Experimential verification of the effect of detergent on surface tension

To study the effect of detergent on surface tension by observing the capillary rise.


Apparatus required
Ø  Traveling microscope
Ø  Beaker
Ø  Detergent
Ø  Cork(with pin)
Ø  Clamps and stand
Ø  Capillary tube
Ø  Glass rod
Ø  Weight measurement


Theory

Surface tension is a phenomenon in which the surface of a liquid, where the liquid is in contact with gas, acts like a thin elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air). If the surface is between two liquids (such as water and oil), it is called "interface tension."
Causes of Surface Tension
Various intermolecular forces, such as Vander Waals forces, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid, as shown in the picture to the right.

Units of Surface Tension

Surface tension is measured in SI units of N/m (Newton per meter), although the more common unit is the cgs unit dyne/cm (dyne per centimeter).

When an object falls onto the surface, it has to push the water molecules apart. If the effect of the weight of the object is insufficient to match the attractive forces between molecules in the surface layer, the object will not enter the surface.
There is a net force of attraction between the molecules of water (or any other liquid) holding the molecules together. For a molecule in the middle of the liquid, these forces, acting equally in all directions, more or less balancing out. For a molecule in the surface layer of the liquid, the forces do not balance out, and all the molecules in the surface layer are pulled towards each other and towards the bulk of the liquid. This brings these molecules closer to their neighbors until increasing forces of repulsion create a new balance, and gives rise to the phenomenon of surface tension.
  In order to consider the thermodynamics of the situation, it is sometimes useful to consider it in terms of work per unit area. The SI unit in that case is the J/m2 (joules per meter squared). The cgs unit is erg/cm2.

These forces bind the surface particles together. Though this binding is weak - it's pretty easy to break the surface of a liquid after all - it does manifest in many ways.
Examples of Surface Tension
Drops of water. When using a water dropper, the water does not flow in a continuous stream, but rather in a series of drops. The shape of the drops is caused by the surface tension of the water. The only reason the drop of water isn't completely spherical is because of the force of gravity pulling down on it. In the absence of gravity, the drop would minimize the surface area in order to minimize tension, which would result in a perfectly spherical shape.
Expression for surface tension on the basis of excess pressure consideration

Let the capillary tube of radius r is dipped vertically in a liquid (e.g. liquid) which wet the wall of the tube and has the concave meniscus. we know that there always exists an excess pressure,
Equal to 2T/R on the concave side of liquid-air interface. Where T is the surface tension of liquid and R is the radius of the curvature of the meniscus. Therefore the pressure just below the meniscus is less than the pressure just above it by 2T/R


Since the capillary tube is open to atmosphere so that pressure on the upper side of the meniscus is atmospheric pressure p and the pressure just below the meniscus is (p-2T/R). This the liquid rise inside the tube to a height h till the hydrostatic pressure due to liquid column in the tube on the surface of the liquid equals this excess pressure
         i.e. 2T/R= hρg
     


 Procedure
           
        We did a simple experiment on, the effect of the detergent on surface tension. We added detergent continuously in the water and noted the every decrease in the height of the liquid inside the capillary tube.
    Since we know that,
               Except ‘h’ all are constant. So we can conclude that,
 Surface tension is directly proportional to the height of liquid inside the capillary tube.
               i.e.  T α h


Observation

  From our experiment we observed that,


Mass of detergent (gm)
Height of liquid in the capillary  tube (cm)
Resultant (decrease in height) (cm)
Total mass
Added mass
0
0
0.81
0.81
1.77
0.96
2.66
1.7
3.48
0.82
4.44
0.96
M.S
V.S
Total

3.6
0.35
3.95
3.50
0.35
3.85
3.45
0.30
3.75
3.35
0.30
3.65
3.33
0.25
3.58
3.30
0.20
3.5
Decreased by

0
0.1
0.1
0.1
0.07
0.08



Data analysis
 As we start to add the mass the height of the liquid goes on decreasing inside the capillary tube. It is because detergent decreased the surface tension so that due to less surface tension the height of the liquid inside the capillary tube goes on decreasing. The decrease rate is 0.1 but at last due to more concentration the rate of decrease in height varies from  0.1.

