How Everything Works
How Everything Works How Everything Works

Question 899

Why does an artificial sponge absorb more water than a natural sponge? — JH, Angleton, TX
Water is drawn into a sponge in part because of an attraction between the water molecules and the sponge's surface and in part because of water's tendency to minimize its own surface area. When you put a drop of water on a waxy surface, the water beads up. That's because water and wax don't bind well to one another and the water molecules pull toward one another instead. The water droplet tries as best it can for form a sphere, since a sphere has the smallest surface area that a given volume of water can occupy. These forces that pull water's surface inward are called surface tension.

But when you put a drop of water on real cellophane (a smooth form of cellulose), the water spreads out. That's because water and cellulose bind strongly to one another and the water will permit its surface area to increase somewhat if that increase allows it to attach to more cellulose. Similarly, water binds well with other forms of cellulose, including paper, cotton, and Rayon. I think that most artificial sponges are either cellulose or a close chemical relative of cellulose.

A sponge absorbs water by allowing that water to cling to an extensive surface that binds well with water. The water spreads out along that surface while trying to minimize the surface area of any water that isn't touching the sponge. The surface of a natural sponge interacts well with water (the sponge lives in water after all), but a natural sponge can't compete with modern technology. A company that makes artificial sponges can adjust the chemical structure of the sponge's plastic so that it binds nicely to water molecules; it can adjust the sizes of the holes in the sponge to attract the water as efficiently as possible with a given mass of plastic; and it can tailor wall thickness to give the sponge the right elasticity. Furthermore, some of the water is brought right into the plastic and that water softens or "plasticizes" the plastic. That's why a sponge is hard when dry and soft when wet—the water molecules are effectively lubricating the plastic molecules so that they can slide past one another.


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