Cationic surfactants, in the form of quaternary ammonium compounds, are the most widely used conditioning agents in commercial products [26-28]. Among the reasons for this are their effectiveness, versatility, availability, and low cost.
Important examples of these quats include stearalkonium chloride, cetrimonium chloride, and dicetyldimonium chloride.
CH2(CH2),4CH3 Dicetyldimottium Chloride
Because of the positive charge on quaternary ammonium compounds such as the above, they are substantive to hair, binding to negative sites on the hair surface. Treatment
with these quats, therefore, results in a hydrophobic coating on the fiber that renders the hair softer and easier to comb . Build-up of static charge (flyaway) is also greatly reduced as a result of this surface modification .
Another consequence of the positive charge on quats is that deposition increases with increasing negative charge on the hair surface. This is seen in Table 1, which shows the results of an experiment in which hair tresses were treated with 1% stearalkonium chloride and then rinsed. Compared with the roots, 22% more quat was found to bind to the tips of virgin hair, while deposition of stearalkonium chloride on bleached hair was found to be more than twice that on untreated fibers.
This result is important because, as previously discussed, damaged portions of the hair, which generally carry a greater amount of negative charge, require a greater amount of conditioning. The fact that cationic surfactants can supply this increased conditioning, makes them effective on a wide variety of hair surfaces. This is a major factor in the widespread use of these types of conditioning agents.
Many important properties of quaternary ammonium conditioners are related to the degree of hydrophobicity of the lipophilic portion of the surfactant. Thus, increasing the length of the alkyl chain of a monoalkyl quat, and therefore making it more hydrophobic, leads to increased deposition [31-36] on hair. Cetrimonium chloride, as a result, deposits on hair to a greater extent than does laurtrimonium chloride. Increasing the number of alkyl chains also increases deposition, so that tricetylmonium chloride exhibits greater deposition than does dicetyldimonium chloride, which, in turn, is more substantive than the monocetyl quat.
This dependence of deposition on degree of hydrophobicity indicates that van der Waals forces play an important role in deposition of quaternary ammonium conditioners . This conclusion is consistent with the entropy-driven deposition demonstrated by Ohbu et al.  and Stapleton  for a monoalkyl quat and a protonated long-chain amine.
Increased hydrophobicity also correlates with increased conditioning by quaternary ammonium compounds [31-34,39]. Thus, cetrimonium chloride provides light to medium conditioning, while dicetyldimonium and tricetylmonium chlorides provide heavier conditioning. Detangling and wet combing, in particular, improve significantly from monocetyl to dicetyl to tricetyl quats; differences in dry combing and static charge among these compounds are not as significant.
Increased conditioning with increased hydrophobicity is probably due, in part, simply to increased deposition of quat on hair. Data from Garcia and Diaz , however, indicate greater improvements in wet combing from heavier conditioning quats even when present on the hair in much lower amounts than less hydrophobic species. The degree of
Type of hair
hydrophobicity of a quat must therefore play a direct role in the conditioning efficacy of these compounds .
Note that on some types of hair, the greater substantivity of higher conditioning quats can lead to build-up and result in limp, unmanageable hair with repeated use. This is especially true, e.g., for untreated, fine hair. Different quats, or mixtures of conditioning agents, are therefore suitable for different uses or different types of hair. A tricetyl quat might be used, e.g., in an intensive conditioner meant only for occasional use.
The length and number of alkyl chains of quats also determines water solubility of these compounds. Monoalkyl quaternaries up to cetrimonium chloride are water soluble, e.g., distearyldimonium chloride is water dispersible, while tricetylmonium chloride is insoluble in water .
Compatib ility with An ion ics
The quaternium compounds normally used in commercial conditioners are not generally found in shampoos because of incompatibility with common anionic detergents . Introducing hydrophilic groups into the quat can increase compatibility with anionics. An example is the class of ethoxylated quaternaries, termed ethoquats. Typical members of this class are PEG-2 cocomonium chloride, where x + y equals 2 and R is a C12 alkyl chain, and PEG-15 stearmonium chloride where x + y equals 15 and R is a C18 chain.
Both of these quats are compatible with typical anionic detergents. As would be expected from this discussion, however, introducing hydrophilic groups decreases the conditioning efficacy of these materials [31,34]. They are therefore suitable only in light-conditioning formulations. Furthermore, conditioning shampoos based on ethoquats would not be expected to be very effective as a result of low deposition of the detergent-soluble ethoquat complex.
