Dynamic Vapor Sorption and its Applications

Dynamic vapor sorption, also known as DVS, determines the amount and rate of absorption of a solvent, by a sample. This gravimetric technique measures the change in mass by altering the concentration of vapor surrounding the sample.

Subsequently, isotherms are plotted, which show the amount of vapor desorbed/absorbed against the decrease/increase in relative humidity. Although a wide range of organic solvents are used for this purpose, water vapor is more commonly utilized.

Types of Sorption Isotherms

Sorption isotherms show the quantity of vapor desorbed or adsorbed at different equilibrium concentrations (partial pressures) in the gas phase. The isotherm shape relies on the interaction between the solid sample and the vapor molecules, and helps in drawing conclusions about the sorption mechanism.

Based on this reason, vapors of different nature can display different shapes of sorption isotherms when adsorbing the same material. This has potential applications in processes where solvents are used like drying, crystallization and wet-granulation.

The BDDT classification often describes the isotherm shapes and differentiates five different kinds of sorption isotherms (type I to V) which was shortly extended by an additional type VI (Figure 1).

Figure 1. Isotherm types according to the BDDT classification.

However, under ambient conditions, type VI isotherms have no significance for vapor sorption and type I isotherms can be seen when chemisorption occurs or when the material is highly microporous. These variations in the desorption or adsorption behavior are the result of variations in solvents’ sorption enthalpy (Figure 2).

Figure 2. Sorption mechanisms of solvents on solid surfaces.

If there is a strong interaction between the vapor molecule and the solid surface with respect to the interaction between two vapor molecules, the vapor is adsorbed as a monolayer on the surface in case of a perfect and uniform surface.

Once the monolayer is completed, multilayer formation takes place, condensation of vapor molecules, on the original monolayer. This behavior is common in molecules with low polarity such as nitrogen or alkanes. As a result, these molecules can be used to establish the specific surface area in accordance with the BET theory.

The reverse case is when the vapor interacts with the solid surface, which is slightly higher than the interaction with another vapor molecule. In other words, the sorption heat is comparable to the condensation heat.

In this case, only a few molecules are adsorbed at low concentration and this is followed by condensation at greater partial pressures on the few originally adsorbed molecules. Such an effect can be explained as a cluster formation as there is no formation of a monolayer.

In fact, both of these mechanisms and the related shapes of sorption isotherms are intense and occur together in view of the heterogeneous nature of actual surfaces. However, it is useful to gain a superior insight into the basic mechanisms driving adsorption on solid surfaces.

Water adsorption as well as a sequence of alcohols on a crystalline α-lactose monohydrate sample demonstrates the variations in adsorption behavior between different solvents.

BET Surface Area Determinations

In many industries, the measurement of the surface properties plays a key role. The surface area existing for adsorption of gas molecules happens to be the most basic of these properties.

While several techniques are being used today, the most effective techniques are built on the Brunauer, Emmett and Teller (BET) method for adsorption of gas onto the surface of a solid. The adsorption method is based on the adsorption of a gas or vapor onto a solid surface. Inert gases such as argon, krypton and nitrogen were generally used for this application prior to this.

Although the nitrogen BET method at low temperatures is of major importance, the advances made in the latest dynamic gravimetric techniques for BET analysis provides better options than that of the conventional approach. Table 1 shows a comparison between the two methods.

Table 1. Comparison of volumetric and gravimetric BET surface area determination

When integrated with conventional nitrogen BET methods, the dynamic gravimetric BET methods help in analyzing the properties of powder and particle surface areas. This makes it possible to examine tiny sample sizes under ambient pressure and temperature with a range of adsorbing vapor species. Moreover, the sensitivity of the ultra-microbalance gravimetric method makes it possible to determine low surface areas.

Heat of Sorption

The DVS technique helps in determining the heat of sorption for different solid-vapor systems easily and accurately. It is important to have a better understanding of the interaction between solid materials and vapors for a range of industries, such as foods, pharmaceuticals, porous materials, flavors, and catalysts. The heat of sorption for a specific vapor-solid system can reveal important data regarding the sorption mechanism.

Diffusion and Permeation Measurements

The rates of vapor diffusion play an important role in many applications. In thin polymer films, moisture diffusion is of increasing importance in different industrial sectors such as membrane technologies and packaging materials. The technique used to measure the diffusion constants for thin films employs diffusion equations, which were initially used by Crank and Park.

A sample of thin film is placed in the DVS and the sorption kinetics for a series of steps in humidity are taken. It is possible to measure the water vapor diffusion coefficient and also its activation energy of diffusion into a pharmaceutical powder by determining water vapor sorption kinetics with a DVS instrument.

Kinetics

There are two major factors that control the moisture sorption kinetics of a solid material: mass transport in the gas phase and diffusion into the sample. The former determines the rate of moisture supplied to the sample, while the latter would be a combination of diffusion via a powder bed and/or diffusion into separate particles.

