Van der Waals forces in alkanes

Nonbonded interactions are the forces be tween atoms that aren t bonded to one another they may be either attractive or repulsive It often happens that the shape of a molecule may cause two atoms to be close in space even though they are sep arated from each other by many bonds Induced dipole/induced dipole interactions make van der Waals forces in alkanes weakly attractive at most distances but when two atoms are closer to each other than the sum of their van der Waals radii nuclear-nuclear and electron-electron repulsive forces between them dominate the fvan derwaais term The resulting destabilization is called van der Waals strain... 

It would, however, appear to be inappropriate for the systems considered by Denkov and coworkers [34, 35] to foUow Vrij and Overbeek [41] by only allowing for van der Waals forces in substitution of the relevant expression for dIlAwo(Wdfi in Equation 3.23. Kellay et al. [42] suggest that in the case where the oil-water inter-facial tension is extremely low, there may, for example, be a positive contribution to the disjoining pressure from the sterically frustrated amplitude of the fluctuations. The presence of anionic surfactant implies a repulsive electrostatic contribution and the Hamaker constants for alkane-water-air films are almost all negative [43], which also implies a positive contribution to the disjoining pressure from van der Waals forces. The latter would, however, mean that pure water should spontaneously wet hydrocarbons (in the absence of surfactant), which is clearly not correct It has been argued therefore that in this case, the so-called short range hydrophobic forces may be responsible for the instability of pseudoemulsion films of pure water on hydrocarbon oils [44]. 

These forces are electrical in nature, and in order to vaporize a substance, enough energy must be added to overcome them. Most alkanes have no measurable dipole moment, and therefore the only van der Waals force to be considered is the induced-dipole/induced-dipole attractive force. 

It is well known that neutral molecules such as alkanes attract one another, mainly through van der Waals forces. Van der Waals forces arise from the rapidly fluctuating dipoles moment (1015 S-1) of a neutral atom, which leads to polarization and consequently to attraction. This is also called the London potential between two atoms in a vacuum, and is given as... 

For example, the melting and boiling points of the alkanes shown in Table 14.1 gradually increase. This is due to an increase in the intermolecular forces (van der Waals forces) as the size and mass of the molecule increases (Chapter 3, p. 49). 

Because unbranched alkanes are neutral, nonpolar molecules, it is difficult to explain the existing intermolecular force between such alkanes that increases as the alkane molecules become larger. We will see that this attractive force is weak and tenuous. These molecules do not become overly friendly with each other. In theory, as atoms within one alkane molecule approach the atoms of another alkane molecule, the electrons around these atoms, for an instant, arrange themselves asymmetrically around the atoms so that instant dipoles are formed—the positive side of one atom attracts the negative side of another atom. This weak intermolecular attractive force is called a London Force. When there is a weak intermolecular attractive force between polar molecules, the force is called a dipole-dipole force. Together, London forces and dipole-dipole forces are called Van der Waals forces. 

Alkanes are nonpolar molecules, hence van der Waals forces are responsible for attractions between the molecules. Increasing the number of carbon atoms causes an increase in the strength of the van der Waals forces. The first four members of the alkanes are gases, those with 5-17 carbons are liquids, and the rest are solids. 

The physisorption of alkanes in zeolites is essentially due to van der Waals forces. The main contributions to the stabilization are the induction energy, roughly speaking the electrostatic interaction of the dipole moment induced by the electric field of zeolite with the framework charge distribution and the... 

Van der Waals forces arise from dipole-dipole interactions. In a dipolar molecule, there is a separation of positive from negative charge. For example, if the molecule contains charges -Fq and —q separated by a distance vector r, then the molecule has a dipole moment of u = rq. Dipolar moments are often measured in units of the Debye D, where 1 D = 3.336 X 10 coulomb-meter. While alkanes have no permanent dipole moments,... 

Another interesting effect seen in alkanes is that increased branching lowers an alkane .s boiling point. Thus, pentane has no branches and boils at 36.1 C, isopentane (2-methylbutane) has one branch and boils at 27.85°C, and neopentane (2,2-dimethylpropane) has two branches and boils at 9.5"C. Similarly, octane boils at 125.7 C, whereas isooctane (2,2,4-trimethylpen-tane) boils at 99.3°C. Branched-chain alkanes are lower-boiling because they are more nearly spherical than straight-chain alkanes, have smaller surface areas, and consequently have smaller van der Waals forces. 

