Water is one of the most important substances on earth, yet it can also be the source of some major headaches throughout the lifecycle of a product. Uptake of moisture from the environment could cause unwanted changes in various materials. If you’ve ever tasted stale chips or seen sugar granules clump up after the package was opened, you’ve had firsthand experience with some of these changes. Understanding how your product might interact with ambient water vapor, how moisture might move between different ingredients, and permeability of the packaging material could help you optimize your process and ensure quality of your product.
In this article, we’ll focus on one essential tool for studying moisture sorption-desorption process: Dynamic Vapor Sorption (DVS).
What does the DVS technique measure?
A DVS instrument contains a highly sensitive micro or ultra-micro balance, housed in a chamber where temperature and %RH can be tightly controlled with programmable changes. This allows the instrument to track the gain (sorption) or loss (desorption) in sample mass as percent relative humidity (%RH) is increased or decreased under constant temperature. Alternately, the temperature might be varied under a constant %RH level.
Sorption encompasses both the adsorption, or the surface interaction between water vapor and the solid, and absorption, or the uptake of water vapor into the bulk of the material. The resulting sorption-desorption isotherm might also include contributions from other modes of interaction between the water vapor and the solid material, e.g. capillary condensation, deliquescence, and crystal hydrate formation.
For thin films or packaging materials, this technique can also be used to monitor the movement of water vapor through the material, or permeability. Water vapor permeability will be discussed in more details in another blog post.
How is a DVS analysis conducted?
Analysis conditions can be tailored towards your goals. A typical DVS analysis of a solid material might consist of some initial drying step, followed by ramping up from 0-90% RH and down from 90-0%RH at 10% increments, at a constant temperature. Sorption or desorption processes are monitored at each stage until either equilibrium is established, as rate of change in sample mass is below the specified criteria, or maximum allowed duration is reached.
To simulate repeated exposure to different humidity levels, analysis may consist of more than one sorption-desorption cycles.
How much sample is needed for analysis?
PTL can accommodate a wide range of sample quantities from as low as 50 mg to more than 100 g.
How long does the analysis take?
For the “typical” analysis conditions as detailed above, an analysis can take almost a week (or even longer if multiple cycles are needed)! While this sounds like a long time, historical methods might take several weeks or even months for one comparable experiment.
Fear not! With the recent addition of the proUmid VSorp system at PTL, up to 11 samples can now be concurrently analyzed under the same experimental conditions.
What does the DVS data look like and what can the data tell me?
Below is an example of the DVS data plots at 25°C for a reference material, following drying on a separate equipment. These change in mass (dm) results were normalized per the dried mass of the material. However, results can also be provided on the “as-received” basis, i.e. using the mass of the sample at the start of the experiment.
With the change in mass over time plot, you can scrutinize whether the change in mass curve reaches a plateau at each RH level. If so, equilibrium has been established. Otherwise, the sorption or desorption level reported for that RH level did not reach equilibrium, but was instead limited by the maximum allowed duration. This time limit is necessary for the experiment as equilibrium conditions might not always be attainable, e.g. at or above the critical RH levels for materials that exhibit deliquescence behavior.
The tendency of a material to take up moisture from the air is termed hygroscopicity. Basic classification of hygroscopicity per the European Pharmacopoeia is given below. While the methodology is not exactly the same, the DVS could provide a higher resolution alternative to this end.
| Classification of hygroscopicity | Weight % of water sorbed after 24 hours of exposure to 80%RH at 25°C |
| Deliquescent | Sufficient water is absorbed to form a liquid |
| Very hygroscopic | ≥ 15 |
| Hygroscopic | 2 – 15 |
| Slightly hygroscopic | 0.2 – 2 |
| Non-hygroscopic (implied) | < 0.2 |
The sorption-desorption isotherm provides the comprehensive look at the equilibrium relationship between the moisture content and the RH of the environment at the analysis temperature. Those in food science might also know the %RH on the X-axis of the isotherm as 100*water activity (Aw) of the sample at equilibrium. Water activity is another interesting topic that will discussed in further detail another blog post.
The moisture isotherm can provide a wealth of information! A change in sorption rate above certain RH levels can inform you of desirable storage conditions, or what to avoid. If several ingredients might be mixed together under the same packaging, the isotherm could offer an insight into how the mixture might behave. When more than one experimental cycle is conducted, the changes (or lack thereof) in the profiles can inform you of whether the changes in the material might be reversible.
Getting the most out of your data typically requires some knowledge of other sample properties. The experts at Particle Technology Labs are always here to guide you every step of the way. Start a conversation with our chat in the lower right. Or learn more about Dynamic Vapor Sorption (DVS) and see a sample DVS report on the site.
By Chorthip Peeraphatdit – Particle Characterization Chemist IV / Team Leader
