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A critical stage is the subsequent drying step, as direct drying of the hydrogel leads to extensive collapse. This is due to the material being relatively soft and structurally weak compared to the significant capillary forces generated on the evaporation of water from the surface of a meniscus (Figure 3.1). These forces act to pull the walls of the pore together, causing collapse if the material is mechanically weak, which is the case with soft polysaccharide gels.

The fact that water has high surface tension and that it forms menisci in nanopores means that direct drying of the hydrogel puts the structure under great stress as the evaporation causes the pore walls to be pulled together – these forces are too great for the porous starch network to survive and structural collapse ensues. Roundabout methods, therefore, need to be utilised in order to avoid these forces and maintain the porosity.

The first methodology utilised solvent exchange – water has a much higher surface tension (72.74 mN m⁻¹ at 20 °C [13]) than most organic liquids (e.g. acetone 23.70 mN m⁻¹, ethanol 22.27 mN m⁻¹, and methanol 22.60 mN m⁻¹ [14]).

Therefore, exchanging water first for ethanol by simple mixing and filtration (×3) led to approximately 12% water in ethanol‐filled gel, which had to be further exchanged with acetone using the same methodology in order to provide an aerogel with porosity intact. What is interesting here is that the gel produced after the ethanol exchanges contains a liquid (c. 12% water in ethanol) which has a surface tension (22.6 mN m⁻¹) [15] very close to that of acetone (23.70 mN m⁻¹) [14], yet fails to lead to a satisfactory material. While this may be due to the evaporation of an ethanol‐rich vapour (meaning that the remaining liquid is enriched in water, leading to the surface tension of the remaining liquid increasing and causing damage), more extensive ethanol washing does not lead to better materials. Similarly, direct exchange with acetone does not provide acceptable materials. Thus, both solvents must be used for a successful material.

Figure 3.1 Role of capillary forces in the collapse of soft porous materials.

A major downside to this approach (apart from cost) is the large volume of mixed solvents that are very difficult to purify and reuse. While the materials obtained are very good, the process lacks environmental credentials.

In order to develop a more efficient, less solvent‐intensive process, Borisova et al. developed a route involving freeze‐drying of hydrogels doped with t‐butanol, a molecule often utilised in freeze‐drying to control the freezing process [16]. This approach avoids surface tension issues as the phase transition is from solid to gas, and the capillary forces that plague direct drying are thus avoided.

The optimal porosity characteristics were achieved at the eutectic points of the water – t‐BuOH system, with a more distinct optimum found at the water‐rich eutectic (c. 23% t‐butanol). At this solvent composition, the macroporosity that dominates in freeze‐drying of the pure water hydrogels is strongly reduced and considerable mesoporosity is developed. While all three polysaccharides display qualitatively similar behaviour, the values of mesopore volume (pectin: 3.1 cm³ g⁻¹; alginic acid: 1.8 cm³ g⁻¹; starch: 1.1 cm³ g⁻¹) are quite different, but nonetheless impressive, in each case (Figure 3.2).

Therefore, compared with a solvent exchange, the quality of the materials (in terms of mesopore content) is excellent, and much less solvent is required as the t‐butanol can be recovered and reused.