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While significant structural and functional differences exist between the various classes, a surfactant, simply put, describes any compound capable of reducing the surface tension between a liquid and one other substance. Surface tension, one will recall, refers to the tendency of liquid molecules to coalesce with one another, thus minimizing05 their collective surface area. This phenomenon is the physical principle underlying a familiar adage, "oil and water do not mix." Phrased more precisely, oils, which contain primarily nonpolar hydrocarbon bonds, are immiscible with aqueous solutions, meaning they will not spontaneously dissolve in water, which consists of highly polar hydrogen-oxygen bonds. Instead, oils will tend to form a film over polar solvents, while10 surface tension serves to stabilize this film at the oil-water interface. Because they are amphiphilic—meaning they possess both polar and nonpolar domains—surfactants may interact with both components of this interface, and interfere with the electrochemical forces that maintain its integrity. Due to this unique property, surface tension lowering agents have found a host of applications in diverse15 commercial products, and are used in particular as emulsifiers, foaming agents, and detergents. An emulsion is merely a mixture of two normally immiscible liquids. The word emulsion derives from a Latin root meaning "to milk," and, as an easily homogenized mixture of fats within an aqueous solution of sugars, proteins, and minerals, milk itself20 is a quintessential example of an emulsion. Moreover, without the surfactant activity of the complex lipids it contains, the fat globules dispersed throughout a given volume of milk would coalesce into a film on its surface. Similarly, the surfactants found in foaming agents decrease the tendency of soap bubbles to coalesce, and are responsible for the lathering effect found in many hygiene products such as toothpaste and25 shampoo. Soap itself, interestingly enough, can also be considered a surfactant. Principally, soap is a salt consisting of a positively charged sodium ion and a negatively charged fatty acid. Importantly, the structure of an ionized fatty acid includes a nonpolar hydrocarbon "tail," and a polar, carboxyl "head." The polar head allows the fatty acid30 to partially dissolve in water, while the nonpolar tail facilitates its interaction with other nonpolar compounds, such as oils. Thus, by interrupting surface tension, soap allows oil to be washed away with water. At the risk of oversimplifying, soaps are created by exposing triglycerides gathered from either plant or animal sources to a strong base in a process called saponification.35 The base hydrolyzes triglycerides to form glycerol and amphipathic free fatty acids. The glycerol, in turn, is removed, and the fatty acids are complexed with sodium. While the words soap and detergent are sometimes used interchangeably in common parlance, one should note that detergents are not synthesized by saponification. Structurally, detergents differ from soaps only in the composition of their polar40 heads. That is, whereas soaps contain an ionized carboxyl group, detergents contain an ionized sulfonate. The significance of this alteration is twofold. First, detergent compounds are far less prone to precipitate and become ineffective in hard water. Hard water, of course, refers to water that is rich in dissolved calcium and magnesium as a result of exposure to limestone, and it is present in an estimated 80% of American45 households. Second, the sulfonate component of detergents makes their degradation products far more toxic to the environment than those of soaps. Owing both to their low cost of production and to their impressive utility, detergents are produced and sold on a scale that dwarfs all other commercially synthesized surfactants. Not surprisingly, this has become a cause of growing concern with regard to the potential50 impacts on aquatic ecosystems, as well as on human health, as exposure to detergent derivatives has been convincingly implicated in several endocrine and reproductive disorders. Though this controversy is heated, complex, and unlikely to be settled in the foreseeable future, it has also sparked significant support for a fascinating field of biotechnology55 that deals with the surfactants produced endogenously by living organisms, and particularly those produced by microbes. With regard to their structure, these so-called "biosurfactants" are highly distinct from both soaps and detergents, and yet several promising preliminary studies have shown them to be functionally viable alternatives to more conventional cleaning products. The advantage lies in the high60 biodegradability and biologically benign character of biosurfactants. The obstacle, of course, lies in the nightmarish logistics of isolating them on a large, industrial scale.
Surface Tension of Soap Bubbles
The graph illustrates changes in surface tension over time in milliseconds (ms) for soap bubbles made with varying concentrations of surfactant. The time label on the x-axis is exponential and is compressed as the values increased.
1. The author most likely uses the phrase "oil and water do not mix" in line 6 to
2. As used in line 13, the word "integrity" most closely means
3. According to the passage, what property of surfactants is most responsible for their widespread human applications?
4. Which option gives the best evidence for the answer to the previous question?
5. It can most reasonably be inferred from the passage that the relative amounts of these man-made surfactants are currently what, from least to greatest?
6. The author's overall description of soaps and detergents is that they are
7. Which option gives the best evidence for the answer to the previous question?
8. As used in line 53, the word "settled" most closely means
9. Based on the information in the graph, if soap bubbles (like the ones measured in the graph) with a concentration of 8.5% surfactant were measured 10 ms after their creation, the surface tension in mN/m would be closest to
10. According to the information in the graph, an increase in surfactant percentage from what to what would most likely result in the largest relative increase in surface tension?
11. What is the most logical reason why the author used a logarithmic scale on the x-axis of the graph?
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