Absorption or adsorption…that is the question.
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Stability testing under various and sometimes extreme conditions is an important component in the final regulatory filing.  These stability tests are performed on both the drug substance and the drug product.  Commonly the physical and chemical stability are reported, but the degree of water sorption is also an essential parameter to take into account.

Water sorption may seem a straightforward phenomenon but it is often complex with detailed investigations into sorption required to understand the process.  Water sorption can be either adsorption or absorption.  At Crystallics we use a humid stage mounted on our X-Ray Powder Diffractometer (XRPD) to observe the water sorption process and distinguish between adsorption and absorption.  In the case of adsorption the diffractogram remains the same whereas during the absorption process the XRPD patterns may change.

Figure 1. Possible ways of insertion (absorption) of water molecules into the crystal.

Adsorption is the physical process that takes place on the surface of the crystals and, as such, has no effect on the crystal structure.  As a consequence, the powder pattern also does not change.  The strength and reversibility of the interaction between the crystal surface and the water molecules highly depends on the dipole-dipole interaction between both partners.  When the affinity of the crystal surface for the water is very high it may even lead to dissolution of the API in the water.

When the crystal surface is poroseous and the adsorbed water pass through the crystal surface, the water molecules may become absorbed by the API molecules.  In contrast to adsorption, absorption will change the structural parameters of the crystal which is reflected in changes in the XRP diffractogram.  Water may interact with the API molecules in different ways as graphically explained in Figure 1.  Figure 1 shows three possible ways in which water molecules can be incorporated in the crystal structure.

Scenario ‘a’ occurs when the absorbed water molecules are interacting with themselves or not interacting with the API molecules. The water molecules line-up in channels and the API molecules are slightly ‘pushed’ aside.  As a result, the symmetry of the crystal cell remains the same but the cell axis will increase.  Usually there is no fixed stoichiometry between the API molecules and the water molecules in the case of these channel hydrates.  In the XRPD pattern this can be seen by small shifts in the peaks particular in the lower q region.  The peak intensities are usually not affected.  Figure 2 shows an example of an anhydrate and channel hydrate XRPD diffractogram of the same API.

Figure 2.  Comparison of the solvated (red) and solvent free (black) powder patterns in situation when solvent molecules are forming channels.

 

Scenario ‘b’ (Figure 1) shows the water molecules inserted between API molecules and bridges the API molecules by the H-bond network. Such structures are usually formed when there is a deficiency of H-bond donors or acceptors.  The water supplements the H-bond donors and acceptors.  Similar to scenario ‘a’, the symmetry is kept the same but the water molecules form now a stoichiometric hydrate. As a consequence, the crystal unit cell will also expand. This expansion is reflected in the XRPD by shifts of the peaks in the lower q region.  However, also the intensities of some peaks may differ and even extra peaks may appear as it is presented in Figure 3.

Figure 3.  Comparison of the solvated (red) and solvent free (black) powder patterns in situation when water molecules bridge the API. The red arrows show two appearing peaks that do not exist in the solvent free form.

 

In the last scenario ‘c’ (Figure 1) a more dramatic rearrangement takes place.  With the absorption of the water molecules into the crystal the API molecule arranges differently to accommodate a more energetically favorable structure together with the water.  The API molecules often adopt a new conformation that allows them to participate in more H-bond interactions that leads to the formation of a new crystal structure with a different symmetry. As a result, the XRPD pattern is totally different from that of the non-solvated crystal (Figure 4).

Figure 4.  Comparison of powder pattern of hydrated (red) and solvent free (black) forms in situation where after absorption of water the new symmetry is formed.

 

In the course of stability testing it is likely that water gets absorbed into a fraction of the crystals and that physical mixtures between hydrated and anhydrated crystals are formed.  Quantification of the different forms may be taken into account when setting specifications with regards to the extent of water sorption.  Such specification could be based on Loss-on-Drying criteria, but XRPD could also be used now that we know where the different XRP diffractograms originate from.  Scenario ‘a’ in Figure 1 represents a non-stoichiometric hydrate and therefore it is almost impossible to determine the ratio between the non-solvated and the solvated form.  The other two possibilities (‘b’ and ‘c’) can be easily quantified using the Rietveld or Whole Powder Pattern Decomposition method. Figure 5 shows the powder pattern of an anhydrous form and of a mixture of hydrated and anhydrous forms. The black line plots the starting material while the red was obtained at 80% of relative humid.  Using the Rietveld method the contribution of the hydrated form in the bulk was calculated to be 84%.

Figure 5.  Comparison of the anhydrous (black) and mixture of anhydrous + monohydrate (red) powder pattern obtained at 80% of relative humid. The latter was quantified using the Rietveld method giving 84% of monohydrate form.

 

The above described techniques for the detection of adsorbed or various ways of absorbed water are also applicable to other (organic) solvents.  In conclusion, next to conventional analytical techniques, XRPD is a powerful technique in the hands of an experienced crystallographer to be used for setting specifications of residual solvent content in production batches of APIs.