There's something that confuses me about lipids. After looking at the structure of things like cholesterol and phospholipids, something struck me as weird. Naturally lipids are hydrophobic, yet according to its structure it implies that it should be polar. The presence of its hydroxyl group is the reason for this. Could it be that its overall electronegativity is what's causing lipids to be hydrophobic, it is there some other aspect I don't understand?
Lipids
- Thread starter Votorx
- Start date
Phospholipids have the special property of having both hydrophobic and hydrophilic parts, although they are still mostly hydrophobic. A molecule that is both hydrophilic and hydrophobic is called amphipathic. The “head” is polar and hydrophilic, whereas the fatty acid “tail” is non-polar and hydrophobic. The amphipathic nature of phospholipids is vital to their ability to make cell membranes. Try searching for a picture of a cell membrane and you will see how a phospholipid bilayer is formed with the hydrophilic heads are on the outside/inside of the cell and the hydrophobic tails are inside the membrane. Sterol derivatives (such as steroids) are entirely hydrophobic, and have their own special structure that does not include fatty acids. 
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Okay, lol I worded my question wrong. I do realize that Phospholipids are both hydro phobic and felic. That was a wrong example to use here. When I look at a phospholipid I can see that its head is hydropfelic because of its phosphate group. What I don't understand though is if you take fats like Lauric Acid, Palmitic Acid, Stearic Acid etc. etc., they each contain a carboxyl group. Rationally you'd think that due to its moderatly polar carbonyl group and its highly polar hydroxyl group, these lipids should be somewhat hydrofelic but they aren't, why? Again is it due to its overall electronegativity?
The carboxyl group is shielded by the hydrogen and carbon atoms, creating a non-polar environment. Carbon-hydrogen bonds do not have much polarity, unlike oxygen hydrogen bonds.
Yes but once again that's a phospholipid and in this case its polarity is due to its phosphate group, not a carboxyl group as I mentioned above. If you look at this picture of a Lauric Acid. You can see towards the right end of the molecule theres a carboxyl group, yet if I'm correct, it is still hydrophobic, why?
P.S - I don't see any carboxyl groups in the example you gave me, and in this case it is not shielded off.
P.S - I don't see any carboxyl groups in the example you gave me, and in this case it is not shielded off.
Whether a molecule is hydrophobic or hydrophilic is based on it's overall polarity. Phosphate and carboxyl groups are both polar groups, but the majority of the molecule is in the long carbon-hydrogen chains, which are non-polar. So, overall, the molecules are hydrophobic. A strong polarity doesn't necessarily mean hydrophilic.
Catastrophe
Registered Senior Member
I think you need take account of the relative proportions of hydrophilic and hydrophobic components.
This is measured by the HLB or hydrophilic lipophilic balance. One method of comparing this is by summing values for different groups. -OH is +1 and -CH2 or CH3 is -1. Carboxylic acid ion is +18. (These are for comparison - not actual HLB values. A constant must be applied).
Thus C12H25OH is -12 +1 = -11.
Note carboxylic acid ion It must be alkaline pH.
Undissociated COOH is +2.
So C12 fatty acid is -11 +2 = -9.
Ionised acid is -11 +18 = +7
HLB was originally designed for ethoxylated nonionic surfactants and was the percentage by weight of the molecule represented by the ethylene oxide chain divided by 5, so the maximum HLB was 20. Later it was applied to ionic surfactants (amphiphiles) by cross calculation.
The HLB of potassium oleate is 18 (but pH dependant).
Glycerides of natural fatty oils have HLB 5-7.
These values are assumed proportional for mixtures (not exact), so a 50/50 mixture of potassium oleate and natural oil glycerides would be assumed to have HLB (0.5 x 18) + (0.5 x 6).
These are used in selecting emulsifiers since HLB requirements for different oils (e.r. mineral oils 9-11) have been determined and can be fitted to known HLBs of emulsifiers.
This is measured by the HLB or hydrophilic lipophilic balance. One method of comparing this is by summing values for different groups. -OH is +1 and -CH2 or CH3 is -1. Carboxylic acid ion is +18. (These are for comparison - not actual HLB values. A constant must be applied).
Thus C12H25OH is -12 +1 = -11.
Note carboxylic acid ion It must be alkaline pH.
Undissociated COOH is +2.
So C12 fatty acid is -11 +2 = -9.
Ionised acid is -11 +18 = +7
HLB was originally designed for ethoxylated nonionic surfactants and was the percentage by weight of the molecule represented by the ethylene oxide chain divided by 5, so the maximum HLB was 20. Later it was applied to ionic surfactants (amphiphiles) by cross calculation.
The HLB of potassium oleate is 18 (but pH dependant).
Glycerides of natural fatty oils have HLB 5-7.
These values are assumed proportional for mixtures (not exact), so a 50/50 mixture of potassium oleate and natural oil glycerides would be assumed to have HLB (0.5 x 18) + (0.5 x 6).
These are used in selecting emulsifiers since HLB requirements for different oils (e.r. mineral oils 9-11) have been determined and can be fitted to known HLBs of emulsifiers.
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Votorx said:
Look at the model I provided along the polar/nonpolar line it clearly shows the carboxyl groups the connect the hydrocarbon tails. In a NMR the electron negativity of those carbon atoms attached to the oxygens would be unusually low for a carboxyl because of the shielding effect of the surrounding hydrogens.
No its a Carboxyl group: the lipid tails when freed from the glycrol forms COOH.
A Carbonyl is characterised by one oxygen double bonded to a carbon atom, Carboxyl is two oxygens, one double bonded to a carbon atom. To be more exact in organic chemsitry both of these are under the family of Carbonyl groups as Ketones (-CO-), Esters(-COO-) and Carboxylic acid (-COOH).
