Have you ever smelled something so repugnant that you had to leave the room? Have you ever disliked a co-worker so much that you could no longer stand to be in the same space as them? If you answered yes to any, or all, of these questions, then you understand how a hydrophobic molecule feels when it is in water.
The energy required to walk out of the stinky area is understandable, even palpable, but the energy associated with a hydrophobic molecule’s distaste for water is actually quantifiable!
To understand how hydrophobic molecules can be compared to one another, we need to ask (and ultimately answer) the question, what is a hydrophobic molecule?
What Is a Hydrophobic Molecule?
The need for the hydrophobic compound (often referred to as a hydrophobe) to separate itself from water is inherent to the structure of the molecule itself, which is why these types of molecules are referred to as hydro-phobic (water-hating). The solubility of a hydrophobe, and even a hydrophilic compound, is a complex phenomenon having both thermodynamic and kinetic components.
A hydrophobe’s inability-to-mix with water is actually a quantifiable energy (the Gibbs Free Energy of the solution) that is associated with many factors, including, but certainly not limited to: i-the dielectric constant (ε) of the solvent, ii-surface tension (γ) of the system, iii-the partition coefficient (or constant), iv-solute mobility as a function of Fick’s law of diffusion and v-the chemical potential (μ) of the solute.[a]
While there are many other factors at play to determine a compound’s solubility in water, a simplistic approach that generalizes all of these quantifiable measurements is to humbly say ‘like-dissolves-like’. This phrase signifies that when a solvent (for instance water) has a similar structure to a solute, then the two will mix (i.e. the solute will dissolve into the solvent).
How Do Hydrophobic Molecules React with Water?
This is the simplistic reason why hydrophilic molecules dissolve in water and hydrophobic molecules do not. Hydrophobic substances are present throughout nature and within the chemical laboratory. They can be extracted from plants, rendered from fat or synthesized from other sources. Some notable hydrophobic examples are hydrocarbons (i.e. gasoline), vegetable oil, fatty alcohols/acids and even the cholesterol in your blood! Oxiteno uses many of these hydrophobes in the synthesis of its surfactants, including (a) fatty alcohols (such as lauryl and stearyl alcohols), (b) fatty acids (such as oleic and stearic acids), (c) tallow amines, (d) branched alcohols, (e) short chained-synthetic alcohols and many more. For a complete list of hydrophobic molecules that Oxiteno uses to produce surfactants, visit here, but for now, let’s better define the differences between hydrophilic and hydrophobic molecules.
What Is the Difference Between Hydrophilic and Hydrophobic?
When in water, these hydrophobic substances behave differently than their hydrophilic counterparts, but what is the difference between a hydrophilic and hydrophobic compound? Hydrophobic molecules come in a wide variety of structures, but a relatively common theme among then is an absence of polarity within the structure itself.
Most, but not all, hydrophobes contain large sections of carbons and hydrogens, two atoms that provide little-to-no polarity within the compound’s structure. The lack of a hydrophobe’s polarity coupled with the relatively high dielectric constant of water (ε =78)a, makes for a very untenable situation between the two components, ultimately resulting in a phase-separated solution, much like you would find with a mixture of oil-and-water.
This concept of polarity can be estimated by the types of atoms in the in compound, their architectural arrangement in the molecule and the presence of charged species within the component. The presence of atoms that carry a charge (ionic species) or that can simply generate a dipole moment in the molecule increases the polarity of the compound.
Let us take water as a good example for a polar molecule, in which the oxygen atom is covalently bonded to two hydrogen atoms, sharing a pair of electrons in each of these ‘O-to-H’ covalent bonds. The oxygen atom in water is a strongly electronegative atom, causing the electrons within the covalent bond between the oxygen-and-hydrogen atoms to be ‘held’ closer to the oxygen.
One can compare electronegativity to a game of tug-of-war, in which the electron’s that create the covalent bond are pulled in the direction of the stronger player, in this case, oxygen.
