groups by a spacer group of varying nature: hydrophilic or hydrophobic, rigid or flexible. These surfactants represent a new class of surfactants that is finding its way into surfactant-based formulations. Dimeric surfactants represent a new class of surfactants. They are made up of two amphiphilic moieties connected at the level of the head groups or very close to the head groups by a spacer group.
4-octylphenol polyethoxylate (Triton X
(n = 9-10)
Hexadecyltrimethylammonium bromide (C
Hydrophobic group
Figure 2.1: Hydrophilic and hydrophobic groups of some surfactant molecules.
10
X-100, C8H17-C6H4-(OC2H4)nOH,C14H22O(C ammonium C16TAB, C19H42BrN)
Hydrophilic group
O(C2H4O)n)
11
2.2 MICELLE FORMATION BY SURFACTANTS
In a phenomenon termed the “hydrophobic effect”, 4 surfactants act the way they do in aqueous systems due to their nature/structure and the nature/structure of water. Once a surfactant monomer is added to water, water forms a “cage” around the hydrophobic carbon chain. This enclosure is driven by the strength of the hydrogen bonds between the water molecules, leading to loss of entropy of the water molecules. It is this entropy loss rather than bond energy that leads to an unfavorable free energy change for the process. Once added to a system, before equilibrium is reached between the surfactant monomers at the interface and those in the bulk, surfactants concentrate at the interfaces, where they gradually decrease the overall free energy or surface tension of the system. Their orientation at the interface varies, depending on the components of the system. At a water/ air interface, the head group is buried in the solution while the tail group extends out of the solution. At oil/air interface, the tail group is buried i...
... middle of paper ...
...chanism of ionic surfactants differs from that of nonionic surfactants.
The schematic of a typical surfactant adsorption isotherm frequently seen for the adsorption of ionic surfactants on oppositely charged surfaces is commonly divided into four regions,9, 10 the shape of a typical adsorption isotherm of a nonionic surfactant follows Langmuir equation.
Unlike ionic surfactants, the adsorption isotherms of nonionic surfactants do not have clear transition points. At very low concentration, nonionic surfactant monomers adsorb via
Admicelle Hemimicelle Micelle (formed in solution)
Inverse Micelle
14
hydrogen bonding between the substrate and the hydrophilic groups. The amount of adsorbed surfactant increases slowly with increasing equilibrium concentration in the bulk phase. After the
CHC or CAC, the slope of the isotherm increases until the CMC and then flattens out.8
Rodriquez, R., & Martin, R. (1999). Exogenous surfactant in newborns. Respiratory Care Clinics of North America 5. (4), 595-616
...this system is increased or decreased, then the time taken to reach equilibrium will not be effected after it was in equilibrium stage.
The water concentration is now even on the inside and out. This process is called osmosis. Part B: Aim: To investigate the action of a differentially permeable membrane. Method: See attached.
a) With the rise in ADH concentration, the permeability of the collecting tubules to water
This increase in potential energy increases the amount of kinetic energy that can be released in the system therefore causes the system to be ...
Most of the water in the beaker travels up through the xylem, and most evaporates through the stomata, into the air.
It is based on physics, and the 2nd law of thermodynamics. A liquid is vaporized through compression, which requires kinetic energy. This draws the energy needed from the direct area; causing a loss in energy and then it
The lipid bilayer is made up of two layers of amphophilic molecules and their main purpose is to act as a barrier for the cell against water molecules. The two layers are composed of a hydrophilic head and hydrophobic tail and they can form spontaneously. The hydrophilic heads are polar an...
The polarity of the water makes it cling to itself. This is also known as cohesion which permits water or H2O to tolerate ecstasy at the time it is being positioned under pressure. For example, oil is not polar and it does not dissolve in water, but it does form droplets. The property of cohesion is also being observed when water arranges in a dome-like shape. Hydrogen bonds are the type of bonds that hold water molecules together and assist them in attraction to on another. Cohesion also aids in the expansion of surface tension. Surface tension is the effect on the surface of a liquid that makes it behave as a stretched elastic membrane; it is caused by unbalanced intermolecular forces. For example, if a small piece of paper was placed on water dewdrops, the paper would float on top the water dewdrops denying the fact that the paper is much denser than the water itself. The weak hydrogen bonds that hold water molecules together is attributed to cohesion and surface tension. Hydrogen bonds can break and reorganize with prodigious occurrence. Surface te...
This is an example of a soap molecule. The hydrocarbon end is non polar and hydrophilic (water hating) and the carboxylate end is polar and hydrophilic (water loving). This the property which allows it to clean, it acts as an emulsifying agent. The soap disperses in water to form miscelles where a negatively charged surface is formed and hydrocarbon chains are in the centre. These miscelles surround droplets of dirt or grease suspending them in the water so they can be washed away.
Aim: To investigate the pattern in the amount by which a hanging piece of elastic stretches when a load force is applied and the amount by which the stretch is reduced when the load is removed.
Because they repel they are as far from each other as possible. The two electrons in each of the oxygen-hydrogen bond are not shared equally. They are more strongly attracted to the oxygen. The bond is polar, it has a 'negative part' (the oxygen) and a 'positive part' (the hydrogen).The hydrogen bond is very weak, about ten times weaker than a single covalent bond.
The rate of evaporation can increase if the gas pressure decreases around the liquid. Heat energy is used to break the bonds that hold water molecules together, that is why water easily evaporates at the boiling point but evaporates much slower at the freezing point. Net evaporation happens when the rate of evaporation surpasses the amount of condensation. Saturation occurs when these two process rates are equivalent when the humidity of the air is at one-hundred percent. On average, a fraction of the atoms in a glass of water has enough heat energy to escape from the liquid. Evaporation from the ocean is the primary device for supporting the surface-to-atmosphere part of the water cycle. Evaporation is the pathway where water moves from the liquid state back into the water cycle as atmospheric vapor. Studies have shown that the major bodies of water are nearly ninety percent of the moisture in the atmosphere from the evaporation of the water, the ten percent left is contributed from plant transpiration. Evaporation can only occur when water is available. It also requires the humidity of the surrounding atmosphere to be less than the evaporating surface. At one-hundred percent relative humidity, there is no more evaporation. The wind chill effect is one of the
There are three main variables that determine whether a liquid will posses capillary action. (Davis, 1995)
a larger one. The bonds between water molecules are so strong that water resists changes in