Physikalisch-chemisches Praktikum,
Departement Chemie, Universität Basel
Experiment: Surface tension
PC Praktikum □ FS / HS
20xx
Date of experiment (to be filled in by the assistant): ……..………………….
Last Name, First Name
□ Chemistry
□ Biology
□ Nanotechnology
Pharmaceutical Sciences
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Email address
PX
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assistant)
1
19.11.09
Surface tension
Group PX :
Name 1
Name 2
2
1. Introduction
Surface tension is a surface property of liquids caused by molecular cohesion. In the
bulk of a liquid, the molecules are pulled in every direction by neighbor molecules ;
at the surface of the liquid, molecules experience asymmetric interactions. They
move towards the aqueous phase and are immediately replaced by other molecules.
In other words, the surface molecules are subject to an inward force which is
balanced only by the liquid’s resistance to compression ; this results in a driving force
to reduce the surface area.
The surface tension can be lowered by surface active compounds – also called
surfactants or tensides – because of their particular structure. Indeed, they are
amphiphilic, meaning they contain a hydrophobic group (their “tail” composed of
hydrocarbons) as well as a hydrophilic group (their ionic “head”) and thus, these
molecules can adsorb at the liquid-gas interface. The surface excess concentration is
then defined as the extra amount of adsorbed substance at the interface as compared
to the equivalent volume of neighboring phase.
With increasing concentration of surfactant, the surface is more and more occupied
by surfactant molecules until a maximum. Beyond, molecules go into the solution
and accumulate to micelles. These are aggregates where the hydrophilic head regions
are in contact with surrounding solvent, sequestering the hydrophobic tail regions in
the micelle centre. The concentration at which tensids begin to form micelles is called
critical micellization concentration or CMC.
The aim of this experiment is to determine the CMC and the Gibbs surface excess of
the surfactant SDS (Sodium dodecyl sulfate) in water using Wilhelmy plate method.
2. Experimental Part
The CMC and the Gibbs surface excess can be calculated thanks to the surface
tension. Several methods can be used to measure this parameter ; their common part
is the – by a tensiometer performed – measurement, which is based on the interaction
of a probe with the surface of interface of two fluids. In our case, by the Wilhelmy
plate method, a thin platinum plate is hung on a balance and brought into contact
with water and later with a surfactant solution tested.
3
In these different cases the height of the meniscus between the plate and the solution
will change ; this phenomenon can be used by the tensiometer to calculate the surface
tension. In fact, it measures the force related to this process and convert it in surface
tension by dividing the force value by the Wilhelmy plate perimeter (as the contact
angle is controlled to be zero).
For our measurement, we first prepared the stock solution, a 0,1 M solution of SDS.
As M=288,38g/mol, we had to weigh 0,432g of SDS in a 25 ml volumetric flask and fill
it with double distilled water. Then we made 9 dilutions :
1) 0,03 M : 3 ml stock solution + 7 ml ddH2O
2) 0,02 M : 2 ml stock solution + 8 ml ddH2O
3) 0,01 M : 1 ml stock solution + 9 ml ddH2O
4) 0,005 M : 500 µl stock solution + 9,5 ml ddH2O
5) 0,002 M : 200 µl stock solution + 9,8 ml ddH2O
6) 0,001 M : 100 µl stock solution + 9,9 ml ddH2O
7) 0,0005 M : 50 µl stock solution + 9,95 ml ddH2O
8) 0,0002 M : 20 µl stock solution + 9,98 ml ddH2O
9) 0,0001 M : 20 µl stock solution + 19,98 ml ddH2O
We measured the surface tension of water and of each diluted solution three times
for a better precision. We dipped the plate in the solution to ensure complete wetting
and tared the tensiometer after hauling it out. After each measurement, we cleaned
the platinum plate with isopropanol and water and dried it by holding it in a flame
until glowing (by doing this, possible surfactant rests could be eliminated).
3. Results
a. Raw data and results
The raw data are given and evaluated in the following tables and graphs.
