Nanolayer made of water-soluble cavitand for extracting alkanes from aqueous media

Xue ShaDepartment of Chemistry Drexel University

Contents:
1. Background
2. Abstract
3. Introduction
3.1.1 History of Cavitand
3.1.2 Improvement of Cavitand
3.1.3 Synthesis of Self-Folding Cavitands
3.2 Hydrophobic Effect
3.2.1 Hydrophobic and Hydrophilic Interactions
3.2.2 Thermodynamic way to explain how hydrophobic effect works
4. How to trap the oil residue in aqueous phase
5. Conclusion
6. Reference

1. Background
A large amount of oil spill in the ocean have been a big issue for years, even though they used booms to collect oil that as in the form of a separate phase floating on the surface of the water, as a result, there is a considerable amount of oil located underneath the surface of the water either completely dispersed or semi-dispersed. This kind of oil can make its way into the mixed salt and fresh water of lakes which connected to the ocean and also can make its way into fresh water that line the coast. Therefore, a new method is definitely need for the long-term removal of these crude oil that residue underneath the surface of the water.

2. Abstract
Extracting the crude oil from natural water is currently a hot issue, nowadays some methods is design to remove the floating oil on the surface of the ocean, but there is still some residue left underneath the water need to be extracted. Thus, there is a new way of designing and constructing of a water-soluble nanolayers that can trap alkanes which are the major components of crude oil from aqueous media has been studied recently. This nanolayers is majorly made from macrocyclic molecule [7]which has two separate parts, one of them presents as open-end hydrophilic hosts, the other possess a cavity of very pronounced hydrophobic character as binding site for non water-soluble guests (alkanes) in aqueous solution.

This macromolecules was synthesized by self-folding cavitand with side group then derivative side group to be water-soluble. Self-cavitand have been designed, improved and applied in many years in different areas. They used this macrocyclic molecules here to be tethered by one end to the surface of the solid to fix the nanolayer on a solid support, in this way, the nanolayer can be positioned to function anywhere within the aqueous medium to trap alkanes.[7]

Hydrophobic effect is the main driving force for synthesize the cavitand, it not only can stablize the folded structure of cavitand, but also give rise to deep cavity of cavitand to form capsular assembly which has a large volume inside result in burial of a large number of hydrophobic residues in the core.[11]

3. Introduction

3.1.1 History of Cavitand
First of all, this thought was discovered during last a few years from biological system, artificial host-guest systems have been built to bind of substrates by enzymes, antibodies, and receptors and the transport of ions across biological membranes mediated by ionophores as natural carriers[1]. The first this kind of cavitand which are open-ended container molecule that act as host for complementary guest was prepared by Cram[2] and used as cancer treatment. The binding properties have been studied and applied in gas phase, solid state and organic solvent, so they are more interested in applied it in aqueous phase to study the intermolecular hydrogen bonding of the host now.[2] The unique part of this kind of artificial host-guest system is the major and stable driving force for binding substrates in aqueous phase is hydrophobic effect[1].

3.1.2 Improvement of Cavitand
According to this thought, a type of artificial hosts was designed with hydrophobic cavitand as collecting binding sites for apolar guests, and complexation reaction in aqueous solution called water-soluble macrocycles of the cyclophane[1].This type of macrocycles was synthesized by two benzidine units to form host-guest complexes.They reported complexes of benzene and dioxane in the formation of stable 1:1 [3] when the macrocycle molecules and with a suitably sized cavity were recrystallized from these solvents. However, it was found that the complexes was not yielded that much in aqueous phase, hydrophobic interaction can not stabilize the conformation of macrocycle molecules and lacked any appropriately positioned functionality.[1] Other type of macrocycles was introduced by using aromatic walls surrounding the cavity by 4,4’-diaminodiphenylmethane units instead of benzene unit, obtained a new series of macrocyclic tetraazacyclophanes.[1] These hosts are water-soluble in their protonated form, and in acidic solution host-guest complexation with apolar guests being enclosed in the cavity. A crystalline 1:1 complex of host with durene as guest was obtained from aqueous solution, and molecular cavity inclusion was demonstrated by a crystal structure anaiysis. However, it need to be done in a high temperature in lower pH. [1]

