About this previous year paper
Course: B.Tech (Textile Chemistry / Textile Engineering)
Subject Code: EC404
Subject Title: Physical Chemistry of Dyeing
Exam Type: Theory
Duration: 3 Hours
Maximum Marks: 100
SECTION – A (10 × 2 = 20 Marks)
Short and conceptual questions testing theoretical definitions
| No. | Question | Explanation |
|---|---|---|
| (a) | Substantivity | The inherent attraction between a fiber and a dye under specific conditions; indicates how well a dye is absorbed and retained. |
| (b) | Affinity in Dye–Fiber System | The thermodynamic tendency of a dye to move from solution to fiber; related to free energy change (ΔG). |
| (c) | EDL and Diffuse Layer | Electrical Double Layer (EDL): the charged interface between dye and fiber surface; diffuse layer contains counter-ions balancing surface charge. |
| (d) | Zeta Potential | The electrical potential at the slipping plane of particles/fibers — affects dye exhaustion and stability of dye baths. |
| (e) | Types of Isotherms | Freundlich, Langmuir, and Nernst isotherms describe adsorption equilibrium between dye and fiber. |
| (f) | Lambert’s Law | The absorbance of light decreases exponentially with thickness of the absorbing medium. |
| (g) | Beer’s Law | Absorbance is directly proportional to concentration and path length of absorbing species. |
| (h) | Chemical Potential | Measure of a substance’s potential to undergo change or reaction; drives diffusion and equilibrium. |
| (i) | Entropy of Dyeing | Indicates randomness or disorder introduced in fiber–dye system during dyeing (units: J/mol·K). |
| (j) | Types of Isotherms Applicable in Dyeing | Primarily Freundlich, Langmuir, and Temkin isotherms. |
SECTION – B (5 × 10 = 50 Marks)
Analytical and descriptive questions combining concepts of thermodynamics and kinetics
(a) Absorption & Light Interaction
Absorption: Process where dye molecules absorb specific wavelengths of light.
Laws: Governed by Lambert–Beer’s law.
Effect of Wavelength: Higher wavelength → lower energy absorption; affects shade and depth of color.
(b) Significance of Beer’s Law
Deviations caused by instrumental errors, chemical interactions, or solution changes (pH, aggregation).
Accurate spectrophotometric measurements depend on maintaining ideal conditions.
(c) Fiber Structure Effect
Cellulosic Fibers (Cotton): Hydroxyl groups form hydrogen bonds with reactive and direct dyes.
Protein Fibers (Wool, Silk): Amine and carboxyl groups interact ionically with acid or basic dyes.
Fiber crystallinity, porosity, and orientation directly influence dye uptake and uniformity.
(d) Equilibrium Dyeing Measurement
Expressed as concentration of dye in fiber vs solution at equilibrium.
Governing equation:
- CfCs=K\frac{C_f}{C_s} = KCsCf=K
where CfC_fCf = concentration in fiber, CsC_sCs = in solution, KKK = equilibrium constant.
(e) Electrical Effects in Dyeing
Electrostatic interactions between charged fiber and dye molecules determine adsorption rate.
For example, cationic dyes have strong attraction to anionic fibers like acrylic.
(f) Chemical Potential in Dyeing
Dyeing occurs when chemical potential of dye in solution > that in fiber.
The process continues until potentials equalize → equilibrium dyeing achieved.
(g) Diffusion Coefficient Measurement
Techniques include Time–lag method, Fiber Sectioning, and Desorption methods.
Diffusion rate depends on fiber porosity, dye size, temperature, and swelling.
(h) Temperature Effect on Dyeing Rate
Temperature increases kinetic energy and diffusion rate.
At higher temperatures, fibers swell more, reducing diffusion resistance.
SECTION – C (2 × 15 = 30 Marks)
In-depth, applied, and theoretical questions
Q3. Thermodynamics of Dyeing
Involves enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) changes.
- ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta SΔG=ΔH−TΔS
Negative ΔG → spontaneous dyeing.
Entropy of dyeing: reflects randomness when dye molecules distribute within fiber.
Q4. Dyeing Rate and Its Limitations
Rate equation:
- dCfdt=k(Cs−Cf)\frac{dC_f}{dt} = k(C_s - C_f)dtdCf=k(Cs−Cf)
Controlled by diffusion and adsorption.
Under equal affinity conditions, the dye rate depends only on diffusion and temperature.
Limitations: Surface saturation, non-uniform fiber structure, aggregation.
Q5. Theories of Dyeing
Pore Model: Diffusion of dye through interconnected pores within fiber structure.
Free Volume Model: Dye molecules migrate through free spaces in polymer matrix; thermal motion creates transient paths.
Both explain dye penetration mechanisms in amorphous polymeric fibers.
Summary
This Physical Chemistry of Dyeing (EC404) paper comprehensively tests:
| Topic | Core Concept |
|---|---|
| Adsorption & Isotherms | Langmuir, Freundlich, Nernst relationships |
| Optical Laws | Lambert’s and Beer’s Laws |
| Thermodynamics | ΔG, ΔH, ΔS, affinity, and entropy of dyeing |
| Kinetics & Diffusion | Temperature effects, rate equations |
| Fiber–Dye Interactions | Electrostatic and hydrogen bonding |
| Dyeing Models | Pore model, free volume model |