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Spreading Coefficient, Adsorption at Liquid Interfaces and Surface Active Agents

Spreading coefficient, Coefficients of Spreading Applications, Adsorption at the liquid interface, Surface active agents (SAA).

Spreading coefficient

Oleic acid spreads immediately on the surface of the water when dropped on it.

Water – Sub layer Liquid (S)

Spreading Liquid (L) - Oleic Acid

According to general theories, spreading occurs when adhesive forces exceed cohesion forces.

Work of Cohesion (Wc) refers to the amount of surface free energy projected from separating two halves of a column of pure liquid.

Increased surface free energy = γ dA

Wc = γL (dA + dA) = 2 γLdA

For this column, the cross-sectional area is 1 cm2.

Wc = 2 γL

Work of adhesion - An immiscible liquid can be separated into two sections by increasing the surface free energy of the interface between them at their boundary, the Work of Adhesion (Wa).

The energy requirements are lower for two sections of immiscible liquids since their boundary already separates them by a factor γLSdA

Wa = γLdA + γSdA - γLSdA.

The column areas are 1 cm2.

Wa = γL + γS - γLS

Cohesion work (W) is the difference between adhesion work and spreading coefficient (S)

S = Wa – Wc = (γL + γS – γLS) - 2γL

= γS – γL – γLS

S = γS – (γL + γLS)

Where a spreading liquid is indicated by γL

and γS - Surface tension of the sublayer liquid

γLS - Interface tension

Positive values of spreading coefficient S result in spreading

γS> (γL + γLS)

for negative spreading coefficient S i.e.

(γL + γLS) >γS

Liquids form globules or floating lenses as they spread. This is why spreading won't happen

In an attempt to reduce the free energy of the sublayers, the spreading of liquids and interfacial tensions with sublayers becomes spontaneous if their free energies are lower than their free energies. In addition, the sublayer liquid may experience changes in surface tension as a result of saturation with the other. After saturation, the spreading coefficient can become negative. The spreading liquid coalesces at the surface and forms a lens. If S is negative after saturation, DUPLEX FILM forms a monolayer over the surface and excess liquid remains as a lens.

As a result of high spreading coefficients, fatty acids and alcohols spread widely. A decrease in spreading coefficient increases with increasing nonpolar chain length in acid or alcohol. ethyl alcohol and propionic acid have high spreading coefficients. Propane does not spread when mixed with water.

Coefficients of Spreading Applications

  • Creams, lotions, et cetera are absorbed into the bloodstream.
  • Emulsions stabilized via physiological means
  • for tablet coating

Adsorption at the liquid interface

Adsorption is the adhesion of gaseous particles, ions, or molecules to a surface whether they originate from liquid or solid.

Activated molecules or ions are partitioned to surfaces or interfaces when added to liquids. This is termed POSITIVE ADSORPTION. In positive adsorption, Molecules are partitioned by surface and interface. There is decreasing surface tension. For example Tweeners, Spanners

It is termed NEGATIVE ADSORPTION when some molecules or ions are partitioned towards the bulk of the liquid. In negative adsorption, the molecule is partitioned towards bulking. Consequently, surface tension increased. Example; glucose and sodium chloride!

As SURFACTANTS (SURFACE-ACTIVE AGENTS), ions or molecules that are adsorbed at surfaces or interfaces reduce surface tension. An amphiphile is a molecule that attaches to another. If they are added to water with a surfactant, the HEAD and tail have a polar structure. Hydrophilic heads are located in the water. Typically, the lipophilic end of a surfactant will face away from water, and the hydrophilic end will face towards water.

An immiscible system is added with the same solute. Heads of hydrophilic particles will orient toward aqueous phases (away from oil). In a lipophilic end (away from water), you will find the oil phase. A lower concentration of surfactants will result in surfaces that are not completely covered with surfactant molecules. Increasing concentrations gradually fill up the surface and this leads to a marked decrease in surface tension when the surface is saturated with surfactant.

As the surface of the liquid is saturated, the molecules in the surface move into the bulk and associate to form small aggregates or MICELLES. Micelles formed are spherical, ranging from 0.001 to 0.5 m in colloidal dimension. There are around 50-100 surfactant molecules in each of the micelles.

The polar groups of the surfactants are oriented towards the aqueous phase in polar solvents and the non-polar groups towards the center of the micelle's spherical structure in aqueous solvents. Non-polar groups of surfactants will be oriented towards the non-aqueous phase (oil) in non-polar solvents, and non-polar groups will be oriented towards the micelle center in non-polar solvents. A CRITICAL MICELLAR CONCENTRATION takes place when a surfactant is at the lowest concentration in bulk fluid before it causes micelles to form.

Surface active agents (SAA)

These surface-active agents (SAAs) work by reducing the surface tension of the liquid in which they are dissolved, giving the liquid better spreading and wetting characteristics. The use of SAAs can be attributed to corrosion inhibition, ore flotation, oil flow through porous rocks, and aerosol production. These devices are used to dissolve dyes and perfumes in aqueous suspensions.

One of the most commonly used compounds in commercial, agricultural, and household activities are surface-active agents, and many of them (and/or their degradation products) are discharged into wastewater treatment plants following their use. Surfactant agents are molecules that act on the surface of liquids.

In surface-active substances, the surface tension and interphase tension of the medium in which they dissolve are lowered, while simultaneously significantly lower interphase tension is achieved and positive adsorption occurs at interfaces such as liquid-vapor interfaces. Many surface coating formulations incorporate surface-active ingredients to impart special properties. The surface-active molecule must be partially hydrophilic (soluble in water) and partially lipophilic (soluble in fats, oils, or lipids). Generally, it is found only at the interfaces between liquid bodies and lipids or oils, acting as an emulsifier and foaming agent. There are other SAAs that is less hydrophilic and more lipophilic that can serve as de-foaming agents or demulsifies.

As a result of the hydrophilic properties of their molecules, surface-active agents are classified as follows:
⦁ Non-ionic - Does not involve ions
⦁ Zwitterionic or amphoteric - The electron pair of the reactant can be positively charged on the cationic site but negatively charged on the anionic site
⦁ Anionic - pH-dependent anions or permanent anions comprise anionic chemistry
⦁ Cationic - A reaction involving primary, secondary, or tertiary amines that are pH-dependent is considered cationic.

The following substances can be used as surface-active agents:
⦁ Fabric softeners
⦁ Inks
⦁ Suspending agents in suspensions
⦁ Leak detectors
⦁ Firefighting
⦁ A polymer containing alkali surfactant (used to mobilize oil from wells)
⦁ Detergents
⦁ Paints
⦁ Anti-fogs
⦁ Ferrofluids
⦁ Emulsifying agents in emulsions
⦁ Pipelines
⦁ Adhesives
⦁ Laxatives
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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