What is Surfactants?
The word Surfactant is coined from the expression “surface active agent”. As the phrase implies, a surfactant molecule possesses surface activity, a property associated with the chemical structure of the molecule. The characteristic feature of a surfactant molecule is its two ends attached by a covalent bond. The two ends have diametrically opposed polarities. The non-polar end is lyophilic (strongly attracted to organic molecules) while the strongly polar end is lyophobic (having little attraction for organic molecules) yet strongly hydrophilic (water loving). Duality of polarity causes the molecule to align itself with respect to the polar nature of the surfaces it contacts.
Surfactants contain two different structural elements:
a. A hydrophilic part that is polar and responsible for molecular interactions with water molecules. Polar groups can bear a positive or negative charge, or contain a number of polar groups (e.g., polyethylene oxides).
b. A hydrophobic part that exhibits low polarity and thus is not able to interact with water molecules. Representatives for nonpolar groups are long chain hydrocarbons, perfluoroalkyl groups and alkylaromates.
Classification of Surfactants:
Surfactants are classified according to use, to ionic charge and to chemical structure.
A. By Use:
Wetting Agents:
The ability of a liquid to spread on a smooth solid surface is dependent on the polar nature of the solid and the surface tension of the liquid. More about the thermodynamic relationships will be brought up in the chapter on water and oil repellents. A non-polar solid surface such as paraffin wax or Teflon will cause a drop of pure water to bead-up and not spread. Water containing surfactants on the other hand will easily spread on paraffin surfaces and have lower contact angles on Teflon. Surfactants used this way are called wetting agents, or penetrating agents when used to wet out repellent fabrics.
Penetration of fabrics is a function of surface wetting, however fabric assemblies have a volume of air entrapped in the void formed spaces. Before a liquid can move in, the air must move out. Penetrants facilitate this process.
Detergents:
Detergents are surfactants that help remove soils from solid surfaces. Over and above reducing water’s surface tension, detergents must adsorb onto the soil’s surface to aid in spontaneous release. Detergents must also keeps the soil suspended to prevent redeposition.
Emulsifying Agents:
Emulsifying Agents are surfactants that convert water-insoluble oils into stable, aqueous suspensions. The lyophilic part of the surfactant molecule is absorbed by the oil droplet and the lyophobic head is oriented outward, surrounding the droplet with a hydrophilic sheath. Ionic surfactants add another dimension to the stability of emulsions, they set up a charge-charge repulsion field which adds to keeping the droplets separated.
Dispersing Agents:
Dispersing Agents function in a manner similar to emulsifying agents. The difference is that solid particulate matter, rather than insoluble oils, is dispersed. The nature of the lyophilic part of the surfactant molecule must be such that it adsorbs onto the particle’s surface. Surfactant molecules must be matched with their intended use.
B. By Ionic Charge:
a. Anionic surfactants: This is the major group comprising almost 70% of commercial surfactants. Those that develop a negative charge on the water solubilizing end. In anionic surfactants, the surface-active molecule contains at least one negatively charged group. The positive counter ion can be, for example, Na+, K+ or a quaternary ammonium ion.
b. Cationic surfactants: Those that develop a positive charge on the water solubilizing end. These surfactants bear a positively charged group in their active molecule part. Counter ions can be Cl− and Br−. These comprise a small group of surfactants that are mainly quaternary ammonium or pyridinium salts.
c. Non-Ionic surfactants: This is the second most important group of surfactants. They are mainly chemicals whose molecules have the usual hydrophobic ‘tail’ connected to a short polyethylene oxide chain. Those that develop no ionic charge on the water solubilizing end. Surface-active properties also can be achieved by the presence of larger molecules that exhibit a certain polarity.
d. Zwitterionic/ Amphoteric surfactants: Those that have both a positive and negative charged group on the molecule. Surfactants that bear a positive and a negative charge in the surface-active part are called zwitterionic and amphoteric, respectively. Such surfactants do not require a small counterion for reasons of charge neutrality as charges of both sign are present in the molecule.
Uses of Surfactants in Textile Industry:
Surface‐active agents are widely used in the manufacture of dyes, in dyeing processes and in preparatory processes for dyeing. Many types of surface‐active agents exist and they have different names depending upon their function:
Detergents are used in washing and scouring processes. Some are called wetting agents and are incorporated into formulations of dye powders to aid wetting out when the powder is being dissolved in water and also to aid the wetting of fibers prior to dyeing operations.
Levelling and retarding agents are also surfactants, these types being used in the application of ionic dyes to fibers.
Dispersing agents are another type of surfactant and are used in the formulation of disperse dyes, whose function is to ensure a stable dispersion of the dyes in the dyebath during the dyeing process, aiding uniform uptake of dye by the fiber.
Although surfactants in the textile wet processing industry are used mainly as wetting agents, detergents or emulsifying and dispersing agents, certain types of surfactants are used as softeners, lubricants, and anti-static and anti-bacterial agents. Their use for controlling the rate of dye uptake, and thus the levelness of the dyeing, has increased considerably, in direct relation to the demands for improved fastness to washing that require the use of dyes of poor migration.
Cationic auxiliaries are also used to aftertreat dyeings with anionic dyes.
In most applications, and particularly in dyeing, the role of auxiliary products is not well understood for a number of reasons:
- It is difficult to evaluate their concentration in a given product;
- Commercial products invariably have unknown formulations and the number and nature of the chemical constituents is rarely clear;
- There has been little fundamental work on the mode of action of various auxiliary products.
References:
- Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar
- Basic Principles of Textile Coloration By Arthur D Broadbent
- Textile Chemistry By Thomas Bechtold and Tung Pham
- An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
- Colorants and Auxiliaries (Second Edition) Volume 2 – Auxiliaries Edited by John Shore
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- What is Stabilizer | Working Process of Stabilizing Agent in Textile Wet Processing
- What is Dispersing Agent | Functions and Trade Names of Dispersing Agents
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
