Wetting solution

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A typical wetting solution molecule consists of a hydrophilic head and long hydrophobic tail. (From top to bottom: non-ionic, anionic, cationic and zwitterionic.) TensideHyrophilHydrophob.png
A typical wetting solution molecule consists of a hydrophilic head and long hydrophobic tail. (From top to bottom: non-ionic, anionic, cationic and zwitterionic.)

Wetting solutions are liquids containing active chemical compounds that minimise the distance between two immiscible phases by lowering the surface tension to induce optimal spreading. The two phases, known as an interface, can be classified into five categories, namely, solid-solid, solid-liquid, solid-gas, liquid-liquid and liquid-gas. [1]

Contents

Although wetting solutions have a long history of acting as detergents for four thousand plus years, the fundamental chemical mechanism was not fully discovered until 1913 by the pioneer McBain. [2] [3] Since then, diverse studies have been conducted to reveal the underlying mechanism of micelle formation and working principle of wetting solutions, broadening the area of applications.  

The addition of wetting solution to an aqueous droplet leads to the formation of a thin film due to its intrinsic spreading property. This property favours the formation of micelles which are specific chemical structures consisting of a cluster of surfactant molecules that has a hydrophobic core and a hydrophilic surface that can lower the surface tension between two different phases. [4]

In addition, wetting solutions can be further divided into four classes; non-ionic, anionic, cationic and zwitterionic. [5]

The spreading property may be examined by adding a drop of the liquid onto an oily surface. If the liquid is not a wetting solution, the droplet will remain intact. If the liquid is a wetting solution, the droplet will spread uniformly on the oily surface because the formation of the micelles lowers the surface tension of the liquid. [6]

Wetting solutions can be applied in pharmaceuticals, [7] cosmetics [8] and agriculture. [9] Albeit a number of practical uses of wetting solutions, the presence of wetting solution can be a hindrance to water purification in industrial membrane distillation. [10]

History

Wetting agent was used as soap for cleansing purposes for thousands of years. The oldest evidence of wetting solution went back to 2800 BC in ancient Babylon. [2] The earliest credible reference of soap is in the writings of Galen, the Greek physician, around 200 AD. [11] Over the following centuries, wetting solutions mainly functioned as detergents due to their wetting properties. Despite the extensive use of wetting solutions, the underlying chemical mechanism remained unknown until the emergence of McBain's proposed theory in 1913. Founded on his research on how the electrical conductivity of a solution of surfactant molecules changed with concentration, he raised the possibility of surfactant molecules in the form of self-assembled aggregates. [3]  Not until Debye published his original hypothesis in 1949 did he described the reason of micelle formation and the existence of finite-shaped micelles. [12] [13] McBain's discovery sparked numerous studies by Hobbs, [14] Ooshika, [15] Reich [16] and Halsey [17] from 1950 to 1956. These scholars intended to correct some of the foundational theories of the description of an equilibrium system, as well as emphasising the role of surface energy which was overlooked in Debye's prototype. In 1976, the fundamental theory for understanding the mechanism of micelle formation was developed by Tanford's free energy model. [18] Apart from integrating all relevant physicochemical elements and explaining the growth of micells, he provided a comprehensive reasoning of why micelles are finite in terms of opposing interactional forces. [19] [20]

Mechanism

Micelles with a hydrophobic core formed by the non-polar tail of wetting solution molecules and are surrounded by a hydrophilic layer arising from the molecules' polar heads. Micelle scheme-en.svg
Micelles with a hydrophobic core formed by the non-polar tail of wetting solution molecules and are surrounded by a hydrophilic layer arising from the molecules’ polar heads.

The chemical structure of wetting solution molecules consist of a hydrophilic head and a long hydrophobic tail. Its distinct amphiphilicity allows it to bury its hydrophilic head in an aqueous bulk phase and hydrophobic part in the organic bulk phase respectively. [6] Wetting solution molecules break the intermolecular forces between each molecule in the organic phase and each water molecule in the aqueous phase by displacement. [5] Due to the lowered attractive forces, the surface tension is reduced. Upon adding more wetting solution, the elevated concentration of wetting solution molecules leads to a further decrease in surface tension and makes the molecules at the surfaces become more crowded. The molecules will be forced to remain in the aqueous phase when there are no more vacancies for them to stay on the surface. At this point, the surface tension is maximally lowered and is termed as the critical micelle concentration (CMC). [21] The lower the CMC, the more efficient the wetting solution is in reducing surface tension. Any additional wetting solution molecules will undergo self-aggregation into several special structures called micelles. Micelles are spheres with a hydrophobic core formed by the non-polar tail of wetting solution molecules and are surrounded by a hydrophilic layer arising from the molecules’ polar heads. [4] Extra wetting solution molecules will be forced to form micelles instead of adhering to the surface, hence the surface tension remains constant. Due to the minimised surface tension, the droplet can now spread thoroughly and form a thin film on the surface. [4]

Classification

Generally, the wetting solution molecules consist of a hydrophilic head and a long hydrophobic tail. The hydrophobic region usually contains saturated or unsaturated hydrocarbon chains, heterocyclic rings or aromatic rings. [5] Despite the similar amphiphilic composition, the molecules can be divided into four classes with respect to the nature of the hydrophilic group, namely, non-ionic, anionic, cationic and zwitterionic.