Water
γ1 [mN/m]
γ2 [mN/m]
γ3 [mN/m]
γMean [mN/m]
72,9
73
72,9
72,93
4
SDS
c (mol/l)
γ1 [mN/m] γ2 [mN/m] γ3 [mN/m]
ln(c)
γMean [mN/m]
0,0001
-9,21034037
69,7
70,1
71,5
70,43
0,0002
-8,51719319
68,3
67,2
69,2
68,23
0,0005
-7,60090246
66,1
67
67,4
66,83
0,001
-6,90775528
59,5
62,4
62,9
61,6
0,002
-6,2146081
50,9
53,2
54,9
53
0,005
-5,29831737
38,95
39,1
41,5
39,85
0,01
-4,60517019
38,9
38,8
38,8
38,83
0,02
-3,91202301
38,5
38,5
38,5
38,5
0,03
-3,5065579
37,7
37,7
38
37,8
Evolution of the surface tension depending on
ln(c)
75
70
γ [mN/m]
65
y = -7,509x + 5,232
R² = 0,874
60
55
50
y = -1,057x + 34,16
R² = 0,957
45
40
35
-10
-9
-8
-7
-6
-5
-4
-3
-2
ln(c)
Calculation of the CMC
To calculate the CMC, we determine the intersection of the two linear curves :
−7,509 ∙ ln(𝑐𝑐) + 5,232 = −1,057 ∙ ln(𝑐𝑐) + 34,16
6,452 ∙ ln(𝑐𝑐) = −28,928
ln(𝑐𝑐) = −4,484
𝐶𝐶𝐶𝐶𝐶𝐶 = 0,011 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
5
Calculation of the Gibbs surface excess and plotting of Gibbs adsorption isotherm
𝛤𝛤 = −
1
∆𝛾𝛾
∙
𝑅𝑅𝑅𝑅 ∆ln(𝑐𝑐)
with :
∆𝛾𝛾 = 𝛾𝛾𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 − 𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 with 𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 = 72,93 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
∆ ln(𝑐𝑐) = ln(𝑐𝑐) , as there are no surfactant molecules in the reference (Water)
𝑅𝑅 = 8,3145 𝐽𝐽 ∙ 𝑚𝑚𝑚𝑚𝑙𝑙 −1 ∙ 𝐾𝐾 −1 : the universal gas constant
T = 295,15 K : the absolute temperature
c (mol/l)
ln(c)
γ [mN/m]
Δγ [N/m]
Γ [mol/m²]
0,0001
-9,21034037
70,43
0,0025
1,10608E-07
0,0002
-8,51719319
68,23
0,0047
2,24865E-07
0,0005
-7,60090246
66,83
0,0061
3,27029E-07
0,001
-6,90775528
61,6
0,01133
6,68366E-07
0,002
-6,2146081
53
0,01993
1,30682E-06
0,005
-5,29831737
39,85
0,03308
2,54418E-06
0,01
-4,60517019
38,83
0,0341
3,01738E-06
0,02
-3,91202301
38,5
0,03443
3,58638E-06
0,03
-3,5065579
37,8
0,03513
4,08242E-06
Gibbs adsorption isotherm
5.00E-06
4.00E-06
Γ [mol/m²]
3.00E-06
2.00E-06
1.00E-06
2.10E-20
0
-1.00E-06
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
Concentration [mol/l]
6
The graph shows us that Γmax is about 4,5 ∙ 10−6 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2 so the area per molecule is :
𝐴𝐴 =
1
𝛤𝛤𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑁𝑁𝐴𝐴
≈
4,5 ∙
10−6
1
≈ 3,7 ∙ 10−19 𝑚𝑚²
∙ 6,022 ∙ 1023
b. Error analysis
The concentration of the stock solution is : c = 0,1 M. Allowing the error propagation :
𝜕𝜕𝑐𝑐
2
𝜕𝜕𝑐𝑐
∆𝑚𝑚
−𝑚𝑚
∆𝑐𝑐0 = ��𝜕𝜕𝜕𝜕0 � ∙ ∆𝑚𝑚2 + ( 𝜕𝜕𝜕𝜕0 )² ∙ ∆𝑉𝑉²=��𝑀𝑀𝑀𝑀 � ² ∙ ∆𝑚𝑚2 + (𝑀𝑀𝑉𝑉 2 )² ∙ ∆𝑉𝑉²
The error of the balance is Δm=0,0001g and the error of the flask is ∆𝑉𝑉0 = 6 ∙ 10−5 𝑙𝑙 so :
∆𝑐𝑐0 = 2,4 ∙ 10−4 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
Next we calculate the error of each concentration of our dilution series :
Allowing the formula 𝑐𝑐0 ∙ 𝑉𝑉0 = 𝑐𝑐 ∙ 𝑉𝑉, we can write : 𝑐𝑐 =
𝜕𝜕𝜕𝜕 2
𝜕𝜕𝜕𝜕 2
𝜕𝜕𝜕𝜕 2
2
2
∆𝑐𝑐 = �� � ∙ ∆𝑐𝑐0 + � � ∙ ∆𝑉𝑉 + �
� ∙ ∆𝑉𝑉02
𝜕𝜕𝑐𝑐0
𝜕𝜕𝜕𝜕
𝜕𝜕𝑉𝑉0
𝑐𝑐0 ∙𝑉𝑉0
𝑉𝑉
; the error propagation :
𝑉𝑉 2
