Molecular Structure Characteristics of Surfactants
Surface Tension and Surface-Active Substances
Solutes that adsorb positively in aqueous solution reduce the surface tension of water, while solutes that adsorb negatively increase the surface tension of water.
Surface active materials (surface active materials),: a large group of substances that can produce positive adsorption in aqueous solution and thus significantly reduce the surface tension of water. For example, ethanol, propionic acid, sodium dodecyl sulphate, and so on.
The nature of such substances to reduce the surface tension of water is called surface activity (surfaceactivity); accordingly can not produce positive adsorption that can not reduce the surface tension of water is called non-surface-active substances, such as inorganic salts, glucose and so on.
Surfactants (surfactants): in the surface-active substances, there is a class of substances in a very low concentration can make the surface tension of water decreased significantly, but when the concentration increased to a certain value, the surface tension will no longer decline or very slowly decrease, people call this class of surface-active substances surfactants (surfactants), such as sodium dodecyl sulfate, dodecyl trimethyl ammonium bromide and nonyl bromide. Surfactants, such as sodium dodecyl sulfate, dodecyltrimethylammonium bromide and nonylphenol polyoxyethylene (9) ether and other substances.
Molecular Structure of Surfactants
The first surfactants used by humans were sodium (or potassium) salts of fatty acids, commonly known as soaps.
In the early days, people mixed plant and animal fats and grass ash aqueous solution and heated to make soap.
Later, with the progress of the chemical industry, caustic soda (NaOH), people through the alkali saponification of fats and oils to produce soap.
In the 1920s and 1930s, World War I led to a shortage of fats and oils, in order to develop soap substitutes, synthetic surfactants were born in Germany, such as alkyl benzene sulfonates, fatty alcohols sulfates, etc. These surfactants have common molecules.
These surfactant molecules have a common molecular structure, that is, the molecules contain both hydrophilic and lipophilic groups, for example, the hydrophilic group in soap is -COONa, and in sodium alkylbenzenesulfonate the hydrophilic group is -SO3Na, and the lipophilic groups are all long alkyl chains.
Nowadays, this kind of molecule is called amphiphilic molecule, in which the hydrophilic group is ionic amphiphilic molecule structure is very similar to matches, the ball part is hydrophilic group, the stalk part is alkyl group, is lipophilic group.
As shown in Figure 1
Figure 1 Schematic diagram of the ball-and-stick model of surfactants and their directional arrangement at the water/air (oil) interface.
The hydrophilic group makes the molecules have certain water solubility.
Lipophilic group makes this type of molecules have certain oil solubility.
When these molecules are in contact with water, the hydrophilic group in the molecule has a strong hydration with water molecules and leads to dissolution, while the lipophilic group in the molecule has a strong tendency to escape from the water environment due to the lack of affinity with water molecules, and these two diametrically opposite effects make the molecules enriched in the water/air interface or the water/oil interface, with the hydrophilic group in the aqueous phase and the lipophilic group in the air phase or the oil phase, in the interface. oriented arrangement, as shown in Fig. 1.
The combined result is a positive adsorption of the surfactant in aqueous solution, resulting in a significant decrease in the surface tension of water or the tension at the oil/water interface.
The molecular structure of surfactants has one thing in common, its molecule consists of two parts: one part is solvent-loving; the other part is solvent-loving.
One part is pro-solvent; the other part is solvent-hating (hydrophobic).
Because surfactants are usually used in aqueous solutions, it is often called the two parts of the surfactant hydrophilic group (polar part) and hydrophobic (hydrophobic) group (non-polar part), hydrophobic group is also called lipophilic group.
As shown in Fig. 2 (a).
Figure 2 Schematic diagram of the molecular structure of surfactants (a)
Size of CH3 (CH2)11SO4 (b)
Taking a common surfactant, sodium dodecyl sulphate [CH3(CH2)11SO4Na], as an example.
In aqueous solution, CH3(CH2)11 SO4Na is ionised to CH3(CH2)11SO4 and Na+, and it is CH3(CH2)11SO4, known as the surface-active ion, that plays the major role.
It is composed of the non-polar CH3(CH2)11-, which is a hydrophobic group (lipophilic), and the polar one SO4, which is a hydrophilic group. Na+, on the other hand, is called the counterion.The magnitude of CH3 (CH2)11SO4 is shown in Fig. 2 (b).
