can you check my calculations are correct and answer the rest of the questions plz. I did the most of calculations
paredan
biosensors.docxBiosensors and biocatalysis
CMNS 5700
Surface tension and contact angles
We used negatively charged surfactant, sodium dodecylsulphonate (SDS) of 0.02 mol . We diluted the stock solution into six solutions of different concentration in the 100 volumetric flask, by using the formula:
M1.V1 = M2.V2
Solution 1 concentration is 0.00125:
0.02 × V2 = 0.00125 × 100
V2 = = 6.25
Solution 2 concentration is 0.0025:
0.02× V2 = 0.0025 × 100
V2 = = 12.5
Solution 3 concentration is 0.005:
0.02× V2 = 0.005 × 100
V2 = = 25
Solution 4 concentration is 0.0075:
0.02× V2 = 0.0075 × 100
V2 = = 37.5
Solution 5 concentration is 0.01:
0.02× V2 = 0.01 × 100
V2 = = 50
Solution 6 concentration is 0.015:
0.02× V2 = 0.015 × 100
V2 = = 75
Next step:
We measured the capillary rise in the two capillaries of diameters 0.7 and 1.2 mm. starting with pure water then the surfactant different concentrations.
Results:
The temperature of all the solutions used was = 20⁰ C
|
Capillary rise
|
0.7 capillary (cm) |
1.2 capillary (cm) |
|
Pure water |
4.2 |
2.4 |
|
SDS 1 |
4.0 |
2.1 |
|
SDS 2 |
3.8 |
1.9 |
|
SDS 3 |
3.1 |
1.5 |
|
SDS 4 |
2.5 |
1.3 |
|
SDS 5 |
2.3 |
1.2 |
|
SDS 6 |
2.1 |
1.1 |
Q1. Explain the reason why the experiments should be carried out in this order?
(measuring pure water then the SDS)
Q2. Plot the capillary rise against the concentration of surfactant for all experiments.
Next step:
Q.3 Calculate the surface tension (ϒ) value versus the concentration of SDS. Also plot the surface tension against In concentration of SDS. Comment on both graphs. From the In plots estimate the CMC value of SDS.
Answer:
The surface tension is calculated by using the equation:
h = 2 ϒ/pgr
Where,
h = height of the capillary column in (cm)
ϒ = surface tension of water in (N )
P = density of water at 20⁰C = 0.99821 g = 9.9821 kg
g = acceleration due to gravity = 9.80665 m
r = the meniscus radius for both capillaries
0.7 capillary radius (r) = 0.7/2 = 0.35 mm
1.2 capillary radius (r) = 1.2/2 = 0.6 mm
so,
ϒ =
Surface tension of the pure water using:
1- 0.7 capillary: ϒ = = 71.94 N
2- 1.2 capillary: ϒ = = 70.48 N
Surface tension of the SDS:
We will use the same equation above and calculate ϒ for both capillaries.
|
Surface tension (ϒ) N |
0.7 capillary |
1.2 capillary |
|
SDS 1 |
68.52 |
61.67 |
|
SDS 2 |
65.09 |
55.79 |
|
SDS 3 |
53.10 |
44.05 |
|
SDS 4 |
42.82 |
38.17 |
|
SDS 5 |
39.40 |
35.24 |
|
SDS 6 |
35.97 |
32.30 |
Next step:
Calculate the surface tension against In concentration of SDS by using Gibbs adsorption isotherm equation, which is:
= - (1/RT).(d ϒ/d In )
= - (1/RT).( ∆ ϒ /∆In )
Where,
= monolyer coverage (mol )
R= gas constant = 8.314 J
T= temperature = 20⁰ C = 293.15 K
ϒ= surface tension
= surfactant concentration (mol )
Calculation:
Monolyer coverage of SDS 1 using 0.7 capillary=
-(1 /8.314 × 293.15) × (68.52/ In 0.00125) =
-(1/2437.2491) × (- 10.25) =
- 4.103 × - 6.684 = 42.06 mol
By the same way we will calculate the for the other SDS concentrations:
|
Monolyer coverage () mol |
0.7 capillary |
1.2 capillary |
|
SDS 1 |
42.06 |
37.85 |
|
SDS 2 |
44.57 |
38.2 |
|
SDS 3 |
41.12 |
34.11 |
|
SDS 4 |
35.90 |
32 |
|
SDS 5 |
35.10 |
31.39 |
|
SDS 6 |
35.14 |
31.55 |
Q.4 prove that the units of surface tension (ϒ ) are N ,and monolayer density are mol .
Q.5 From the Gibbs equation above calculate the monolayer density in mol of SDS on the water surface.
