Gemini surfactants: A novel gift to paint industry

Excerpt: The surfactants are molecules containing both hydrophobic and hydrophilic portion which reduce the surface tension of water by absorbing at the liquid-gas or liquid-liquid interface.


The surfactants or surface active agents can be described as molecules containing both hydrophobic and hydrophilic portion which are able to reduce the surface tension or interfacial tension of water by absorbing at the liquid-gas or liquid-liquid interface. Apart from conventional monomeric cationic surfactants relatively new category of surfactant known as “Gemini surfactants” has been extensively studied in past two decades. Gemini surfactant is the family of surfactant molecules possessing more than one hydrophobic tail and hydrophilic head group. These surfactants usually have better surface-active properties than corresponding conventional surfactants of equal chain length. This review deals with synthesis, structure, critical micellar concentration, surface active properties and uses of geminis.

Key Words: Surfactants, Dimeric surfactant, critical micelle concentration.


SURFACTANTS or 'surface active agents' are organic compounds, which consist of two parts, which are lyophilic (solvent loving) and lyophobic (water loving) group in the molecule. The lyophobic part is usually a long chain, which is called tail, while the lyophilic part usually is much shorter and is called head of surfactant. Depending on their electrical properties surfactants could be ionic (part of molecules is charged) and non-ionic (whole molecule is uncharged) or zwitterionic, where surfactant molecule contain both cationic and anionic group. Figure 1 depicts examples of surfactants with various hydrophilic head groups (1)

Figure 1: Surfactant examples

Surfactants show interesting interfacial and bulk properties (2) and have a wide variety of uses (3), which are mostly met by conventional representatives. Changes in the molecular structure and type to improve upon their properties have attracted the attention of chemists (4,5,6,7). This has led to the preparation of new generation surfactants such as Geminis. Conventional surfactant has a single hydrophobic tail connected to an ionic or polar head group, whereas a gemini has in sequence a long hydrocarbon chain, an ionic group, a spacer, a second ionic group and another hydrocarbon tail. A schematic representation of gemini surfactant along with conventional surfactant are given in Figure 28. Related surfactants with more than two tails are also known to provide greater surface activity, higher adsorption efficiency, superb solubilizing, foaming and wetting properties as compared to conventional surfactants (9), which were later renamed as 'gemini' surfactants. Geminis are considerably more surface-active than conventional surfactants. Menger and Littau10 assigned the name gemini to bis-surfactants with rigid spacer (i.e. benzene, stilbene). The first report on dimeric or gemini surfactants in the scientific literature is that Bunnton et al in 1971. These researchers synthesized bisquaternary ammonium bromide dimeric surfactants and studied the micelle behaviour of these surfactants (11).

Figure 2: Systematic representation of Gemini surfactant (a) and conventional surfactant (b)

The dimeric surfactants act mainly as conventional surfactants, with agnate arrangement of phases and aggregates structures; however, they also demonstrate some intriguing contrasts. They display some exceptional physico-chemical properties that may demonstrate valuable for specialized applications in the future. The current interest in such surfactants arises from three vital properties:

  • Dimeric surfactants are characterized by critical micelle concentrations (CMC) that are one to two orders of magnitude lower than corresponding monomeric or conventional surfactants (12).
  • Dimeric surfactants are significantly more efficient than the conventional surfactants at decreasing the surface tension of water (13,14).
  • Aqueous solution of some dimeric surfactants with short spacers can have astounding rheological properties at relatively low surfactant whereas the solution of the corresponding monomers remains less viscous (15).

There are undoubtedly numerous new dimeric surfactants that will continue to be manufactured later on, with a sturdy thrust towards asymmetric surfactants and functional spacers. Okahara et al emphasized that low CMCs and high effectiveness of anionic dimeric surfactants can be manifested by their ability to decrease the surface tension of water, and modifying micelle structural characteristics16. These potential properties led to a renewed interest of the industrial community in these surfactants. Numerous surfactant-delivering organizations now have progressive research on dimeric surfactants (17,18).

Classification of dimeric surfactants

The dimeric surfactants can also be classified on the basis of charge present on the head groups. The four basic classes of dimeric surfactants are defined as follows:

Anionic dimeric surfactants

The anionic dimeric surfactants are used in most surfactants and detergents based formulations. Commonly used anionic groups are sulfonates, sulfates, carboxylates and phosphates. Anionic dimeric surfactants have two hydrophobic tails and two anionic head groups. The spacer can be polar or nonpolar, short aliphatic chain or methylene units. The very first anionic dimeric with bis phosphate head groups was reported by Menger and Littau (1991), as shown in Scheme 1.

