The World Can be Divided Into Three Types of Molecules: Ionic, Polar and Non-Polar
All the molecules in the world are divided into two categories: ionic compounds, polar molecules and non-polar molecules. Ionic compounds are a vast 3D grid of charged particles that are attracted to all oppositely charged particles that surround them, creating a network of highly connected particles. Polar molecules have a positive and a negative side, and are like the Earth with a north and a south pole. Each atom in a polar molecule has a tendency to want to be either positively charged or negatively charged molecules. Non-polar molecules are like most other objects, they are made up of uncharged atoms, they do not have a negative and positive side, and are overall neutral.
Molecules are Attracted to Each Other
Molecules interact with each other in non-covalent ways, these are divided into three types of intermolecular forces: ionic bonding, hydrogen-bonding, and Van der Waals forces which includes subsets of: London dispersion forces (which also goes by induced dipole-induced dipole forces), dipole-dipole forces, and dipole-induced dipole forces. These attractions are electrostatic in nature, and stem from interactions in the electron clouds of valence electrons of the molecules in question.
All molecules are attracted to each other by Van der Waals forces. Different types of molecules interact in different ways. The strength of the interactions depends on the charges on the atoms within the molecules and the bonds inside each molecule. The greater the polarity or charge on the individual molecules, the more energy is given off by attaching them to each other, and the stronger the intermolecular interaction.
Each type of intermolecular attraction has an optimum distance where the attraction is maximized and repulsion is minimized, this is the most energy efficient state for these two molecules.
The optimum distance is called bond length, and generally the stronger the attraction, the lower the energy state, and the shorter the bond length. Distances that are shorter than optimum create a repulsive force between the molecules. Distances that are greater than the optimum still have attractive forces, but forces are decreased due to an inverse squared relationship with the distance between the molecules (see the explanation of Coulomb’s Law later in this unit.)
Ionic bonding occurs when a positively charged molecule is nearby a negatively charged molecule. These oppositely charged molecules, or ions, have an electrostatic attraction to each other. Positively charged molecules are those that have fewer electrons than protons. Negatively charged molecules are the opposite, and have more electrons than protons.
For example, an ammonium molecule (NH
) has one fewer electron than protons. Nitrogen contributes 7 protons and 7 electrons (5 of the electrons are valence electrons), and each of the four hydrogens contributes 1 proton, and 1 valence electron. There should then be 9 valence electrons in this molecule, but it only has space for 8 valence electrons, according to the Octet Rule and the electron requirements of each atom. One of the valence electrons is not used, and is essentially lost. This leaves the molecule with a total of 10 electrons (including valence and kernel), even though it has 11 protons. Protons are positive, and the overall balance of charge is +1.
A hydroxide ion (OH
) has one more electron than it has protons. Oxygen contributes 8 protons and 8 electrons (6 of the electrons are valence electrons), and the hydrogen contributes 1 proton, and 1 valence electron. There should then be 7 valence electrons in this molecule, but it has space for 8 valence electrons, and according to the Octet Rule and the electron requirements of each atom, it needs one more valence electron. It will take an electron from a nearby atom or ion that is able to donate one. This leaves the molecule with a total of 10 electrons (including valence and kernel), even though it has only 9 protons. Electrons are negative, and the overall balance of charge is -1.
The charges on molecules often have a particular location in the molecule where they settle, this is due to atoms pulling on their electrons with greater strength, a property known as electronegativity. Positive charges settle on the atoms with the lowest electronegativity, while negative charges settle on the atoms with the highest electronegativity. The relative electronegativities of the atoms in our example molecules are H < N < O. The (+1) for the ammonium ion will be located on one of the hydrogens, and the (-1) for the hydroxide ion will be located on the oxygen. This creates the positive and negative ends of the molecule, essentially a (+) and (-) pole, which is a property also known as polarity, or having a dipole moment. According to the equation for Coulomb’s law, F=k((q1q2))/r^2 the force of attraction between the charged objects (the NH
ions in our case) is proportional to the charge on the objects multiplied by each other (q1 & q2) divided by the distance between the objects (r) squared. Basically, the larger the charges on the ions, the larger the force of attraction between them, and the closer the molecules are to each other, the larger the force of attraction between them.