Then Diederich and Dick[3] made a few improvements of constructing macrocyclic hosts of the cyclophane type which have a cavity of well-defined size and shape and very hydrophobic are water soluble not only at room temperature but also in neutral pH. In the macrocyclic skeleton, diphenylmethane units in the hosts are applied as bridged should function as rigid aromatic spacers and cavity walls. The hydrophobicity of the binding site should not be perturbed by strongly hydrated ionic centers built into the macrocyclic skeleton surrounding the cavity. In order to lower the pH in aqueous media, they introduced quaternary ammonium residues for water solubility cavitand at neutral pH.[3] They even changed the spatial arrangement by combining piperidinium rings, and introduced eight methyl groups at diphenylmethane unit to enhance the hydrophobic property of the cavity and lead to increase binding with apolar guests.Their cavitand can also accelerate via host-guest complexation the transports of aromatic hydrocarbons through an aqueous media along the concentration gradient.[3]

Based on the improvement, Saito, Nuckolls,and Rebek reported in 2000 [4]a rapid and efficient synthetic access to the vessels which could produce cavity with different kinds of shapes, structures and sizes. The incorporation of a variety of nonracemic groups into the structure’s upper rim was introduced in this new structrual idea. The stereocenters of vessels quickly and effectively transfer their asymmetry to the flexible walls of the cavitand and result in handed spaces with increased stability in aqueous media.[4]

Far, Shivanyuk and Rebek reported in 2001 [5]that they derivative resorcinarene which doesn't have strong and deep conformation inside cavity and no intramolecular hydrogen bonds to a novel energy-minimized structure of tetrabenzimidazoles in vaselike shape, this container not only can hold a large number of guest but also provides four execellent binding site for hydroxyl-containing molecules and would stabilized the vase conformation.

In 2005, Hooley, Van anda and Rebek [6]published another paper about a even more interesting design of cavitand: open-end can be rotated over by attaching a "door" to the cavitand, The doors reduce the rate of exchange of various small guests into and out of the cavitand because there is van der waals interactions between host and guest since guests are surrounded by surfaces made of aromatic subunits. Thus, the conformation changed by attraction, so they coil to make a better contact with cavity and reduce to living outside with aqueous environment. The door also increase selectivity for small guests because large guest can force the door away from cavitand.

According to the improvment and new design of synthesizing cavitand, the nanolayer for extracting and trapping alkanes from aqueous medium were made of low molecular weight polymer has major part is cavitand which have both hydrophobic and hydrophilic character. We called "Polymer-Bound Cavitand" was achieved by Rebek.[7] This two character of a cavitand by making a portion of the length of the hydrophobic which we can call it as "host" part and the remaining portion of the length of hydrophilic.The host have a cavity of well-defined size and shape. Strong inclusion complex formation with 1: 1 host-guest stoichiometry[3] should occur in aqueous solution with alkane guests, especially with aromatic hydrocarbons, and hydrophobic interactions should be the major driving force for complexation.

The host should be good models for the hydrophobic pockets located at the active site of certain solid surface, so a functional group at the end of the host was used to tether to the polystyrene[7], this placed the hydrophobic portions of the macromolecules closet to the solid support to form, so the cavity which we can say as pocket acts like sublayer that serve as a stable trap for alkane molecules. The hydrophilic portions of the macrocylic molecules hang freely into the aqueous phase to from as overlayer that confers water-solubility on the nanolayer as a whole.(see Figure 1) Therefore, the result of this nanolayer can be an immobilized, stratified that can extract and trap hydrophobic particles from natural water . Dissolved alkane molecules in the range of 10-50 hydrophilic overlayer and enter the hydrophobic pocket sublayer.[21]
external image moz-screenshot-2.pngc.JPG
Figure 1. Conformation of Cavitand [7]

From figure 1 we can see that the pocket in this case is made of aromatic rings, so it can trap alkanes which used as guest molecules, and in most of cases the host and guest molecules are only soluble in organic solvents, but many others were rendered water-soluble by the derivation with charged functional groups ( above is carboxylic acid salt) or water-soluble oligomers to the top of hydrophobic pockets.[7]

3.1.3 Synthesis of Self-Folding Cavitands
Self-folding Cavitands are synthetic cavitand, the way to make it conformational dynamic is due to intramolecular hydrogen bonding and solvent effect.It also depends on the size and shape of the cavitand.[8] (See Figure 2)
sf.JPG
Figure 2. Cartoon representations of self-folding cavitands (top) and self-folding nanoscale containers (bottom)

