Nonionic surfactants

A POE nonionic, by itself, showed very weak adsorption onto positively charged alumina (Somasundaran, 1983), while dodecyl-p-o-maltoside adsorbed strongly (Zhang, 1997), presumably because of the negative charge on the latter. 

pH Change This usually causes marked changes in the adsorption of ionic surfactants onto charged solid substrates. As the pH of the aqueous phase is lowered, a solid surface will usually become more positive, or less negative, because of adsorption onto charged sites of protons from the solution, with consequent increase in the adsorption of anionic surfactants and decrease in the adsorption of cationics (Van Senden, 1968 Connor, 1971). The reverse is true when the pH of the aqueous phase is raised. These effects are shown markedly by mineral oxides, such as silica and alumina, and by wool and other polyamides. 

Nonionic surfactants do not dissociate into ions in water. As a consequence, nonionics are less sensitive to electrolytes and pH changes. Nonionic fluorinated surfactants are soluble in an acid or an alkaline medium and are compatible with ionic and amphoteric species. Unlike ionic surfactants, nonionic fluorinated surfactants are not preferentially adsorbed on charged surfaces. 

The hydrophile of nonionic surfactants is usually a polyoxyethylene chain or consists of polyoxyethylene and polyoxypropylene segments. The length of the hydrophilic chain can be conveniently varied to modify the hydrophile-lipophile balance (HLB) of the surfactant, a property which affects interfacial behavior and the stabilization of emulsions. 

The solubility of nonionic surfactants decreases with increasing temperature, and at the cloud point (see Section 6.4), the aqueous solution becomes turbid, In genera], nonionic fluorinated surfactants are more soluble in organic solvents than ionic fluorinated surfactants. 

The polyoxyethylene hydrophile is less stable chemically than carboxylate or sulfonate hydrophiles. Hence, nonionic fluorinated surfactants cannot be used in strongly oxidizing media. 

Examples of nonionic fluorinated surfactants are given in Table 1.7. Typical nonionic fluorinated surfactants are oxyethylated alcohols, amines, or thiols (mercaptans). 

The amphiphilic nature of nonionic surfactants is often expressed in terms of the balance between the hydrophobic and hydrophilic portions of the molecule. An empirical scale of ffLB (hydrophile-lipophile balance) numbers has been devised (see Chapter 7, section 7.3.2). The lower the ffLB number, the more lipophilic is the compound and vice versa. ffLB values for a series of commercial nonionic surfactants are quoted in Tables 6.7 and 6.8. The choice of surfactant for medicinal use involves a consideration of the toxicity of the substance, which may be ingested in large amounts. The following surfactants are widely used in pharmaceutical formulations. 

Commercial products are mixtures of partial esters of sorbitol and its mono- and dianhydrides with oleic acid. They are generally insoluble in water and are used as water-in-oil emulsifiers and as wetting agents. The main sorbitan esters are listed in Table 6.7 together with a space-filling model of a representative component of sorbitan palmitate. 

Commercial products are complex mixtures of partial fatty acid esters of sorbitol and its mono- and dianhydrides copolymerised with approximately 20 moles (usually) of ethylene oxide for each mole of sorbitol and 

Cremophor EL is a polyoxyethylated castor oil containing approximately 40 oxyethylene groups to each triglyceride unit. It is used as a solubilising agent in the preparation of intravenous anaesthetics and other products. 

Poloxamers are synthetic block copolymers of hydrophilic poly(oxyethylene) and hydro-phobic poly(oxypropylene) with the general formula E ,P E , where E = oxyethylene (OCH2 CH2) and P = oxypropylene (OCH2CHCH3) and 

Vegetable oils and natural fats are traditional raw materials for the production of soaps and other surfactants. Coconut oil, palm and palm kernel oil, rape oil, cotton oil, tall oil, as well as the fats of animal origin (tallow oil, wool wax), present renewable raw sources. Linear paraffins and olefins (with terminal or internal double bond), higher synthetic alcohols, and benzene are fossil sources for surfactant production which are obtained from oil, natural gas and coal. Other auxiliary materials are required to construct amphiphilic surfactant structure, such as ethylene oxide, sulphur trioxide, phosphorous pentaoxide, chloroacetic acid, maleic anhydride, ethanolamine, and others. 

