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Vitamin Solubility

Molecular Basis for Water Solubility and Fat Solubility

The solubility of organic molecule is frequently summarized through the phrase, "like dissolves like." This way that molecule with countless polar teams are an ext soluble in polar solvents, and also molecules with few or no polar groups (i.e., nonpolar molecules) are more soluble in nonpolar solvents. (You encountered these concepts in the "Membranes and also Proteins" experiment and also the associated tutorial, "Maintaining the Body"s snucongo.org: Dialysis in the Kidneys".) Hence, vitamins room either water-soluble or fat-soluble (soluble in lipids and also nonpolar compounds), relying on their molecule structures. Water-soluble vitamins have plenty of polar groups and are hence soluble in polar solvents such together water. Fat-soluble vitamin are mostly nonpolar and hence space soluble in nonpolar solvents such together the fatty (nonpolar) organization of the body.

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What makes polar vitamins soluble in polar solvents and also nonpolar vitamins dissolve in nonpolar solvents? The answer to this question lies in the species of interactions that occur between the molecule in a solution. Solubility is a complex phenomenon that counts on the readjust in free energy (ΔG) of the process. Because that a process (in this case, a vitamin dissolve in a solvent) to it is in spontaneous, the adjust in complimentary energy should be an adverse (i.e., ΔG


Thermodynamics of dissolution (Solubilization)

The resolution of a problem (solute) have the right to be separated right into three steps:
The solute particles have to separate indigenous one another. The solvent particles must separate sufficient to make room for the solute molecules to come in between them. The solute and also solvent corpuscle must connect to kind 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) the a procedure ( ΔG=ΔH-TΔS, whereby T is the pure temperature). The enthalpy and also entropy alters that take place in the dissolution process are shown in figure 2, below. In the dissolution process, steps 1 and also 2 (listed above) require energy because interactions between the corpuscle (solute or solvent) room being broken. Action 3 usually release energy because solute-solvent interactions space being formed. Therefore, the change in enthalpy (ΔH) for the dissolution procedure (steps 1 with 3) can be either positive or negative, depending upon the amount of energy released in action 3 family member to the amount of energy required in actions 1 and 2. In terms of the change in entropy (ΔS) that the dissolution process, most dissolution processes cause a better randomness (and therefore rise in entropy). In fact, because that a huge number of dissolved reactions, the entropic impact (the change in randomness) is much more important than the enthalpic effect (the change in energy) in identify the spontaneity that the process.


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Figure 2

The figure on the left schematically shows the enthalpy transforms accompanying the three procedures that must take place in order because that a solution to form: (1) separation of solute molecules, (2) separation the solvent molecules, and (3) communication of solute and solvent molecules. The overall enthalpy change, ΔHsoln, is the sum of the enthalpy transforms for each step. In the example shown, ΔHsoln is slightly positive, although it have the right to be optimistic or negative in other cases.

The figure on the appropriate schematically reflects the large, positive entropy change, ΔSsoln, the occurs when a equipment is formed. (Although ΔSsoln is generally positive, this value can be negative in certain situations entailing the resolution of strong ions.)


In general, if the solute and solvent interactions space of comparable strength (i.e., both polar or both nonpolar), then the energetics of procedures 1 and also 2 are comparable to the energetics of step 3. Therefore, the rise in entropy identify spontaneity in the process. However, if the solute and solvent interactions space of differing strength (i.e., polar through nonpolar), then the energetics of measures 1 and also 2 are much greater than the energetics of step 3. Hence, the boost in entropy that can happen is not enough to overcome the large increase in enthalpy; thus, the dissolution procedure is nonspontaneous.

To highlight the importance of ΔH and ΔS in determining the spontaneity of dissolution, let us consider three feasible cases:
The polar solute molecules are organized together by strong dipole-dipole interactions and hydrogen bonds between the polar groups. Hence, the enthalpy change to break this interactions (step 1) is large and positive (ΔH1>0). The polar solvent molecule are additionally held together by solid dipole-dipole interactions and hydrogen bonds, for this reason the enthalpy readjust for step 2 is also huge and positive (ΔH2>0). The polar groups of the solute molecules can communicate favorably with the polar solvent molecules, causing a large, an adverse enthalpy change for action 3 (ΔH31+ΔH2+ΔH3) is small. The little enthalpy adjust (ΔH),together through the positive entropy readjust for the process (ΔS), an outcome in a negative complimentary energy adjust (ΔG=ΔH-TΔS) because that the process; hence, the dissolution wake up spontaneously.

