How Do Hair Conditioners Work on Different Hair Types?

ACCM Concepts

• IV. C. Intermolecular forces can be categorized based on the permanence and structural details of the dipoles involved.

• IV. C. 2. a. In covalent bonding, electron-pair sharing is not always equitable and the model of electronegativity differences allows one to explain when polar covalent bonds will form.

• X. C. 2. b. Concentration gradients lead to diffusion or osmosis.

Have you ever found yourself showering at a friend’s house and borrowing some of their conditioner, only for it to have no effect on your hair? Or maybe, it even made your hair look worse. Everyone has different hair structures that work best with different types of chemicals.

Figure 1: Different Hair Types

Background: Electronegativity and Intermolecular forces

A dipole moment occurs when a molecule has a center of positive and a center of negative charge. Dipole moments arise due to differences in electronegativity levels (the pull that molecules have on electrons).^[1] The greater the difference in electronegativities, the greater the dipole moment in molecules. These moments can occur between ionic or covalent bonds. The dipole moment is a measure of the polarity of a molecule.^[1]

A molecule is polar when the dipole moments created by its bonds do not cancel each other out. Some bonds may be polar but have no dipole moments because another bond in the molecule counteracts the charge. For example CO2 has a polar bond with C — O because Oxygen is more electronegative, however, because it forms a linear structure (O = C = O), the opposing bonds are canceled and no dipole moment occurs.

Intermolecular forces(IMFs) are the electrostatic attractions between oppositely charged molecules, ions, or atoms.

One type of bonding resulting from IMFs are Dipole-Dipole interactions which occur when two molecules with dipole moments interact. A dipolar molecule has one positive and one negative end.^[2] During a dipole-dipole bonding, the positively charged sides are attracted to each other and interact (positive interacts with negative side). 

Another type of bonding that results from IMFs are Hydrogen bonds. They are dipole-dipole bonds that are much stronger than regular dipole-dipole bonds because they have a much larger electronegativity difference. Hydrogen Bonding occurs when a hydrogen atom bonds to a very electronegative atom, mainly N, O, or F. This results in a high electronegativity difference between N, O, or F and the hydrogen creates a large dipole that would therefore create a strong dipole-dipole interaction.^[4] An example would be water molecules; the lone pairs on the oxygen atoms that are negatively charged attract the positive (δ+) Hydrogen. These bonds are much stronger than a normal covalent bond and each water molecule forms other hydrogen bonds with other water molecules near them.^[4]

Lastly, the weakest of the IMFs, London dispersion forces, are the attractions from small charge centers created by random electron movement.

The Chemistry of Conditioners

Shampooing strips away dirt and the hair’s natural oils which then leaves the hair dull and frizzy. The hair appears dull because it is no longer surrounded by a shiny substance. It appears frizzy because the negatively charged hair shaft is now exposed to other negatively charged hair strands which repel each other.^[5] Additionally, without the hair’s natural oils, the water content of the hair is no longer sealed in. It therefore evaporates and leaves the hair dry. 

Along with other minor ingredients, conditioners work to solve these main issues with oils, cationic surfactants, and humectants. The nonpolar oils rest on the hair strands, creating sheen. Cationic surfactants are substances that have a polar, hydrophilic head, and a nonpolar tail. The polar heads bond to cysteine, the amino acid in keratin that is responsible for hair’s slight negative charge.^[5] This makes the conditioner more resistant to removal during rinsing since the London dispersion forces between the water and the nonpolar tails cannot overpower the dipole-dipole bonding between the polar head and the cysteine molecules. Meanwhile, the nonpolar tail works like an oil, in that it appears shiny and rests on the hair shaft.  In this way, cationic surfactants create a shiny protective envelope for the hair. Lastly, humectants work by bonding to water and diffusing into the hair shaft, returning the moisture that was lost with shampooing.^[5] A common humectant in skin and hair products is glycerol (CH2OH-CHOH-CH2OH). Glycerol has three hydroxyl groups, so it has three regions where form a hydrogen bond with water. Due to this, it is highly soluble in water and therefore helps draw water into the hair shaft.

                                                           

                                                                    Figure 2: Glycerol 

Conditioners for Curly Hair

Though shape is the most apparent difference among curly and straight hair, they also differ in their porosity, the ability of the hair shaft to absorb and release moisture. Curly hair is usually low porosity–more difficult to absorb water while straight hair is usually high porosity––easier to absorb water.^[6] Additionally, sebum, the hair follicle’s natural oils, can easily travel down straight hair, but does not easily travel down curly hair.^[6] Because of this difference in porosity and lubrication, curly hair and straight hair conditioners have different chemical makeups to cater to the different hair textures. Two of the key differences are that curly hair conditioners have heavier oils and its ingredients are in higher concentrations because curly hair requires more moisturizing. A key ingredient difference is the use of Shea butter in curly hair conditioners. Shea butter is a 50% saturated fat, meaning half of its molecular makeup involves saturated carbons (carbons surrounded by Hydrogen).^[7] This makes the substance thicker since the molecules are less kinked by double bonds between carbons allowing the molecules to pack more closely to each other. This thicker butter surrounds curly hair strands without weighing them down or making them appear greasy as it would for straight hair.

