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The shapes of molecules also affect the magnitudes of the dispersion forces between them. For example, boiling points for the isomers n -pentane, isopentane, and neopentane (shown in [link] ) are 36 °C, 27 °C, and 9.5 °C, respectively. Even though these compounds are composed of molecules with the same chemical formula, C 5 H 12 , the difference in boiling points suggests that dispersion forces in the liquid phase are different, being greatest for n -pentane and least for neopentane. The elongated shape of n -pentane provides a greater surface area available for contact between molecules, resulting in correspondingly stronger dispersion forces. The more compact shape of isopentane offers a smaller surface area available for intermolecular contact and, therefore, weaker dispersion forces. Neopentane molecules are the most compact of the three, offering the least available surface area for intermolecular contact and, hence, the weakest dispersion forces. This behavior is analogous to the connections that may be formed between strips of VELCRO brand fasteners: the greater the area of the strip’s contact, the stronger the connection.

Three images of molecules are shown. The first shows a cluster of large, gray spheres each bonded together and to several smaller, white spheres. There is a gray, jagged line and then the mirror image of the first cluster of spheres is shown. Above these two clusters is the label, “Small contact area, weakest attraction,” and below is the label, “neopentane boiling point: 9.5 degrees C.” The second shows a chain of three gray spheres bonded by the middle sphere to a fourth gray sphere. Each gray sphere is bonded to several smaller, white spheres. There is a jagged, gray line and then the mirror image of the first chain appears. Above these two chains is the label, “Less surface area, less attraction,” and below is the label, “isopentane boiling point: 27 degrees C.” The third image shows a chain of five gray spheres bonded together and to several smaller, white spheres. There is a jagged gray line and then the mirror image of the first chain appears. Above these chains is the label, “Large contact area, strong attraction,” and below is the label, “n-pentane boiling point 36 degrees C.”
The strength of the dispersion forces increases with the contact area between molecules, as demonstrated by the boiling points of these pentane isomers.

Geckos and intermolecular forces

Geckos have an amazing ability to adhere to most surfaces. They can quickly run up smooth walls and across ceilings that have no toe-holds, and they do this without having suction cups or a sticky substance on their toes. And while a gecko can lift its feet easily as it walks along a surface, if you attempt to pick it up, it sticks to the surface. How are geckos (as well as spiders and some other insects) able to do this? Although this phenomenon has been investigated for hundreds of years, scientists only recently uncovered the details of the process that allows geckos’ feet to behave this way.

Geckos’ toes are covered with hundreds of thousands of tiny hairs known as setae , with each seta, in turn, branching into hundreds of tiny, flat, triangular tips called spatulae . The huge numbers of spatulae on its setae provide a gecko, shown in [link] , with a large total surface area for sticking to a surface. In 2000, Kellar Autumn , who leads a multi-institutional gecko research team, found that geckos adhered equally well to both polar silicon dioxide and nonpolar gallium arsenide. This proved that geckos stick to surfaces because of dispersion forces—weak intermolecular attractions arising from temporary, synchronized charge distributions between adjacent molecules. Although dispersion forces are very weak, the total attraction over millions of spatulae is large enough to support many times the gecko’s weight.

In 2014, two scientists developed a model to explain how geckos can rapidly transition from “sticky” to “non-sticky.” Alex Greaney and Congcong Hu at Oregon State University described how geckos can achieve this by changing the angle between their spatulae and the surface. Geckos’ feet, which are normally nonsticky, become sticky when a small shear force is applied. By curling and uncurling their toes, geckos can alternate between sticking and unsticking from a surface, and thus easily move across it. Further investigations may eventually lead to the development of better adhesives and other applications.

Three figures are shown. The first is a photo of the bottom of a gecko’s foot. The second is bigger version which shows the setae. The third is a bigger version of the setae and shows the spatulae.
Geckos’ toes contain large numbers of tiny hairs (setae), which branch into many triangular tips (spatulae). Geckos adhere to surfaces because of van der Waals attractions between the surface and a gecko’s millions of spatulae. By changing how the spatulae contact the surface, geckos can turn their stickiness “on” and “off.” (credit photo: modification of work by “JC*+A!”/Flickr)

Questions & Answers

what is the property of pressure
Sheyanna Reply
Hello everyone, anyone who about precursor?
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What is an electron affinity
Emmanuel Reply
Electron affinity is d amount of energy absorbed in the process in which an electron is added to a neutral isolated gaseous atom
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very brief information on collision theory
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what is matter
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matter is anything that has mass and occupied space.
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list mixtures and their components
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describe how Ethene reacts with bromine
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welcome
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Onah
the principles and applications of extraction chromatographic methods in the isolation and purification of organic compounds
Precious Reply
what is the dislocation of CH³COOH
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another name for alkane ?
adeyeye Reply
alkene
Kelechi
alkyne
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alkanols
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parafins
Ademola
what is the chemical formula for benzene
Emmanuel Reply
C6H6.
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correct
Kelechi
c6c6
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Benzene C6H6
Ademola
what does each group on the periodic table stand for
Chidera Reply
what's lanthanide series
kimah Reply
lanthanide series are element in the 4f block from 57 to 70
Ummulkhairi
lanthanide series consist of F block element in the first series in the end of the periodic table..they are placed separately in the periodic table.because they possess different properties than rest of the elements!
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whats is Dalton's law
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Practice Key Terms 8

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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