29 April 2014 | 17:26
Dr Jürgen Dienstmaier

Water vapor is one of the components of the Earth’s atmosphere. Although it is present in relatively small quantities when compared to other gases, it is found everywhere on the planet, even in the driest of places

[1]. Hence how its presence and concentration will affect different materials is important across many industries and applications.,/p>

Materials differ in the proportion of water they can absorb or adsorb from their surrounding environment. Not all materials absorb water in its bulk structure; on some of them water can only adsorb on their surface. Consequently, if a material only adsorbs water, its water mass uptake will be much less when compared to a material which can absorb.

Water being absorbed can diffuse into the bulk of a material. The rate at which this process takes place can be experimentally determined using a Dynamic Vapor Sorption (DVS) instrument. Experiments done for this purpose yield data that in turn can be evaluated using known mathematical equations, and thus obtain relevant information about this process: if it takes place at all or not, and at what rate.

A case in point is the use of this method to determine the rate at which water diffuses into a polyimide film sample [2], i.e. the diffusion constant. For this purpose, a sample of the film is placed inside a DVS, which will constantly monitor the change of weight while the relative humidity (RH) level inside the instrument is set to a determined value. Starting with 0% RH until the sample is completely dry; the RH is then set to a higher value. This RH level is kept inside the DVS until the sample has achieved equilibrium –defined as when the rate of change of weight is zero or close to it–, and then it is set back to 0%. This procedure is graphically depicted in the next picture for several different RH levels.

Graph 1a

For each one of the above depicted changes in RH, known as single steps, diffusion constant is found. This constant is evaluated by plotting Mt/M∞ vs. t1/2/d and finding D from the linear part of the graph. This relationship is derived from the equation.

Formula

where Mt = amount adsorbed at time t, M∞ = amount adsorbed at thermodynamic equilibrium, d = thickness of the sample, and D = diffusion constant [2].

Graph 2a

This methodology can be applied to film materials, like those used in packaging or as membranes. However, it is not only limited to films but can also be used for materials in the form of powders, as long as the mean particle diameter is known [3]. It has also the added advantage of being a rapid experimental technique.

References[1] J. DiRuggiero, J. Wierzchos, C. K. Robinson, T. Souterre, J. Ravel, O. Artieda, V. Souza-Egipsy, and C. Ascaso, Microbial colonization of chasmoendolithic habitats in the hyper-arid zone of the Atacama Desert. Biogeosciences Discuss., 9, 15603–15632, 2012[2] Surface Measurement Systems Ltd. Application Note 16. Calculation of Diffusion Constants in Thin Polymer Films Using DVS.[3] Surface Measurement Systems Ltd. Application Note 30. Calculation of Diffusion Constants in a Pharmaceutical Powder Using DVS.

About the author:
Dr Jürgen Dienstmaier studied Chemistry at the Pontifical Catholic University of Peru, followed by a Master in Material Science at the Technische Universität Ilmenau in Germany. Thereafter a Ph. D. at the Ludwig-Maximilians- Universität in Munich concluded in 2013. He is currently Surface Measurement Systems Lead Application Scientist for DVS products, working on various DVS research studies and applications.

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