Unit 5: Chemistry of Materials
Soaps and Detergents
Definitions
- Soaps: Soaps are the sodium (Na+) or potassium (K+) salts of long-chain fatty acids (e.g., Sodium Stearate, C17H35COO-Na+). They are made by saponification (hydrolysis of fats/oils with an alkali like NaOH).
- Detergents: Synthetic (man-made) cleansing agents that have a similar structure (hydrophilic head, hydrophobic tail) but are not soaps (e.g., sodium lauryl sulfate). They work in hard water, whereas soaps form scum.
Their Mode of Action
Both soaps and detergents work by emulsification, which allows non-polar grease and oil to mix with polar water.
- Structure: The molecule is amphiphilic:
- Hydrophobic Tail: A long, non-polar hydrocarbon chain (e.g., C17H35-) that is "water-fearing" and "oil-loving".
- Hydrophilic Head: A polar or ionic group (e.g., -COO-Na+) that is "water-loving".
- Micelle Formation: In water, the molecules cluster to form a sphere called a micelle. The hydrophobic tails hide on the inside, and the hydrophilic heads face the outer water.
- Cleansing Action:
- The hydrophobic tails of the detergent molecules dissolve into the non-polar grease/oil spot on fabric.
- They surround the grease, forming a micelle with the grease trapped in the center.
- The outside of the micelle is covered in the hydrophilic heads, which are water-soluble.
- Agitation (scrubbing) lifts this micelle (with the grease inside) off the fabric, and it is washed away by the water.
Biofuels
Definition: A fuel that is produced from biomass (organic matter from plants or animals), rather than from the slow geological processes that form fossil fuels.
Production of Biofuels
- Bioethanol: Produced by fermentation. Sugars and starches (from corn, sugarcane, switchgrass) are fermented by yeast to produce ethanol, which is then distilled.
- Biodiesel: Produced by transesterification. Vegetable oils or animal fats are reacted with an alcohol (like methanol) in the presence of a catalyst (NaOH) to produce fatty acid methyl esters (biodiesel) and glycerol (a byproduct).
Utility as Alternative Fuel Source
- Renewable: Biomass can be grown relatively quickly, unlike fossil fuels.
- Reduced Emissions: They typically burn cleaner, producing less sulfur oxides (SOx) and particulate matter.
- Carbon Neutral (in theory): The CO2 released when burning is offset by the CO2 absorbed by the plants when they grew. (This is debated).
- Biodegradable: Less environmentally damaging if spilled.
- Energy Security: Reduces dependence on foreign oil imports.
Fibers
Definition
A fiber is a thread-like strand of material, significantly longer than it is wide. In chemistry, fibers are long-chain polymers.
Classification
- Natural Fibers: Obtained from plant, animal, or mineral sources.
- Artificial Fibers: Man-made fibers.
- Semi-Synthetic: Made from natural polymers (like cellulose) that are chemically modified (e.g., Rayon).
- Synthetic: Synthesized completely from simple chemical monomers (e.g., Nylon, Polyester).
Natural Fibers (Source and Application)
| Fiber |
Source |
Application |
| Cotton |
Plant (Cellulose from cotton boll) |
Clothing (denim, t-shirts), towels, bedsheets. |
| Wool |
Animal (Protein from sheep fleece) |
Clothing (sweaters, suits), blankets, carpets. |
| Silk |
Animal (Protein from silkworm cocoon) |
Luxury clothing (dresses, ties), parachutes. |
| Rayon |
Semi-synthetic (Regenerated cellulose from wood pulp) |
Clothing (artificial silk), industrial applications. |
Artificial Fibers (Application)
- Polyamides: (e.g., Nylon). A strong, tough, and elastic synthetic polymer.
- Application: Ropes, carpets, parachutes, clothing (stockings), bristles for brushes.
- Acrylic (from Acrylic Acid/Polyacrylonitrile):
A synthetic polymer with a soft, wool-like feel.
- Application: Artificial wool (sweaters, blankets), tents, awnings.
- PVC (Polyvinyl Chloride):
A versatile plastic polymer. While not a traditional textile fiber, it is used in "pleather" (artificial leather), raincoats, and waterproof coatings on other fabrics.
Biodegradable Polymers
Definition and Benefit
- Definition:
Polymers that can be broken down (decomposed) by the action of living microorganisms (like bacteria or fungi) into natural substances like water, CO2, and biomass.
- Benefit:
They offer a solution to the environmental persistence of plastic waste. Instead of lasting for hundreds of years in landfills, they can decompose and return to the natural carbon cycle.
Application of Cellulose, Cellulose Acetate, Cellophane
These polymers are all based on cellulose, a natural, biodegradable polymer found in plants.
- Cellulose:
- What: The most abundant natural polymer on Earth (a polysaccharide of glucose).
- Application:
Paper, cardboard, cotton (textiles), wood (construction).
Cellulose Acetate:
- What: A semi-synthetic polymer made by treating cellulose with acetic anhydride.
- Application:
Photographic film (replacing flammable celluloid), fibers (acetate rayon), cigarette filters, spectacles frames.
Cellophane:
- What: A semi-synthetic polymer made from regenerated cellulose (viscose process).
- Application:
A clear, transparent film used for food packaging (e.g., candy wrappers) and adhesive tape (Scotch tape).