The words hydrophobic or hydrophilic are used to describe the behavior of a molecule or chemical compound in water, the well-known Universal solvent, which is essentially polar in nature.
Hydro denotes water, while the suffix phobic means to fear or aversion to. Thus, hydrophobic molecules are water-repellant.
Contrarily, the suffix philic stands for affinity or attraction, which implies that hydrophilic molecules are water-loving, i.e., they have a natural affinity towards water or water-based (aqueous) environments.
As per the concept of polarity, like attracts like. So what do you think are polar molecules hydrophobic or hydrophilic?
Oxygen, being more electronegative, attracts the O-H electron cloud to a large extent and gains a partial negative charge (δ–), while both the H-atoms obtain strong partial positive charges (δ+).
The O-H dipole moments stay uncancelled due to the bent shape of H2O w.r.t the central O-atom to yield an overall polar H2O molecule (net µ = 1.85 D).
Polar H2O molecules strongly attract other polar molecules, such as HCl, H2SO4, NaCl, NH3, NaOH, CH3CH2OH, CH3COOH, C6H12O6, and the list goes on and on.
Hence, polar molecules are hydrophilic as they readily dissolve in water.
In contrast, non-polar molecules such as H2, O2, CO2, CH4, diethyl ether (C2H5)2O, hydrocarbons (CH3CH2-R), lipids, oils, etc., are all hydrophobic in nature.
A fatty acid ester (soap) possesses both a polar (hydrophilic) head and long non-polar (hydrophobic) tails. It is thus known as amphiphilic.
What makes a molecule polar or non-polar?
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule is considered non-polar.
The Cl-atom strongly attracts the shared electron cloud and gains a permanent negative charge (Cl–), while the Na-atom attains a permanent positive charge (Na+). The Na+Cl–dipole moments stay uncancelled to yield a strongly polar ionic bond and, thus, a polar NaCl crystal lattice.
When added to water, polar H2O molecules strongly attract NaCl unit cells and break each Na+Cl– bond to release oppositely charged ions.
6 H2O molecules attract each Na+ ion using their negatively charged ends. Conversely, each Cl– ion is surrounded by 6 H2O molecules via their positively charged ends. This process is called hydration.
In this way, the H2O molecules break the intermolecular forces of attraction between Na+ and Cl– ions and dissolve the NaCl salt within no time.
A high electronegativity difference of 1.24 units is present between an oxygen and a hydrogen atom in the O-H bond. Conversely, an electronegativity difference of 0.89 units exists between a carbon (E.N = 2.20) and an oxygen in the C-O bond of ethanol.
The unequal C-O and O-H dipole moments stay uncancelled due to the bent, angular or V-shape of ethanol w.r.t the O-atom. Consequently, CH3CH2OH is overall polar (net µ > 0).
It is hydrophilic as the polar CH3CH2OH molecules can develop strong Van der Waal’s forces of attraction with polar H2O molecules, i.e., hydrogen bonding (H-bonding).
The partial negative end (Oδ-) of an ethanol molecule attracts the partial positive end of an H2O molecule (Hδ+) and vice versa.
Aqueous solutions of hydrophilic alcohols can be prepared in different concentrations by dissolving the right amount in water.
Glucose (C6H12O6)
Glucose (C6H12O6) is a polar carbohydrate molecule as it is a polyhydroxy aldehyde. It comprises multiple polar O-H bonds and a polar carbonyl (C=O) functional group.
The strong dipole moments of individually polar bonds stay uncancelled due to the bent shape of the molecule w.r.t each O-atom.
The charged electron cloud stays non-uniformly spread to yield an overall polar glucose (C6H12O6) molecule.
Polar sugar molecules readily dissolve in water as they are strongly hydrophilic in nature. Glucose dissolves in water by forming multiple H-bonds by using its OH groups.
Now let’s see an interesting example of an amphiphilic molecule that exhibits both hydrophilic and hydrophobic characteristics at the same time.
The simultaneous hydrophilic and hydrophobic cleaning action of soap
A fatty acid is a carboxylic acid molecule. The carboxyl (COOH) functional group is attached to a long, non-polar hydrocarbon chain.