Because the electrons are closer to the oxygen atom in the water molecule, and because electrons carry a negative charge, the oxygen atom has a slight negative charge associated with it. This has the consequence of creating a slight positive charge on the hydrogen atoms in the water molecule. Because the entire molecule now has an area that is slightly negative and an area that is slightly positive, water can be considered to have polar characteristics (much like a magnet carries a positive and negative terminal). Recalling the title from Paula Abdul’s infamous song, “Opposites Attract”, the positive area of the water molecule is ‘attracted’ to a different water molecule’s negative portion.
The water molecules will align together, forming a network of water molecules, often referred to as hydrogen bonding. It is this hydrogen bonding network of water molecules that causes water to have an unusually high boiling point for such a small molecule, and the reason that ice expands during those cold winter nights… causing your pipes to crack, which creates a massive water leak under your house, causing you to call the plumber at 1 a.m., for which he charges an extra fee for the late-night call, waking up the entire neighborhood… all because of the existence of a slight polarity in H2O’s structure… but I digress, sorry.
Conversely, hydrophobic molecules do not contain areas of polarity within their structure, and thus do mix with water. Hydrophobes do not have the correct physical properties to disrupt water’s hydrogen bonding network, and thus are relegated to aggregating within the aqueous solution. This is where physics and geometry take over. Evoking our inner-Archimedes, the geometric shape with the highest volume and lowest surface area is a sphere, which is why hydrophobic molecules tend to form spherical shapes when placed in water. These ‘oil-droplets’ adopt this spherical shape in order to minimize contact with the polar solvent, thus reducing the energy at the surface (recall that the lowest energy state of a system is always favored).
Surfactant and Hydrophobic Examples
While physics and geometry have guided us to this point, clever chemistry is what has taken these hydrophobes into new area of surface science. Nature, as well as brainy scientists, have taken advantage of a hydrophobe’s tendency to aggregate in water by modifying the hydrophobe’s molecular architecture.
If a hydrophobe is covalently linked to a hydrophilic entity, then a new type of molecule is formed that can co-exist at the interface between oil-and-water. These types of compounds that have distinct sections of hydrophobicity and hydrophilicity are considered to be agENTS that are ACTive at the SURFace of two (or more) opposing substrates. Due to these properties, these compounds are commonly referred to as SURF-ACT-ANTS.
The structure of a surfactant allows the hydrophobic portion to interact with other hydrophobes in the solution, while the hydrophilic area of the surfactant can be dispersed in the aqueous media. This has profound improvements on the solubility of a hydrophobic compound in water, as surfactants help to lower the energy between the two immiscible substrates.
Two of the best-known methods to measure the energy at the surface is to determine the
- contact angle of a solution on a particular surface or
- interfacial tension of two immiscible liquids/solutions.
Both methods relate the geometrical shape (remember Archemedes’ geometry) of the energy of entire system, whereas solutions with higher surface energy (i.e. no surfactant) will adopt a more spherical shape and solutions with lower surface energy will have an increasingly more oval shape (‘ovality’ increases with the amount or strength of the surfactant).
Oxiteno Is a Leading Surfactant Innovator
For more than 30 years, Oxiteno has dedicated itself to the development of better, greener and safer surfactants for uses throughout the industrial and commercial markets. Our knowledge of surface chemistry coupled with our ability to meet the industrial volumes of surfactants has placed Oxiteno’s products in a variety of market segments, including surfactants for Oil & Gas, Agricultural surfactants and even into your home (in the form of cosmetics and cleaning products).
Oxiteno stresses the importance of producing a safe product for all to use, while also finding new ways to further minimize our environmental footprint. Sourcing for renewable, biodegradable and eco-friendly raw materials is an onus that Oxiteno places upon itself to sustain a future for the surfactant industry that has helped so many areas of society.
Visit our website today to learn more about Oxiteno’s product line and team of surfactant manufacturers.
[a] Anslyn, Eric V.,Dougherty, Dennis A.. (2006) Modern physical organic chemistry /Sausalito, CA : University Science, Chapter 3.