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Vitamin Solubility

Molecular Basis for Water Solubility and Fat Solubility

The solubility of organic molecules is often summarized by the phrase, "like disappear like." This method that molecule with many polar teams are an ext soluble in polar solvents, and also molecules with couple of or no polar groups (i.e., nonpolar molecules) are much more soluble in nonpolar solvents. (You encountered these principles in the "Membranes and Proteins" experiment and the connected tutorial, "Maintaining the Body"s cg-tower.com: Dialysis in the Kidneys".) Hence, vitamins are either water-soluble or fat-soluble (soluble in lipids and nonpolar compounds), relying on their molecular structures. Water-soluble vitamin have many polar groups and also are therefore soluble in polar solvents such as water. Fat-soluble vitamin are primarily nonpolar and also hence room soluble in nonpolar solvents such as the fat (nonpolar) organization of the body.

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What makes polar vitamins soluble in polar solvents and nonpolar vitamins soluble in nonpolar solvents? The answer come this concern lies in the species of interactions the occur in between the molecule in a solution. Solubility is a complex phenomenon that relies on the change in free energy (ΔG) that the process. Because that a process (in this case, a vitamin dissolving in a solvent) to it is in spontaneous, the adjust in complimentary energy must be negative (i.e., ΔG


Thermodynamics of dissolved (Solubilization)

The dissolution of a substance (solute) deserve to be separated into three steps:
The solute particles need to separate from one another. The solvent particles should separate sufficient to make space for the solute molecule to come between them. The solute and also solvent corpuscle must connect to form the solution.
The free energy (G) describes both the energetics (i.e., the enthalpy H) and the randomness or probability (i.e., the entropy S) of a procedure ( ΔG=ΔH-TΔS, whereby T is the pure temperature). The enthalpy and also entropy alters that occur in the dissolution process are presented in figure 2, below. In the dissolution process, measures 1 and 2 (listed above) require energy since interactions between the corpuscle (solute or solvent) space being broken. Action 3 usually releases energy since solute-solvent interactions are being formed. Therefore, the change in enthalpy (ΔH) for the dissolution procedure (steps 1 through 3) deserve to be either hopeful or negative, depending upon the lot of power released in action 3 loved one to the quantity of energy required in actions 1 and 2. In terms of the change in entropy (ΔS) of the resolution process, most dissolution processes lead to a better randomness (and therefore rise in entropy). In fact, because that a big number of resolution reactions, the entropic result (the readjust in randomness) is much more important 보다 the enthalpic result (the adjust in energy) in determining the spontaneity that the process.


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Figure 2

The number on the left schematically reflects the enthalpy changes accompanying the three procedures that must occur in order for a solution to form: (1) separation that solute molecules, (2) separation of solvent molecules, and also (3) interaction of solute and solvent molecules. The overall enthalpy change, ΔHsoln, is the sum of the enthalpy alters for every step. In the example shown, ΔHsoln is contempt positive, return it have the right to be confident or negative in other cases.

The number on the ideal schematically reflects the large, hopeful entropy change, ΔSsoln, that occurs when a systems is formed. (Although ΔSsoln is usually positive, this value could be negative in details situations involving the dissolution of strong ions.)


In general, if the solute and also solvent interactions room of similar strength (i.e., both polar or both nonpolar), climate the energetics of steps 1 and also 2 are comparable to the energetics of step 3. Therefore, the increase in entropy identify spontaneity in the process. However, if the solute and solvent interactions are of differing toughness (i.e., polar through nonpolar), climate the energetics of measures 1 and also 2 space much better than the energetics of action 3. Hence, the rise in entropy the can take place is not enough to conquer the large increase in enthalpy; thus, the dissolution process is nonspontaneous.

To show the importance of ΔH and ΔS in determining the spontaneity the dissolution, let us think about three possible cases:
The polar solute molecule are held together by solid dipole-dipole interactions and also hydrogen bonds between the polar groups. Hence, the enthalpy change to break these interactions (step 1) is large and confident (ΔH1>0). The polar solvent molecule are likewise held with each other by strong dipole-dipole interactions and also hydrogen bonds, therefore the enthalpy adjust for step 2 is also large and positive (ΔH2>0). The polar teams of the solute molecules can connect favorably v the polar solvent molecules, resulting in a large, an unfavorable enthalpy adjust for action 3 (ΔH31+ΔH2+ΔH3) is small. The small enthalpy adjust (ΔH),together with the hopeful entropy adjust for the process (ΔS), result in a negative free energy change (ΔG=ΔH-TΔS) for the process; hence, the dissolution occurs spontaneously.