The esterification of fatty acids with polyols (alcohols containing more than 1 OH group) yields a fatty acid ester. The chemical reaction of the fatty acid ester with an alkali (NaOH) gives soap.
Therefore, soaps comprise hydrophobic, hydrocarbon tails and a hydrophilic head. Soap forms lather in water by developing strong, attractive forces with polar H2O molecules by using its hydrophilic head.
Meanwhile, the hydrophobic tails of soap solubilize non-polar, hydrophobic grease (fat or oil) stains.
In this way, soap act as a mediator between non-polar grease and polar water molecules; otherwise unlikely to interact.
FAQ
Are polar molecules hydrophobic?
No. Polar molecules are generally hydrophilic in nature.
Hydrophobicity refers to the water-fearing or water-repellant nature of a chemical compound or molecule, while hydrophilicity denotes affinity with water.
Like dissolves like so, polar molecules use their oppositely charged poles to develop an affinity with water (a polar solvent) and get solubilized.
Is acetone hydrophilic or hydrophobic?
Acetone (CH3COCH3) is a polar molecule, so it is hydrophilic.
CH3COCH3 consists of a polar carbonyl (C=O) bond as per an electronegativity difference of 0.89 units between the covalently bonded carbon and oxygen atoms.
The strong C=O and small C-H dipole moments stay uncancelled due to the bent or V-shape of acetone w.r.t the central O-atom.
Acetone dissolves in water via dipole-dipole interactions. Oδ- of the carbonyl group attracts Hδ+ of H2O, while Cδ+ gets attracted to Oδ- of water.
Is collagen hydrophilic or hydrophobic?
Collagen is hydrophilic. It is a fibrous protein composed of three polypeptide chains, tightly coiled in a triple-helix structure.
The polypeptide chains contain polar amino acids such as hydroxyproline, hydroxylysine, arginine, serine, threonine, etc. These amino acids possess polar bonds such as O-H, N-H, C=O, etc.
Thus, collagen protein is overall polar (net µ > 0). It develops a strong affinity with water. Polar H2O molecules can break down the protein’s structure by surrounding the constituent amino acids via opposite charge attraction.
Are triglycerides hydrophilic or hydrophobic?
Triglycerides are hydrophobic. Triglycerides are a sub-class of lipids. A triglyceride molecule consists of long non-polar hydrocarbon chains (containing C-C, C=C and C-H bonds) and three ester (COO) functional groups.
Each ester group is polar; however, the major non-polar part overwhelms the small polar part.
Therefore, triglycerides (lipids) are generally considered non-polar. Thus, they are hydrophobic molecules that stay largely insoluble in water, which is why oil and water don’t mix.
Is there a difference between polarity and hydrophobicity?
Yes.Polarity refers to the unequal charge distribution in a molecule. Contrarily, hydrophobicity or hydrophilicity denotes the molecule’s interaction with water.
As polar molecules are generally hydrophilic, while non-polar molecules are essentially hydrophobic in nature thus, the two terms are often used interchangeably in the respective order.
Summary
Hydrophilic molecules have a good affinity with water, while hydrophobic molecules are water-repellant.
Polar molecules are generally hydrophilic in nature.
Non-polar molecules are typically hydrophobic.
Polar molecules dissolve readily in water by developing strong Van der Waal’s forces of attraction with polar H2O molecules.
Examples of polar, hydrophilic molecules are NaCl, HCl, H2SO4, alcohols (CH3OH, CH3CH2OH), sugars (C6H12O6, C12H22O11), carboxylic acids (CH3COOH), etc.
Soap is an amphiphilic molecule that can develop attraction with both polar and non-polar molecules using its hydrophilic head and hydrophobic tails, respectively.
References
StudySmarter. ‘What is the relationship between polarity and hydrophobicity’’. https://www.studysmarter.us/textbooks/biology/fundamentals-of-biochemistry-5th-edition/water/q5-what-is-the-relationship-between-polarity-and-hydrophobic/
Study.com. ‘Hydrophobic Effect. Hydrophobic Molecules and Examples’’. https:// study.com/learn/lesson/hydrophobic-effect-molecules-examples.html
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