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...
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...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
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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
Osmosis is the facilitated diffusion of water across the cell membrane of a cell. The inside layer of the cell membrane is hydrophilic, meaning water cannot easily pass through the membrane. The cell membrane has to have aquaporins, which are water channel proteins, that move the water across the membrane. If there is a water and salt solution outside the cell, the salt can enter the cell by diffusion, but the cell membrane is not permeable to the water. Because there is more solute solution inside the cell, there is less water. The aquaporins move the water across the membrane until equilibrium is reached.
a) With the rise in ADH concentration, the permeability of the collecting tubules to water
Osmosis is a type of diffusion which is only applied on water and is a passive process which does not require an input of energy from the cell; this is because materials are moving with the concentration gradient. Osmosis is a process that occurs at a cellular level, which entails the spontaneous net movement of water through a selectively permeable membrane, from a region of high to low water concentration, in order to equalise the level of water in each region. This form of diffusion takes place when the molecules in a high concentration are too large to move through the membrane. The term semi-permeable or selectively permeable means that some substances can easily pass through the cell membrane, whereas others cannot. The significance of osmosis to cells is great, since it is the osmotic pressure that maintains the shape of an animal cell and provides support in the plant cells. Many factors affect the rate of osmosis including size of particles and temperature however in this particular experiment the factor investigated is the concentration of sodium chloride. Tubes of potatoes will be used to demonstrate the fact...
Most of the water in the beaker travels up through the xylem, and most evaporates through the stomata, into the air.
Although the experiment produced varying results amongst the pairs of test tubes in each of the water temperatures, the Mean calculations proves that the temperature rising will increase the amount of kinetic energy in the movement of the Phosphate and Lipids in the cell membrane as well as breaking the hydrogen bonds of the proteins in the cell membrane,
When in solution, the hydronium and chloride ions formed will be partially surrounded by water molecules via ion-dipole bonds, an electrostatic force of attraction that exists between charges in the ions and the partial charges in the water molecules. Water molecules surrounding ions is called hydration.
contains for stresses; there is a strong caesura in the middle of the lines and
However, as carbon dioxide is released in the reaction it can dissolve into the water and lower the pH. So it is important that the pH is monitored so that it doesn’t make the water too acidic. The addition of coagulants reduces the charge on the colloid and means that there is less repulsion between adjacent particles. The next step in the process is known as flocculation where the particles begin to clump together. The particles will be more strongly attracted to each other and can form much larger clumps. These larger clumps can be removed much more easily than the smaller particles. As the colloids have a neutral charge the force dominating the attraction of the particles is van der Waals forces of attraction. As the particles come within a close proximity an induced dipole is generated on an adjacent particle and they begin to clump together. Once the clumps are large enough they can be separated by sedimentation as they settle at the bottom of the
Osmosis is also another type of diffusion where water is transferred from a higher concentration to a lower concentration. Osmosis will then come into play when a membrane that differs in solute concentration breaks in two; the water will move from the hypotonic solution to a hypertonic solution. Initially, the hypotonic solution has a lower concentration, and the hypertonic solution has a higher concentration. The water will then continue its journey down its concentration gradient until it reaches equilibrium; that means the water will have the same solute concentration on both sides. When the solute reaches equilibrium on both sides of the semipermeable membrane, the solution then becomes isotonic. Isotonic solutions are when the solute concentration is the same on the inside and outside of the cells, and that is the reason most cells live in the isotonic state. Additionally, a semi-permeable membrane only permits specific substances to enter, usually opening for a solvent but not most solutes. The substances dissolved in a solution are referred to as a solute; and the solvent is the substance that dissolves a solute in a solution. Therefore, the mixture of a solvent and a solute
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
Another system is called the spray dry system. A slurry of alkali is njected into the flue gases. The heat in the flue gases causes water to evaporate. The sulfur dioxide reacts with the drying alkali solution to form a solidi reaction product. There is no waste water from this process.
Aim: To determine the effect of temperature change on the height of water in capillary action at 8 seconds.
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...
• Phosphates are used as part of toothpaste formulas because they help the product to leave the teeth looking white and feeling clean.