Polar molecules are built by connecting atoms with differing electronegativities. When atoms with high electronegativity are bonded to atoms with low electronegativity, the electrons are shared unevenly. Electrons gather more closely to the atom with high electronegativity. This causes a partial negative charge at one end of the bond, and a partial positive charge at the opposite end of the bond. This results in a similar polarity to ionic bonds, but not as strong. It is also referred to as polarity, and dipole moment. Polar molecules can have multiple polar bonds. Often the polar bonds line up asymmetrically, sometimes causing one partial positive side and one partial negative side, but other times causing more than one area of partial positive and partial negative charge.
Non-polar molecules exist in two different categories. The first type consists of connecting atoms with very similar or the same electronegativity. Bonds between atoms with similar pull on electrons means the electrons are shared relatively evenly, and have insignificant charge separation, or perfectly evenly, and creates a neutral bond.
The second type consists of connecting atoms with differing electronegativities in extremely symmetrical shapes. Polar bonds exist between pairs of atoms because they have significantly differing electronegativities, but since the dipole moments line up in opposite directions, the dipoles (which are basically electromagnetic forces) cancel each other out. They are like evenly matched teams in tug-o-war, stalemate occurs. The dipoles cannot be felt by neighboring molecules. So, even though there are polar bonds within the molecule, the overall molecule is non-polar.
London Dispersion Forces/Induced Dipole-Induced Dipole Forces
London dispersion forces are forces of attraction between two molecules. When one molecule gets close to another molecule, the electrons in each molecule repel each other, inducing temporary polarity in both molecules. The temporarily positive end of one molecule is attracted to the nearby temporarily negative charge on the neighboring molecule. This induced dipole–induced dipole interaction is the weakest of the types of intermolecular interactions, and is the only type of interaction that occurs between two non-polar molecules. Non-polar molecules are therefore not strongly attracted to each other when compared to other types of molecules, and their corresponding interactions.
Van de Waals interactions are always present, but other types of interactions can compete with and overshadow them.
Dipole-dipole forces are forces of attraction between two polar molecules. When one polar molecule gets close to another polar molecule, the molecules orient themselves so the partial positive of one molecule is close to the partial negative of the neighboring molecule. They are then attracted to each other. This dipole–dipole interaction is the strongest of the types of Van der Waals interactions, but is not as strong as ionic bonding. Polar molecules are therefore significantly attracted to each other when compared to other types of molecular interactions.
Dipole-Induced Dipole Forces
Dipole-induced dipole forces are forces of attraction between one polar and one non-polar molecule. When the polar molecule gets close to the non-polar molecule, a temporary polarity is induced in the non-polar molecule, much like with London dispersion forces. The permanently charged end of one molecule is attracted to the nearby temporarily charged end on the neighboring molecule. This dipole–induced dipole interaction is the second weakest of the types of intermolecular interactions. Non-polar molecules are therefore not strongly attracted to polar molecules and other types of interactions can compete with them, and overshadow them.
Hydrogen bonding occurs when a polar molecule has a very electronegative atom (N, O, F, S) with a non-bonding lone pair of electrons, and a neighboring molecule has a very electronegative atom bonded to a hydrogen atom.
The hydrogen can loosen its covalent bond with its atom, and interact with both its own electronegative atom, and the unbonded electrons on the neighboring electronegative atom. Hydrogen bonds usually are asymmetrical, with the hydrogen having a shorter and stronger connection to the atom in its own molecule, but occasionally can be perfectly symmetrical.
Hydrogen bonds are known to stabilize many types of molecules.
Polymers can be stabilized by impermanent cross-links made of hydrogen bonds. The complimentary bases in DNA are connected via hydrogen bonding, which provides a stable interaction to allow DNA to “zip-up”.
This bond is significantly weaker than the covalent bond between bases in the DNA sequence, allowing DNA to more easily “un-zip” than be completely broken.
