December 13, 2022
Density in cosmetics, one of the essential physicochemical properties in cosmetic formulation.
The physicochemical properties of our finished products are determining factors in their cosmeticity. They are sometimes overlooked or only questioned when quality controls are performed on finished products, but it is important to understand them in order to know our products well.
In this post we will try to find out what this measure is and reveal its importance in formulation, packaging and quality control.
Density is an intrinsic property of matter that began to be known already in antiquity; Archimedes already performed the first experiments with this property as explained in the story of the defalcated gold crown of Hieron II.
In short, it is a scalar quantity of matter, in its different states, which is defined as the ratio between the mass of a substance and the volume it occupies at a given temperature and pressure. As the temperature increases, the density decreases and as the pressure increases, the density increases. This change is significantly noticeable in the case of gases, but not so much in the case of solids and liquids.
There are different types of densities:
We will focus on absolute density as an intensive quantity. This will be expressed in the units of the international system kg/m3. The formula for absolute density is represented as:
As we can see, density is inversely proportional to volume, therefore, the greater the volume occupied by the substance per unit mass, the lower the density. For example, if we take the density of water as a reference (1 kg/m3), we can foresee that almond oil with its density of 0.91 kg/m3will have the same volume value as water, but weighs less than water. And that is why the oil floats on water.
In illustration 1 we can observe a graduated cylinder containing different liquids of different densities and colors. This separation is due to buoyancy, which is eradicated by the difference in absolute densities of each liquid. The substance at the top will be the least dense and the substance at the bottom will be the most dense of all.
It is important to understand that the density of heterogeneous mixtures uses another equation, very similar to that of the absolute density, but with a slight change:
For example, seawater has an absolute density of 1.03 kg/m3, which is higher than distilled water because it has more solute dissolved in its total volume. Alcohol has an absolute density of 0.81 kg/m3, pure glycerin 1.27 kg/m3, among many other substances.
In the case of cosmetic emulsions and creams, density is a factor that varies greatly, depending on the composition.
A very viscous texture is usually associated with a very dense texture, but this is not always the case.
For example, for a gelatin that is a totally solid mixture, which does not flow, but the density is not necessarily high, it can be around 1. For a solution of water with a lot of sugar, it is a liquid solution and the density is very high. That is why we should not confuse viscosity with density.
Viscosity could be defined as the ability to flow. As we have explained, density is a property which determines how much mass there is per unit volume. For example, we have vegetable extracts that have a higher density than some cosmetic products, such as a gel-cream or a hydroalcoholic gel. On the other hand, Lasar paste, toothpaste, the density is very high. Therefore, density is not a factor that measures how viscous or solid or not very fluid a substance is.
Density is a property that we usually forget when talking about cosmetic products, especially those with a high amount of water.
However, this property is an important factor for our skin. The density is related to the amount of each of the phases, it is likely that the fatty phase is more emollient, leaving a greater greasy residue on the skin and a greater barrier on the skin.
For liquid products, it is a parameter that is widely used since it largely conditions the packaging.
For the two-phase is a determining factor, since there has to be a very marked separation between the two phases and for that the density of the phases has to be very different between one and the other.
It is also important to understand that density is a crucial factor, especially in the case of emulsions, as it can influence the stability of these galenic forms.
As we already know, emulsions are unstable heterogeneous mixtures of two or more immiscible phases of which one of them is a dispersed phase in the other continuous phase. In order for the emulsion to be stable, both densities have to be as similar as possible. For emulsions where the dispersed phase is denser than the continuous phase (sometimes w/o emulsions) the process of sedimentation can occur whereby both phases separate and the dispersed phase ends up precipitating at the bottom. In o/w emulsions we can see that a creaming process occurs, the particles that have a lower density than the dispersion medium tend to rise to the surface of the emulsion. Because of this difference in densities we can also see that the coalescence process generates a phase separation and that the internal phase ends up separating at the top.
It is important that our finished products have a specification to follow and in this specification we will determine the analytical controls to be performed in order to validate the batch of cosmetic product.
These parameters are given by the formulator and depend on the dosage form being controlled and the components it contains. Density is a physicochemical property of each substance and makes the difference between some dosage forms.
To determine the density specification, it must be determined with which apparatus the density is determined and at what temperature.
This is the simplest parameter to determine, but no less important for that reason.
There are different procedures that allow us to determine the density of our products directly or indirectly. Currently the determinations are focused on obtaining the result directly and the commonly used methods are:
The packaging is a crucial point in obtaining our cosmetic product, in addition there is a legal quantity that must be declared following the current legislation, therefore, it is of vital importance that the filling corresponds to the volume declared on the product.
The content that a container can contain is normally defined in volume units, but when we manufacture cosmetics, the controls are carried out in mass units. It is important to differentiate that the filling, if it is in volume, we have to make a conversion factor in order to be able to correctly fill the container with the mass quantity that can fit in that container.
For example, when we pack an oil in a 15-milliliter container, 15 grams can never fit, since the density of an oil is less than 1 kg/m3.
Knowing the density of our product will allow us to pack it properly and control its quality and stability.
Find out more about Verónica Díez Ojeda, R&D Formulation Technician at MuttuLab, author of this article.
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