Refractometry in Chemistry and Industry

Surface Tension

• A physical phenomenon where the surface of a liquid behaves like a stretched elastic sheet
• This is due to an imbalance of forces at the liquid's surface
• Molecules at the surface experience unbalanced cohesive forces from neighboring molecules, pulling them inwards, with no corresponding molecules above to balance these forces
• Defined as the force experienced at a right angle per unit length of the liquid's surface
• Represented by the Greek letter sigma (σ)
• Measured in newtons per meter (N/m)
• Causes surface area to reduce

Intermolecular Forces

• The primary cause of surface tension
• Various types contributing include:
  • Hydrogen bonding: Occurs between molecules with a hydrogen atom covalently bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. Crucial for water's high surface tension
  • Dipole-dipole interactions: Occur between molecules with permanent electric dipole moments, responsible for polar liquids' surface tension (e.g., ethanol, acetone)
  • Van der Waals forces: Arises from temporary dipoles, responsible for non-polar liquids' surface tension (e.g., hexane, benzene)
• Imbalance of intermolecular forces at the surface creates a net inward force, pulling surface molecules towards the liquid's interior, this is the fundamental cause of surface tension.

Molecular Arrangement

• Molecules at the surface are in a higher energy state compared to those in the bulk of the liquid
• They arrange themselves to maximize interactions with neighboring molecules
• This energetically favorable arrangement minimizes the overall energy of the system, leading to surface tension

Examples of Surface Tension

• Water striders walking on water
• Rain droplets forming spherical shapes due to surface tension.
• Needles floating on water

Factors Affecting Surface Tension

• Temperature: Surface tension decreases with increasing temperature as molecular kinetic energy increases, reducing intermolecular forces
• Pressure: Higher pressure slightly increases surface tension, this effect is more notable in gases than liquids.
• Composition: Different types and concentrations of chemicals present affect surface tension

Optical Activity

• Refers to the ability of certain substances to rotate the plane of polarized light
• It is due to molecular asymmetry (chirality). A molecule (or object) is chiral if it is not superimposable with its mirror image
• Chirality is essential in drug development and other fields. Different enantiomers of a substance may have different properties and effects on the body and how it is metabolized.
• Polarimeter is the device to measure Optical activity

Types of Optical Activity

• Dextrorotatory: Rotates polarized light clockwise.
• Levorotatory: Rotates polarized light counterclockwise.
• Racemic mixtures: Equal amounts of enantiomers cancel each other's rotation.

Measurement of Optical Activity

• Polarimetry is used for measurement.
• Using a polarizer, sample tube, and analyzer.
• Quantitative measurement of optical activity is recorded.

Factors Affecting Optical Rotation

• Wavelength of light: Different wavelengths can result in differing degrees of rotation due to varied molecular absorption.
• Nature and concentration of the sample: Greater concentration usually results in higher rotation.
• Temperature: Temperature changes can impact the structure of molecules, potentially affecting rotation.
• Path length of the sample: Longer path through the sample usually impacts the extent of rotation.
• Solvent used: Different solvents affect the degree of rotation by influencing molecular interactions.

Applications of Optical Activity

• Pharmaceuticals: Crucial for ensuring the correct dosage and efficacy of pharmaceuticals, as different enantiomers often have vastly different effects on the body.
• Food industry: Verification of product authenticity, distinguishing between synthetic and natural products; flavor enhancement in foods.
• Other Applications: Used in materials science, protein folding studies, high-throughput drug discovery, and more.

Density

• Higher Density Liquids: Higher surface tension (e.g., mercury) because their molecules pack more closely together
• Lower Density Liquids: Weaker surface tension (e.g., alcohol) because their molecules are further apart

Viscosity

• A measure of a fluid's resistance to flow, measured in pascal-seconds (Pa·s).
• Dynamic Viscosity: Represents tangential force per unit area needed to move one fluid layer relative to another.
• Kinematic Viscosity: Ratio of dynamic viscosity to fluid density.
• Depends on temperature, pressure, and chemical composition of the fluid

Factors Affecting Viscosity

• Temperature: Viscosity decreases with rising temperature in liquids, while in gases it increases.
• Pressure: Usually slightly increases viscosity in liquids and gases, the effect is generally more pronounced in gases than liquids.
• Chemical composition: Fluid's inherent viscosity depends on chemical structure and composition.

Applications of Viscosity

• Engineering: Used in lubrication systems, fluid transport, and more
• Medicine: Characterizing blood flow
• Food Industry: Analyzing properties of syrups and oils
• Chemical Industry: Understanding and controlling fluid behavior

Vapour Pressure

• Pressure exerted by a liquid's vapours at a particular temperature.
• Tendency of a material to change state from liquid to gas.
• Equilibrium between liquid and gaseous state.

Factors Affecting Vapour Pressure

• Intermolecular forces: Stronger forces mean lower vapour pressure.
• Temperature: Directly proportional; increased temperature equals increased vapour pressure.
• Nature of liquid: Liquids with weaker intermolecular forces (lower boiling points) have higher vapour pressures.

Interconversion of Matter

• Processes involving transformations between solid, liquid, and gaseous states.
• Includes melting, freezing, evaporation, condensation, sublimation, and deposition.
• Energy changes (heat absorption or release) accompany these transitions

Properties of Solids

• Fixed shape and volume.
• High density (particles tightly packed).
• Low compressibility (cannot be easily compressed).
• Strong intermolecular forces (particles vibrate in fixed positions).

Properties of Liquids

• No fixed shape but definite volume; fluid.
• Moderate density (higher than gases but lower than solids).
• Surface tension (molecules form a "skin").
• Viscosity (resistance to flow)

Properties of Gases

• No fixed shape or volume; fills container
• Low density (particles far apart)
• High compressibility
• Diffusion and effusion

Kinetic Molecular Theory

• Particles in matter are in constant random motion.
• Collisions between particles are elastic (no energy loss).
• Particle size is negligible compared to container volume.
• Higher temperature leads to faster particle motion.

Real Gas Theory

• Intermolecular forces, volume, shape, compressibility, expansibility, diffusion, density are properties of gases

Liquid and Solid Properties

• Important characteristics that affect substances' interactions.

Plasma

• Fourth state of matter; ionized gas
• High energy and temperature
• Conducts electricity
• Found in stars, neon lights, and lightning.
• Most abundant state in the universe.

Water

• Sources: Rain, ground water, surface water
• Water Cycle: Evaporation, condensation, precipitation, collection
• Phases: Solid (ice), liquid (water), gas (water vapor)
• Unique properties: High specific heat capacity, cohesion and adhesion, excellent solvent, high heat of vaporization.
• Importance: Crucial for life, ecosystems, and various industrial processes.

Structure of Water

• Bent molecular geometry: Two hydrogen and one oxygen molecule
• Polarity: Water is a polar molecule due to differences in electronegativity, making it a good solvent.
• Hydrogen bonding: Enables water to interact strongly with other polar molecules and itself (explains many of water's unique properties).

Solubility of Ionic Compounds

• Ionic compounds dissolve in water due to electrostatic attraction between water molecules and ions in the solid.
• This process is called dissociation.
• "Like dissolves like" principle applies (polar solutes dissolve in polar solvents)

Notes on solid, liquids, and gases, relating to non-polar and polar substances

• Non-polar substances lack significant intermolecular forces compared to polar substances.
• Polar substances dissolve polar substances
• Non-polar substances dissolve non-polar substances
• Water is a polar substance that readily dissolves ionic and other polar substances.