Determining the Moisture Desorption and Diffusion Constants of Creams using Dynamic Vapor Sorption

Many applications, ranging from cosmetics such as suntan cream and hand cream through to medical uses such as antiseptic cream, widely use cream formulations.

The moisture content of creams and the rate of moisture loss under different conditions are important for determining factors such as the texture, the feeling on the skin and the efficacy of the cream.

This article shows how DVS can be used to measure moisture (and organic vapor) loss in creams and how this can be applied for calculating diffusion coefficients.

Method

Two different commercial creams: one a hand cream and the other a baby moisturizing lotion were selected and the moisture (and organic vapor) loss of each was measured on a DVS Adventure at 25 °C.

A 100 µm thick film of cream was uniformly deposited onto a microscopic slide which was placed horizontally into the DVS system. The sample was subsequently subjected to decreasing steps in humidity (ranging from 95% down to 0% RH). The time period of each step was set to 24 hours.

Results

Moisture Content at Different Humidity Values

The kinetics of desorption for the two creams are shown in Figure 1 for steps from 95% RH to 60%, 40%, 20% and 0% RH.

The green and red lines depict the change in mass referenced to the dry mass (i.e. the mass toward the end of the analysis) for the hand cream and baby moisturizing lotion respectively, while the blue line shows the humidity profile as a function of time.

Moisture desorption kinetics at 25 °C for two different creams (— hand cream and — baby moisturizing lotion)

Figure 1. Moisture desorption kinetics at 25 °C for two different creams ( hand cream and baby moisturizing lotion)

The results show that except for the first step at 95% RH for the baby moisturizing lotion, both samples have attained equilibrium state at each humidity step. Table 1 lists the moisture contents at equilibrium (with reference to the dry mass) of the creams at different humidities.

Table 1. Equilibrium moisture contents for hand cream and baby lotion

Target RH (%) Moisture Content (%)
Hand Cream Baby Lotion
Initially 134.9 839.0
95 70.8 <50
60 11.2 3.2
40 5.5 1.8
20 2.4 0.9
0 0.00 0.00

Initially, the baby moisturizing lotion contained a lot more moisture (and organic vapor) compared to the hand cream.

However, whatever the humidity considered (for RH <95%), the moisture content at equilibrium was higher for the hand cream when compared to the baby moisturizing lotion. This means that the hand cream retains moisture more easily. The two creams exhibit a completely different isotherm.

Diffusion Coefficients

For a single step in humidity and a one sided thin film of thickness d, the initial kinetics of desorption may be described by the equation (1),

(1)

where: Mt indicates the mass of the sample at time t, M∞ is the mass of the sample at thermodynamic equilibrium and D represents the diffusion constant. Equation (1) is generally valid for values of Mt/M∞ values < 0.4, where a plot of Mt/M∞ against t1/2/d should be linear.

For each discrete step in humidity, a linear least squares analysis is carried out on the initial slope (Mt/M∞ < 0.4). Figure 2 shows an example of a Mt/M∞ versus t1/2/d (red trace) along with the linear fit (blue trace).

Diffusion plot for 60% RH to 40% RH step in humidity at 25 °C on a 100-µm film of hand cream.

Figure 2. Diffusion plot for 60% RH to 40% RH step in humidity at 25 °C on a 100-µm film of hand cream.

From this type of plot, values of the initial diffusion coefficients are measured for both cream samples, and are shown in Table 2.

Table 2. Initial diffusion coefficients for the hand cream and baby cream samples

Previous RH (%) Target RH (%) Hand Cream Baby Lotion
Diff. Coeff. (cm2/s) R-sq. (%) Diff. Coeff. (cm2/s) R-sq. (%)
95 60 2.87 * 10-8 99.53 - -
60 40 1.87 * 10-8 99.95 4.87 * 10-8 99.76
40 20 7.93 * 10-9 99.76 2.73 * 10-8 100.00
20 0 3.45 * 10-9 99.85 - -

It is observed that the values of the initial diffusion coefficients decrease with decreasing humidity values, for the same size of humidity steps.

The values of the initial diffusion coefficient are higher for the baby moisturizing cream when compared to the hand cream, at least for the two steps in humidity that were considered. Thus, it can be concluded that the rate of loss of moisture for the baby moisturizing cream is higher than that of the hand cream.

Conclusion

The DVS methodology described above can be used to study the desorption isotherm and the kinetics of moisture loss of creams quickly. The analysis of the experimental data can be carried out with the help of the Advanced Data Analysis add-in suite, which enables fast and reliable evaluation of experimental data.

Both creams analyzed in the article show considerable disparities in their initial moisture content, in their diffusion coefficients and their moisture content at equilibrium under different humidity conditions. These differences clearly show the creams’ differences in texture and feeling on the skin, and explain their application for different purposes.

Acknowledgement

SMS thanks Ms. S. Reutenauer for her contributions to the article.

This information has been sourced, reviewed and adapted from materials provided by Surface Measurement Systems Ltd.

For more information on this source, please visit Surface Measurement Systems Ltd.

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