   Reason for the decrease in surface tension
         Molecules of most detergents and soaps are long chain hydrocarbon molecules with an ionic group at one end, usually carrying a negative charge, thus making it an anion. This charge is balanced by the opposite charge of a soluble cation, for example Na+. The long hydrocarbon chains do not interact well with water molecules, and many of them are effectively ‘squeezed out’ to the interfaces between the water and the air or the glass sides of the beaker. The effect of these molecules on the water surface is to considerably weaken the forces between water molecules there, thus lowering the surface tension.







In hard water there is a significant concentration of calcium, Ca2+, and/or magnesium, Mg2+, cations. These cations form an insoluble compound with soap anions, so instead of forming a surface layer, they are precipitated out, leaving the surface tension little changed.
2 /\/\/\/\/\/\/COO−(aq) + Ca2+(aq) → (/\/\/\/\/\/\/COO)2Ca(s)
However, the calcium and magnesium salts of many detergent molecules are soluble, so detergents still lower the surface tension of hard water.

Conclusion
  
    Finally at last we conclude that, the addition of the detergent on the surface tension decreases the surface tension. Due to this detergents are used for making the soft water, washing the clothes, and as a cleansing agent.



A news article on England’s newspaper “London Times” entitled ‘Effect of detergent on surface tension”

                                                                                         -BY John Riddick

The Effect of Detergent on Surface Tension of Water Environment
The purpose of this paper is to examine the effect of detergent on the surface tension of water through weighing water drops.]
What is surface tension of water? How is it important in terms of water itself and plants or living thing in water?
As we look at water strider, it can walk safely on water surface to catch food without sinking. It is not because of it has smaller density than water, but it can float because of there is a support from surface tension of water. Water can form a tension on the surface which acts like a thin layer. This tension causes by the "cohesion" interaction between molecules of water on the surface. The molecules are pulled by the other molecules at the side and below the surface, and since there are no any molecules above the surface of water, this interaction becomes stronger. The interaction is similar to when we tight the balloons together with the strings. The balloons are connect each other from one to one. This also causes water drop a spherical shape.
How ever, by giving a small drop of detergent to water, water strider is soon sink into water. This because surface tension of water is lose in the present of detergent. Here, how detergent affects on the surface tension of water is solved by weighing water drops. The different samples with different concentrations of detergent are prepared: 0%, 0.001%, 0.0025%, 0.005%, 0.01%. Then, ten drops of each samples are weighed. By comparison, an interesting result is found that the weight of the drops becomes decreased while the amount of detergent increased. Another ways to say, the size of the drops gets smaller when the amount of detergent increases.
From this result, it is clear to say that detergent is a substance that decreases the surface tension of water significantly. It can be explained that, if the surface tension of water is stronger, it can keep the drops in bigger size with heavier weight. But in the result, in the present of detergent, the drop becomes smaller or lighter. This means detergent can reduce surface tension of water.
Therefore, it is proved that detergent is not good for water living thing especially the group of insects that depend on the surface of water such as water strider. With the enough amount of detergent in water, those insects can not survive. On the other hand, water condition must also changes. It could cause Eutrophication Phenomenon in the water if too much detergent is contributed to the water.


According to [California professor]
Martha Croll
====================================================
The surface tension is determined by what molecules are present in the one
or so layers of molecules at the surface of the interface between a liquid
and its vapor or air from the atmosphere.
           If the attraction between these molecules is strong, as is the case for water due to hydrogen bonding, the surface tension will be higher. For water it is about 72 dyne x cm = 72 erg / cm^2. I prefer the units of erg / cm^2 because it says that it take 72 ergs of energy to create a new square cm. of surface, which I find easier to visualize.
 Now molecules that are surface active (surf-act-ants) or detergents have two properties that cause them to reduce the surface tension of water. First, there is an excess concentration of these molecules at the interface (called the surface excess concentration) and the forces between these molecules is smaller than, or interfere with, the bonding between the water molecules at the surface. So it is easier (takes less energy) to "stretch" the surface of the interface. It turns out that a lot, but not all by any means, of such molecules have a hydrophilic polar end, and a hydrophobic end. The polar end sticks into the water phase and the hydrophobic end (often a hydrocarbon) sticks up into the vapor or air phase.
 They are "standing on their heads"! Since these hydrocarbon ends are more
Weakly bonded to one another they are easier to stretch. As the surface is
Stretched and the surface density of the "tails" decreases there are new
ones below in the water phase ready to take their place on the surface. In
fact, within the water phase these surfactants form bodies called micelles
where the hydrophilic (water loving) heads point into the water phase and
the hydrophobic tails are on the "inside" of these microscopic globules.

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