Other detergent-soluble quats have been produced. These include alkylamidopropyl dihydroxypropyl dimonium chlorides , lauryl methyl gluceth-10 hydroxypropyl dimo- g nium chloride , and even a hydrolyzed ginseng-saponin quaternary derived from Ko- |
rean ginseng saponin . Although certain advantages have been claimed for these sur- i|
factants, particularly low irritation, they all suffer from much the same conditioning limitations as the ethoquats.
Other Cationic Surfactants ts
In addition to the aforementioned examples, numerous other cationic surfactants are in use or have been proposed for commercial products. One example of a compound that has been receiving increasing use recently is the behentrimonium (C22) quat. This quat 2
exhibits significantly reduced eye and skin irritation compared with the corresponding C18 conditioner. In addition, superior conditioning and thickening properties have been claimed .
Another interesting example is hydrogenated tallow octyl dimonium chloride . This material is quite substantive and provides high conditioning as a result of its two hydrophobic chains. Unlike conventional dialkyl quats, however, this particular conditioner is soluble in water as a result of branching (2-ethylhexyl) in the octyl moiety. This makes the compound much easier to formulate into a commercial product.
Stearamidopropyl dimethylamine is another conditioning agent that is found in many commercial conditioners. This material is cationic at the pHs normally used in conditioning products and therefore acts as a cationic emulsifier and, also, as a secondary conditioning agent.
Concern for the environment has led to the synthesis of ester quats that exhibit increased biodegradability and environmental safety. One such example is dipalmitoyl-ethyl hydroxyethylmonium methosulfate, an ester quat based on a partially hydrogenated palm radical .
Other cationic surfactants used in conditioners include quats derived from Guerbet alcohols  (low to high conditioning depending on length of the main and side alkyl chains), distearyldimonium chloride (high conditioning), and the quaternized ammonium compounds of hydrolyzed milk protein, soy and wheat protein, and hydrolyzed keratin (varying conditioning efficacy depending on alkyl chain length).
Quaternary ammonium surfactants in commercial products are almost never used alone. Instead they are used in combination with long-chain fatty conditioners, especially cetyl and stearyl alcohols . These fatty materials are added to boost the conditioning effects of the quaternary compounds . In one study, e.g., addition of cetyl alcohol to cetrimo-nium bromide nearly doubled the observed reduction in wet combing forces on hair . In another study, using a novel hydrodynamic technique, Fukuchi et al.  found that the addition of cetyl alcohol to a behentrimonium chloride formulation resulted in significantly reduced surface friction.
Several investigators have studied combinations of cationic surfactants and fatty alcohols. Under the right conditions, these mixtures have been found to form liquid crystal mesophases and gel networks [50-54] that can greatly increase viscosity and, at the same time, confer stability upon emulsions. As a result of reduced repulsion between cationic head groups when long chain alcohols are interposed, liquid crystal formation has been observed even at low concentrations [53,54]. The ready formation of these extended struc- ^
tures between quats and cetyl and stearyl alcohols, along with the low cost, stability, and compatibility with cosmetic ingredients of the latter are important reasons why these alcohols are so ubiquitous in conditioning formulations.
Other lipids found in commercial products include, e.g., glycol distearate, triglycer- ^
ides, fatty esters, waxes of triglycerides, and liquid paraffin.
Cationic Polymers °
There are numerous cationic polymers that provide conditioning benefits, especially improved wet combing and reduced static charge. Important examples of these polymers are Polyquaternium-10, a quaternized hydroxyethylcellulose polymer; Polyquaternium-7, a -c copolymer of diallyldimethylammonium chloride and acrylamide; Polyquaternium-11, a copolymer of vinylpyrrolidone and dimethylaminoethyl methacrylate quaternized with dimethyl sulfate; Polyquaternium-16, a copolymer of vinylpyrrolidone and quaternized vi-nylimidazole; and Polyquaternium-6, a homopolymer of diallyldimethylammonium chlo-
By virtue of their cationic nature, these polymers are substantive to hair. The particular conditioning effectiveness of any of these materials depends on the polymer structure. In one set of studies, deposition on hair was found to be inversely proportional, roughly, to cationic charge density [55,56]. This has been explained by the observation that the higher the charge density, the lower the weight of polymer needed to neutralize all of the negative charge on the hair. Once deposited, however, multiple points of electrostatic attachment makes these polymers harder to remove, especially if charge density is high [30,57]. Care must be taken, therefore, in formulating conditioners containing these materials to avoid overconditioning as a result of build-up with continued use.
As with the preceding monofunctional cationics, deposition of polyquaterniums increases on treated, or damaged, hair [30,57,58]. Unlike common monofunctional quats, however, the first four of these polymers are compatible, to varying degrees, with anionic surfactants [57-61]. As a result, they are used more often in shampoos than in standalone conditioners, although they find some use in leave-in conditioners.
