**3. The way of interaction with sorbents**

The possibility to form hydrogen bonds suggests that phenols should have good water solubility, but this is not the rule. The unsubstituted phenol is relatively well soluble in water (83 g L−1), while its substituted derivatives are not. For most of them, solubility does not exceed 30 g L−1. Alkyl and halogen groups enhance hydrophobic character of aromatic ring resulting in decrease of water solubility. For phenols possessing functional groups with strong polar character, differences in ability to dissolve are more pronounced. Even for isomers of the same compound, they can be significant. The close proximity of the –OH group to substituents such

coordinated the hydroxyl group becomes less active in the solvation process. Due to this fact, solubility of ortho-isomer is lower than meta or para ones (e.g. nitrophenols, hydroxylbenzoic

Very interesting behavior demonstrates benzenediols and benzenetriols. Increasing number of hydroxyl groups implies their solubility should be better in comparison to phenol. It is indeed, but hydroquinone and phloroglucinol are exceptions. Solubility of these two compounds is lower and amount 72 and 10 g L−1, respectively. This apparently abnormal behavior is the result of the presence of hydrogen bonds whose strength are additionally enhanced by symmetry of the molecules. The adjacent molecules form a kind of network in which each of them is strongly bonded with others by infinite chains of hydrogen bonds (**Figure 5**) [13–15]. In this way, a compact structure is created that prevents the penetration of solvent/water molecules into the interior. For this reason, dissolution process is significantly hindered. In

**Figure 5.** Hydrogen bonds enhanced by symmetrical structure of the molecules in (a) hydroquinone and (b)

and -COOH lead to formation of intramolecular hydrogen bonds. In this way

as -NO2

acids).

, -NH2

12 Phenolic Compounds - Natural Sources, Importance and Applications

phloroglucine. Based on Refs. [13–15].

Great varieties of physicochemical properties make phenols very difficult compounds to adsorb or extract, especially they occur at extremely low level of concentration. Although so many different sorbents are commercially available, none of them is universal. For this reason, researchers are constantly working to develop new, better and more efficient materials for phenols sorption processes. All accessible sorbents due to their origin and chemical structure can be divided as the following classes:


Independently of the above division, in each class, the materials can have hydrophobic or hydrophilic character due to the lack or presence of polar functional groups.

Retention of phenols from aqueous solution by nonpolar reversed phase adsorbent, for example, silica sorbents (RP-C<sup>8</sup> , RP-C18, RP-cyclohexyl, RP-phenyl), is the result of apolar Van der Waals forces that play the key role in reversed phase mechanisms. Differences in the polarity and solubility of the phenols between the aqueous and the solid apolar phase cause the mass transfer leading to partition process. For this reason, the efficiency of sorbent is related to the octanol-water partition coefficient, Log P (**Tables 1** and **2**), which is a measure of hydrophobicity (or lipophilicity) of the compound. The smaller is the value of Log P, the more limited is sorbent efficacy towards the compound. Conversely, increase in Log P provides its better affinity to the sorbent.

In sorption of phenols using hydrophobic polymers (e.g. PS-DVB (copolymer styrenedivinylbenzene) or hypercrosslinked resins) and graphitized carbons, the possible interactions are also hydrophobic ones. The partition process occurs due to combination of apolar Van der Waals and π–π electrons interactions of the aromatic rings of the adsorbate and adsorbent. Therefore, the retention is the result of the reversed phase mechanism, too [18].

Unfortunately, for very polar phenols, the described interactions in many cases are insufficient to carry out the process of sorption efficiently. In order to facilitate the retention, two methods can be applied:


Resulting materials still retain the high capacity towards less hydrophilic compounds and gain possibility to specifically interact with polar molecules due to stronger interactions including: dipole-dipole, hydrogen bonding or even ionic if sulfonic or carboxylic or amine groups are present. Additionally, polar surface is more wettable and consequently support mass transfer of the more polar species from aqueous solution to the sorbent [18], thus influencing the selectivity of sorption process and the capacity of the sorbent [26].

In a situation when a solution consists of phenols with significantly different properties, one sorbent may be insufficient to trap all the compounds quantitatively. In such cases, the possibility to achieve high retention and selectivity for all analytes is the preparation of device containing mixed or layered sorbents with different chemical characters [18, 20, 21].