−𝑐𝑐0 ∙ 𝑉𝑉0 2
𝑐𝑐
2
2 + ( 0 )² ∙ ∆𝑉𝑉 ²
�
∆𝑐𝑐 = � � ∙ ∆𝑐𝑐0 + �
�
∙
∆𝑉𝑉
0
𝑉𝑉0
𝑉𝑉 2
𝑉𝑉
Again with the error propagation we can calculate the error of ln(c) :
1
∆ ln(𝑐𝑐) = �( )² ∙ ∆𝑐𝑐²
𝑐𝑐
1) The concentration of the first diluted solution is 𝑐𝑐1 = 0,0001 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
The volume of the diluted solution : 𝑉𝑉1 = 0,02 𝑙𝑙
The volume of the stock solution we needed : 𝑉𝑉0 = 2 ∙ 10−5 𝑙𝑙
The error of the flask : ∆𝑉𝑉 = 6 ∙ 10−5 𝑙𝑙, the error of the pipette ∆𝑉𝑉0 = 10−7 𝑙𝑙
∆𝑐𝑐1 = 6,3 ∙ 10−7 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐1 ) = 6,3 ∙ 10−3
7
2) The concentration of the second diluted solution is 𝑐𝑐2 = 0,0002 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉2 = 0,01 𝑙𝑙 ; 𝑉𝑉0 = 2 ∙ 10−5 𝑙𝑙 ; ∆𝑉𝑉 = 2,5 ∙ 10−5 𝑙𝑙
∆𝑐𝑐2 = 1,2 ∙ 10−6 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐2 ) = 6,08 ∙ 10−3
3) The concentration of the third diluted solution is 𝑐𝑐3 = 0,0005 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉3 = 0,01 𝑙𝑙 ; 𝑉𝑉0 = 5 ∙ 10−5 𝑙𝑙 ; ∆𝑉𝑉 = 2,5 ∙ 10−5 𝑙𝑙
∆𝑐𝑐3 = 2 ∙ 10−6 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐3 ) = 4 ∙ 10−3
4) The concentration of the fourth diluted solution is 𝑐𝑐4 = 0,001 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉4 = 0,01 𝑙𝑙 ; 𝑉𝑉0 = 0,00011 𝑙𝑙 ; ∆𝑉𝑉 = 6 ∙ 10−5 𝑙𝑙
∆𝑐𝑐4 = 6,5 ∙ 10−6 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐4 ) = 6,54 ∙ 10−3
5) The concentration of the fifth diluted solution is : 𝑐𝑐5 = 0,002 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉5 = 0,01 𝑙𝑙 ; 𝑉𝑉0 0,0002 𝑙𝑙 ; ∆𝑉𝑉 = 2,5 ∙ 10−5 𝑙𝑙
∆𝑐𝑐5 = 7 ∙ 10−6 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐5 ) = 3,5 ∙ 10−3
6) The concentration of the sixth diluted solution is : 𝑐𝑐6 = 0,005 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉6 = 0,01 𝑙𝑙 ; 𝑉𝑉0 = 0,0005 𝑙𝑙 ; ∆𝑉𝑉 = 6 ∙ 10−5 𝑙𝑙
∆𝑐𝑐 = 3,23 ∙ 10−5 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐6 ) = 6,47 ∙ 10−4
7) The concentration of the seventh diluted solution is : 𝑐𝑐7 = 0,01 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉7 = 0,01 𝑙𝑙 ; 𝑉𝑉0 = 0,001 𝑙𝑙 ; ∆𝑉𝑉 = 6 ∙ 10−5 𝑙𝑙
∆𝑐𝑐7 = 6,46 ∙ 10−5 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐7 ) = 6,46 ∙ 10−3
8) The concentration of the eighth diluted solution is : 𝑐𝑐8 = 0,02 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉8 = 0,01 𝑙𝑙 ; 𝑉𝑉0 = 0,002 𝑙𝑙 ; ∆𝑉𝑉 = 4 ∙ 10−5 𝑙𝑙
∆𝑐𝑐8 = 9,33 ∙ 10−5 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐8 ) = 4,67 ∙ 10−3
9) The concentration of the nineth diluted solution is : 𝑐𝑐9 = 0,03 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
𝑉𝑉9 = 0,01 𝑙𝑙 ; 𝑉𝑉0 = 3 ∙ 10−3 ; ∆𝑉𝑉 = 6 ∙ 10−5
∆𝑐𝑐9 = 1,9 ∙ 10−4 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑙𝑙 −1
∆ ln(𝑐𝑐9 ) = 6,46 ∙ 10−3
8
Calculation of the maximal error of the surface tension :
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾 = 𝛾𝛾𝑚𝑚𝑚𝑚𝑚𝑚 − 𝛾𝛾𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 where γmax is the maximal deviant value from the mean value
of surface tension.