 

Exercise 1.A

The following diagrams are the chemical structures of Behentrimonium Methosulfate and Cysteine. Behentrimonium Methosulfate is a common cationic surfactant in hair conditioners. Cationic surfactants work by coating the hair shaft and creating an envelope that remains even after rinsing. This protective envelope adds sheen and protection to the hair. Cysteine is an amino acid on the surface of keratin that cationic surfactants interact with.     

                                                 

                   Figure 3: Behentrimonium Methosulfate                                        Figure 4: Cysteine

Use the Pauling electronegativity values from the chart to calculate the electronegativity differences among the C-O, O-H, H-S, and N-H bonds in the cysteine molecule.^[3]

Element	Hydrogen	Carbon	Nitrogen	Oxygen	Sulfur
Pauling Electronegativity Values	2.1	2.5	3.0	3.5	2.5

 

Solution

To calculate the electronegativity difference for a given bond, simply subtract the smaller electronegativity value from the larger value. This difference is an important value because a large difference would indicate that one atom has a much stronger pull on the electrons whereas a smaller difference would indicate that the electrons are shared more equally. 

|     C - O : 1.0     |     O - H : 1.4    |     H - S: 0.4     |     N - H : 0.9    |

Exercise 1.B

Use the calculations from Exercise 1.A to predict where the negative pole is located in the cysteine molecule. 

Solution

Though the asymmetry of the cysteine molecule suggests some polarity in all these regions, the largest electronegativity difference (1.4) is from the O-H bond, suggesting that the O-H bond is where the electrons are shared most unequally, and thus where the greatest dipole moment occurs. Specifically, the slight negative charge will be on the oxygen because it is the more electronegative atom in the bond.

Exercise 1.C

Based on your conclusion from Exercise 1.B, use your knowledge of polarity to suggest how the cationic surfactant reacts with keratin and how the molecules of  Behentrimonium in direct contact with the hair remain in place even while rinsing with water.

Solution

The end of Behentrimonium with the positively charged nitrogen group will bond to the negative region of the Cysteine molecule (the O in the O-H bond) with a dipole-dipole interaction. This will create the protective envelope around the hair shaft. The envelope will remain even while rinsing because the rest of the Behentrimonium ion is a carbon chain with symmetrically bonded Hydrogens. This symmetry means there are no net poles, and so the rest of the molecule is therefore nonpolar. The polar water molecules and the nonpolar tail of the Behentrimonium will therefore not interact with strong enough intermolecular forces to pull the Behentrimonium away from the hair shaft. 

Conclusion

In conclusion, the socially accepted hygiene routine of shampooing leads to the reliance on another product, conditioner, to artificially return the hair to its a moisturized and electrically neutral state. In short, conditioners do this by getting moisture into the hair shaft with humectants, and then seal and neutralize the hair shaft with oils and cationic surfactants. Because of structural differences, curly hair requires more highly concentrated conditioners and those that contain heavier oils and butters. 

References

(1) Bond Polarity and Dipole Moments https://chem.libretexts.org/@go/page/89490 (accessed Nov 11, 2022).

(2) Dipole-Dipole Interactions https://chem.libretexts.org/@go/page/1658 (accessed Nov 11, 2022).

(3) Donald J. DeCoste; Steven S. Zumdahl. Chemical Principles, 8th ed.; Cengage Learning, 2017

(4) Hydrogen Bonding https://chem.libretexts.org/@go/page/1660 (accessed Nov 11, 2022).

(5) Gavazzoni Dias, M. F. Hair Cosmetics: An Overview. International Journal of Trichology 2015, 7 (1), 2. https://doi.org/10.4103/0974-7753.153450. 

(6) Hessefort, Y. Z.; Holland, B. T.; Cloud, R. W. J. Cosmet. Sci.,59, 263-289 (July/August 2008) True Porosity Measurement of Hair: A New Way to Study Hair Damage Mechanisms. International Journal of Cosmetic Science 2009, 31 (4), 324–324. https://doi.org/10.1111/j.1468-2494.2009.00510_4.x. 

(7) Shea Butter: An Opposite Replacement for Trans Fat in Margarine https://www.longdom.org/open-access/...ine-34682.html (accessed Nov 10, 2022). 

Image Credit:

Figure 1: https://stock.adobe.com/171842215

Figure 2: https://stock.adobe.com/search?...glycerol...69732424

Figure 3: https://pubchem.ncbi.nlm.nih.gov/compound/Behentrimonium-methosulfate#section=2D-Structure

Figure 4: https://pubchem.ncbi.nlm.nih.gov/compound/Cysteine