The dissolved of a nonpolar solute in a polar solvent.

The nonpolar solute molecules are hosted together only by weak van der Waals interactions. Hence, the enthalpy readjust to break this interactions (step 1) is small. The polar solvent molecules are organized together by solid dipole-dipole interactions and hydrogen bonds as in example (a), for this reason the enthalpy readjust for action 2 is big and hopeful (ΔH2>0). The nonpolar solute molecules perform not type strong interactions with the polar solvent molecules; therefore, the an adverse enthalpy change for action 3 is little and can not compensate for the large, positive enthalpy change of action 2. Hence, the overall enthalpy change (ΔH1+ΔH2+ΔH3) is big and positive. The entropy readjust for the process (ΔS) is not big enough to get over the enthalpic effect, and also so the overall free energy change (ΔG=ΔH-TΔS) is positive. Therefore, the dissolution does not occur spontaneously.


The nonpolar solute molecules are organized together only by weak valve der Waals interactions. Hence, the enthalpy adjust to break these interactions (step 1) is small. The nonpolar solvent molecules are likewise held with each other only by weak van der Waals interactions, therefore the enthalpy readjust for step 2 is likewise small. Also though the solute and solvent particles will additionally not form strong interactions with each other (only van der Waals interactions, for this reason ΔH3 is additionally small), over there is very tiny energy forced for procedures 1 and also 2 that have to be overcome in action 3. Hence, the overall enthalpy change (ΔH1+ΔH2+ΔH3) is small. The little enthalpy adjust (ΔH), together with the confident entropy change for the procedure (ΔS), result in a negative complimentary energy change (ΔG=ΔH-TΔS) for the process; hence, the dissolution wake up spontaneously.

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The principles outlined in the green box above explain why the interactions between molecules favor options of polar vitamin in water and nonpolar vitamin in lipids. The polar vitamins, and also the polar water molecules, have strong intermolecular pressures that must be conquer in order because that a equipment to be formed, requiring energy. Once these polar molecules interact with each other (i.e., once the polar vitamins are liquified in water), solid interactions are formed, publication energy. Hence, the in its entirety enthalpy change (energetics) is small. The small enthalpy change, coupled through a far-reaching increase in randomness (entropy change) once the systems is formed, enable this solution to form spontaneously. Nonpolar vitamins and also nonpolar solvents both have actually weak intermolecular interactions, for this reason the as whole enthalpy adjust (energetics) is again small. Hence, in the case of nonpolar vitamins dissolving in nonpolar (lipid) solvents, the tiny enthalpy change, coupled v a significant increase in randomness (entropy change) as soon as the solution is formed, allow this systems to kind spontaneously as well. For a nonpolar vitamin come dissolve in water, or because that a polar vitamin come dissolve in fat, the power required to conquer the early stage intermolecular forces (i.e., in between the polar vitamin molecule or in between the water molecules) is big and is not counter by the energy released once the molecules connect in equipment (because over there is no strong interaction between polar and also nonpolar molecules). Hence, in these cases, the enthalpy readjust (energetics) is unfavorable come dissolution, and the size of this unfavorable enthalpy readjust is too big to be offset by the boost in randomness the the solution. Therefore, these remedies will not kind spontaneously. (There space exceptions to the rule "like dissolves like," e.g., when the entropy decreases when a equipment is formed; however, this exceptions will certainly not be questioned in this tutorial.)

In general, it is possible to predict whether a vitamin is fat-soluble or water-soluble by analyzing its structure to identify whether polar groups or nonpolar teams predominate. In the structure of calciferol (Vitamin D2), shown in number 3 below, we uncover an –OH team attached come a bulky plan of hydrocarbon rings and chains. This one polar group is not enough to compensate for the much bigger nonpolar region. Therefore, calciferol is classified as a fat-soluble vitamin.


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Figure 3

This is a 2D ChemDraw depiction of the framework of calciferol, Vitamin D2. Return the molecule has one polar hydroxyl group, the is taken into consideration a nonpolar (fat-soluble) vitamin due to the fact that of the predominance of the nonpolar hydrocarbon region.