The following table shows the composition, special features of the corresponding classes and common examples of various forms of the respective wetting solutions.

Wetting solutionCompositionSpecial featuresCommon examples
Non-ionicapoly (oxyethylene) chain as the hydrophilic region, in the absence of ionic groups [5] Synthetic forms of fatty alcohols, fatty acids, fatty amines, alkylphenols and polymers [22] alkylphenolethocylate, Tween 80 [22]
Anionic sulphate, carboxylate, sulfonate or phosphate ionic head accompanied by sodium or potassium as counterions to enhance aqueous solubility [5] sodium and potassium ions can be substituted by magnesium or calcium ions for higher oil solubility [5] sodium dodecyl sulphate (SDS), alkylbenzene sulfonate [5]
Cationic amine or charged quaternary ammonium cation as the head group [5] amine-containing molecules can only be regarded as wetting agents under low pH after protonation [23] alkyltrimethylammonium bromide, cetylpyridinium chloride (CPC) [23]
Zwitterionicmore than one polar ionic head of opposite charges, the positively charged head group is mostly ammonium cation while the negative charge is carried by a carboxylate anion [5] N-alkyl derivatives of amino acids like glycine and aminopropionic acid [24] Alkyl betaine, phosphatidylcholine (lecithin) [24]

Applications

Generally, wetting solution is applied in pharmaceuticals, [7] cosmetics [8] and agriculture. [9] McBain’s research on maximising the application of wetting solutions have an important role in enabling a range of options to both manufacturers and consumers and improving product performance in the respective areas of application, such as modifying the stability of pharmaceuticals, delivery of drugs, effectiveness of cleansing products and water retention in soils.

Pharmaceuticals

Specific properties of different wetting solutions are able to alternate drug delivery which is beneficial in improving drug safety and patients' experiences . For example, solulan C-24, a non-ionic wetting solution, forms large bilayers of wetting solution molecules known as discosomes that have a lower risk of causing systemic adverse effects. [7] [25] Non-ionic wetting solutions are found to have a wider usage and are more efficient in reducing surface tension compared to ionic wetting solutions which have higher toxicity and CMC value in general. [7] To ensure the safety, efficacy and quality of the preparations, toxicity and interaction profiles of the choice of wetting solutions are carefully investigated. [7]

Dosage form: Suspensions

Suspension preparation is a liquid dosage form that contains insoluble solid drug particles. [7] The suspension preparation is ideal if solid particles that have become compacted together during storage can re-disperse throughout the liquid vehicle readily with gentle shaking for a period of time that is sufficient for measuring the required dosage. [26]

Solid particles have a natural tendency to aggregate and eventually cause caking due to the presence of air film coating. [7] A solution to this is using a wetting solution as the liquid vehicle for suspension preparation. [7] Wetting solution increases the dispersal ability of the solid particles by replacing the air film to increase steric hindrance and minimise interactions between solid particles and resulting in a decreased rate of aggregation. [7] [27]

Topical ophthalmic solutions

Wetting solutions lowers the surface tension of topical ophthalmic solutions and induces instant spreading when applied onto the cornea by increasing the interaction between the two. [7] The instant spreading increases the amount of drug molecules that are exposed to the cornea for absorption and therefore a quicker onset of action. [7] The increased interaction allow the topical ophthalmic solutions to remain on the corneal surface for a longer period of time to maximise the amount of drug that can diffuse from the applied topical ophthalmic solution layer to the corneal epithelium through tear film, the protective layer of the cornea from the external environment. [7] [28]

Cosmetics: Skin cleansing products

Skin cleansing products including facial cleanser, body wash and shampoo consist of wetting solutions. [8] Wetting solutions allow efficient spreading and wetting of the surface of skin and scalp by reducing the surface tension between the hydrophobic sebum secreted by the sebaceous gland in our skin. [8] An efficient wetting solution penetrates the skin and clears any topical applications, body fluids including sebum secreted via openings of hair follicles, dead skin cells and microbes. [8]

Non-ionic wetting solutions have a low risk of causing skin irritation and are efficient in reducing surface tension between different ingredients, for example, fragrance and essential oils extracted from plants, in skin cleansing products to produce a consistent liquid formula. [8] However, non-ionic wetting solutions are of higher cost than the other types of wetting solutions hence are less favourable for commercial products. [8]

