Intermolecular Forces and Properties of Liquids: Hydrogen Bonding, Surface Tension, and Wetting

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Module 7: Intermolecular Forces and Properties of Liquids

Overview

This module explores the nature of intermolecular forces, with a focus on hydrogen bonding, and examines how these forces influence the physical properties of liquids, such as boiling point, viscosity, surface tension, and wetting behavior.

Hydrogen Bonding in Water

Structure and Arrangement

Hydrogen Bonding: Each water molecule forms hydrogen bonds with up to four neighboring water molecules, resulting in a tetrahedral arrangement.
Bond Distances: Each hydrogen atom is situated along a line joining two oxygen atoms, but is closer to one (100 pm) than the other (180 pm).
Ice Crystal Structure: In ice, hydrogen atoms lie between pairs of oxygen atoms, closer to one O atom than the other. Oxygen atoms are arranged in bent hexagonal rings in layers, giving rise to the hexagonal shapes of snowflakes.

Example: The hexagonal pattern of snowflakes is a direct result of the arrangement of water molecules in ice.

Density Differences: Liquid vs. Solid Water

Liquid Water: Hydrogen bonds are transient, constantly forming and breaking, resulting in a random distribution of molecules.
Solid Ice: Hydrogen bonds form rigid lattices, with each water molecule bonded to four others. These lattices contain relatively large gaps, making ice less dense than liquid water.
Packing Efficiency: Water molecules pack about 10% more densely in the liquid phase than in the solid phase.

Example: Ice floats on water due to its lower density.

Boiling Point Trends and Intermolecular Forces

Boiling Point Comparison

Boiling Point: The temperature at which a liquid turns into a vapor. It is influenced by the strength of intermolecular forces.
Hydrogen Bonding: Molecules capable of hydrogen bonding (e.g., ethanol, ethylene glycol) have much higher boiling points than those with only London dispersion forces (e.g., ethane, butane).
Trend: As the number of hydrogen bonds increases, so does the boiling point.

Name	Boiling Point (°C)
Ethane	-89
Butane	-1
Ethanol	78
Ethylene Glycol	197
CS2	46
Additional info: Table includes more alkanes and their boiling points.

Example: Ethylene glycol (with two OH groups) has a much higher boiling point than ethanol (one OH group).

Biological Importance of Hydrogen Bonding

DNA Structure

Hydrogen Bonds in DNA: Hydrogen bonds hold complementary strands of DNA together.
Base Pairing: Thymine pairs with adenine via two hydrogen bonds; cytosine pairs with guanine via three hydrogen bonds.
Specificity: The position and number of hydrogen bonds determine the specificity of base pairing.

Example: The stability of the DNA double helix is due to hydrogen bonding between base pairs.

Other Properties Affected by Hydrogen Bonding

Viscosity

Definition: Viscosity is a measure of a liquid's resistance to flow.
Influence of Intermolecular Forces: Stronger intermolecular forces (e.g., hydrogen bonds) result in higher viscosity.
Molar Mass: Higher molar mass also increases viscosity.
Units: 1 Poise (P) = 0.1 Pa·s; 1 cP = 1 mPa·s.
Glycerol (C3H8O3): Has three OH groups, leading to extensive hydrogen bonding and high viscosity.

Substance	Viscosity (cP)
Water	1.00
Ethanol	1.20
Ethylene glycol	16.1
Glycerol	945
Additional info: Table includes more substances and their viscosities.

Example: Glycerol is much more viscous than water due to its ability to form multiple hydrogen bonds.

Summary of Non-Covalent Interactions

Types and Energies

Force	Energy (kJ/mol)	Example	Model
London dispersion	0.05–40	CH4–CH4	Molecular models
Dipole-induced dipole	2–10	CH3Cl–CH4	Molecular models
Ion-induced dipole	3–15	I−–C6H6	Molecular models
Dipole-dipole	5–25	SO2–SO2	Molecular models
Hydrogen bond	10–40	CH3OH–CH3OH	Molecular models
Ion-dipole	40–600	Na+–H2O	Molecular models
Ion-ion	600–4000	Na+–Cl−	Molecular models
Metallic	Additional info: Energies vary widely	Ag–Ag	Metal lattice

Additional info: These interactions are responsible for many physical properties of substances, such as boiling point, melting point, and solubility.

Properties of Liquids

Cohesive and Adhesive Forces

Cohesive Forces: Intermolecular forces between like molecules (e.g., water-water).
Adhesive Forces: Intermolecular forces between unlike molecules (e.g., water-glass).

Surface Tension

Definition: Surface tension is the energy or work required to increase the surface area of a liquid.
Cause: Molecular attractions at the surface are weaker than in the interior, leading to a tendency to minimize surface area.
Units:

Example: Water forms droplets with a spherical shape due to surface tension.

Properties of Liquids: Surface Tension

Molecular Explanation

Interior Molecules: Experience more attractive intermolecular interactions than surface molecules.
Minimization of Surface Area: Liquids tend to exhibit minimum surface area, often forming spherical droplets.
Elastic Film Analogy: The surface behaves as if tightened into an elastic film.

Equation: Surface tension () is defined as: where is the force and is the length over which the force acts.

Consequences of Surface Tension

Examples and Applications

Floating Objects: Steel paper clips can float on water due to surface tension, despite being denser than water.
Water Striders: Insects can walk on water because their legs do not break the surface tension.
Mercury: Mercury has a much higher surface tension than water ( vs. ), allowing a person to sit on its surface.

Additional info: High surface tension in mercury is due to strong metallic bonding and high cohesive forces.

Surface Wetting and Contact Angles

Wetting Behavior

Surface Wetting: The behavior of a liquid droplet on a surface depends on the surface energy and the intermolecular forces involved.
Contact Angle: The angle between the surface and the tangent to the droplet at the point of contact. Lower angles indicate better wetting (hydrophilic), higher angles indicate poor wetting (hydrophobic).
Superhydrophilic: Contact angle < 10°
Hydrophilic: Contact angle < 90°
Hydrophobic: Contact angle > 90°
Superhydrophobic: Contact angle > 150°

Example: Water droplets bead up on waxy or superhydrophobic surfaces, while they spread out on glass.

Surface Wetting with Water: Material Comparisons

Hydrophilic vs. Hydrophobic Surfaces

Glass: Hydrophilic, water spreads out.
Polydimethylsiloxane: Hydrophobic, water beads up.
Amine Modified Surface: Hydrophilic, water spreads out.
Polytetrafluoroethylene (Teflon): Hydrophobic, water beads up.

Additional info: The type of intermolecular forces (hydrogen bonding, van der Waals, etc.) determines the wetting behavior of surfaces.