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I have fabricated an extruded starch film, impregnated with calcium carbonate. After addition of calcium carbonate the contact angle of the film has risen suggesting a more hydrophobic surface. I have gone through various literature sources and the behaviour of calcium carbonate with water is very confusing. Some researchers have stated that calcium carbonate is hydrophobic and some have stated it is hydrophilic. Please suggest a relevant theory behind the increase in hydrophobicity of the starch films after calcium carbonate is added.

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$\ce{CaCO3}$ itself is hydrophylic, though it might be less so than starch.

However, surface texture may make a substance hydrophobic. Simply forming a surface into nano-pillars can make it water-repellent. There are some "hairy" materials in nature so water-repellent that a layer of air is maintained even when fully submerged: the Salvinia effect.

Examine the surface carefully under high magnification, with various types of lighting (e.g., bright-field and dark-field illumination) to look for surface "fuzz". If you have access to an electron microscope, give that a try.

Perhaps your technique could be used to make a useful product, e.g., slow-release starch capsules for medicines... or not-so-useful, e.g., water-repellent pasta -- boil for hours and it stays dry.

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