Surface tension and interfacial energy are crucial concepts in colloid science. They explain why liquids form droplets, insects can walk on water, and how surfactants work. These phenomena arise from molecular interactions at interfaces between different phases.

Understanding surface tension helps us grasp many everyday occurrences and industrial processes. From capillary action in plants to emulsion stability in foods, these principles shape our world. We'll explore measurement techniques, factors affecting surface tension, and applications in various fields.

Definition of surface tension

• Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible due to the cohesive forces between liquid molecules
• It is a property of the interface between two phases, such as liquid-gas or liquid-liquid interfaces
• Surface tension is responsible for the shape of liquid droplets, the formation of bubbles, and the ability of some insects to walk on water

Molecular origin of surface tension

• Surface tension arises from the imbalance of intermolecular forces experienced by molecules at the surface compared to those in the bulk of the liquid
• Molecules in the bulk are surrounded by other molecules and experience attractive forces in all directions, resulting in a net force of zero
• Molecules at the surface experience a net inward force, as there are fewer liquid molecules above the surface to provide balanced attractive forces
• This imbalance of forces leads to a higher energy state for surface molecules and a tendency to minimize the surface area

Factors affecting surface tension

Effect of temperature on surface tension

• Surface tension generally decreases with increasing temperature
• As temperature increases, the kinetic energy of molecules increases, weakening the cohesive forces between them
• The reduced cohesive forces lead to a decrease in surface tension
• This relationship is described by the Eötvös rule, which states that surface tension is a linear function of temperature

Effect of solutes on surface tension

• The presence of solutes can either increase or decrease the surface tension of a liquid
• Solutes that increase surface tension are called positive surface-active agents or "surfactants" (soaps, detergents)
• Solutes that decrease surface tension are called negative surface-active agents or "co-surfactants" (alcohols, fatty acids)
• The effect of solutes on surface tension depends on their concentration and how they interact with the liquid molecules at the surface

Measurement techniques for surface tension

Du Noüy ring method

• The Du Noüy ring method uses a platinum-iridium ring that is submerged in the liquid and then slowly pulled out
• The force required to detach the ring from the liquid surface is measured and used to calculate the surface tension
• This method is suitable for liquids with low to medium viscosity and can be used for both liquid-gas and liquid-liquid interfaces

Wilhelmy plate method

• The Wilhelmy plate method involves using a thin plate (usually made of platinum or glass) that is partially submerged in the liquid
• The force acting on the plate due to surface tension is measured and used to calculate the surface tension
• This method is suitable for liquids with a wide range of viscosities and can be used for both liquid-gas and liquid-liquid interfaces

Pendant drop method

• The pendant drop method involves forming a drop of liquid at the end of a capillary tube and analyzing its shape
• The shape of the drop is determined by the balance between surface tension and gravitational forces
• By measuring the drop's dimensions and using the Young-Laplace equation, the surface tension can be calculated
• This method is suitable for liquids with low to medium viscosity and is commonly used for liquid-gas interfaces

Spinning drop method

• The spinning drop method is used to measure interfacial tension between two immiscible liquids
• A drop of the lighter liquid is injected into a rotating capillary filled with the denser liquid
• The drop elongates along the axis of rotation due to centrifugal forces, and its shape is determined by the balance between centrifugal and interfacial tension forces
• By measuring the drop's dimensions and the rotation speed, the interfacial tension can be calculated

Surface tension vs interfacial tension

• Surface tension refers to the tension at the interface between a liquid and a gas (usually air)
• Interfacial tension refers to the tension at the interface between two immiscible liquids
• Both surface tension and interfacial tension arise from the imbalance of intermolecular forces at the interface
• The main difference is the nature of the two phases involved: liquid-gas for surface tension and liquid-liquid for interfacial tension

Thermodynamics of interfaces

Gibbs free energy of interfaces

• The Gibbs free energy of an interface is the work required to create a unit area of the interface at constant temperature and pressure
• It is a measure of the thermodynamic stability of the interface
• The Gibbs free energy of an interface is related to the surface or interfacial tension by the equation: $dG = \gamma dA$, where $G$ is the Gibbs free energy, $\gamma$ is the surface or interfacial tension, and $A$ is the area of the interface

Young's equation and contact angle

• Young's equation describes the balance of forces at the contact line between a liquid, a solid, and a gas
• It relates the contact angle $\theta$ to the surface tensions of the solid-gas ($\gamma_{SG}$), solid-liquid ($\gamma_{SL}$), and liquid-gas ($\gamma_{LG}$) interfaces: $\gamma_{SG} = \gamma_{SL} + \gamma_{LG} \cos \theta$
• The contact angle is a measure of the wettability of the solid by the liquid
• A contact angle less than 90° indicates that the liquid wets the solid, while a contact angle greater than 90° indicates that the liquid does not wet the solid

Laplace pressure and curved interfaces

• Laplace pressure is the pressure difference across a curved interface between two fluids, such as a liquid and a gas or two immiscible liquids
• It arises from the surface or interfacial tension and the curvature of the interface
• The Laplace pressure is given by the Young-Laplace equation: $\Delta P = \gamma (1/R_1 + 1/R_2)$, where $\Delta P$ is the pressure difference, $\gamma$ is the surface or interfacial tension, and $R_1$ and $R_2$ are the principal radii of curvature
• Laplace pressure is responsible for the spherical shape of bubbles and droplets, as well as the capillary rise of liquids in narrow tubes