Cationic dimeric surfactants

Cationic dimeric surfactants generally contain two chains and two quaternary ammonium compounds. These surfactants are more expensive to produce and therefore used in more specialized applications, for example, in disinfectant formulations, fabric softeners, corrosion inhibitors and so forth. Commonly, cationic dimeric surfactants are prepared by heating with alcoholic solutions of alkyldimethylamine and a, -alkylene-dihalide or of N,N,N,N-tetramethyl-a, -alkylenediamine and alkyl halide in order to accomplish the quaternization of the amine (Scheme 2).

Nonionic dimeric surfactants

Another important class of dimeric surfactants is nonionic dimeric surfactants. These amphiphilics are ranked after the anionics in terms of industrial significance and often comprise repeating ethylene oxide units or carbohydrates as a hydrophilic group. A dimeric surfactant with sugar moiety and an ester linkage as a spacer group is depicted in Figure 3.

Zwitterionic dimeric surfactants

Zwitterionic dimeric surfactants are also known as amphoteric dimeric surfactants that possess both cationic and anionic groups, and normally induce lower skin irritation and exhibit nice compatibility with other amphiphilics (Figure 4).

Various properties of anionic dimeric surfactants

Dimeric surfactants show unusual surface and micellar properties and have a wide variety of uses, which are mostly superior to conventional representatives. The main focus of this work is on anionic dimeric surfactants; therefore some vital properties of anionic dimeric surfactants are briefly discussed here under:

Surface properties (surface tension, CMC and C20 value)

Surface tension is defined as the force that acts on the surface of a liquid and tends to minimize the surface area. The cohesive forces among liquid molecules are responsible for the phenomenon of surface tension. The molecules of a liquid attract each other due to hydrogen bonding. In the bulk liquid a molecule senses the same attractive and repulsive forces in all directions, while for a molecule at the surface those forces are lacking in one direction. This asymmetry of forces is the source of the surface energy or equivalent to the surface tension21. There are three major methods for determination the surface tension of air-liquid interfaces and the interfacial tension of oil-water interfaces; force methods, pressure methods and shape methods22. Force methods i.e., Du Noüy method and Wilhelmy plate method are commonly used to measure the surface or interfacial tensions23. The surface tension of water i.e. 72 dyne/cm at 25ºC is decreased to 30-40 dyne/cm at the CMC of surfactant. Surface tension has broad industrial applications and anionic dimerics are much surface active than traditional surfactants. Because of higher surface activity of dimerics, they are most beneficial for industrial uses, such as detergency and emulsification etc. Several anionic dimeric surfactants, e.g., (CH2)2[N(COCnH2n+1) CH(COOH)CH2COOH]2NaOH, (n+1 = 8,10,12,14 and 16) were studied24 and compared to those of the monomeric surfactant, sodium- Ndodecanoylsarconsinate (SDSa). They found that the surface tension at CMC of dimeric surfactants was much lower than that of SDSa. Yoshimura et. Al. also observed that anionic dimeric, N,N-di(3-perfluoroalkyl-2- hydroxypropyl)-N,N-diacetic acid ethylenediamine showed 18.4 mN/m surface tension at CMC (20).

One of the most significant properties of surfactants is their capacity to aggregate in solutions. The critical micelle concentration (CMC) is the concentration above which surfactant molecules suddenly assemble into aggregates called micelles. Davis and Bury (1930) coined the term CMC, characterizing it as the threshold concentration at which micelles first appear in solution. The determination of the CMC value for a surfactant can be made using the break point in the surface tension/electrical conductivity/light scattering, or fluorescence spectroscopy versus surfactant concentration curves. The determination of the CMC via numerous different methods was illustrated by Preston's classic graphs [Preston (1948)] shown in Figure 1.5. The CMC has also been measured from the change in the spectral characteristics of some dyestuff added to the surfactant solution when the CMC of the latter is reached; this method is known as Dye solvatochromism method [Christian and Thomas (2010)]. The CMC is useful as it demonstrates the tendency of surfactants to assemble in water. Due to the greater amount of hydrocarbon per molecule, the CMCs of dimerics are typically one or more order of magnitude smaller than those of the corresponding conventional surfactants. One review article [Hait and Moulik (2002)] has also demonstrated that various types of anionic dimerics showed maximum reduction in the CMC values and the CMC of anionic dimerics reduces with increasing spacer length. Yoshimura et al. (2004) prepared anionic dimerics 1,2-bis(N-ß-carboxypropanoyl-N-alkylamino)ethane with two carboxylate groups and found that anionic dimerics show much less CMC and high efficiency in lowering the surface tension. There are various factors that affect the CMC value of a surfactant. These include the structure of the surfactant or amphiphile chain length, the structure of the head group, the presence of added salts and alcohol in the solution, the presence of various organic compounds, the temperature of the solution, the structure of the alkyl chain and polar additives.