Nonionic surfactants are amphiphilic compounds the lyophilic (in particular hydrophilic) part of which does not dissociate into ions and hence has no charge. However, there are nonionics, for example such as tertiary amine oxides, which are able to acquire a charge depending on the pH value. Even polyethers, such as polyethylene oxides, are protonated under acidic conditions and exist in cationic form. Long-chain carboxylic acids are nonionic under neutral and acidic conditions whereas they are anionics under basic conditions. So, the more accurate definition is as follows nonionics are surfactants that have no charge in the predominant working range of pH. The main part of nonionics can be classified into alcohols, polyethers, esters, or their combinations. 

The importance of nonionics is that their total world production reaches two million tons per year and they occupy up to a quarter of the total surfactant production. In addition, this is the most diverse type of surfactants with respect to properties, structure and fractional composition. 

It thus must be noted that when a surfactant is needed for any application one must consider the solubility characteristics, besides other properties, which should conform to the experimental conditions. Their area of application characterizes thus the surfactants, which are available in the industry. In fact, the detergent manufacturer is in constant collaboration with washing industry and tailor-made surfactants are commonly developed in collaboration. In detergent industry one, has a whole spectra of molecules, which can be manipulated, depending on the application area. 

Anionic surfactants are used in different areas while cationic surfactants are used in completely different systems. For instance, anionic surfactants are used for shampoos and washing, while cationics are used for hair conditioners. Hair has negative (-) charged surface and thus cationic 

FIGURE 1.26 Solubility of a nonionic surfactant in water (CP) (schematic) (dependent on temperature). 

Cationic detergent (positive charge) + hair (negative charge)  

Alkyl group (cationic detergent)-polar group/(+)hair(-) 

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After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

Fig. XIV-10. The correlation between the HLB number and the phase inversion temperature in cyclohexane of nonionic surfactants. (From Ref. 71.)...

Fig. XIV-16. A photomicrograph of a two-dimensional foam of a commercial ethox-ylated alcohol nonionic surfactant solution containing emulsified octane in which the oil drops have drained from the foam films into the Plateau borders. (From Ref. 234.)...

P. Becher, in Interfacial Phenomena in Apolar Media, H. Eicke and G. D. Parfitt, eds., Marcel Dekker, New York, 1987 Nonionic Surfactants Physical Chemistry, M. J. Schick, ed., Marcel Dekker, New York, 1987. 

Schemes for classifying surfactants are based upon physical properties or upon functionality. Charge is tire most prevalent physical property used in classifying surfactants. Surfactants are charged or uncharged, ionic or nonionic. Charged surfactants are furtlier classified as to whetlier tire amphipatliic portion is anionic, cationic or zwitterionic. Anotlier physical classification scheme is based upon overall size and molecular weight. Copolymeric nonionic surfactants may reach sizes corresponding to 10 000-20 000 Daltons. Physical state is anotlier important physical property, as surfactants may be obtained as crystalline solids, amoriDhous pastes or liquids under standard conditions. The number of tailgroups in a surfactant has recently become an important parameter. Many surfactants have eitlier one or two hydrocarbon tailgroups, and recent advances in surfactant science include even more complex assemblies [7, 8 and 9].

The Kraft point (T ) is the temperature at which the erne of a surfactant equals the solubility. This is an important point in a temperature-solubility phase diagram. Below the surfactant cannot fonn micelles. Above the solubility increases with increasing temperature due to micelle fonnation. has been shown to follow linear empirical relationships for ionic and nonionic surfactants. One found [25] to apply for various ionic surfactants is ... 

