In our exploration of modified starches, Figure 4.4 emerges as a key player, showcasing the intricate dance between these starch variants and moisture over time in a consistent 75% humidity environment. The revelations are profound, shedding light on the hydrophilic and hydrophobic tendencies that define the behavior of starch in various forms.
Hydrophilic Symphony: Natural Starch and Ionic Groups (AS or CS)
The visual narrative in Figure 4.4 paints a vivid picture of the hydrophilic nature of natural starch and starches modified with ionic groups, such as AS or CS. These starches eagerly embrace moisture, reaching an equilibrium value of 50 wt%. Natural starch, with its abundant hydroxyl groups, has an inherent affinity for water, a characteristic we explored in Chapter II. However, this affinity comes with a price—mechanical properties of natural starch-based products are in constant flux as they absorb water from the surroundings. This high water content also puts a cap on using starch as a conventional thermoplastic material.
Surprisingly, starches modified with ionic groups (AS or CS) elevate this hydrophilic dance, outperforming even natural starch in moisture absorption. The values of DS (degree of substitution) for AS and CS hover around 0.1, indicating a subtle substitution of hydroxyl groups by ionic groups. Yet, this small tweak amplifies their hydrophilic nature, accentuating their affinity for water. A noteworthy revelation from Table 4.2 is that both AS and CS showcase solubility in cold water, a trait absent in natural starch. This dual evidence—enhanced moisture absorption and cold water solubility—solidifies the notion that AS and CS stand as more hydrophilic contenders in the starch arena.
Hydrophobic Transformation: Hexanoyl and Benzoyl Modified Starches
In stark contrast, the plot thickens for starch modified with hexanoyl or benzoyl groups, revealing a marked departure from the hydrophilic tendencies witnessed earlier. Figure 4.4 unveils a significant decrease in moisture absorption for these chemically modified starches, signaling a transformation from hydrophilic to hydrophobic material. The magic wand here is the substitution of functional groups, a strategic move that reduces the number of hydroxyl groups—the avid water enthusiasts.
The rationale is clear: hydroxyl groups possess a strong affinity for water, and by curtailing their numbers through chemical modification, the starches transition into a hydrophobic realm. Moreover, the hydrophobic substituents, be it hexanoyl or benzoyl groups, act as gatekeepers, restricting the access of available hydroxyl groups to moisture.
In essence, Figure 4.4 serves as a visual chronicle of this dynamic dance between starches and moisture. It unravels the hydrophilic allure of natural starch and ionic-modified starches (AS or CS) while spotlighting the hydrophobic transformation achieved through the strategic substitution of functional groups in hexanoyl and benzoyl modified starches. The starch saga continues, each variant weaving its unique tale in the intricate tapestry of hydrophilicity and hydrophobicity.