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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 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 molecules at the interface in a liquid system?
What happens to the surface free energy of a liquid when its surface is increased?
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?
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?
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?
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?
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?
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?
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?
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.
