Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Physical Chemistry → Surface and colloidal chemistry ↓


Surface tension and wetting


Surface tension and wetting are fundamental concepts in surface and colloidal chemistry. These phenomena are important for understanding a wide range of processes in the physical sciences and have practical applications in a variety of industries, including cosmetics, pharmaceuticals, and materials engineering.

Understanding surface tension

Surface tension is the force that causes the surface of a liquid to shrink and behave like a stretched elastic membrane. It is caused by cohesive forces between liquid molecules, which are particularly strong at the surface. In a bulk liquid, molecules experience forces from all directions that are balanced. However, molecules at the surface are subject to net inward forces because there are fewer neighboring molecules on the outside.

Consider a surface molecule:

    inside the liquid molecule on the surface
    Ooo Ooo
    O^O^ O^^^
    Ooo Ooo

The inward forces acting on the surface of a liquid give rise to a phenomenon known as surface tension. This is why small objects, or even some insects, can float on the surface of water without sinking if they do not significantly break the cohesive forces of the liquid.

Factors affecting surface tension

Surface tension can be affected by several factors:

  • Temperature: An increase in temperature generally decreases surface tension, because increased molecular motion reduces the cohesive forces.
  • Impurities: The presence of impurities or surfactants, which reduce cohesive forces, can significantly reduce surface tension.
  • Nature of liquid: Every liquid has its own intrinsic surface tension which depends on its molecular structure. For example, water has a high surface tension due to hydrogen bonding.

Example: The surface tension of water at 20°C is about 72.8 mN/m, which is much higher than most organic liquids. This explains why raindrops are spherical; liquids minimize their surface area for a given volume, forming spheres.

Wet

Wetting is the ability of a liquid to maintain contact with and spread on a solid surface, resulting from intermolecular interactions at the interface between the liquid and the solid. Wetting describes how the liquid interacts with the surface and whether it spreads into a thin film or forms droplets.

Wettable Surface Non-wettable Surface
    ,
     ,
    ,

    Detailed: 
    Wetting occurs when the adhesive forces between a liquid and a solid are stronger than the cohesive forces within the liquid.

Wetting and contact angle

The degree of wetting is usually described by the contact angle (θ), which is the angle made by the liquid at the three-phase boundary where the liquid, gas, and solid intersect. A lower contact angle indicates better wetting.

  • High contact angle (θ > 90°): Indicates poor wetting (e.g., water on wax).
  • Low contact angle (θ < 90°): Indicates better wetting (e.g., water on glass).

Cos(θ) = (γ SG - γ SL ) / γ LG , where:

γ SG = surface free energy between solid and gas
    γ SL = surface free energy between solid and liquid
    γ LG = surface tension between the liquid and the gas

Young's equation correlates these surface energies with the contact angle, helping to understand and predict wetting behavior.

Wetting agents and surfactants

Surfactants or wetting agents are compounds that reduce the surface tension between two liquids or between a liquid and a solid. Surfactants consist of two parts: a hydrophobic tail and a hydrophilic head. The hydrophilic head is attracted to water, while the hydrophobic tail repels water and forms an interface with air or oils.

  • Applications: Surfactants are used in detergents, emulsification, and foaming.
  • Effect: By reducing surface tension, they increase the wettability of surfaces.

Example: Soap is a common surfactant. When soap is added to water, it reduces the surface tension of the water, allowing it to spread and penetrate the pores of clothing and other materials more easily.

Practical applications of surface tension and wetting

Surface tension and wetting have numerous applications in industrial and everyday situations:

  • Cosmetics: Moisturizers maximize skin moisture and hydration by reducing the interfacial tension of the skin.
  • Printing: In inkjet printing, the surface tension of the ink affects its spreading and adhesion to various surfaces.
  • Painting: Paint formulations are adjusted to ensure suitable wetting and adhesion properties on various surfaces.
  • Metal treatments: In processes such as electroplating, surface tension determines the thickness and uniformity of the coating.

Examples and calculations

Let us consider the following example to illustrate wetting. Suppose you have oil droplets on a metal surface, and a surfactant is added to improve wetting.

Example calculation:

given:
    γ SG = 50 mN/m, γ SL = 20 mN/m, γ LG = 30 mN/m

    Solve for the contact angle (θ):
    Cos(θ) = (γ SG - γ SL ) / γ LG
    Cos(θ) = (50 - 20) / 30
    Cos(θ) = 30 / 30
    Cos(θ) = 1
    θ = 0° (complete wetting)

Calculations show that the surface is fully wettable with the surfactant, indicating efficient spreading and contact of the liquid on the solid surface.

Challenges and environmental considerations

Although many applications have benefited from the use of surfactants, it is necessary to consider the environmental impacts. Some surfactants are non-biodegradable and can contribute to water pollution. The development of green and biodegradable surfactants is necessary to minimize these impacts.

Conclusion

Surface tension and wetting are important for understanding various chemical and physical interactions at surfaces and interfaces. With the historical development of surface and colloidal chemistry, these phenomena continue to inspire advances in technology, materials science, and environmental studies. The search for advanced material properties and environmentally friendly formulations remains a vibrant area of research and development.


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Surface tension and wetting

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