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Questions and Answers
What is the primary reason for the existence of interfacial tension at the boundary between two phases?
What happens to the molecules at the interface in a liquid system?
What happens to the surface free energy of a liquid when its surface is increased?
What is the relationship between the surface free energy and the size of the free surface?
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What is the result of attempting to reverse the spontaneous movement of molecules from the interface to the bulk phase?
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What is the unit of interfacial tension in the SI system?
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What is the primary reason for the increase in energy of a liquid when its surface is increased?
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What is the characteristic of the force of interfacial tension?
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What is the result of the reduction in actual contact area between dissimilar molecules at the interface?
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What is the definition of surface?
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Study Notes
Attractive Forces between Molecules
- Attractive forces between molecules occur due to variations in electron distribution in a molecule.
- These forces are responsible for the state of substances, whether solid, liquid, or gas.
- The strengths of intermolecular forces are generally weaker than ionic or covalent bonds.
Types of Attractive Intermolecular Forces
- Van der Waals forces:
- Dipole-dipole attractions (Keesom forces)
- Dipole-induced dipole attractions (Debye forces)
- Induced dipole-induced dipole attractions (London forces or dispersion forces)
- Hydrogen bonding (H-bonding)
- Ion-dipole and ion-induced dipole forces
- Ion-ion interactions
Dipole-Dipole Attractions
- Occur between the dipoles of two polar molecules.
- Caused by the permanent, uneven distribution of electrons due to electro-vity differences of atoms in the molecule.
Dipole-Induced Dipole Attractions
- Occur between a polar molecule and a non-polar molecule.
- The non-polar molecule temporarily becomes polarized.
Induced Dipole-Induced Dipole Attractions
- Force in which non-polar molecules induce instantaneous polarity in one another.
- Electrons are shifted to overload one side of an atom or molecule.
Ion-Dipole and Ion-Induced Dipole Forces
- Account in part for the solubility of ionic crystalline substances in water.
- The cation attracts the relatively negative oxygen of H2O, and the anion attracts the hydrogen atoms of the dipolar H2O molecules.
- Ion-induced dipole forces are involved in the formation of the iodide complex.
Ion-Ion Interactions
- A cation on one compound will interact with an anion on another compound, giving rise to an intermolecular association.
- May be intermolecular (e.g., a hydrochloride salt of a drug) or intramolecular (e.g., a salt-bridge interaction between counter ions in proteins).
Hydrogen Bonding (H-Bonding)
- A force between hydrogen and an electronegative atom (O, N, and F).
- The strongest type of dipole-dipole attraction.
- Occurs in ice and liquid water, accounting for many unusual properties of water, including:
- High dielectric constant
- Abnormally low vapor pressure
- High boiling point
Phases
- A phase is a homogeneous, physically distinct portion of a system that is separated from other portions of the system by bounding surfaces.
- The three primary phases of matter (solid, liquid, and gaseous) are defined individually under different conditions, but in most systems, we encounter phases in coexistence.
Phase Rule
- Formulated by J. Willard Gibbs, the phase rule states that the least number of intensive variables required to define the state of a system is F = C - P + 2.
- F is the number of degrees of freedom of the system.
- C is the number of components, and P is the number of phases present.
Applications of Phase Rule
- Examples of applying the phase rule to different systems, including a mixture of ice, liquid water, and water vapor.
- Importance of phase rule in understanding the behavior of pharmaceutical systems, such as the formulation of phenol-water solutions.
Interface and Surface Tension
- Interface: the boundary between two phases.
- Surface: the interface between a liquid and a gas or a solid and a gas.
- Surface tension: the force that causes a liquid to behave as if it has an "elastic skin" at its surface.
- Surface free energy (G) is proportional to the size of the free surface.
Importance of Interfacial Tension
- Interfacial tension plays a significant role in medicine, affecting:
- Emulsion formation and stability
- Dispersion to form suspensions
- Adsorption of drugs onto solid adjuncts in dosage forms
- Penetration of molecules through biological membranes
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Description
Learn about the attractive forces between molecules, responsible for the state of substances, and compare their strengths to ionic and covalent bonds.
