Intermolecular Interactions

Intermolecular interactions are the fundamental attractive and repulsive forces acting between neighbouring molecules. In this thesis, a variety of intermolecular interactions are employed in order to demonstrate the adsorption behaviour of surfactant exfoliated graphene particles. These include electrostatic interactions, Van der Waals forces and hydrogen bonding interactions. Electrostatic interactions derive from the Coulombic force between charged chemical species whilst the Van der Waals force and hydrogen bonding interactions originate from quantum mechanical effects. All three types of interactions can be used to facilitate coupling with adjacent surfaces and molecules, with their range of influence being determined to a large extent by the nature of chemical species involved in the interaction, in addition to their molecular mobility.

2.2.1

Electrostatic Interactions

Electrostatic intermolecular interactions appear exclusively as a consequence of Coloumbic attraction and repulsion. These forces arise from interactions between electric fields, which emanate from a range of species including charges, ions, dipoles and charged surfaces separated by a distance, 𝑟. Often, electrostatic forces manifest as strong, long range intermolecular interactions, decaying at a rate of 1_𝑟 for isolated charge-charge interactions or _𝑟41 for interactions between charges and induced molecular dipoles which experience free rotation, such as those in bulk solutions. Examples of electrostatic intermolecular interactions include those between the cationic polyelectrolyte, polyethyleneimine and negatively charged graphene particles discussed in Chapter 5.

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2.2.2

Van der Waals Interactions

Van der Waals forces are weak, ubiquitous intermolecular interactions which arise as a consequence of temporal fluctuations in the electronic polarisation of molecules. They are comprised of three different types of polar interactions which include London dispersion forces (instantaneous dipole-instantaneous dipole), Keesom interactions (permanent dipole-permanent dipole) and Debye interactions (permanent dipole- induced dipole).

London dispersion forces are a type of weak, attractive, intermolecular force that exists between all molecules, regardless of chemical composition. They arise from the instantaneous dipole moment caused by oscillating electron charges, and are able to interact with the electronic polarisability of other molecules by propagating an oscillating electric field. Affected molecules redistribute their electron cloud in response to this electric field, forming an instantaneous dipole that propagates a corresponding in-phase electric field. The combination of these fields results in an overall attractive intermolecular interaction. Dispersion forces are short range interactions which remain effective for distances of tens of nanometres, decaying at a rate of _𝑟61 for isolated molecules and for bulk materials. Dispersion forces appear throughout the adsorption experiments in this thesis and also occur in graphite. More specifically, dispersion forces are the primary attractive forces that hold the graphene sheets in the lamellar structure and arise chiefly due to the system of delocalised π- electrons along the carbon monolayer. They are also responsible for π-π interactions, which occur between neighbouring π-electron rich systems such as aromatic molecules.

Dipole-dipole (Keesom) and dipole-induced dipole (Debye) interactions are two other components of the van der Waals force, which exist as a result of at least one polar

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group being present. The Keesom force occurs when the permanent electric fields surrounding at two molecular dipoles interact, causing either repulsion or attraction depending on molecular orientation. In contrast, Debye interactions are caused by electric fields emanating from permanent dipoles, which perturb the distribution of electron clouds surrounding nearby molecules, inducing polarization. Both Keesom and Debye interactions are stronger than dispersion forces, and decay at a rate of _𝑟61 assuming the molecules are in an environment, such as a bulk liquid phase in which they are free to rotate.

2.2.3

Hydrogen Bonding Interactions

Hydrogen bonding is a special category of dipole-dipole (Keesom) interactions that involves molecules containing hydrogen being directly bound to highly electronegative atom such as fluorine, oxygen or nitrogen. The proximity of the hydrogen atom to the electronegative species in these molecules causes an overall shift in the electron cloud away from the hydrogen atom. As a consequence, the bond between the two atoms becomes highly polarised, with the hydrogen atom acquiring a partial positive charge while the adjacent atom acquires a corresponding negative charge. The dipole is then able to interact with dipoles present in other molecules, leading to attractive hydrogen bonding interactions. The resultant interactions are generally strong and remain effective over short ranges, decaying at a rate of _𝑟21. The interactions are also highly orientation dependent, and therefore exist for only short periods in solutions due to rapid molecular motion. A variety of systems involving hydrogen bonding interactions appear in this thesis, including those between the polyelectrolyte, PAA, and the polyethylene oxide groups present on the surfactant Pluronic F108 discussed in Chapter 6.

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Hydrogen bonding is also an important interaction that influences the intermolecular forces acting in aqueous systems. The hydrogen bonds formed between neighbouring water molecules in a pure liquid are highly directional and give rise to a structured, yet dynamic molecular network. In aqueous solutions however, the presence of solutes often affects the local structuring of water molecules. For instance, polar solutes are able to associate with water molecules through favourable dipole-dipole interactions, promoting solubility of these species in water. In contrast, solutes containing non-polar groups tend to either aggregate in the bulk or accumulate at interfaces in order to minimise interactions between polar and non-polar species whilst maximising the energetically favourable hydrogen bonding interactions between water molecules. This behaviour is known as the hydrophobic effect and is responsible for a variety of colloidal and surface phenomena examined in this thesis, including the surface affinity of surfactants and micelle formation.