APPLIED CHEMISTRY

SURFACE TENSION, SURFACTANTSTYPES OF SURFACTANTS & THEIR USES

IN TEXTILE PROCESSING

Lecture No. 13 & 14

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

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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

Surface Tension….

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• 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

Unit of Surface Tension

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Liquid Temperature °CSurface tension,

γAcetic 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

Surface Tension (dynes/cm)

Surface tension decreases with increase intemperature

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• 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

Interfacial Tension

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SURFACTANTS

The word “surfactant” is derived from theterm “surface active agents”

The surfactants are molecules thatreduce:

a) surface tension of a liquid

b) interfacial tension between twoliquids

c) interfacial tension between a liquidand a solid

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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 (C8 to C18), 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 naturalfats and oils or petroleum fraction (synthetic polymers from petrochemicals)

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a) Anionic Surfactants

b) Cationic Surfactants

c) Non-ionic Surfactants

d) Amphoteric Surfactants

Types of Surfactants

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• Anionic surfactants contain –vely chargedhead (hydrophilic group)

• These are most widely used type ofsurfactants in industrial applications due totheir low cost of manufacturing and usedalmost all surfactant applications.

• Most commonly used hydrophilic groups arecarboxylates, sulphates, sulphonates andphosphates. Their general formula are givenbelow:

Anionic Surfactants

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• Cationic surfactants contain +velycharged head (hydrophilic group)

• Use of these surfactants is smallcompared to anionic and nonionicsurfactants.

• Owing to their ability to adhere to andmodify solid surfaces, they are also usedas corrosion inhibitors, germicides andin hair conditioners.

Cationic Surfactants

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• Nonionic surfactants contain unchargedhead (hydrophilic group)

• Their basis is long chain alcohols,reacted with ethylene oxide producing apolymeric ether chain ending in anhydroxyl (OH) group, giving highsolubility and stability in both acetic andalkaline conditions.

• General formula of nonionic surfactantsis given below:

Nonionic Surfactants

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• Amphoteric surfactants contain both an anionic and cationic groups

• In acetic media they tend to behave as cationic agents and in alkaline mediaas anionic agents.

• Commonly at pH=7, lies isoelectric point (point at which anionic and cationicproperties are counterbalanced).

• At this isoelectric point the molecule is said to be zwitterionic and itssurfactant properties tend to be at their lowest.

Amphoteric (zwitterionic) Surfactants

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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

Uses of Surfactants in Textile Processing

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Detergents

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In water detergents have two purposes:

a) Lower Surface Tension

b) Remove Grease & Dirt

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.

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Jatoi Department of Textile Engineering, MUET

• 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

Wetting Agents

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• 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.

Dispersing Agents in Disperse Dyeing

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• 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

Retarding Agents and Leveling Agents

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Retarding Agents and Leveling Agents

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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

Fiber Substantive v/s Dye SubstantiveRetarding Agents

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