New SAT Reading Practice Test 80: Surfactants

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Surfactants

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 minimizing
05 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, while
10 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 diverse
15 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 itself
20 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 and
25 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 acid
30 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 polar
40 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 American
45 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 potential
50 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 biotechnology
55 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 high
60 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

  • A. explain the process in which two immiscible substances are emulsified.
  • B. imply that most laypeople cannot understand the topic of this article.
  • C. show that substances that do not have triglycerides cannot undergo saponification.
  • D. connect the esoteric analysis to a commonly understood phenomenon.

2. As used in line 13, the word "integrity" most closely means

  • A. rectitude.
  • B. solidarity
  • C. cohesion.
  • D. decadence.

3. According to the passage, what property of surfactants is most responsible for their widespread human applications?

  • A. They are produced by microbes.
  • B. They are amphiphilic.
  • C. They are an emulsion.
  • D. Their saponification

4. Which option gives the best evidence for the answer to the previous question?

  • A. Lines 13-16 ("Due to . . . detergents")
  • B. Lines 17-20 ("An emulsion . . . emulsion")
  • C. Lines 33-35 ("At the . . . acids")
  • D. Lines 53-56 ("Though . . . microbes")

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?

  • A. Biosurfactants, detergents, soaps
  • B. Biosurfactants, soaps, detergents
  • C. Soaps, detergents, biosurfactants
  • D. Detergents, soaps, biosurfactants

6. The author's overall description of soaps and detergents is that they are

  • A. commonly thought of as interchangeable, but having important differences.
  • B. one and the same insofar as their chemical properties, such as molecular structure.
  • C. different with respect to their capacity to mix in emulsions.
  • D. major obstacles to the widespread acceptance of biosurfactants.

7. Which option gives the best evidence for the answer to the previous question?

  • A. Lines 16-20 ("The word . . . emulsion")
  • B. Lines 28-31 ("Importantly . . . oils")
  • C. Lines 37-38 ("While . . . saponification")
  • D. Lines 56-59 ("With regard . . . products")

8. As used in line 53, the word "settled" most closely means

  • A. firm.
  • B. resolved.
  • C. disturbed.
  • D. mobilized.

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

  • A. 30.
  • B. 40.
  • C. 50.
  • D. 60.

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?

  • A. From 0.5% to 1.5%
  • B. From 4.5% to 5.5%
  • C. From 8% to 9%
  • D. All of these would result in equivalent surface tension increases.

11. What is the most logical reason why the author used a logarithmic scale on the x-axis of the graph?

  • A. It helps give a more accurate compilation of the data than would a typical linear scale.
  • B. It enables readers to more easily see the directly proportional relationship between the variables.
  • C. It impresses the reader because of the author's obvious mastery of advanced mathematical reasoning.
  • D. It makes it easier to visualize the changes in surface tension over the ever-increasing orders of magnitude of time.