Efficiency and effectiveness of a surfactant can be measured by another concentration. e.g., C20 value that signify the surfactant concentration in the aqueous medium that produces a 20 dyne/cm reduction in the surface tension of the solvent25. Micellization behavior of dimeric surfactants is schematic presented in Figure 5.

Several anionic dimeric surfactants with different spacer groups26 were investigated and it was found that, these anionic dimerics exhibited lower C20 and CMC values are compared with those conventional ones and showed higher surface activity.

Foaming ability

The foam performances of the products according to the specific demands during the application play a major role. In detergents or cleaning process, systems with rather low or no foaming abilities are required. On the other hand, in the cosmetics and toiletries products, the foam performance of hair and body care products have to be optimized so that consumer will experience a most affirmative and sensorial impression over the product. Surfactants are very famous ingredients for the manufacturing of toothpaste; in toothpaste these compounds are used to create foam while teeth are brushed. Various parameters such as surface tension, surface viscosity and surface elasticity are amenable for the adjustment of the required foam properties of the system [Kumar and Tyagi (2013)]. Anionic dimeric surfactants having carboxylate head groups of structure (C11H23)2C(OCHCOONa)2 demonstrated higher foam ability than its conventional analogue. C11H23COONa [Ono et al (1993)]. Anionic dimerics of structure [C10H21N(COCH2CH2COO-Na+)CH2]2CHOH, had shown much better foaming behaviour at 0.1% concentration of the sodium laurate and no irritation to the skin27. Acharya et al (2005) documented that small concentration of anionic dimeric, sodium 2,3-didodecyl-1,2,3,4-butane tetracarboxylate in aqueous solution of monomeric surfactant (SDS) increases the foam stability markedly and results in a dry foam that lasts even after 24 hrs.

Effect of salts

It is well known that salts affect the physio-chemical properties of aqueous mixtures of surfactants solution. These interactions promote relationship of surfactant ions that encourage formation of micelles, leading to higher surface activity [Villeneuve et al. (1999)]. Different factors acting on salt addition, the formation and development of micelles are primarily supported by the screening of electrostatic repulsion among the polar head groups and movement of the hydrophobic alkyl chains far from the aqueous environment. This is confirmed by a decrease in CMC28. If salts are added into surfactant solution, salting-out phenomenon often occurs; this is the result of the preferable movement of water molecules from coordination shells of surfactant molecules to those of salts [Grover and Ryall (2005) and Wattebled and Laschewsky (2007)]. The impact of inorganic salts on surfactant solutions have been usually discussed in terms of electrostatic interactions and changes in the water structure. The organic salts have strong tendency to affect the micelle behavior as compared to the inorganic electrolytes. Organic electrolytes containing benzene ring infiltrate into micelles by impelling strong hydrophobic interaction and consequently reducing electrostatic repulsion between the hydrophilic head groups, which offer climb to tight packing and possible reduced curvature of surfactant aggregates. Organic salts typically known as hydrotropes are ordinarily short amphiphilic molecules that increase the solubility of a variety of hydrophobic compounds in water29. Numerous salts with hydrophobic counter-ions for instance sodium tosylate, sodium salicylate and sodium benzoate are significantly effective to micellar growth even at less concentration.