For each surfactant solution of our dilution series :
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾1 = 1,07 m𝑁𝑁 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾2 = 0,97 m𝑁𝑁 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾3 = 0,57 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾4 = 2,10 m𝑁𝑁 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾5 = 2,10 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾6 = 1,65 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾7 = 0,07 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾8 = 0,00 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾9 = 0,20 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
Calculation of the error of surface excess :
Based on the formula : 𝛤𝛤 = −
We use the error propagation :
1
𝑅𝑅𝑅𝑅
∙
∆𝛾𝛾
∆ln (𝑐𝑐)
=−
1
𝑅𝑅𝑅𝑅
∙
𝛾𝛾𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 −𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊
ln ⁡(𝑐𝑐)
2
𝜕𝜕𝜕𝜕
𝜕𝜕𝜕𝜕 2
𝜕𝜕𝜕𝜕 2
2
�
� ∙ (∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾)² + �
� ∙ (∆𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 ) + �
� ∙ (∆𝑙𝑙𝑙𝑙𝑙𝑙)²
∆𝛤𝛤 = �
𝜕𝜕𝛾𝛾𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀
𝜕𝜕𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊
𝜕𝜕𝜕𝜕𝜕𝜕𝜕𝜕
(𝛾𝛾𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 − 𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 )2 ∙ ∆ ln(𝑐𝑐)
−𝛾𝛾𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 ∙ ∆𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 2
𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 ∙ ∆𝑚𝑚𝑚𝑚𝑚𝑚 𝛾𝛾 2
∆𝛤𝛤 = ��
� +�
� +(
)²
𝑅𝑅 ∙ 𝑇𝑇 ∙ 𝑙𝑙𝑙𝑙𝑙𝑙
𝑅𝑅 ∙ 𝑇𝑇 ∙ 𝑙𝑙𝑙𝑙𝑙𝑙
𝑅𝑅 ∙ 𝑇𝑇 ∙ (𝑙𝑙𝑙𝑙𝑙𝑙)2
with : 𝛾𝛾𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊𝑊 = 72,93 ± 0,07 𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−1
𝛤𝛤1
𝛤𝛤2
𝛤𝛤3
𝛤𝛤4
𝛤𝛤5
𝛤𝛤6
𝛤𝛤7
𝛤𝛤8
𝛤𝛤9
= 1,09 ∙ 10−7 ± 3,45 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 2,23 ∙ 10−7 ± 2,45 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 3,24 ∙ 10−7 ± 2,24 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 6,62 ∙ 10−7 ± 9,03 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 1,30 ∙ 10−6 ± 1,00 ∙ 10−8 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 2,5 ∙ 10−6 ± 9,73 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 2,99 ∙ 10−6 ± 4,25 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 3,55 ∙ 10−6 ± 4,28 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
= 4,04 ∙ 10−6 ± 7,68 ∙ 10−9 𝑚𝑚𝑚𝑚𝑚𝑚 ∙ 𝑚𝑚−2
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4. Discussion
In our experiment we determined the critical micelle concentration of sodium
dodecyl sulfate (SDS) to be 0,011mol/l, estimated maximal surface excess and the
mean area per molecule at the boundary until it’s saturated is 4,5*10-6mol*m-2 and
3,7*10-19m2, respectively. Deviation of the CMC value of SDS obtained in our
experiment from the value presented in the literature (CMC=0,0082mol/l), is due to
several reasons.
