APPLIED CHEMISTRY SURFACE TENSION, SURFACTANTS TYPES OF SURFACTANTS & THEIR USES IN TEXTILE PROCESSING Lecture No. 13 & 14
2 Surface Tension This property of liquids arises from the intermolecular forces of attraction. A molecule in the interior of a liquid is attracted equally in all directions by molecules around it. A molecule in the surface of a liquid is attracted only sideways and towards the interior. The forces on the sides being counterbalanced, the surface molecule is pulled only inward the liquid. Thus there is a tendency on the part of the surface molecules to go into the bulk of liquid. The liquid surface is, therefore, under tension and tends to contract to the smallest possible area in order to have the minimum number of molecules at the surface. Its for this reason that in air, drops of a liquid assume spherical shape; for a given volume a sphere has the minimum surface area
Surface Tension. Surface Tension if defined as: the force in dynes acting along the surface of a liquid at right angle to any line 1cm in length Example: a) Surface tension helps insects to walk on water b) Surface tension helps dew drops stick to the grass and prevents them from spreading c) Surface tension prevents a paper clip from sinking d) Surface tension keeps the rain drops spherical shaped in air 3
Unit of Surface Tension The unit of surface tension in CGS system is dynes per centimeter (dyne/cm) Dyne: the force required to accelerate a mass of one gram at a rate of one centimeter per second squared The SI unit of surface tension is Newton per meter (N/m) 1 Dynes/cm = 0.001 N/m or 1 N/m = 1000 dynes/cm 1 Dyne = 10 micro Newton 4
5 Surface Tension (dynes/cm) Surface tension decreases with increase in temperature Liquid Temperature C Surface tension, γ Acetic acid 20 27.6 Ethanol 20 22.27 Glycerol 20 63 Mercury 15 487 Water 0 75.64 Water 25 71.97 Water 50 67.91 Water 100 58.85
6 Interfacial Tension The forces involved in interfacial tension are adhesive forces (tension) between the liquid phase of one substance and either a solid, liquid or gas phase of another substance
7 SURFACTANTS The word surfactant is derived from the term surface active agents The surfactants are molecules that reduce: a) surface tension of a liquid b) interfacial tension between two liquids c) interfacial tension between a liquid and a solid
8 Surfactant Molecule The surfactant molecule consists of hydrophilic head (water loving/soluble) and hydrophobic tail (water fearing/insoluble) The hydrophobic tail is usually equivalent of 8 to 18 carbon hydrocarbon (C 8 to C 18 ), and can be an aliphatic, aromatic, or a mixture of both. The hydrophilic head group give the primary classification to surfactants, and are anionic, cationic, nonionic and amphoteric in nature. The sources of surfactants are either natural fats and oils or petroleum fraction (synthetic polymers from petrochemicals)
9 Types of Surfactants a) Anionic Surfactants b) Cationic Surfactants c) Non-ionic Surfactants d) Amphoteric Surfactants
Anionic Surfactants Anionic surfactants contain vely charged head (hydrophilic group) These are most widely used type of surfactants in industrial applications due to their low cost of manufacturing and used almost all surfactant applications. Most commonly used hydrophilic groups are carboxylates, sulphates, sulphonates and phosphates. Their general formula are given below: 10
Cationic Surfactants Cationic surfactants contain +vely charged head (hydrophilic group) Use of these surfactants is small compared to anionic and nonionic surfactants. Owing to their ability to adhere to and modify solid surfaces, they are also used as corrosion inhibitors, germicides and in hair conditioners.
Nonionic Surfactants Nonionic surfactants contain uncharged head (hydrophilic group) Their basis is long chain alcohols, reacted with ethylene oxide producing a polymeric ether chain ending in an hydroxyl (OH) group, giving high solubility and stability in both acetic and alkaline conditions. General formula of nonionic surfactants is given below: 12
Amphoteric (zwitterionic) Surfactants Amphoteric surfactants contain both an anionic and cationic groups In acetic media they tend to behave as cationic agents and in alkaline media as anionic agents. Commonly at ph=7, lies isoelectric point (point at which anionic and cationic properties are counterbalanced). At this isoelectric point the molecule is said to be zwitterionic and its surfactant properties tend to be at their lowest. 13
14 Uses of Surfactants in Textile Processing Surfactants in textile process serve generally following purposes: a) Detergents b) Wetting Agents c) Dispersing Agents d) Retarding agents and Leveling agents for Dyeing Some of the other surfactant applications include, softeners, antistatic agents and water repellent etc
In water detergents have two purposes: a) Lower Surface Tension b) Remove Grease & Dirt Detergents The grease and dirt removing process can be referred as head and tail detergency and can be systematically represented as: a) Detergent molecules form micelles at their Critical Micelle Concentration (C.M.C) and set in their tail (hydrophobe) in the dirt or grease b) The hydrophilic heads repel each other and the lower surface tension allows water to penetrate between fiber and grease. This is made easier if there is some agitation occurring to cause fibers to bend and grease surface to be disrupt c) The droplet of emulsified grease separates from the fiber due to the net repulsion of the surfactant head groups on the dirt particles and the fiber surface, resulting in complete and lasting separation. 15
Applied Chemistry Abdul Wahab Jatoi Department of Textile Engineering, MUET 16
17 Wetting Agents Water without surfactant is poor in wetting textile materials due to high surface tension. If some textiles (such as polyester or polyester and cotton blends; PC) are dipped in to the water and removed, they may not be uniformly wet. If a surfactant is added into the water then the fabric will be uniformly wet. This is because the surfactant has lowered the surface tension of the water thus enabling thus enabling the water to wet the surfaces. Thus the surfactant molecules reduce surface tension of water so that the materials (textile fabrics) can be easily wet. Hence, it can be said that wetting agents, reduce surface tension of water and increase its tendency to be absorbed in the different surfaces. Textile materials treated with wetting agents are generally more absorbent. NOTE: All wetting agents are surfactants but all surfactants are not wetting agents
18 Dispersing Agents in Disperse Dyeing Many disperse dyes have low solubility in water and it is not possible to obtain solution of suitable concentration for dyeing (adequate dispersion) without dispersing agents. However, the dye must be introduced to the fiber as a fine, stable dispersion so as to obtain levelness and full colour yield. In this context, dispersing agents serve two purposes: a) to facilitate the breakdown of aggregated dye particles during dye manufacturer s grinding or milling of the dyes (dye manufacturing) b) to act as stabilizing agents in maintaining the dispersion of the dyes in the dye bath during the dyeing. The aqueous dispersion is so fine that they almost have the appearance of true solution The dispersing agents act to prevent agglomeration or precipitation of undissolved dyestuffs and hence produce level dyeing.
Retarding Agents and Leveling Agents Retarding agents are used in dyeing in order to achieve level dyeing. They slow down the exhaustion rate of dye molecules from liquor to fiber and prevent unlevel dyeing They are classified into two categories: a) Fiber Substantive b) Dye Substantive 19
Retarding Agents and Leveling Agents 20
Fiber Substantive v/s Dye Substantive Retarding Agents Dye Substantive Charge on the dye ion and retarder molecule is opposite Some of the dyes are complexed with retarder As dyeing proceeds uncomplexed dye concentrations reduces due t absorption of the dye by fiber Equivalent proportion of the dye/retarder complex break down to restore the dye concentration in the dye bath Dye Substantive Charge at the dye site in the fiber and charge on the retarder are opposite Retarder ion attaches with the dye site in the fiber, and few sites are available for dye molecule 21