Lecture 13: Intermolecular Interactions

Lecture 13: Intermolecular interactions Summary This lecture focused on intermolecular forces (IMFs), which are interactions between molecules weaker than ionic or covalent bonds, but that on a larger scale take on an enormous role in giving materials their properties. First, we defined a dipole: a pair of charges, one positive and one negative, separated by a distance. We have already seen dipoles in this class—a covalent bond that involves a difference in electronegativities, for example if one atom is electropositive and the other electronegative, HCI dipole diagram forms a dipole. The H-Cl molecule is a simple example shown here, where removed due to the arrow in the diagram points towards the more electronegative chlocopyright restrictions. rine, and the δ + / δ − indicate an excess of positive/negative charge. The arrow is the direction of the dipole. It is possible for a molecule with polar covalent bonds to have no net dipole, as shown for CO2 below. This is because its two dipoles cancel. Water has a net dipole only in the y-direction. The presence of a net dipole means that the dipoles of the molecule will feel attraction to opposite charges. This other charge could be an ion, or another dipole. The former is called an ion-dipole interaction, and the latter a dipole-dipole interaction. These are attractions between molecules rather than bonds within them. It is still possible for molecules with no net dipole to participate in intermolecular bonding, because even nonpolar molecules can experience temporary dipoles due to fluctuations in the electron cloud. These temporary dipoles can happen due to interactions with surrounding charges, in which case they are called induced dipoles. These can interact with ions or permanent dipoles or other induced dipoles. These fluctuations in the electron cloud can also be a result of temperature. When these temporary dipoles interact with each other, the attractive forces are called London dispersion forces (LDF). It is easier to temporarily deform the electron clouds of some molecules compared to others. They have differences in polarizability. For example, larger atoms are more polarizable than smaller atoms due to their outer electrons being less affected by nuclear pull. The more polarizable a molecule is, the stronger its LDF. The LDF also depends on the surface area of the molecule, since greater surface area means more electron cloud that can be temporarily induced to shift, which in turn means higher LDF. Finally, we discussed hydrogen bonds, which occur when a hydrogen atom attached to a highly electronegative element in a 22 Middle right figure by B.A. Averill and P. Eldridge in General Chemistry. License: CC BY-NCSA. Source: Open Textbook Library. Lower left figure © source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/fairuse. molecule (such as nitrogen, oxygen, or fluorine) is attracted to a negatively charged region, like a lone pair of electrons. Why this matters Weak bonds aren’t actually all that weak, especially when there are a lot of them. The gecko is a perfect example of this. It’s a remarkable animal that can walk up and down walls and even across the ceiling without breaking a sweat! The reason is that its toes are padded with microscopic hairs (called “setae”), and each of these in turn has hundreds of nanoscale branches. One gecko toe can have as many as a billion little hairs! These hairs hold the key to its seemingly magFigure of gecko structures from ical adhesive abilities and the reason there are removed due to copyright restrictions. so many of them is precisely because of what we learned in this lecture: weak interactions like van der Waals get stronger with more surface area. A billion little nano-strands on each toe gives the gecko’s foot a whole lot of surface area, especially when those strands lie nearly flat against whatever surface the gecko is climbing or sticking to. When the strand runs parallel to a surface, it maximizes the amount of the strand that can engage in van der Waals attraction with that surface. When the strand is more perpendicular, the force is dramatically reduced. Herein lies the secret of the Gecko, because it’s not enough but it also needs to be able to alternate between super-adhesive and not adhesive at all. Otherwise, it would stick to the wall and not be able to move! So, the Gecko clearly knows all about the dependence of van der Waals attraction on surface area contact, and each time it takes a step it adjusts toe adhesion by changing the angle of its billions of toe hairs. for it to have a super-adhesive toe, Photo of man climbing glass wall wearing gecko gloves removed due to copyright restrictions. This matters not only because it’s such a cool illustration of how amazing nature can be, but also because of how relevant improved reversible adhesives would be for a broad range of applications. Yes, this includes scaling buildings like Tom Cruise in Mission Impossible 4, but it also would impact areas ranging from advanced manufacturing to treating wounds. Just think about what it could mean to have a tape that’s 1000 times stronger than current tape, and can be applied and removed thousands of times 23 without loss of stickiness. Researchers and companies alike have been trying to mimic the Gecko’s adhesive abilities for decades, and still we’re not at a place where we can quite do it, although improved control over the chemistry and nanostructure of materials is getting us closer than ever. It all comes down to engineering the van der Waals attraction. Why this employs The last weak interaction we discussed in this chapter was that of the hydrogen bond. This bond is incredible in so many ways, not the least of which being the employment opportunities it provides. Hydrogen bonding is prevalent (and often dominant) in determining the way in which proteins fold and unfold, and in the way in which the helices of DNA hold together. It is the reason paper is possible, and the reason fish don’t freeze in the winter in a pond, to name only a few examples. But the example I want to focus on for this Why This Employs is that of surface coatings. The way a surface interacts with water can be engineered by modifying either the way in which the water hydrogen-bonds to the surface, the way in which water hydrogen-bonds to itself, or both. Coatings that repel water are needed all over the place, from keeping car windshields clear, to making clothes that stay dry in a rainstorm, to preventing condensation in power station turbines. The chemistry that goes into these coatings holds the key to controlling the hydrogen bonding, and there are a lot of companies interested in hiring people to work with such chemistry. This ranges from medium-sized materials companies like NEI Corporation, which makes a “superhydrophobic” coating called Nanomyte, or UltraTech, which makes spill-containment coatings, or Aculon, which states on their website, “Where there is a surface that has a problem, we like to think we can help solve that problem,” to larger companies like DuPont that makes specialized coatings for the Oil and Gas industry, to small start-ups like DropWise, founded by our very own Course 2 Professor Kripa Varanasi. These are only a handful of examples of the many companies building products around controlling hydrogen bonds. Extra practice 1. At T=25°C, F2 and Cl2 are gases, Br2 is a liquid, and I2 is a solid. This is because (choose one): A. London interactions increase with molecular size. B. Dipole-induced dipole interactions increase with molecular size. C. Dipole-dipole interactions increase with molecular size. D. Polarizability increases with molecular size. E. London interactions increase with molecular size and polarizability increases with molecular size 24 2. Based on the following information: CF4 , Molecular Weight 87.99, Normal Boiling Point -182°C CCl4 , Molecular Weight 153.8, Normal Boiling Point -123°C The intermolecular forces of attraction in the above substances are described by which of the following (choose one): A. gravitational forces B. London forces C. ion-dipole forces D. dipole-dipole forces (permanent dipoles) E. repulsive forces 3. Acrylic acid is pictured below. What is the dominant intermolecular force in a solution of acrylic acid? (also, what’s the hybridization around every atom?) 25 Solutions 1. E 2. B 3. H-bonding (all atoms are sp2 except for the O-H, which is sp3) 26