Cationic wetting solutions cause more severe skin irritation problems hence are not used in skin cleansing products. [8] They are used in hair conditioners that are only applied to the second half hair length and washed off after a short period of time. [8]

Anionic and amphoteric wetting solutions are often used as a mixture in body wash and shampoo. [8] The anionic wetting solutions formulated into skin cleansing products have often undergone chemical modification as they often contain sulphur which triggers skin irritation by causing collagen in skin cells to swell and sometimes cell death. [8] [29] Examples of modified anionic wetting solutions include ammonium laureth sulphate and modified sulfosuccinates, both reported to exhibit low skin irritation. [8] [29]

Agriculture

Wetting solutions are widely used in Agriculture to increase crop yield which is affected by the degree of infiltration and penetration of water, nutrients and chemicals such as fertilisers and pesticides. [9] [30] Wetting solutions reduce surface runoff of water and nutrients and enhance water infiltration in water repelling soil by reducing surface tension. [31] Wetting-solution-treated soil has shown to retain high water content and an even distribution of nutrients in the root zone that are in deep soil areas, benefiting crop yield and improving water efficiency. [31]   Examples of wetting solutions used in agriculture are modified alkylated polyol, mixture of polyether polyol and glycol ether and mixture of poloxalene, 2-butoxyethanol. [30]

Industrial concerns

Membrane distillation is a water purification process that utilises a hydrophobic membrane with pores to separate water vapour from contaminants, for example, oil and unwanted chemicals. [10] The filtration efficiency and stability of the membrane can be diminished by wetting. [10] [32] Wetting of the hydrophobic membrane is resulted from the presence of wetting solution in sewage due to its increasing large variety of usage in different fields, for example, pharmaceuticals, cosmetics and agriculture. [10] A possible solution is to pretreat the sewage to remove wetting solutions, limiting the amount of wetting solution in contact with the membrane. [32] Other possible solutions to lengthen durability of the membrane include modification of the membrane material repellent to water and oil, air-backwashing and membrane surface geometry modification. [10] [32] [33] These solutions are costly and require further research and development to optimise the durability and efficiency of membrane distillation. [32]

Related Research Articles

<span class="mw-page-title-main">Emulsion</span> Mixture of two or more immiscible liquids

An emulsion is a mixture of two or more liquids that are normally immiscible owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion should be used when both phases, dispersed and continuous, are liquids. In an emulsion, one liquid is dispersed in the other. Examples of emulsions include vinaigrettes, homogenized milk, liquid biomolecular condensates, and some cutting fluids for metal working.

<span class="mw-page-title-main">Detergent</span> Surfactants with cleansing properties

A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more soluble in hard water, because the polar sulfonate is less likely than the polar carboxylate to bind to calcium and other ions found in hard water.

<span class="mw-page-title-main">Surfactant</span> Substance that lowers the surface tension between a liquid and another material

Surfactants are chemical compounds that decrease the surface tension or interfacial tension between two liquids, a liquid and a gas, or a liquid and a solid. The word "surfactant" is a blend of surface-active agent, coined c. 1950. As they consist of a water-repellent and a water-attracting part, they enable water and oil to mix; they can form foam and facilitate the detachment of dirt.

<span class="mw-page-title-main">Micelle</span> Group of fatty molecules suspended in liquid by soaps and/or detergents

A micelle or micella is an aggregate of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension. A typical micelle in water forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre.

Lipophilicity is the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such compounds are called lipophilic. Such non-polar solvents are themselves lipophilic, and the adage "like dissolves like" generally holds true. Thus lipophilic substances tend to dissolve in other lipophilic substances, whereas hydrophilic ("water-loving") substances tend to dissolve in water and other hydrophilic substances.

<span class="mw-page-title-main">Hydrophobic effect</span> Aggregation of non-polar molecules in aqueous solutions

The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and to be excluded by water. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar substances, which maximizes the entropy of water and minimizes the area of contact between water and nonpolar molecules. In terms of thermodynamics, the hydrophobic effect is the free energy change of water surrounding a solute. A positive free energy change of the surrounding solvent indicates hydrophobicity, whereas a negative free energy change implies hydrophilicity.

An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of the twentieth century. A wide variety of synthetic membranes is known. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. Most commercially utilized synthetic membranes in industry are made of polymeric structures. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. The chemical and physical properties of synthetic membranes and separated particles as well as separation driving force define a particular membrane separation process. The most commonly used driving forces of a membrane process in industry are pressure and concentration gradient. The respective membrane process is therefore known as filtration. Synthetic membranes utilized in a separation process can be of different geometry and flow configurations. They can also be categorized based on their application and separation regime. The best known synthetic membrane separation processes include water purification, reverse osmosis, dehydrogenation of natural gas, removal of cell particles by microfiltration and ultrafiltration, removal of microorganisms from dairy products, and dialysis.