The n-decane-urea complex can be demonstrated as shown in the hotograph with the models developed by one of us and a sheet of cellulose acetate to represent the inside of the complex (Fig. U-1). The cylinder fits snugly over the model and defines the total space, not that actually occupied, since van der Waals forces prevent actual contact and keep the atoms at a certain stand-off distance. The diameter represented is 14.3x0.2 = 2.86 A, but X-ray measurements indicate the true diameter to be 5.30 A. Half the difference, or 1.22 A, then represents the stand-off distance all the way around the circumference. This estimate applies only to the complexes of normal alkanes and may be a maximal value. Thus 3-nonyne forms a urea complex but the model cannot be filled inlo Ihe 14.3-cm. cylinder. It does CHjCHjC SCCHjCHjCHjCHjCH, C=C-CSCCHj... 

Koddermann et al. calculated the heats of vaporization for imidazolium-based ILs [Cnmim][NTf2] with n = 1, 2, 4, 6, 8 by means of MD simulations [81], The authors applied a force field which they had developed recently. Within this force field the authors reduced the Lennard-Jones parameters in order to reproduce experimental diffusion coefficients [81], The refined force field also led to absolute values of heats of vaporization as well as their increase with the chain length of the imidazolium cation such that this quantities were described correctly. The overall heats of vaporization were split in several contributions and discussed in detail. The authors observed that with increasing alkyl chain length, the Coulomb contribution to the heat of vaporization remained constant at around 80 kJ mol 1, whereas the van der Waals interaction increased continuously. The calculated grow of about 4.7 kJ mol"1 per CH2-group of the van der Waals contribution in the IL exactly matched the increase in the heats of vaporization for n-alcohols and n-alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions [81],... 

In principle, the adsorption of alkane molecules on C(OOOl) would appear unlikely because of the inert character of the substrate. In this case, however, besides van der Waals forces, other contributions come into play and can make energy adsorption reach values of up to lOOkJ/mol, which are comparable to those of chemisorption processes. This enables determining the structure of aliphatic hydrocarbons adsorbed on C(OOOl) by AFM or STM because the adsorbate withstands tip—sample interaction forces. 

The elution of analytes from reversed-phase sorbents is a rather simple process and consists of choosing a nonpolar solvent to disrupt the van der Waals forces that retain the analyte. Because the sorption process is a partitioning process, it is usually only necessary to allow the eluting solvent to have intimate contact with the bonded phase (e.g., C-18) in order to elute the analytes from the sorbent. Because the bonded phases consist of a silica matrix, they have an increased polarity compared to the original hydrophobicity of the C-18 alkane. Thus, the elution solvent must be capable of mutual solubility with the silica surface, as well as with the C-18 or other bonded phase. 

The variation of y of a large variety of liquids (more than a hundred) is available in literature. The different homologue series will provide information about the stabilizing forces in these fluids. For example, while alkanes are stabilized mainly by van der Waals forces, the alcohols would be mainly stabilized by both van der Waals forces and hydrogen bonds the latter is stronger than the former. 

Surface tension of any fluid can be related to various interaction forces, e.g., van der Waals, hydrogen bonding, dipole, and induction. The above analyses of the alkanes thus provide information about the van der Waals forces only. In other homologous series, such as alcohols, we can expect that there are both van der Waals and hydrogen bonding contributions. We can thus combine these two kinds of homologous series of molecules and analyze the contribution from each kind of interaction. 

The small differences in stability between branched and unbranched alkanes result from an interplay between attractive and repulsive forces within a molecule (intramolecular forces). These forces are nucleus-nucleus repulsions, electron-electron repulsions, and nucleus-electron attractions, the same set of fundamental forces we met when talking about chemical bonding (see Section 1.12) and van der Waals forces between molecules (see Section 2.14). When the energy associated with these interactions is calculated for all of the nuclei and electrons within a molecule, it is found that the attractive forces increase more than the repulsive forces as the structure becomes more compact. Sometimes, though, two atoms in a molecule are held too closely together. WeTl explore the consequences of that in Chapter 3. 

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