Early method of synthesized cavitand by Cram[2] is complicated that the key step the condensation of haloaromatic compounds with octol. 1% yield of the important starting materials provided by an improved synthesis of octol from 2-methylresorcinol and hexanal. Halides have in common two vicinal halogens substituted in aromatic rings which readily undergo nucleophilic aromatic substitution reactions. Four new nine-membered ring was remarkably high yielded which usually ranged between 86% for these reactions and built by making and breaking of eight bonds between oxygen and aryl carbons, and products were easily isolated and charactized.Preparation of dimethylpyrazine in the synthesis of 2,3-dichloroquinoxaline was oxidized (KMnO4) to give 2,3-dichloropyrazine-5,6-dicarboxylic acid is similar to the sequence of preparation of Diethylpyrazine. This diacid when mixed with SOCl2 gave the corresponding anhydride, which when heated with CH3NH2-HCI gave imide.

Rebek's method of making cavitand are hightly applied during these years [8,9].They coupled resorcinarenes and their 2-methyl derivative with 1,2-difluoro-4,5-dinitrobenzene in DMA in the presence of bright yellow octanitro compounds Et3N and yield in 25-40% product. Product with SnCl2.H2O in boiling EtOH for reduction reaction of the octanitro derivatives and concentrated HCl afforded the relatively unstable and oxygen-sensitive octaamines. In the presence of K2CO3 gave octaamides as colorless solids under subsequent acylation with appropriate acyl chlorides under Schotten-Baumann conditions [8,9]in EtOAc-H2O. They also isolated and characterized the incompletely acylated products, heptaamides From this reaction. Acylation of 1,2-diamino-4,5-dimethoxybenzene with octanoyl and chloroacetyl chloride, respectively to prepare model compounds for spectroscopic comparisons.

3.2 Hydrophobic Effect
hydrophobic effect is a very unique organzing driving force, base on repulsion of solute and solvent instead of their attraction at the site of organization, then lead to not rigid conformation.[10,11] However, it is considered to play the main role of folding the cavitand even though there are some other driving force exist such as van der waals or electrostatics interaction, so it not only can stablize the folded structure of cavitand, but also give rise to deep cavity of cavitand to form capsular assembly which has a large volume inside result in burial of a large number of hydrophobic residues in the core.[12]
he.JPG
Figure 3. Schematic diagram of an amphiphilic solute in aqueous solution. The strong at- tractive forces between the polar end and the surrounding water molecules probably neutralize the CH2 group immediately adjacent to the polar group.[13]

3.2.1 Hydrophobic and Hydrophilic Interactions
All types of interaction will be described below contribute to the balance of forces responsible for the native structure of cavitand. [13]Generally, there are two major way to fold the cavitand: first of all, hydrophobic effect, which decrease the unfavorable interaction between hydrophobic residue and water molecules in terms of driving the molecules toward a more compact and 3-dimensional structure. Second, entropy effect, which give more favorable conformational freedom between nonpolar solute and cavitand.[14]

It is not expected to be energetically favorable that either the interaction between nonpolar molecules surround with polar molecule under normal condition or similar interaction that in a polar aqueous environment such as water, noncovalent interactions occur between macrocyclic molecule, including electrostatic interactions, hydrogen bonds, or Van der Waals forces.[1] For example, hydrophobic residues of proteins or remaining oil underneath of water, because this kind of molecules can not form hydrogen bonds in an aqueous media so it can be one of major interaction of relatively unfavorable interaction between water and nonpolar molecules.[15] Thus, here comes the hydrophobic interaction, which interaction between nonpolar groups is so much easier than the lack of hydrogen bond between nonpolar molecules and water, however latter is a main factor contributing to the structural stability of proteins, nucleic acids, and membranes. [14] Water, compared to organic solvents, is a poor solvent for apolar molecules, which have many anomalous physical properties in an aqueous environment.It requires a large number of energy to interact water molecules with cavity where will fit the hydrophobic solute, once cavity extract the solute, the solvent will change their configuration to decrease the total free energy of system, so the process can occur.[13]