Hydrogen bonds in smaller molecules make liquids and crystalline solids more stable, viscous and cohesive than similar small molecules that do not hydrogen bond.
The classic example of this is water. Comparing the properties of molecules of C, N, O, and F bonds with H, only water has the highest melting and boiling point, and extremely strong cohesive properties.
is incapable of hydrogen bonding, because carbon is not electronegative enough, NH
has only one lone pair available for hydrogen bonding, and HF has only one hydrogen available for bonding, but water has two hydrogens available, and two lone pairs available for hydrogen bonding, doubling the hydrogen bonding capacity, making it very stable and cohesive.
Polar molecules can mix with and dissolve other polar molecules and ionic molecules. Non-polar molecules can mix with and dissolve other non-polar molecules. Polar molecules and non-polar molecules do not mix, and behave as if they even repel each other.
Molecules That Have Polar and Non-Polar Ends
Some very large molecules have different sections, sometimes with a polar section and a non-polar section. There are several very common groups of linked atoms that are attached to molecules of atoms that are called functional groups, or for short ‘R’ groups. Some R-groups are polar, like alcohols, aldehydes, and carboxylic acids while others are non-polar like methyl groups, (CH
It is possible to covalently bond a polar functional group on one side and a non-polar functional group on another side of a molecule. Having one end of a molecule that is polar, and one end non-polar means you can get one end to stick to other polar molecules (called hydrophilic), and the other to stick to non-polar molecules (called hydrophobic).
These molecules are called surfactants, and they have many uses. (Students should think about and brainstorm ideas about what a molecule like this might be able to do.)
The hydrophobic effect is a result of many energy factors present when non-polar molecules attempt to mix with water. Non-polar molecules are only weakly attracted to polar water, but water is strongly attracted to itself because it is an excellent hydrogen-bonder. It is more favorable for the water to stick to itself by dipole-dipole and hydrogen-bonding interactions than to the non-polar molecules by only dipole-induced dipole bonding.
These conditions create a situation where it is favorable for non-polar molecules to aggregate in pockets away from the water.
Both the non-polar molecules and the water can bond to themselves, but not to each other. It is also worth noting that the water molecules will bond with each other around the surface of the surfactant pocket.
To break these bonds takes energy, this creates a sort of barrier.
Once they are in this configuration, gaining the energy to break free can be difficult. If the non-polar substance has a different density than that of water, and there is enough quantity the solvents will repel each other enough to form layers, one on top of the other.
In the case of large molecules like proteins, the hydrophobic effect causes molecules to fold and to orient themselves so the polar or charged ends are on the outside of the pocket, and their non-polar ends are folded inside.
The non-polar groups then weakly bond with themselves by London dispersion forces, the polar groups bond to the water molecules by whatever means they are able, dipole-dipole, hydrogen-bonding, ionic bonding.
The water will still construct a hydrogen-bond energy barrier on the outside of the pocket, but it is compounded frustrated landscape proteins move in.
They are composed of long chains, sometimes with branches, and have very limited mobility, and can often get stuck in particular configurations because they are difficult to move. Once in a favorable configuration it is very stable.
Surfactants Attenuation With Water: Do Surfactants Prefer to Dissolve in Water or Oil?
Since the scope of this unit encompasses intermolecular forces in biological systems, attenuation with water needs to be considered. When comparing the interactions of polar or ionic portions of surfactants in aqueous systems, and in non-aqueous, ionic bonding in aqueous systems are significantly weaker.
When dissolved in water, the hydrophilic ends of the surfactant can interact with water by dipole-dipole, hydrogen bonding (if it can accept or donate hydrogen), or ionic bonding with OH
ions in neutral water. Bonding to the water is favorable, it increases the options for what the polar portions can bond to, and the choice between bonding to the water or to itself is muddled. Water therefore obstructs the dipole-dipole and ionic interactions, and diminishes their strength.
Meanwhile, the hydrophobic (non-polar) sections will fold inward due to the hydrophobic effect and bond to itself via induced dipole-induced dipole forces.