This particular structure of surfactants is called amphiphilic structure (hydrophilic group is hydrophilic and hydrophobic group is lipophilic). Thus surfactants are a class of amphiphilic compounds.
The hydrophobic group of a surfactant is generally composed of a long-chain hydrocarbon group, dominated by a hydrocarbon chain, while the hydrophilic group (polar group, head group) has a wide variety of groups, including charged ionic groups and uncharged polar groups.
All molecules of surface-active substances have an amphiphilic molecular structure. In terms of aqueous solution, the hydrophilicity of the hydrophilic group and the lipophilicity of the lipophilic group should be basically matched in order to have significant surface activity, either side is too strong or too weak will significantly weaken the amphiphilic molecules of surface activity.
When the number of carbon atoms is below 8, the hydrophilicity is too strong, for example, formic acid, acetic acid, propionic acid, butyric acid have amphiphilic structure, but only have surface activity, can not be surfactant.
The hydrophobic chain of a surfactant in the general sense should be large enough, generally above 8 carbon atoms (there is no strict limit).
On the contrary, when the number of carbon atoms is above 20, the lipophilicity is too strong, the solubility in water is very small, and it can not be a typical surfactant. For example, in the case of soap, fatty acids become good surfactants when their carbon atoms are in the range of 8 to 20.
Basic functions of surfactants
Surfactants are functional fine chemicals.
There are two most basic functions of surfactants:
The first is
Adsorption on the surface (boundary) surface, forming adsorption film (generally monomolecular film);.
The second is
Self-polymerisation within the solution, forming many types of molecularly ordered assemblies.
From these two functions, a variety of other
application functions.
Adsorption of surfactants on the surface results in a reduction of the surface tension and a change in the surface chemistry of the system.
Thus the surfactant has the functions of foaming, defoaming, emulsifying, demulsifying, dispersing, flocculating, wetting, spreading, penetrating, lubricating, antistatic, and bactericidal.
Surfactants self-polymerise within the solution to form various forms of molecularly ordered assemblies, such as micelles, anticolloids, vesicles, liquid crystals and so on. These molecularly ordered assemblies exhibit a wide variety of application functions.
The most basic of these is the solubilising (also known as solubilising) function of micelles.
Based on the solubilising effect of micelles and other molecularly ordered assemblies, functions such as micelle catalysis, formation of microemulsions, and use as spacer reaction media and microreactors, drug carriers, and so on, have been derived.
The detergent function of surfactants is also largely related to the solubilising effect of micelles on oil.
The size of surfactant molecularly ordered compositions or the thickness of aggregated molecular layers is close to the nanometre scale, which can provide a suitable place and conditions for the formation of ultrafine particles with “quantum size effect”.
Therefore, surfactant molecularly ordered assemblies can be used as templates (template function) for the preparation of ultrafine particles (e.g. nanoparticles).
The special structure of molecularly ordered assemblies makes them optimal for modelling biofilms.
Molecularly ordered assemblies can also be rearranged to form advanced ordered structures (supramolecular structures), whose solutions exhibit novel and complex phase behaviour or unusual rheological properties, optical properties, chemical reactivity and so on. Thus, they have other special application functions.
Principles of surfactant application in detergents
Application of surfactants in traditional fields
Surfactants have a series of superior properties such as wetting, emulsifying, dispersing, solubilising, foaming and defoaming, penetrating, washing, antistatic, bactericidal, etc. They have a wide range of applications in traditional civil fields such as detergents, cosmetics, personal hygiene products, etc., as well as in industrial and technological fields such as textiles, food, medicine and pesticides, paints and coatings, construction, mineral flotation, energy, pulp and paper, tanning and other industries.
In recent years, the development of high and new technology is changing day by day, and surfactants with their unique functions in nanotechnology, environmental protection; new materials, life sciences and other high-tech fields become indispensable products.
Application principle of surfactants in detergents
Detergents are daily-use chemicals that are formulated from surfactants, detergents and auxiliaries to remove dirt from the surface of objects and achieve the purpose of cleaning and cleaning.
Currently, detergents commonly used in the civil field mainly include laundry detergent and fabric softener for cleaning clothes, detergent and dishwashing liquid for cleaning tableware, fruits and vegetables, grease remover for the kitchen, toilet bowl cleaner for toilet cleaning, and other major categories.