Polyquaternium-10 (PQ-10) and Polyquaternium-7 (PQ-7) are two of the most frequently used polymers in commercial shampoos. Both of these polymers form negatively charged complexes [57,59] with excess anionic surfactant, resulting in reduced deposition because of repulsion by the negatively charged hair surface. The magnitude of this effect depends on the particular anionic used, and on the anionic surfactant/polymer ratio. In all cases, however, conditioning from shampoos is significantly less than from stand-alone conditioners.
Despite reduced deposition, Hannah  has reported that polyquaternium association complexes formed with SLS resist removal from hair. Build-up and a heavy, coated feel on the hair can therefore result from conditioning shampoos containing polyquats unless they are carefully formulated.
The use of silicones in haircare products has increased considerably in the past two decades, although their first incorporation into commercial products dates back to the 1950s. Different types of silicones find use as conditioning agents in a wide variety of products, including conditioners, shampoos, hair sprays, mousses, and gels . One of the most widely used silicones is dimethicone, which is a polydimethylsiloxane. Other important silicones are dimethiconol, which is a dimethylsiloxane terminated with hydroxyl groups, and amodimethicone, which is an amino-substituted silicone.
Most silicones used in haircare products, including those previously mentioned, are insoluble and must therefore be emulsified. To increase ease of product manufacture, many suppliers offer silicones as preformed emulsions, in addition to the pure material. The factors affecting deposition of silicones from such emulsions have been reported by Jachowicz and Berthiaume [64,65].
Conditioning Properties of Silicones
Silicones used in haircare products possess a range of unique properties including lubricity, low intermolecular forces, water insolubility, and low surface tension. These properties permit the silicones to spread easily on the hair surface, forming a hydrophobic film that provides ease of combing, and imparts a smooth, soft feel to the hair without greasiness.
The relative conditioning efficacy of silicones compared to other conditioners was demonstrated by Yahagi , who found that dimethicone lowered frictional coefficients and surface energy of virgin hair to a greater extent than did a series of cationic surfactants, including distearyldimonium chloride, a very effective conditioning agent. Dimethicones with molecular weights greater than 20,000 were found to be most effective in reducing surface tension.
Nanavati and Hami  measured conditioning on slightly bleached European hair treated with dimethicone fluids and dimethiconol gums. Both types of silicones were found to significantly reduce combing forces on hair. Ease of wet combing was roughly the same for the two silicone treatments, while dimethiconol was found to be more effective in reducing dry combing forces.
Interestingly, under the treatment conditions used (exposure to silicone solutions for 30 sec followed by drying without rinsing), deposition of all silicones studied was found to nearly double if tricetyldimonium chloride was present in the treatment solution. Reduction in combing forces was also doubled, roughly, when silicones were deposited in the presence of quat. This latter effect was found to be synergistic, i.e., it depended on deposition of both silicone and quat, and its magnitude was greater than the sum of the individual conditioner contributions.
Wendel et al.  used electron spectroscopy for chemical analysis (ESCA) to demonstrate that the presence of amino groups in silicones considerably increases substantivity of these materials. This is a result of the positive charge developed by these groups at the pHs commonly found in commercial products.
Comparison of conditioning effects of a series of silicone emulsions on bleached and virgin hair was carried out by Hoag et al. . Most of the silicones were dimethicones or amodimethicones, while emulsions were anionic, neutral, or cationic in nature. Diluted emulsions were applied directly to the hair and combing forces measured both before and after rinsing. Prior to rinsing, reduction of combing forces by most emulsions was greater than 80%. This number was decreased after rinsing as a result of partial removal of deposited silicone. Unsurprisingly, the least change in ease of combing was found for cationic emulsions, especially those containing amodimethicone. Combing forces on virgin hair increased less than on bleached hair after rinsing, indicating that the silicones were more substantive to this type of hair. This is also unsurprising considering the hydrophobic nature of these conditioning agents.
Further effects of amodimethicones can be seen in work reported by Berthiaume et al. , who studied a series of amodimethicone emulsions in a prototype conditioner formulation. Deposition on hair from the conditioner was found to increase with increasing amine content in the silicone. This increased deposition was found, in half-head tests, to correlate with conditioning efficacy, including wet and dry combing, softness, and detan-gling. A microemulsion in the test series that provided high conditioning was also shown to significantly reduce the color fading caused by shampooing of temporarily dyed hair.