The resolution of a nonpolar solute in a polar solvent.

The nonpolar solute molecule are organized together only by weak van der Waals interactions. Hence, the enthalpy adjust to break these interactions (step 1) is small. The polar solvent molecules are hosted together by strong dipole-dipole interactions and hydrogen bonds together in instance (a), therefore the enthalpy change for action 2 is huge and positive (ΔH2>0). The nonpolar solute molecules do not form strong interactions through the polar solvent molecules; therefore, the negative enthalpy readjust for action 3 is tiny and can not compensate for the large, hopeful enthalpy readjust of step 2. Hence, the overall enthalpy readjust (ΔH1+ΔH2+ΔH3) is big and positive. The entropy change for the process (ΔS) is not big enough to get over the enthalpic effect, and also so the overall totally free energy adjust (ΔG=ΔH-TΔS) is positive. Therefore, the dissolution does not take place spontaneously.


The nonpolar solute molecules are held together only by weak van der Waals interactions. Hence, the enthalpy adjust to break these interactions (step 1) is small. The nonpolar solvent molecules are likewise held together only by weak valve der Waals interactions, so the enthalpy adjust for action 2 is additionally small. Also though the solute and also solvent corpuscle will additionally not type strong interactions with each other (only van der Waals interactions, so ΔH3 is also small), there is very small energy compelled for measures 1 and also 2 that must be get rid of in action 3. Hence, the overall enthalpy adjust (ΔH1+ΔH2+ΔH3) is small. The little enthalpy change (ΔH), in addition to the confident entropy change for the procedure (ΔS), an outcome in a negative free energy readjust (ΔG=ΔH-TΔS) because that the process; hence, the dissolution occurs spontaneously.

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The ethics outlined in the environment-friendly box over explain why the interactions in between molecules favor options of polar vitamins in water and also nonpolar vitamins in lipids. The polar vitamins, and the polar water molecules, have strong intermolecular pressures that need to be overcome in order because that a systems to be formed, request energy. As soon as these polar molecules communicate with each other (i.e., once the polar vitamin are liquified in water), solid interactions are formed, release energy. Hence, the in its entirety enthalpy adjust (energetics) is small. The tiny enthalpy change, coupled through a far-ranging increase in randomness (entropy change) once the equipment is formed, enable this systems to type spontaneously. Nonpolar vitamins and also nonpolar solvents both have weak intermolecular interactions, therefore the all at once enthalpy adjust (energetics) is again small. Hence, in the case of nonpolar vitamins dissolving in nonpolar (lipid) solvents, the small enthalpy change, coupled v a significant increase in randomness (entropy change) as soon as the equipment is formed, enable this systems to kind spontaneously together well. Because that a nonpolar vitamin to dissolve in water, or for a polar vitamin to dissolve in fat, the power required to get rid of the early intermolecular pressures (i.e., in between the polar vitamin molecules or between the water molecules) is big and is not counter by the power released when the molecules communicate in systems (because over there is no solid interaction between polar and nonpolar molecules). Hence, in this cases, the enthalpy readjust (energetics) is unfavorable come dissolution, and the size of this unfavorable enthalpy change is too huge to be balance out by the rise in randomness that the solution. Therefore, these options will not kind spontaneously. (There space exceptions come the principle "like disappear like," e.g., once the entropy decreases when a equipment is formed; however, this exceptions will not be discussed in this tutorial.)

In general, it is feasible to predict whether a vitamin is fat-soluble or water-soluble by evaluating its framework to recognize whether polar groups or nonpolar groups predominate. In the structure of calciferol (Vitamin D2), displayed in number 3 below, we find an –OH team attached come a bulky arrangement of hydrocarbon rings and also chains. This one polar team is not enough to compensate because that the much bigger nonpolar region. Therefore, calciferol is classified together a fat-soluble vitamin.


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Figure 3

This is a 2D ChemDraw representation of the structure of calciferol, Vitamin D2. Back the molecule has one polar hydroxyl group, it is thought about a nonpolar (fat-soluble) vitamin because of the predominance of the nonpolar hydrocarbon region.