A lipid bi-layer is made up of molecules that are both “Hydrophobic” and “Hydrophillic”. When the bi-layer forms are one of these forces stronger then the other?
If the forces aren’t balanced, the membrane will change until they are balanced. This may entail the dissolution of the membrane.
I’m not sure what you mean by “stronger.” The lipid molecules have a hydrophilic polar head and a hydrophobic tail. This ensures that the heads orient toward the outside of the membrane (next to the aqueous solution) while the tails form the inner core of the membrane.
Some basic explanations are here.
I guess what I was trying to ask is if in an aqueous solution do bi-layers form because of repulsion or attraction to water? Does one force drive the other?
Attraction, as much as anything else. There’s no such thing as a hydrophobic or hydrophilic “force”. There are van der Waals interactions, hydrogen bonds, and dipole-dipole interactions.
What happens is that the hydrophilic heads preferentially enter into transient hydrogen bonds and dipole-dipole interactions with the water. This is a low-energy state. The hydrophobic tails cannot do this. Since water in the liquid phase prefers to form a network of hydrogen bonds, what will happen is that over time, the hydrophilic heads will tend towards being part of those. This means that they will get pointed out towards the water.
Simultaneously, the long hydrophobic tails want to rub against each other. The van der Waals attractive forces between them grow larger with the contact area. So over time, the amount of contact area between them will tend towards the maximum.
There are several configurations of lipids that simultaneously fulfill both of these states. One is the micelle, a spherical structure made of lipids with hydrophilic heads out, and hydrophobic tails in. Another is the bilayer, made of two planar layers of lipids.
Because the “hydrophilic” property of a substance really is dependent on how many hydrogen bonds it can form with water, and because there’s a fundamental limit to how many hydrogen bonds a set of water molecules can form with the solute (resulting in the so-called “water cage” bond network), the extent of impact that the hydrophilic head has is limited to that. It can be less, but it cannot be more.
The “hydrophobic” property, on the other hand, is dependent on the surface area of a molecule. The greater the surface area of a non-polar molecule, the more hydrophobic it is. It will want to maximize van der Waals interactions with itself, or similar molecules, forming tight aggregates. I imagine that if the hydrophobic tail was sufficiently long, and if the packing characteristics were set up sufficiently weirdly that it would not want to pack with other molecules like itself, the “curl up onto yourself” interaction would dominate. What’d happen then is that there’d be no membrane, and instead you’d have tightly conformed hydrophobic tails floating in the water, with a little polar head sticking out. These structures would preferentially form aggregates in the water, so what you’d eventually have is something that maximized the amount of little polar heads contacting the water, and had very tightly packed and contorted masses of tails inside, sorta like an uber-micelle.
SO! Back to the OP. Bilayers form because there exists a statistical tendency for the hydrophilic heads to remain contact the water, and simultaneously there exists a statistical tendency for the tails to remain in contact with other tails. As I explained previously, there’s a fundamental limit to how strong the hydrophilic tendency of an uncharged molecule can be, while the hydrophobic tendency can go up and up and up. A bilayer will form when the hydrophilic tendency of the head moeity is high enough proportionally to the hydrophobic tendency of the tail moeity that they get oriented properly, and self-interaction is minimal.
The answer, thus, is “it depends”. 