Studies of micellar catalysis of himolecular reactions of uncharged substrates have not been frequent" ". Dougherty and Berg performed a detailed analysis of the kinetics of the reaction of 1-fluoro-2,4-dinitrobenzene with aniline in the presence of anionic and nonionic surfactants. Micelles induce increases in the apparent rate constant of this reaction. In contrast, the second-order rate constant for reaction in the micellar pseudophase was observed to be roughly equal to, or even lower than the rate constant in water. 

Many solutions of common nonionic surfactants and water separate into two phases when heated above a certain temperature (the cloud point), and some investigators call the phase of greater surfactant concentration, a microemulsion. Thus, there is not even universal agreement that a microemulsion must contain oil. 

Eigure 6 illustrates how the three tensions among the top, middle, and bottom phases depend on temperature for a system of nonionic surfactant—oil—water (38), or on salinity for a representative system of anionic surfactant—cosurfactant—oil—water and electrolyte (39). As T approaches from lower temperatures, the composition of M approaches the composition of T, and the iaterfacial teasioa betweea them, goes to 2ero at T =. ... 

The surfactants used in the emulsion polymerization of acryhc monomers are classified as anionic, cationic, or nonionic. Anionic surfactants, such as salts of alkyl sulfates and alkylarene sulfates and phosphates, or nonionic surfactants, such as alkyl or aryl polyoxyethylenes, are most common (87,98—101). Mixed anionic—nonionic surfactant systems are also widely utilized (102—105). 

An FEP copolymer dispersion is available as a 55-wt % aqueous dispersion containing 6% nonionic surfactant (on a soflds basis) and a small amount of anionic dispersiag agent. Its average particle size is ca 0.2 p.m. 

Nonionie Detergents. Nonionic surfactants rarely are used as the primary cleansing additives ia shampoos. They are generally poor foaming, but have value as additives to modify shampoo properties, eg, as viscosity builders, solubilizers, emulsifiers, and conditioning aids. 

Baby Shampoos. These shampoos, specifically marketed for small children, feature a non-eye stinging quaHty. The majority of the products in this category are based on an amphoteric detergent system a system combining the use of an imidazoline amphoteric with an ethoxylated nonionic surfactant has been successfiiUy marketed (15,16). The sulfosuccinates also have been suggested for baby shampoo preparation because of thek mildness... 

Solvents. The most widely used solvent is deionized water primarily because it is cheap and readily available. Other solvents include ethanol, propjdene glycol or butylene glycol, sorbitol, and ethoxylated nonionic surfactants. There is a trend in styling products toward alcohol-free formulas. This may have consumer appeal, but limits the formulator to using water-soluble polymers, and requires additional solvents to solubilize the fragrance and higher levels of preservatives. 

Many different types of foaming agents are used, but nonionic surfactants are the most common, eg, ethoxylated fatty alcohols, fatty acid alkanolamides, fatty amine oxides, nonylphenol ethoxylates, and octylphenol ethoxylates, to name a few (see Alkylphenols). Anionic surfactants can be used, but with caution, due to potential complexing with cationic polymers commonly used in mousses. 

Iodine is extensively used in a variety of forms as both an antiseptic and a disinfectant. lodophors, usually nonionic surfactants (qv) complexed with iodine, were developed for more readily usable iodine-based antiseptics and disinfectants. These are used as disinfectants in dairies, laboratories, and food processing (qv) plants, and for sanitation of dishes in restaurants. The reaction product of lanolin and iodine shows utiHty as a germicide (149). 

Three generations of latices as characterized by the type of surfactant used in manufacture have been defined (53). The first generation includes latices made with conventional (/) anionic surfactants like fatty acid soaps, alkyl carboxylates, alkyl sulfates, and alkyl sulfonates (54) (2) nonionic surfactants like poly(ethylene oxide) or poly(vinyl alcohol) used to improve freeze—thaw and shear stabiUty and (J) cationic surfactants like amines, nitriles, and other nitrogen bases, rarely used because of incompatibiUty problems. Portiand cement latex modifiers are one example where cationic surfactants are used. Anionic surfactants yield smaller particles than nonionic surfactants (55). Often a combination of anionic surfactants or anionic and nonionic surfactants are used to provide improved stabiUty. The stabilizing abiUty of anionic fatty acid soaps diminishes at lower pH as the soaps revert to their acids. First-generation latices also suffer from the presence of soap on the polymer particles at the end of the polymerization. Steam and vacuum stripping methods are often used to remove the soap and unreacted monomer from the final product (56). 

