Amphiphiles as surfactants: Definitions

Adsorption

The retention of molecules as the surface of an interface. To gather on a surface in a condensed layer.

Hydrophilic repulsion

The actual nature of the repulsive is very complex. It can be naively understood as the cumulative effect of effect of electrostatic, hydration and steric interactions.

Surface Tension

Energy per unit area.
A force resulting from inter-molecular forces that is responsible for bubbles forming (as a consequence of being energetically favorable), water drops hanging from faucets and a meniscus forming when a column of water rises in glass tubes. It causes a reduction in the ratio of surface area to volume of a geometrical object - to a minimum in the absence of other competing effects.
Note: A sphere is the shape that a 3d object must have in order to have the smallest possible surface area for a given fixed volume. As a consequence of having the smallest surface area it also has the smallest energy associated with the surface (as Surface energy = surface tension * surface area).

Interface

The interface is formed between two `immiscible' liquids when they are saturated with each other.

Interfacial Tension (Y)

Defined as the rate of change of the Helmoltz free energy with respect to change in surface area at a constant temperature (T) and volume (V) and number of molecules (N) of each component present.
Y*dS = work that must be done to increase the surface area S by dS while keeping the volume fixed.

Colloid

A substance made of particles with linear dimensions in the range of about 10^-7 to 5*10^-7 cm dispersed in a continuous gaseous, liquid or solid medium whose properties depend upon the large specific surface area. The particles can be large molecules like proteins or solids, liquids, gaseous aggregates and remain dispersed indefinitely.

Colloidal Suspension

A system consisting of small particles kept dispersed (i.e. prevented from clustering together) by molecular motion in the surrounding medium, the solvent.

Emulsions

They are colloidal systems :
A liquid in a liquid colloidal system is called an emulsion.
usually the Liquid/Liquid preparation consist of two otherwise completely immiscible liquids one of which exists as minute globules coated by another substance (the surfactant) and is dispersed throughout the other.
Emulsions are not usually very stable.
Milk is an example of an oil-in-water emulsion with relatively low levels of oil.
Surfactants play an important role in their manufacture and their subsequent stability. Surfactants are used to enhance the stability of desired emulsions and also used to break down (degrade) unwanted emulsions e.g. the dispersed water present in crude oil.

Foam

Foam is a gas in a liquid colloidal system.
Surfactants play an important role in their manufacture and their subsequent stability. Surfactants are used to enhance the stability of desired foams and also used to break down unwanted foams e.g. the foams formed in waste and runoff streams.

Phospholipids

A class of molecules containing a polar head group that contain phosphorus atom and two non-polar hydrocarbon chains. They are the primary constituents of liposomes (lipid vesicles). There are many phospholipids due to the various possible types of head groups and hydrocarbon chains of different lengths.

Liposomes (lipid vesicles)

Spherical closed structures of curved lipid bilayers (vesicles) which entrap in their interior a part of the solvent in which they freely float.They may be composed of one or several concentric bilayer membranes.
Their sizes ranges in the range from 20nm to several dozens of micrometers while the thickness of their membranes is about 4 nm.
Lecithin is a common example.

Lipids

Any of a group of organic compounds that are

1. greasy to the touch,
2. insoluble in water
3. soluble in alcohols and ether.

Lipids are the main constituents of fats and other esters.They are important as major fuels in cell metabolism. Along with water, proteins, nucleic acids and carbohydrates lipids are essential biomolecules in the structure and function of living matter.
Polar lipids have amphiphilic properties and are the building blocks of cell membranes and liposomes.

Microemulsions

Can be considered as swollen micelles - but not all micellar solution can be swollen to the extent of forming microemulsions.
Microemulsions are apparently homogeneous mixtures of water and oil with large amounts of surfactants. They are thermodynamically stable (as opposed to emulsions) are formed spontaneously and contain particles that are extremely small. Droplet diameters in micro-emulsions typically range from 10-100 nm. Microemulsions are usually transparent/translucent as opposed to emulsions which are dirty or muddy looking.

Ternary System:

Three phase systems - water, oil and surfactant.

Lyotropic

Systems (usually in the context of liquid crystals ) whose properties (e.g. phase) are dependent on their concentration rather than on the temperature.

Note how a change in the concentration of the molecules affects the shape and type of structures formed may naively be understood by realizing that a change in the concentration not only effects how a structure (e.g. micelle) interacts with another structure but also affects the forces between molecules within each aggregate thereby modifying the size and shape of the structures. This is because the forces that hold the amphiphilic molecules together in these structures are not covalent bonds but are the weaker hydrophobic, hydrogen-bonding and electrostatic screening interactions.

Polar Molecule

Capable of acquiring effective charge.
As a consequence of the nature of binding present amongst the atoms there is an intrinsic uneven distribution of charge- one part is positively charged whilst another part is negatively charged (even in the absence of an external field).Water is a polar molecule.

Non-dipolar Molecule

Molecules that do not have an intrinsic dipole moment, e.g. oil and most hydrocarbons.

Types of Surfactants

They can be divided into 4 categories based on the type of hydrophilic head: anionic, cationic, non-ionic and zwitter-ionic.

Critical Micelle Concentration

Depending upon the relative strengths of the hydrophilic repulsion and the hydrophobic attraction and the temperature, this is the minimum concentration (number of molecules per unit volume) needed for the resultant micelle to be stable.

Energetically favorable

If a ball is placed at the top of a hill and given a very small push it will tend to move to the bottom of the hill. This is because at the bottom the potential energy is at a minimum and hence more stable. In this example, we would say that it is more energetically favorable for the ball to be at the bottom of the hill.

Enhanced Oil Recovery

Advanced process used to recover oil

Rheology

Study of the deformation and flow of matter.

p.p.m.

Parts per million, a commonly way to describe concentrations in chemistry.

Ester

Compounds that are formed by the reaction of alcohols and acids with the elimination of a water molecule (e.g. ethyl-acetate).

Contact angle

The contact angle describes the shape of a tiny drop of fluid placed on an interface with some other material. If the fluid drop spreads out so it is completely flat then the contact angle is 0 degrees. If it forms a meniscus whose sides are perpendicular, then the contact angle is 90 degrees. If the fluid were perfectly immiscible then it would stay in a perfect spherical bead sitting on the surface and only touch it at one single point and the contact angle would be 180 degrees. On adding surfactant to the oil-water interface, the contact angle theta decreases - cos(theta) ~1/Y: as surfactants reduce the interfacial tension, Y, cos(theta) increases thus theta decreases.

Capillary action

Assuming the radius of a capillary is small and the fluid inside forms a hemispherical meniscus at the surface - further motion through the capillary is resisted because of a pressure gradient across the interface . delta P is ~ Y/R :(Laplace's Formula)- smaller the radius of the pore (capillary) greater the resisting pressure .

Thus the force (= pressure gradient) preventing further motion of the drop can be decreased by

• decreasing the interfacial tension ( Y )
• increasing the radius of the capillary pore (R).

Pressure gradient

Change of pressure across unit length. Think of pressure gradient as a force.

in vivo

(relating to a biological process). Occurring or made to occur within a living organism or natural setting. Opposite to in vitro.

in vitro

(relating to a biological process). Made to occur in a laboratory vessel or other controlled experimental environment. Opposite to in vivo.