When dissolved in oil, the hydrophilic ends of the surfactant will fold inward, and while isolated from water, hydroxide and hydronium ions, will bond to each other by either dipole-dipole or ionic bonding.
This happens as if in a vacuum, and the attraction between the positive and negative poles becomes very strong due to the very small distance between them (refer to the explanation of Coulomb’s Law.)
Simultaneously, induced dipole-induced dipole bonds occur between the non-polar ends of the molecule and the non-polar solvent molecules.
When comparing all the factors and types of bonding in both situations, dissolving in oil is more favorable than dissolving in water.
Ionic bonding and dipole-dipole bonding are stronger than induced dipole-induced dipole bonding, so choosing the situation that maximizes the stronger type bonds is more favorable. Surfactants can and will dissolve in water, but if given the option, due to stronger intermolecular interactions it prefers to be dissolved in a non-polar solvent.
Surfactants: Plant Oil + Base = Soap
Surfactants can be produced by reacting fatty acids with lye (sodium hydroxide).
Adding sodium hydroxide solution to oils causes detachment of the fatty acids. The oxidized fatty acid becomes an anion, and can bond to the sodium cation, allowing a network of large clumpy non-polar molecules to bond ionically at one end. This means it can form a tangled network. As the soap cools, it becomes a solid.
The fatty acids have a hydrophobic and a hydrophilic end, allowing them to stick to both polar and non-polar molecules.
Soaps are made from plant oils, which are triglycerides. Triglycerides are 3 long chains of carbons, either saturated or unsaturated with hydrogen atoms, and connected to each other by a glycerol.
They are esters, but the carbon chains are so much larger than the ester they are essentially non-polar molecules.
These molecules can by hydrolyzed with a base. Sodium hydroxide is often used, and produces 3 non-polar chains with an ionized oxygen at one end (fatty acid), all ionically bonded to a sodium ion, and a glycerin molecule.
Paint: Plant Oil+ Pigment + Binder = Paint
Paints are similar in starting materials to surfactants, triglycerides. Because triglycerides are very large molecules they have a lot of London dispersion forces, they are fairly well bonded to each other for non-polar molecules, and are a fairly thick liquid at room temperature.
Most plant oils are unsaturated (have some C=C double bonds). As they out in the presence of oxygen, the carbon atoms that are double-bonded to each other start to bond to other unsaturated carbons in double bonds in nearby chains. Each unsaturated carbon that bonds leaves a carbon at the other end of its previous double-bond destabilized.
The destabilized carbon then forms a cross-link bond to another unsaturated carbon in another nearby triglyceride molecule.
This chain reaction keeps going until essentially all the unsaturated carbons have bonded to other unsaturated carbons. This essentially turns the thick liquid into one huge flat molecule with many connections. It is now a web of connected chains, which is an ideal solid to trap pigment molecules in place.
Plant oils take a fairly long time to complete this drying process they are highly desired for paints.
Lake pigments were developed in renaissance times, and are essentially oil paints that hold a water soluble pigment.
Oil paints are desirable for artists because they dry slowly, staying wet for long periods of time.
This property allows the painter to keep working on a section, blending, changing shade or hue over several days or weeks, allowing for a lot of flexibility. Artists do want the oils to dry eventually, so they are permanent, but not too fast, so, not all oils are created equal, linseed is a favorite.
The problem is that oil based pigments don’t often occur in nature in robust colors, the robust colors are often salts that are soluble in water, and not in oil.
Water soluble salts are polar and will not mix with non-polar oils. They will form large pockets or layers above or below the drying oil. This is not ideal for painting. It could cause uneven color, bubbling, cracking, or prevent the oil from drying altogether. Some polar pigment particles can be coaxed into diffusing in oil even though they would rather not dissolve. This can be done primarily with aluminum salts.
Aluminum potassium sulfate (commonly known as alum) will bond to polar pigments, and creates small globular particles.
These particles will diffuse evenly in the oil, and get stuck in the web of crosslinking fatty acids as the oils dries. These type of pigments are called Lake pigments.