Marangoni effect and surface tension gradients

• The Marangoni effect is the mass transfer along an interface between two fluids due to a surface tension gradient
• Surface tension gradients can arise from temperature gradients (thermal Marangoni effect) or concentration gradients of surface-active agents (solutal Marangoni effect)
• In the presence of a surface tension gradient, liquid flows from regions of low surface tension to regions of high surface tension
• The Marangoni effect is responsible for the "tears of wine" phenomenon and plays a role in various industrial processes, such as crystal growth and welding

Applications of surface tension

Capillary action in porous media

• Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity
• It is driven by the combination of surface tension and adhesive forces between the liquid and the walls of the narrow space
• Capillary action is important in various natural and industrial processes, such as water transport in plants, wicking in textiles, and oil recovery from porous rocks

Wetting and spreading of liquids

• Wetting refers to the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together
• The degree of wetting is determined by the balance between adhesive and cohesive forces, which is related to the contact angle
• Spreading is the process by which a liquid deposited on a solid substrate spreads out to form a thin film
• The spreading of liquids is important in various applications, such as coating, printing, and lubrication

Emulsion stability and Ostwald ripening

• Emulsions are dispersions of one liquid in another immiscible liquid, stabilized by surface-active agents (emulsifiers)
• The stability of emulsions is influenced by various factors, including droplet size, surface tension, and the presence of emulsifiers
• Ostwald ripening is a phenomenon in which larger droplets grow at the expense of smaller ones due to the difference in their solubility
• Ostwald ripening can lead to the destabilization of emulsions over time

Foams and foam stability

• Foams are dispersions of gas bubbles in a liquid or solid medium, stabilized by surface-active agents (foaming agents)
• The stability of foams depends on various factors, such as bubble size, surface tension, and the presence of foaming agents
• Foam stability is important in various applications, such as firefighting, food production, and personal care products

Flotation processes in mineral separation

• Flotation is a process used to separate minerals from gangue (waste material) based on their surface properties
• In flotation, air bubbles are introduced into a mixture of water, minerals, and surface-active agents (collectors)
• Hydrophobic mineral particles attach to the air bubbles and are carried to the surface, while hydrophilic gangue particles remain in the water
• The efficiency of flotation depends on factors such as particle size, surface tension, and the type of collector used

Surfactants and their effect on surface tension

Classification of surfactants

• Surfactants are surface-active agents that adsorb at interfaces and lower the surface or interfacial tension
• They are classified based on the charge of their head group: anionic (negatively charged), cationic (positively charged), nonionic (no charge), and zwitterionic (both positive and negative charges)
• Examples of surfactants include sodium dodecyl sulfate (SDS, anionic), cetyl trimethylammonium bromide (CTAB, cationic), and polyoxyethylene glycol alkyl ethers (nonionic)

Micelle formation and critical micelle concentration (CMC)

• Micelles are aggregates of surfactant molecules that form in solution above a certain concentration called the critical micelle concentration (CMC)
• In micelles, the hydrophobic tails of the surfactant molecules are oriented towards the interior, while the hydrophilic head groups are in contact with the aqueous medium
• The formation of micelles is driven by the reduction of the free energy of the system, as it minimizes the contact between the hydrophobic tails and water
• The CMC is an important characteristic of surfactants, as it determines their effectiveness in various applications

Adsorption of surfactants at interfaces

• Surfactants adsorb at interfaces due to their amphiphilic nature, with the hydrophobic tails oriented towards the non-polar phase (air or oil) and the hydrophilic head groups towards the aqueous phase
• The adsorption of surfactants at interfaces leads to a reduction in surface or interfacial tension
• The extent of adsorption depends on factors such as the surfactant concentration, temperature, and the nature of the interface
• The adsorption of surfactants at interfaces is important in various applications, such as detergency, emulsification, and foam stabilization

Experimental methods for studying interfacial phenomena

Surface pressure-area isotherms

• Surface pressure-area isotherms are used to study the behavior of insoluble monolayers at the air-water interface
• The surface pressure is the reduction in surface tension caused by the presence of the monolayer and is measured as a function of the area per molecule
• The isotherm provides information about the phase behavior and compressibility of the monolayer
• Surface pressure-area isotherms are commonly used to study the behavior of lipids, proteins, and other surface-active molecules

Brewster angle microscopy (BAM)

• Brewster angle microscopy is an optical technique used to visualize monolayers at the air-water interface
• It is based on the principle that p-polarized light is not reflected at the Brewster angle of the interface
• The presence of a monolayer at the interface changes the refractive index and causes some of the p-polarized light to be reflected
• BAM provides information about the morphology and homogeneity of the monolayer with a resolution of a few micrometers

Langmuir-Blodgett (LB) films

• Langmuir-Blodgett films are thin films of organic molecules deposited on solid substrates by transferring insoluble monolayers from the air-water interface
• The monolayer is first compressed to a desired surface pressure on the water surface and then transferred onto the substrate by dipping it through the monolayer
• LB films can be deposited as single layers or multilayers, with precise control over the film thickness and composition
• LB films are used in various applications, such as molecular electronics, sensors, and biophysical studies of membrane proteins