Application of anionic dimeric surfactants

Anionic dimeric surfactants are outstandingly superior to conventional surfactants in characteristic aspects. They display strong adsorption at air- liquid, liquid-liquid and liquid-solid interfaces and are particularly effective at decreasing the surface tension of aqueous solution. These surfactants have superb solubilizing and foaming properties and may have low Krafft temperatures. They are about three orders of magnitude more dynamic in reducing the surface tension of water and more than two orders of magnitude more effective in interfacial performances than conventional surfactants. Hence, a very low quantity of anionic dimeric surfactant can have artistic effect in applications. These advantages led many researchers, scientists, and manufacturers to evolve modern varieties of dimeric surfactants for industrial, agricultural, and daily uses. Some anionic dimerics with specific performances have been introduced in the market as occupational products, individually or mixed with other surfactants.

Detergents industries

Surfactants play enormous role as a cleaning agent in detergent industry. The primary traditional applications of surfactants are their use as soaps and detergents for a wide variety of cleaning processes. Dimeric surfactants have the potential to reduce the environmental impact of detergents, because much lesser magnitude is needed to achieve the same function. Also, their excellent capacity and chemical utility will be of numerous conveniences in formulating new detergents. The low CMC values of dimeric surfactants mean that the concentration of non-micellized dimeric surfactant in the solution is also much lesser. This may result in lower irritancy and toxicity and also in a greater capacity in the solubilization of water-insoluble material. Closer packing of hydrophobic chains of dimeric surfactants at interfaces will result in more laterally cohesive interfacial films and thus, in superior emulsifying, dispersing, and foaming properties30. Dimeric surfactants in which the alkyl chains are short and branched have linkage between the hydrophilic groups display magnificent dynamic wetting properties. The dicarboxylateanionic dimeric [C10H21OCH2CH(OCH2COO- Na+)CH2]2Y, where Y is OCH2CH2O, have Draves skein wetting times at 0.1% concentration as 30 sec, in comparison to 226 sec for C11H23COO-Na+31. Dimeric surfactant i.e. (RC6H3SO3-M+)2O enhanced the solubilization of water-insoluble matter, which can remove stains from paints, inks, and other coloring materials from clothing and skin32 [Rosen and Tracy (1998), and Kumar and Tyagi (2014)]. The disodium phosphate geminis [C10H21OCH2CH[OP(O) (ONa)(OH)]CH2]2Y, where Y= (OCH2CH2), has excellent foaming properties, in contrast the tetra sodium salt of the same surfactant [C10H21OCH2CH{OP(O)(ONa)2}CH2]2]2Y showed almost no foaming when tested at 0.1% (33).

Alkylated diphenyloxidedisulfonates (ADPODS) as shown in Figure 7, offer excellent solubility and stability in acidic, alkaline, and other oxidizing systems, and are available with one or two alkyl chains varying from C6 to C1634. These surfactants are applied in cleaning formulations and in textile processing. They are well-known for, good detergency and bleach stability at low temperatures. The detergency performance of ADPODS peaks at an equivalent molecular weight of 327 g/mol in the built formulation. Due to the disulfonated structure of the ADPODS, superb solubility and dispersibility are shown at low temperatures. The disulfonation is necessity for hard-water washing and dialkylated disulfonates are better hydrotropes than dialkylatedmonosulfonates (DAMS). Anionic dimeric surfactants display excellent soap dispersing efficiency. These surfactants due to their low sensitivity to Ca++ and Mg++ can be used as a cleaning agents and other industrial processes where the presence of the hard water make the traditional surfactants ineffective [Rosen and Zhu (1993)]. The cost of such type of dimeric surfactants is greater than for commercial conventional surfactants. They could be used in low amounts in mixed surfactants for a specific requirement.