<span class="mw-page-title-main">Amphiphile</span> Hydrophilic and lipophilic chemical compound

An amphiphile, or amphipath, is a chemical compound possessing both hydrophilic and lipophilic (fat-loving) properties. Such a compound is called amphiphilic or amphipathic. Amphiphilic compounds include surfactants. The phospholipid amphiphiles are the major structural component of cell membranes.

<span class="mw-page-title-main">Dipalmitoylphosphatidylcholine</span> Chemical compound

Dipalmitoylphosphatidylcholine (DPPC) is a phospholipid (and a lecithin) consisting of two C16 palmitic acid groups attached to a phosphatidylcholine head-group.

<span class="mw-page-title-main">Surfactin</span> Chemical compound

Surfactin is a cyclic lipopeptide, commonly used as an antibiotic for its capacity as a surfactant. It is an amphiphile capable of withstanding hydrophilic and hydrophobic environments. The Gram-positive bacterial species Bacillus subtilis produces surfactin for its antibiotic effects against competitors. Surfactin showcases antibacterial, antiviral, antifungal, and hemolytic effects.

A hydrotrope is a compound that solubilizes hydrophobic compounds in aqueous solutions by means other than micellar solubilization. Typically, hydrotropes consist of a hydrophilic part and a hydrophobic part, but the hydrophobic part is generally too small to cause spontaneous self-aggregation. Hydrotropes do not have a critical concentration above which self-aggregation spontaneously starts to occur. Instead, some hydrotropes aggregate in a step-wise self-aggregation process, gradually increasing aggregation size. However, many hydrotropes do not seem to self-aggregate at all, unless a solubilizate has been added. Examples of hydrotropes include urea, tosylate, cumenesulfonate and xylenesulfonate.

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<span class="mw-page-title-main">Lyotropic liquid crystal</span>

Lyotropic liquid crystals result when amphiphiles, which are both hydrophobic and hydrophilic, dissolve into a solution that behaves both like a liquid and a solid crystal. This liquid crystalline mesophase includes everyday mixtures like soap and water.

Sodium lauroyl sarcosinate (INCI), also known as sarcosyl, is an anionic surfactant derived from sarcosine used as a foaming and cleansing agent in shampoo, shaving foam, toothpaste, and foam wash products.

Hydrophobicity scales are values that define the relative hydrophobicity or hydrophilicity of amino acid residues. The more positive the value, the more hydrophobic are the amino acids located in that region of the protein. These scales are commonly used to predict the transmembrane alpha-helices of membrane proteins. When consecutively measuring amino acids of a protein, changes in value indicate attraction of specific protein regions towards the hydrophobic region inside lipid bilayer.

<span class="mw-page-title-main">Peptide amphiphile</span>

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<span class="mw-page-title-main">Hydrophile</span> Molecular entity that is attracted to water

A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.

Paint has four major components: pigments, binders, solvents, and additives. Pigments serve to give paint its color, texture, toughness, as well as determining if a paint is opaque or not. Common white pigments include titanium dioxide and zinc oxide. Binders are the film forming component of a paint as it dries and affects the durability, gloss, and flexibility of the coating. Polyurethanes, polyesters, and acrylics are all examples of common binders. The solvent is the medium in which all other components of the paint are dissolved and evaporates away as the paint dries and cures. The solvent also modifies the curing rate and viscosity of the paint in its liquid state. There are two types of paint: solvent-borne and water-borne paints. Solvent-borne paints use organic solvents as the primary vehicle carrying the solid components in a paint formulation, whereas water-borne paints use water as the continuous medium. The additives that are incorporated into paints are a wide range of things which impart important effects on the properties of the paint and the final coating. Common paint additives are catalysts, thickeners, stabilizers, emulsifiers, texturizers, biocides to fight bacterial growth, etc.

Polyelectrolytes are charged polymers capable of stabilizing colloidal emulsions through electrostatic interactions. Their effectiveness can be dependent on molecular weight, pH, solvent polarity, ionic strength, and the hydrophilic-lipophilic balance (HLB). Stabilized emulsions are useful in many industrial processes, including deflocculation, drug delivery, petroleum waste treatment, and food technology.

In colloidal chemistry, the critical micelle concentration (CMC) of a surfactant is one of the parameters in the Gibbs free energy of micellization. The concentration at which the monomeric surfactants self-assemble into thermodynamically stable aggregates is the CMC. The Krafft temperature of a surfactant is the lowest temperature required for micellization to take place. There are many parameters that affect the CMC. The interaction between the hydrophilic heads and the hydrophobic tails play a part, as well as the concentration of salt within the solution and surfactants.

References

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