Although most greases offer some inherent protection against msting, additives, eg, amine salts, sodium sulfonate, cycloparaffin (naphthenate) salts, esters, and nonionic surfactants (qv), are often used to provide added protection against water and salt-spray corrosion. A dispersion of sodium nitrite has been particularly effective in some multipurpose greases. 

Emulsifiers. Removing the remover is just as important as removing the finish. For water rinse removers, a detergent that is compatible with the remover formula must be selected. Many organic solvents used in removers are not water soluble, so emulsifiers are often added (see Emulsions). Anionic types such as alkyl aryl sulfonates or tolyl fatty acid salts are used. In other appHcations, nonionic surfactants are preferred and hydrophilic—lipophilic balance is an important consideration. 

A detergent that is compatible with the remover formula must be developed for water rinse removers. Anionic or nonionic surfactants should be selected, depending on the pH and intended application of the remover. 

Low molecular cationic polymers or alum can also be used to flocculate pitch, ie, bind up the pitch so that it is retained in the sheet, to minimize pitch deposition on machine surfaces and fabrics (35,36). Alum is used commonly in newsprint operations (34). The addition of a nonionic surfactant with a hydrocarbon solvent to the wet end has shown some utility in preventing deposits of adhesive recycled furnish contaminants from forming on the paper... 

To overcome these difficulties, drilling fluids are treated with a variety of mud lubricants available from various suppHers. They are mostly general-purpose, low toxicity, nonfluorescent types that are blends of several anionic or nonionic surfactants and products such as glycols and glycerols, fatty acid esters, synthetic hydrocarbons, and vegetable oil derivatives. Extreme pressure lubricants containing sulfurized or sulfonated derivatives of natural fatty acid products or petroleum-base hydrocarbons can be quite toxic to marine life and are rarely used for environmental reasons. Diesel and mineral oils were once used as lubricants at levels of 3 to 10 vol % but this practice has been curtailed significantly for environmental reasons. 

In the 1990s, the thmst of surfactant flooding work has been to develop surfactants which provide low interfacial tensions in saline media, particularly seawater require less cosurfactant are effective at low concentrations and exhibit lower adsorption on rock. Nonionic surfactants such as alcohol ethoxylates, alkylphenol ethoxylates (215) and propoxylates (216), and alcohol propoxylates (216) have been evaluated for this appHcation. More recently, anionic surfactants have been used (216—230). 

Studies carried out on anionic, cationic, and nonionic surfactants bave shown tbat tbe aromatic and bydropbilic portions of molecules are easily oxidi2ed, wbereas tbe long hydrocarbon chains are converted at slower rates. Surfactant activity does, however, disappear upon loss of the aromatic portion, thereby reducing the nuisance of the reactants (32). Total mineraLi2ation to CO2 has been demonstrated for nonionic polyethoxylated 4-nonylphenols having average numbers of 2,5, and 12 ethoxy units (33). 

Inositols, ie, hexaliydrobenzenehexols, are sugars that have received increasing study and are useful in the treatment of a wide variety of human disorders, including vascular disease, cancer, cirrhosis of the Hver, frostbite, and muscular dystrophy (269). Myoinositol esters prepared by reaction with lower fatty acid anhydrides are useful as Hver medicines and nonionic surfactants the aluminum and ammonium salts of inositol hexasulfate are useful anticancer agents (270). Tetraarjloxybenzoquinones are intermediates in the preparation of dioxazine dyes (266,271). The synthesis of hexakis(aryloxy)benzenes has also beenpubUshed (272). 

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