A Carbonyl is characterised by one oxygen double bonded to a carbon atom, Carboxyl is two oxygens, one double bonded to a carbon atom. To be more exact in organic chemsitry both of these are under the family of Carbonyl groups as Ketones (-CO-), Esters(-COO-) and Carboxylic acid (-COOH).
its a Ester to be exact, calling it a carbonyl is like calling a person a mammal.
Catastrophe
Registered Senior Member
Why not consider a simple example?
Take a strong group like sulphate instead of a weak carboxylic acid group the strength of which varies with pH.
Sodium methyl sulphate is water-soluble. (High HLB). As you increase the alkyl chain length the water-solubility decreases. Sodium octyl sulphate is still water-soluble. By the time you get to sodium lauryl sulphate the Krafft temperature is agout 8 deg. C. That means it comes out of solution at that temperature. Actually the Krafft Point (or temperature) gives a straight line relationship with carbon chain length.
Gu, T and Sjoblom, J. (1992)
Colloids and surfaces, 64(1), 39-46
You can see the same with soaps, but water-solubility varies with pH. In acid pH you have the low water-solubility fatty acid. At high pH, the more water-soluble soap. Sodium acetate is water-soluble and the water-solubility decreases with increase in fatty chain length (at constant pH).
Take a strong group like sulphate instead of a weak carboxylic acid group the strength of which varies with pH.
Sodium methyl sulphate is water-soluble. (High HLB). As you increase the alkyl chain length the water-solubility decreases. Sodium octyl sulphate is still water-soluble. By the time you get to sodium lauryl sulphate the Krafft temperature is agout 8 deg. C. That means it comes out of solution at that temperature. Actually the Krafft Point (or temperature) gives a straight line relationship with carbon chain length.
Gu, T and Sjoblom, J. (1992)
Colloids and surfaces, 64(1), 39-46
You can see the same with soaps, but water-solubility varies with pH. In acid pH you have the low water-solubility fatty acid. At high pH, the more water-soluble soap. Sodium acetate is water-soluble and the water-solubility decreases with increase in fatty chain length (at constant pH).
Well of course its solubility will varie with PH, but this isn't what I'm referring to. I was just simply asking why, with such a polar group like a carboxyl group, is fats so hydrophobic. I just accepted the fact now that it's it overall polarity, yet I still wonder why this isn't true for Phospholipids. What is more polar? A phosphate group or a carboxyl group? Does anyone know? And what the difference between the 2 groups?
the phosphate group has a constant ionic charge, which makes it more polar the a carboxyl group that only has a resonant ionic charge when deprotonized. The ester is even less polar.
Catastrophe
Registered Senior Member
Please distinguish between a carboxylic acid group which is ionic at alkaline pH and the presence of such a carbonyl group in an ester. The contribution towards water-solubility of an ester group is almost nonexistent.
Distinguish between ionic surfactants where by definition there is a negative or positive charge (or both as in amphoterics) and nonionic surfactants. The commonest water-solubilising group in nonionics is the ethoxylate chain (polyoxyethylene chain) produced by addition of ethylene oxide to a hydrophobe such as fatty alcohol, fatty acid etc.. In nonionics water-solubility is achieved by hydrogen bonding of water to slightly polar groups, often involving oxygen e.g., -OH. Because the contribution towards water-solubility per group is low, many such groups are needed. The number of groups to equate to one ionic group is around 7-11.
Distinguish between ionic surfactants where by definition there is a negative or positive charge (or both as in amphoterics) and nonionic surfactants. The commonest water-solubilising group in nonionics is the ethoxylate chain (polyoxyethylene chain) produced by addition of ethylene oxide to a hydrophobe such as fatty alcohol, fatty acid etc.. In nonionics water-solubility is achieved by hydrogen bonding of water to slightly polar groups, often involving oxygen e.g., -OH. Because the contribution towards water-solubility per group is low, many such groups are needed. The number of groups to equate to one ionic group is around 7-11.
Catastrophe
Registered Senior Member
"Let's not forget Micelle's "
OK. The hydrophobic groups in surfactants tend to congregate in water so that the hydrophilic groups interfaces with the water. The simplest micelle is spherical with the hydrophobes congregating in the centre. Typically around 60 molecules are involved in spherical micelles.
There is a repulsion between the head (hydrophilic groups) even in nonionic surfactants and the hydrophobes appear to attract one another - although this is actually an entropic effect in that the hydrophobes tend to avoid the water.
An interesting sidelight on micelles is that there is a decrease in entropy on forming a more ordered structure. This must be balanced by an increase in entropy which is brought about by the structured hydrophobes (in the micelle) release water which would otherwise be more structured by their presence in bulk solution. Releasing water molecules as free water provides a larger increase in entropy than the apparent structure of the micelle decreases entropy.
"OK. The hydrophobic groups in surfactants tend to congregate in water so that the hydrophilic groups interfaces with the water. The simplest micelle is spherical with the hydrophobes congregating in the centre. Typically around 60 molecules are involved in spherical micelles.
There is a repulsion between the head (hydrophilic groups) even in nonionic surfactants and the hydrophobes appear to attract one another - although this is actually an entropic effect in that the hydrophobes tend to avoid the water.
An interesting sidelight on micelles is that there is a decrease in entropy on forming a more ordered structure. This must be balanced by an increase in entropy which is brought about by the structured hydrophobes (in the micelle) release water which would otherwise be more structured by their presence in bulk solution. Releasing water molecules as free water provides a larger increase in entropy than the apparent structure of the micelle decreases entropy.