In the DVS instrument, the first factor is improved by having a dynamic flow of 200sccm of humidified gas across the sample. This allows rapid delivery of moisture to the sample surface. The second factor is a function of the overall sample mass and the normal particle size of the sample (Figure 3).

Figure 3. The effect of particle and sample size on moisture diffusion kinetics.

Amorphous Content

During pharmaceutical formulations, amorphous materials present complex issues with respect to the processing, performance, and storage. Such materials may be required or not based on the favorable or unfavorable properties of the amorphous state.

Moreover, during the processing of pharmaceutical solids, crystal defects and amorphous regions may be produced. In fact, even low levels of amorphous material can affect the manufacturability, stability, and dissolution properties of a drug product. Hence, it is important to determine the amount of amorphous material during the development of pharmaceutical powders.

Most low molecular weight amorphous materials above the glass transition will remain in a stable, crystalline state. Usually, the amorphous material will exhibit a greater water vapor sorption capacity when compared to the crystalline material because of increased free energy, void space, and surface area. Gravimetric techniques can be applied to determine the amorphous contents that are less than 1%.

When the material changes from an amorphous to crystalline state, there would a significant reduction in the capacity of water sorption. This, in turn, leads a total mass loss as surplus water is desorbed during crystallization. Hence, this mass loss can be utilized to track the transition from an amorphous state to crystalline state.

Water Activity and Shelf Life

The measurement of water activity, or Aw, is a critical factor in a wide range of industries such as cement, food, etc. It not only helps in assessing the quality of ingredients, but also aids in predicting the potential shelf life of a product.

One way to determine Aw is to measure the water vapor pressure arising from a material housed in a closed chamber. In the food sector, when the end product is packaged, it starts to lose its desired qualities, which are usually related to its Aw or internal water content. Properties like taste, color, texture, smell, shelf life, etc. largely depend on the water activity and the amount of water present in the sample.

Glass Transition Temperature

As an amorphous material passes through the glass transition it often transforms from a glassy, hard, brittle material to a less viscous, ‘rubber’ state , thus the water content in amorphous foods, polymers, and pharmaceutical materials can have a significant lowering effect on the glass transition temperature.

For these materials, the glass transition is a direct function of relative humidity. Additionally, there is a shift in the molecular mobility of amorphous compounds at the glass transition.

Above the glass transition, the molecular mobility increases as evidenced by a decrease in viscosity and increasing flow. Temperature often forces the transformation at a characteristic temperature or temperature range (i.e. Tg). Plasticisers, often at a lower molecular weight than the bulk, can decrease the Tg. The extent of Tg depression depends on the concentration of the plasticiser and its interaction with the amorphous material.

Water is a common plasticiser for a range of materials, thus the water content in amorphous foods, polymers, and pharmaceutical materials can have a significant lowering effect on the glass transition temperature. For these materials, the glass transition is a direct function of relative humidity.

Below the glass transition, water sorption will typically be limited to surface adsorption. As the material passes through the glass transition, molecular mobility increases, allowing bulk water absorption. Therefore, the shift in sorption characteristics can be used as a measure of the glass transition.

As in determining the Tg, the glass transition RH depends on the time scale of the experiment. Faster RH ramping rates will yield higher glass transition RH values. If a series of experiments are completed over a range of relative humidity ramping rates, then the critical RH for a glass transition can be plotted versus ramping rate.  

Surface Energy

It is possible to measure the powders’ surface energy by determining the adsorption isotherm of organic vapors through an automated gravimetric vapor sorption analyser. This technique eliminates the constraints associated with other available methods.

Vapor Pressure Measurements

The vapor pressure of solid materials can help in predicting thermodynamic stability. One way to determine the enthalpies of vaporization and solid vapor pressures consistently is to use the DVS-vacuum system.

The vapor pressure of solid compounds reflects both the shelf life and thermodynamic stability of various products.

Porosity Distribution Modelling

The pores in natural and synthetic systems can have a major effect on the way materials respond and behave in environmental conditions. The DVS method helps in detecting and defining porosity in these kinds of systems. It is also used in a variety of applications, such as water vapor flux determination, true density, caking issues and hydrate formation.

Conclusion

The DVS technique is used in a range of research and development activities, from bulk and surface sorption effects of organic vapors and water through to compound stability and polymorphism studies.

An important part of quality control analysis method for production and scale-up, the DVS method can also be found in the packaging area for determining permeability, efficacy, and the effects of temperature and humidity on the samples.

Surface Measurement Systems offers a range of DVS instruments that have been specifically developed to analyze different types of materials across different industries. These instruments make it possible to control the pre conditioning requirements, the amount of solvents utilized, as well as the experimental temperature and relative humidity pressure required for such analysis.

Dynamic Vapor Sorption (DVS) for Materials Characterization Run Time 38:57mins

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

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