Two important silicones not covered in the preceding section are dimethicone copolyol, which is a dimethylsiloxane containing polyoxyethylene and/or propylene side chains, and cyclomethicone, which refers to a class of cyclic dimethyl polysiloxanes ranging from trimer to hexamer. The most commonly used variant is the pentamer. CHj CH3 CH3 CH3
Most commercial dimethicone copolyols are soluble in water and are therefore not very effective in rinse-off products. These silicones find important application, however, in leave-on products, including hair sprays, styling mousses, and gels.
Cyclomethicone is volatile and would not remain on dry hair, especially after blow-drying. It helps other conditioning agents disperse, however, and form films on hair. It also helps improve wet combing and provides transient shine.
Silicones find important application as the primary conditioning agents in 2-in-1 conditioning shampoos. These shampoos, upon their introduction in the latter part of the 1980s, represented a major advance in haircare technology, providing a significantly higher degree of conditioning than was then the norm for conditioning shampoos and, at the same time, leaving a desirable, soft, smooth feel on the hair.
Conditioning from 2-in-1 shampoos is expected to occur primarily at the rinsing stage during which time the shampoo emulsion breaks, releasing the silicone for deposition on hair. This separation of cleaning and conditioning stages permits the shampoo to perform both functions efficiently.
The conditioning agent used most frequently in 2-in-1 shampoos is dimethicone. This silicone can provide good performance in shampoo formulations without building-up excessively on the hair . The level of conditioning from these types of shampoos is lower than that from stand-alone conditioners. This is especially true for treated hair because the greater the degree of negative charge on the hair surface, the lower the substan-tivity of a hydrophobic material like dimethicone. Many 2-in-1s contain polyquats, which might be expected to increase conditioning on damaged hair. In shampoos with high levels of anionic detergent, however, polyquat performance on treated hair may be no better than dimethicone as a result of formation of the negatively charged polymer complexes discussed in the section on cationic polymers (see p. 338).
Yahagi  studied the performance of dimethicone, amodimethicone, and dimethicone copolyols in 2-in-1 shampoos. Ease of combing was found to be similar on hair treated with shampoos containing dimethicone or amodimethicone. Unsurprisingly, soluble dimethicone copolyols did not perform well; insolubility, or at least dispersibility, was required for adequate silicone deposition. In the latter case, dimethicone copolyols were found to provide a somewhat lower level of conditioning than the other two silicones studied, especially once blowdrying was begun. Yahagi also studied silicone effects on foam volume. In these studies dimethicone was found to significantly reduce foam volume in a model shampoo formulation, while amodimethicone and dimethicone copolyol had a minimal effect on foam.
A number of ingredients besides conditioning actives are added to commercial conditioners for functional, aesthetic, and marketing purposes . These include fragrances, dyes, preservatives, thickeners, emulsifying agents, pearlizers, herbal extracts, humectants, and vitamins. Some of these are discussed in the following sections; the literature also contains many examples [28,73-77].
Preservatives are necessary to insure the microbiological integrity of a conditioning product. If the product contains high concentrations of ethyl alcohol (generally 20% or above), additional preservatives are not needed and the product is described as self-preserving.
For other products, a wide variety of preservatives are available; in general, combinations of different preservatives provide the broadest possible protection. Every commercial product that is not self-preserving must be carefully tested over time for adequacy of preservation. Most of the preservatives used in personal-care products are described in the Cosmetic Preservatives Encyclopedia .
The section on lipophilic conditioners described thickening as a result of liquid crystal formation in those products containing common quaternary ammonium compounds and fatty alcohols. Cationic conditioning polymers (see p. 338) can also act as thickeners. Many formulations may require additional thickening agents. Hydroxyethylcellulose, a nonionic cellulose ether compatible with cationic surfactants and stable over a wide pH range, is the most common thickening agent added to conditioning products . In addition to providing increased viscosity, this material stabilizes viscosity over time.
Polyamides may also be used to thicken formulations. A commercial product, Sepigel, which contains polyamide, laureth-7, and isoparaffin, can be used to emulsify and thicken lotion or cream conditioners. Other thickeners are described in Ref. 76.
Many conditioners contain humectants, which are used to attract moisture. Examples are propylene glycol, glycerine, honey, chitosan, and hyaluronic acid. These materials are not expected to be very effective in rinse-off products.
As previously discussed, the fatty alcohol, quat combinations found in common conditioners confer stability on product emulsions. If necessary, other emulsifiers may be added to improve stability. Information on emulsions and emulsifiers may be found in the literature [77,78], as well as from manufacturers' technical bulletins. Most emulsifiers used in conditioners are nonionic, including ethoxylated fatty alcohols, ethoxylated fatty esters, and ethoxylated sorbitan fatty esters.
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