Surfactant is the main ingredient of detergent, so detergent and surfactant, in addition to excellent washing and decontamination ability, but also has good wetting, foaming, emulsification, dispersion, solubilisation ability.
Early detergents often used a single surfactant, such as sodium alkylbenzene sulfonate. However, nowadays detergents generally use complex surfactants, such as anionic/anionic complex or anionic/nonionic complex.
Due to the synergistic effect between the surfactant blends, the detergents sometimes have good detergent detergency at lower surfactant contents.
The process of detergent decontamination and the related mechanism have been briefly introduced. The core of the process is to separate the dirt from the surface of the object to be cleaned through the physicochemical action of surfactants and the mechanical and aqueous action of the water flow, and to be carried away by the water flow.
The adsorption of the surfactant on the surface of the dirt and the substrate is the key, and this leads to the following series of basic actions.
(1) Penetration and wetting
During the washing process, surfactant molecules are able to adsorb onto the surface of the article and dirt, reducing the interfacial tension between the medium (generally water) and the surface of the article, as well as between the medium and the surface of the dirt, so that the medium is able to penetrate between the surface of the article and the dirt and penetrate into the interior of the article. This action is called the wetting penetration action of the detergent.
The wetting of the washing liquid on the washing article is a prerequisite for washing, if the washing liquid can not wet the article well, there is no good washing and decontamination effect.
The wetting penetration effect of the detergent solution not only reduces the attraction between the surface of the article and the surface of the dirt, but also reduces the attraction between the particles of the dirt, which can be broken into fine particles and dispersed in the medium when appropriate external force is applied.
Picture
(2) Emulsification and dispersion
In the washing process, with the help of the physical and chemical effects of surfactants and mechanical agitation, the oil is emulsified to form an O/W emulsion. Most aqueous solutions of detergents with good performance have low surface tension and oil/water interfacial tension.
In the lower interfacial tension at the same time, the surfactant in the oil / water interface to form a certain strength of the interfacial film, can prevent the aggregation of oil beads, is conducive to the stability of the emulsion, so that the oil stains are not easy to deposit on the surface of the goods again. The lower interfacial tension is conducive to the emulsification of the liquid dirt, and therefore is conducive to the removal of liquid dirt.
Of course, during the washing process, the liquid dirt is not directly dissolved in the medium, but under the action of the surfactant, it is firstly “coiled”, and then detached from the surface of the article under the action of the water flow and emulsified, and suspended in the medium.
Picture
(3) Solubilising effect
When the surfactant concentration is higher than the critical micelle concentration (cmc), micelles are formed in the solution. Some insoluble or slightly soluble substances in the aqueous medium will diffuse into the micelles, thus increasing their solubility in the medium significantly, this effect is called solubilisation of micelles.
Picture
In addition to emulsification, solubilisation of oil by micelles may be the other main way of removing liquid dirt from solid surfaces.
Non-polar oils are generally solubilised in the non-polar core of the micelles, while polar oils may be solubilised in the polar group region of the micellar shell depending on their polarity and molecular structure; in the case of amphiphilic oils, the polar group of the oil molecule will be “anchored” to the micelle surface, while the non-polar hydrocarbon chain will be inserted into the micellar core.
It has been shown that when the surfactant concentration is greater than cmc, the increase in detergency is very limited, so solubilisation is not a major factor in the washing process.
However, in the local washing process (such as clothing local smeared with soap or other detergents scrubbing, as well as washing the face and hands with soap, etc.), the amount of detergent is large, the surfactant concentration is very high, at this time the solubilisation of oil in the micelles will be the main mechanism of oil removal.
(4) Foaming effect
Detergent reduces the interfacial tension between the medium / air, so that the air can be dispersed in the medium and the formation of bubbles. At the same time the surfactant in the bubble surface to form a solid layer of directional arrangement of the film, to maintain the stability of the foam, which is foaming, bubble stabilisation.
Although the foam and washing effect has no corresponding direct relationship, but it can adsorb the dirt has been dispersed so that it gathered in the foam, and dirt to the surface of the media solution.
However, in the industrial washing process or family washing machine washing process, the emergence of foam gives the impression that it is not rinsed clean, the need to increase the number of rinsing, so the machine washing is often low-foam type detergent.