3.2.2 Thermodynamic way to explain how hydrophobic effect works
According to the famous equation,
Delta G = Delta H - T Delta S ,
Delta G = Delta H - T Delta S ,
, if we want the process of extraction and trapping alkanes from aqueous medium works or takes spontaneously,
 Delta G < 0 ,
Delta G < 0 ,
, so both the enthalpy and entropy of the process must be considered. [13] As we talked above, interaction between two hydrophobic sites, or hydrophobic and hydrophilic sites is all really weak compared to the interaction with two hydrophilic sites, this means that alkane group is not reactive in both organic or aqueous solution. So it is not favorable to break the interactions between the alkane and water or forming the same number of interactions between the alkane and organic solvent, that makes the change of enthalpy is positive, whereas the net gain of hydrogen bonds as the water released from the molecular surface of the alkane mixes with the bulk water can give rise to negative enthalpy change.[13] In a word, net enthalpy change can be either positve or negative when chemical structure involved, so it doesn't play the main role in extracting process. However, entropy change in the opposite way of the same process can be sufficiently large and positive so that it drives the reaction spontaneously. The significant gain in entropy in this reaction that reactants in the release of the water molecules from the surfaces of individual hydrophobic molecules as the latter move into contact.[14] Once released into bulk water, the previously bond and ordered water molecules has many more configurations available to it and makes more dispersal engergy, so greater the entropy changes. This is the thermodynamic way to explain the term of hydrophobic effect.

when the sum of newly formed attractions between hydrophobic sites and the new hydrogen bonds formed when bound water is released into bulk water has a large enough negative value for enthalpy changes, then it already is the main driving force to make the process spontaneously.[16] So we all need to be very clear that the area of the hydrophobic surfaces moving into contact and the amount of bound water that can be released from the surfaces all determined by the molecular structure of the hydrophobic molecule being trapped from water as well as by the structure of its hydrophobic host, changing of either enthalpy or entropy is depending on that.[17]

4. How to trap the oil residue in aqueous phase
The mechanism of extracting the alkanes from natural water, first of all, it must be accessible to alkane molecules diffusing through an aqueous medium and be able to serve as a stable host to them. hydrophobic macromolecules would merely collapse onto the solid surface to form a thin, compact coating when exposed to water. The interior of such a collapsed layer would be inaccessible to free alkane molecules on a reasonable time-scale.[17] Therefore, the transfer of alkane molecules from aqueous solution into a compacted hydrophobic layer would be so slow kinetically that the collapsed layer would be useless as a trap. [18] The hydrophilic overlayer will prevent the full collapse of the hydrophobic sublayer and will make it accessible to the individual alkane molecules dissolved in the aqueous phase. This configuration will reduce the kinetic barrier to the transfer of the dydropcarbon molecules from the aqueous solution into the cavitand layer, sllowing them to enter progressively and swell the sublayer.[19]

Alkanes dissolve as individual molecules in very small of concentration in aqueous phase, additional alkane molecules will be supplied to the medium from the micelles. Individual alkane molecules, diffusing through the aqueous medium, will randomly encounter the stratified nanolayer.[20] They will diffuse first through the hydrophilic overlayer and then will encounter the hydrophlbic sublayer. The first arrivals will interact with the hydrophobic oligomers that comprise the framework of the sbulayer. [21]Additional alkanes will be stabilized by the hydrophobic interactions with the alkanes already there.

The accumulation of alkane molecules witin the hydrophobic sublayer of the stratified nanolayer is favored by two factors. First, burial of hydrophobic molecule surfaces against each other is a favorable process, the most obvious example of which is the spontaneous separation of oil and water into two phases. Second, alkanes trapped in a micelle, unable to tumble, have higher chemical potential than the same alkane molecules dispersed in bulk alkane, where they can tumble freely.[21,22] So an alkane molecule in a micelle has a higher chemical potential than it does in bulk liquid, so that transfer will occur from micelle to hydrophobic sublayer, where a separate phase analogous to bulk alkane will be formed progressively. The fluidity of the hydrophobic sublayer will allow it to expand to hold numberous alkane molecules. At some point the sublayer will reach capacity, probably due to a growing entropy cost: inability of the hydrophobic oligomers-and therefore of the sublayer-to expand further.[23]

5. Conclusion
Extraction of crude oil which released of petropeum into occean from underwater became a popular theme recently. A new way which build a water-soluble nanolayer has been designed to trap the alkanes( major content of crude oil). This thought was coming from biological system, and nanolayer is composed of hydrophobic and hydrophilic portion, mainly conducted by macrocyclic molecules which we was called cavitand . Water-soluble cavitand is deep vessel like shape derived from resorcinareneshas act like host can bind small molecules such as alkanes of complementary size and shape. Hydrophobic effect is the major driving force for this process. In this way, by making variations in the lengths and chemical identities of the hydrophobic and hydrophilic portions of macromolecules, it can be able to probe the relative role of entropy and enthalpy in the extraction process, drive the process spontaneously.

6. Reference
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