Toiletries and personal care products

Anionic dimeric surfactants are often used for the manufacturer of shampoos, cosmetics, body lotions, and personal care products because of their pulpy effect, mildness, and absence of skin irritation. Sulfoesters type anionic geminis of structure [C12H25CH(SO3-Na+ COOCH2]2 were tested on human arms by protein denaturation and were found to be superior to single chain sulfomonoester. This type of dimeric had shown lesser protein denaturation by factor of 10 and 5 % solution applied to human arms for five days resulted in no skin scar or redness35. Sulfated dimeric surfactants of structure [C12H25N(COCH2CH2OSO3Na)CH2]2CHOH tested on the abdominal region of guinea pigs are claimed to show no irritancy [Fujikura et al (1998)], while methylene bis sulphate of structure [C8H17C3H6O(C2H4O)7O3Na]2CH2 tested as mild surfactants [Tracy et al. (1998)]. Deodorants are used for personal hygiene to remove body rot which occurs because of the bacterial decomposition of sweat, common in the under arm zone. Smell can be curbed by using a formulation which prevents the secretion of sweat or its rottenness. The formulations containing a dimeric surfactant viz. alcoholethersulfate, dimeric alcoholsulf ate, or trimeric alcohol- trisethersulfate an esterase resistor, an antiseptic agent, and a bacteriostatic agent are excessively effective. These surfactants also improve the reconcilability between skin and cosmetics and stop the action of esterolytic enzymes, besides this very minor quantity is used in cosmetic formulations. They can also be employed for making hair lotions, hair shampoos, bubble baths, and cream gels or lotions. The efficiency of the formulation was characterized by its inhibition of esterase after 15 minutes with the concentration of 100-6000 ppm, at pH 6, as compared with a non-inhibited control. The results revealed a reduction of esterase activity from 100% to 0% upon the addition of 10-100 ppm of dimeric alcoholsulfate, whereas the activeness remained at 100 % with the concentration of 2000 ppm aluminum cholorohydrate in the absence of dimeric36. Toiletries and personal care products are generally based on emulsions of oil-in-water (o/w). The durability and viscosity of the emulsion depend heavily on the structure of the emulsion. Anionic dimeric surfactant viz. Sodiumdicocoylethylenediamine PEG-15 could be used for o/w emulsion for the end use of toiletries and personal care products for skin care, face lotion, sun protection, skin moisturizing, and hair care. This dimeric surfactant is traded in a blend with co-surfactants under the trade mark Ceralution. Ceralution H is a reasonable blend of the dimeric surfactant, glyceryl stearate, glyceryl citrate, and behenyl alcohol. It can be used to prepare a category of stable dispersions of micro-pigments like Titanium dioxide in water, as applied for the preparation of sunburn cream37,38. Anionic type geminis for instance [C11H23CONHCH2CH2NCH2CH2COO-Na+]2CH2CH2 demonstrate good mildness to skin and are eco-friendly39. Dimeric surfactants i.e., disodium2,3-dialkyl-1,2,3,4-butanetetracarboxylates, can be applied as new winsome ingredients in cosmetics, toiletries and personal care products [Villa et al. (2013).

Surface coatings

It is probably not surprising to find that dimeric surfactants are required in many capacities in the production of paints and in related coating systems. Anionic dimerics are widely applied in polymer dispersions for making paints and protective coatings, as surfactants still play an important role in almost all coating and paint formulations. Various companies concentrate on building more efficient environmentally referred technologies, such as waterborne formulations, but conventional surfactants limit the performance of water-borne polymers when compared to solvent- borne formulations. They are also applied in water-borne systems in automotive industry, industrial maintenance, and overnight varnishes, wood coatings, emulsion, printing inks, and paints. These types of surfactants are also used in food and beverage and are prepared using a clean process technology [Kumar and Tyagi (2014)].

Objectives of the present study

In the text explained above, the synthesis of novel surfactants in the pursuit of intriguing and exceptional properties is advantageous activity. The general aim of this research work is to proceed for the investigation in the field of surfactants with new anionic dimeric surfactants having carboxylate head group. Anionic surfactants are of the highest practical interest amongst all the general class of surfactants. The worldwide consumption of anionic surfactants is still maintaining their supremacy in detergents applications. Anionic dimeric surfactants with sulphonate, sulphate and phosphate head groups have been widely studied, but there have been few studies of anionic dimeric surfactants with carboxylate head groups. Carboxylate anionic dimerics are modern members of the dimeric surfactants category. Thereby, this work incorporates the systematic study and discussion of the vital properties of carboxylate anionic dimeric surfactants. The anionic dimerics with sulphonate, sulphate and phosphate head groups have been investigated as their synthesis is quite complex, requires costly raw materials and time consuming. Hence, the proposed research work is focused to develop cost-effective carboxylate anionic dimeric surfactants by simple synthetic route, based on commercial raw materials and require lesser time in synthesis. Besides this, their higher stability and more solubility in hard water make them further attractive as compared to conventional anionics and corresponding gemini surfactants.


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Author Details

Santosh Bhuva

M/s. Kansai Nerolac Paints Ltd. Nerolac House, Ganpatrao Kadam Marg, Lower Parel, Mumbai 400 013.