First of all, the real concentration of the aqueous stock solution of SDS deviated from
the target one due to the errors in the determination of the weight of the surfactant
powder (precision of the balance) as well as of the amount of the dubble destilled
water (precision of the pipette and of the flasks, human factor by the determination
of the position of the meniscus, leading to the faulty measurement of the volume of
the solvent).
Consequently, during the preparation of the dilution series the further error of the
solution concentration has been added.
Another source of error was the tensiometer, in particular the platinum plate, which
has been dipped into solution, in order to measure the surface tension. This plate was
attached to a tiny wire in such way, that small deformations of the last resulted in
some inclination of the plate and thus in the modification of the cross-section being
in contact with solution. The fact that the plate itself was deformed could also have
an influence on the precision of the measurements.
Furthermore, the temperature instability of the solutions could occur, originating
from the annealing step of the platinum plate. Namely, we didn’t measure the
temperature of the plate before immersing it into solution, which could still be at
elevated temperature. However, taking into account the size of the plate and the
amount of the solution in the experiment and the high specific heat of water as well,
this factor should have minor influence on the magnitude of the surface tension.
Additionally, the high sensitivity of the device complicated the read-out of the
measurement values, since the smallest vibrations of the plate have been disturbing
the measurements too.
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5. Exercises
(1) Describe in detail one method that can be used for surface tension measurement.
Because surface tension manifests itself in various effects, it offers a number of paths
to its measurement. Here we focus on the ring method, also called Du Noüy method.
The principle is approximately the same just that the probe is not a plate but a ring,
which hangs on a torsion dynamometer. The ring has to be submerged below the
interface and subsequently raised upwards. As the ring is being moving upwards it
raises a meniscus of the liquid, which corresponds to the force indicated by the
dynamometer. The value increases until just before the meniscus is torn from the
ring. The surface tension can be calculated by dividing this maximal force by the area
of the ring.
(2) How does the CMC change when we use water/ethanol mixture, instead of pure
water?
Tensids are better dissolvable in ethanol than in water that’s why we need a higher
quantity of tensid to reach the CMC.
(3) How does the CMC change with temperature and why ?
The CMC value is proportional to the temperature : an increase (decrease) of the
temperature results in an acceleration (deceleration) of the movement of molecules
and consequently in lower (higher) interaction between them ; this leads finally to a
better (worse) solubility of the molecules and a higher (lower) CMC.
(4) Which detergent concentration (below or above CMC) should be best used to reach the
highest efficiency for dishwashing/laundry ? Why ?
Detergents are used in dishwashing/laundry because of their ability to catch fat
molecules due to their structure. Indeed, detergents stay first at the surface of the
solution ; from a certain concentration, the CMC, they begin to build micelles,
aggregates where the hydrophilic head regions are in contact with surrounding
solvent, sequestering the hydrophobic tail regions in the micelle centre, where fat
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molecules can be retained. On this note, detergent concentration has to be above
CMC so that the system can unfold its effect.
(5) Small objects like paper clips can swim on water surface, even though they are heavier
than water. Some insects can “walk” on water. These phenomena are related to surface
tension of water. What happens to a paper clip / insect, if we add some soap to water ?
Why ?
Paper clips as well as small insects can walk on water because of its surface tension,
which gives the water an elastic similar behavior. If we add soap to water, the surface
tension will decrease because the tensid molecules will have replaced the water
molecules at the surface. So the paper clips or insects won’t be able any more to swim
on the water (and consequently will sink).
(6) The maximum size of water droplets is always bigger than for an aqueous solution of a
detergent. Why ?
Due to surface tension, a water droplet has the smallest possible surface area to
volume ratio. This ratio decreases with an increasing of the radius respectively the
diameter of the sphere. As a consequence, water droplets are bigger than droplets of
an aqueous solution, whose area to volume ratio is bigger.
6. Literatur
Mortimer/Müller, Chemie, 8. Auflage, Thieme Verlag, 2003
Physikalisch-chemisches Praktikum, 3. Auflage, Wiley-VCH, 1997
Atkins, Physikalische Chemie, 3. Auflage, Wiley-VCH, 2001
www.wikipedia.org
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