Intermolecular force term

Kutzelnigg W and Maeder F 1978 Natural states of interacting systems and their use for the calculation of intermolecular forces. III. One-term approximations of oscillator strength sums and dynamic polarizabilities Chem. Phys. 35 397... 

The term polymer is derived from the Greek words poly and meros, meaning many parts. We noted in the last section that the existence of these parts was acknowledged before the nature of the interaction which held them together was known. Today we realize that ordinary covalent bonds are the intramolecular forces which keep the polymer molecule intact. In addition, the usual type of intermolecular forces—hydrogen bonds, dipole-dipole interactions, and London forces—hold assemblies of these molecules together in the bulk state. The only thing that is remarkable about these molecules is their size, but that feature is remarkable indeed. 

Dispersive Interactions. For pairs of nonpolar polymers, the intermolecular forces are primarily of the dispersive type, and in such cases the energy of interaction between unlike segments is expected to be closely approximated by the geometric mean of the energies of interaction between the two like pairs (98). In this case, the Flory-Huggins interaction energy between this polymer pair can be expressed in terms of the solubiUty parameters 5 of the pure components. 

The distribution coefficient is an equilibrium constant and, therefore, is subject to the usual thermodynamic treatment of equilibrium systems. By expressing the distribution coefficient in terms of the standard free energy of solute exchange between the phases, the nature of the distribution can be understood and the influence of temperature on the coefficient revealed. However, the distribution of a solute between two phases can also be considered at the molecular level. It is clear that if a solute is distributed more extensively in one phase than the other, then the interactive forces that occur between the solute molecules and the molecules of that phase will be greater than the complementary forces between the solute molecules and those of the other phase. Thus, distribution can be considered to be as a result of differential molecular forces and the magnitude and nature of those intermolecular forces will determine the magnitude of the respective distribution coefficients. Both these explanations of solute distribution will be considered in this chapter, but the classical thermodynamic explanation of distribution will be treated first. 

Molecular interactions are the result of intermolecular forces which are all electrical in nature. It is possible that other forces may be present, such as gravitational and magnetic forces, but these are many orders of magnitude weaker than the electrical forces and play little or no part in solute retention. It must be emphasized that there are three, and only three, different basic types of intermolecular forces, dispersion forces, polar forces and ionic forces. All molecular interactions must be composites of these three basic molecular forces although, individually, they can vary widely in strength. In some instances, different terms have been introduced to describe one particular force which is based not on the type of force but on the strength of the force. Fundamentally, however, there are only three basic types of molecular force. 

Most nonpolar substances have very small water solubilities. Petroleum, a mixture of hydrocarbons, spreads out in a thin film on the surface of a body of water rather than dissolving. The mole fraction of pentane, CsH12, in a saturated water solution is only 0.0001. These low solubilities are readily understood in terms of the structure of liquid water, which you will recall (Chapter 9) is strongly hydrogen-bonded. Dissimilar intermolecular forces between C5H12 (dispersion) and H2O (H bonds) lead to low solubility. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

Account for the following observations in terms of the type and strength of intermolecular forces, (a) The melting point of solid xenon is —112°C and that of solid argon is — 189°C. 

What Do We Need to Know Already This chapter develops the concepts of physical equilibria introduced in the context of thermodynamics (Chapters 6 and 7) and assumes a knowledge of intermolecular forces (Sections 5.1-5.5). The compositions of some of the solutions discussed are expressed in terms of mole fraction (Section 4.8). 

The van der Waals equation adds two correction terms to the ideal gas equation. Each correction term includes a constant that has a specific value for every gas. The first correction term, a fV, adjusts for attractive intermolecular forces. The van der Waals constant a measures the strength of intermolecular forces for the gas the stronger the forces, the larger the value of a. The second correction term, n b, adjusts for molecular sizes. The van der Waals constant b measures the size of molecules of the gas the larger the molecules, the larger the value of b. 

C12-0057. A drop in temperature accompanies the escape of CO2 from a carbonated beverage. Explain this phenomenon in terms of intermolecular forces. 

The difference between the log P of a given compound in its neutral form (log P ) and its fully ionized form (log P ) has been termed dialog P ) and contains series-specific information, and expresses the influence of ionization on the intermolecular forces and intramolecular interactions of a solute [44, 51, 52]. 

In this form Eq. (86) cannot be integrated without a relation between P and V, because die second term on the right-hand side involves both variables. However, in the special case in which the gas is ideal, PV = RT for one mole and (dEfdV)r = 0 (see problem 6). The latter relation implies the absence of intermolecular forces. Then, Eq. (86) becomes... 

When describing the effect of an external force, we must first define the force itself. A lay person s definition of a force is the amount of effort to get the desired effect. As scientists, we need a more precise definition of force. With a precise definition we can understand and quantify the effect of an applied force on a polymeric material. The mathematical definition of force is the work (which is a form of energy) required to move an object over some distance. Another way to define a force is in terms of the acceleration it creates when applied to some object of a mass m. In our everyday experiences, the first explanation is a simple idea to relate to. When we push a stalled car we exert a force on it. We could easily quantify the force from the weight of the car, the slope of the hill it is sitting on, and how far we must push it. Once we begin to talk about forces in polymer systems, the ideas become a bit more complicated. For example, the force required to open a bag of candy is defined by the work required to deform the bag until it ruptures by overcoming the intermolecular forces which hold the plastic together. 

KEY TERMS hydrogen bonding intermolecular forces surface tension... 

KEY TERMS intermolecular forces hydrogen bonding polymer... 

KEY TERMS intermolecular forces proteins colloidal dispersion... 

KEY TERMS intermolecular forces amino acids hydrogen bonds hydrophobic... 

The term molecular crystal refers to crystals consisting of neutral atomic particles. Thus they include the rare gases He, Ne, Ar, Kr, Xe, and Rn. However, most of them consist of molecules with up to about 100 atoms bound internally by covalent bonds. The dipole interactions that bond them is discussed briefly in Chapter 3, and at length in books such as Parsegian (2006). This book also discusses the Lifshitz-Casimir effect which causes macroscopic solids to attract one another weakly as a result of fluctuating atomic dipoles. Since dipole-dipole forces are almost always positive (unlike monopole forces) they add up to create measurable attractions between macroscopic bodies. However, they decrease rapidly as any two molecules are separated. A detailed history of intermolecular forces is given by Rowlinson (2002). 

In van der Waals equation, it is the term n2a/V2 that is of interest in this discussion, because that term gives information about intermolecular forces. Specifically, it is the parameter a that is related to inter-molecular forces rather than the number of moles, n, or the volume, V. It should be expected that the... 

The viscosity of 1-chloropropane is only about one-seventh that of 1-propanol. Explain this difference in terms of intermolecular forces. 

Supramolecular aggregations are commonly referred to by a variety of terms, including adduct, complex, and van der Waals molecule. In this chapter we shall primarily employ the more neutral term cluster, which may, if desired, be qualified with the type of intermolecular interaction leading to clustering (e.g., H-bonded cluster ). General and specific types of intermolecular forces are discussed in the following sections. 

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