces of the mouth are continuously coated by components originating from saliva.

The so-called acquired pellicle forms rapidly on the enamel and contains numerous salivary proteins and glycoproteins. It is not simply dirt: it participates in the interaction between the enamel and the oral environment.

It has been experimentally observed that different toothpaste formulations can modify the properties of films formed by salivary proteins. [6]

After exposure to the toothpaste, saliva can deposit proteins again and progressively rebuild that film. However, different combinations of detergents and fluorinated compounds can produce different behaviours.

Once again, this does not by itself demonstrate clinical harm.

But it shows why it is overly simplistic to imagine brushing as an interaction between a product and an inert mineral surface.

The oral environment responds.

The mouth is also the beginning of the digestive tract

Digestion does not begin in the stomach.

Chewing physically fragments food while saliva mixes it and certain enzymes already begin its transformation. Salivary α-amylase, for example, initiates the breakdown of starches.

For this reason it is legitimate to investigate whether the ingredients we repeatedly introduce into the mouth can temporarily modify the activity of its enzymes.

There are experimental data on the interaction of certain surfactants with natural enzymes. For example, the effect of sodium lauroyl sarcosinate — a surfactant different from SLS — on salivary amylase and various digestive enzymes has been studied experimentally. This is, however, old experimental evidence that does not allow us to conclude that normal use of a toothpaste alters human digestion in a clinically relevant way. [7]

The difference is essential:

a demonstrated molecular interaction does not automatically equate to a relevant physiological consequence.

But nor should it be ignored.

A complete formulation, not a list of independent ingredients

All of this raises a broader problem.

When we read the composition of a toothpaste, we tend to analyse each ingredient separately in our minds:

fluoride, surfactant, abrasive, humectant, preservative, flavour...

But in the mouth they do not arrive separately.

They arrive simultaneously, within a formulation, and they encounter saliva, proteins, enzymes, microorganisms, particles, mucosae and dental surfaces.

For this reason, knowing the safety or the function of each individual ingredient is necessary, but it does not necessarily answer all the questions about the interactions between the formulation and the oral ecosystem.

And precisely here it is always worth distinguishing three levels:

what we know, what we can explain through a plausible mechanism, and what has not yet been demonstrated.

SLS and recurrent mouth ulcers

The relationship between SLS and recurrent aphthous stomatitis has also been studied.

A systematic review of randomised clinical trials found four crossover studies, with a total of 124 participants. Overall, SLS-free toothpastes were associated with improvements in the studied parameters of the ulcers, although the authors noted that better-designed studies are needed to strengthen the available evidence. [8]

And the individual results help to understand why absolute conclusions should be avoided.

In a trial with 90 participants, eliminating SLS did not significantly reduce the number of ulcers or the number of episodes, but a significant decrease was observed in the duration of the ulcers and in pain during the period of use of the SLS-free toothpaste. [9]

For this reason it would be incorrect to state that:

"SLS causes mouth ulcers."

The available evidence allows a more limited statement:

in people with recurrent aphthous stomatitis, SLS-free toothpastes may be beneficial for some parameters of the disease, although the evidence is still limited.

And the particles present in a toothpaste?

Toothpastes can contain solid materials, including different forms of silica used for their physical and abrasive properties. [10]

Here it is worth carefully separating what is known from what still constitutes a question.

The fact that a material is described as micronised does not by itself allow us to know its entire particle size distribution. Nor does it automatically allow us to conclude that it is a nanomaterial. [10]

To establish this, it would be necessary to know data such as the granulometric distribution, the size of the primary particles and their state of aggregation or agglomeration.

And then another question appears.

If certain experimental conditions show that a surfactant can modify the barrier properties of the oral mucosa, can it also modify its interaction with other particulate components present simultaneously in the formulation?

It is a scientifically legitimate question.

But it is not equivalent to an answer.

The evidence we have examined does not allow us to state that the SLS used in a toothpaste causes the systemic penetration of the particles present in it.

Confusing a plausible hypothesis with a demonstrated effect would be as incorrect as ignoring the question because we do not yet know the answer.

What we know and what we do not yet know

We know that surfactants serve technological and detergent functions.

We know that foam influences consumer perception, but does not by itself constitute a measure of cleaning efficacy. In a clinical trial, an SLS-free toothpaste obtained results comparable to the one containing SLS for plaque and gingival bleeding. [1]

We also know that SLS can interact with the oral mucosa and that there is evidence of irritation and desquamation under certain conditions. [3]

And there are clinical indications that some people with recurrent mouth ulcers may benefit from SLS-free toothpastes. [8]

What we cannot conclude from these data is that the surfactants in toothpastes cause systemic diseases, metabolic alterations, or that they necessarily facilitate the entry of particles into the body.

For those statements we would need different evidence.
The question, therefore, may not be whether a toothpaste contains "chemical" ingredients — everything around us is chemistry — but how much we really know about the interactions between those ingredients and a biological environment as complex as the mouth.

A simple question

For decades we have learned to associate a mouth full of foam with a clean mouth.

Chemistry explains why that foam appears.

Research allows us to study what the ingredient that produces it actually does.

And when we distinguish one thing from the other, a much simpler question remains:

how much of the foam we expect when brushing responds to a need of our mouth, and how much responds to what we have learned to expect from a toothpaste?