Hydrophilic interactions, polymer-water

Hatakeyama, H. and Hatakeyama, T. 1998. Interaction between water and hydrophilic polymers. Thermochimica Acta 308 3-22. 

The structure of this interface determines fhe sfabilify of PEMs, the state of water, the strength of interactions in the polymer/water/ion system, the vibration modes of side chains, and the mobilities of wafer molecules and protons. The charged polymer side chains contribute elastic ("entropic") and electrostatic terms to the free energy. This complicated inferfacial region thereby largely contributes to differences in performance of membranes wifh different chemical architectures. Indeed, the picture of a "polyelectro-lyfe brush" could be more insighttul than the picture of a well-separated hydrophobic or hydrophilic domain structure in order to rationalize such differences. ... 

The water solubility of R-(EO)n types of nonionic emulsifiers is derived from the weak interaction between the ether oxygen of EO unit and water. It was suggested that each EO unit in the PEO chain, requires three molecules of water to form a hydrated complex [35]. This hydrogen bond complex is destroyed if the solution is taken above the melting point of the PEO. Water usually acts as plasticizer when present in hydrophilic PEO polymers and Tg values decrease with increasing water contents [36]. This phenomenon in the PEO-water system is observed up to 1 mol water/ether group. Beyond this a rise in Tg is observed and water acts as an antiplasticizer. 

In addition to the silica-based materials mentioned above, modem polymers are widely used for TTA and QTA sample preparation allowing SPE not impaired by undesirable silanol activities. HLB Oasis (Waters) is the tradename for a hydrophilic-lipophilic balance reversed-phase sorbent enabling lipophilic interaction to benzene moieties and hydrophilic interactions to pyrrolidone groups as present in the macroporous copolymer of poly(divinylbenzene-co-iV-vinylpyrrolidone). Elution of analytes is often performed with solvents containing MeOH or ACN. Applying this adsorbent TA such as atropine and scopolamine were extracted from human viscera [15], human serum [97-99], human urine [12] as well as from rat plasma and brain microdialysate [77], Furthermore, this hydrophilic-lipophilic balance phase was also suitable for extraction of the QTA trospium from human and rat plasma [77, 84] and methyl scopolamie from rat plasma [77] (Table 4). 

Liquid water as contacting medium (dealt as a solvent) interacts with polymer molecules in different degrees some dissolve polymers, some swell polymers, and some are merely adsorbed at the polymer/water interface with no significant alteration of physical properties of polymers. The enthalpy term of the interaction could be represented by Flory-Huggins s interaction parameter x- In order to distinguish the molecular interaction from the interfacial interaction, the molecular interaction could be termed x interaction and the interfacial interaction y interaction. When a solid polymer is in contact with water (solvent), the extent of x interaction depends on the hydrophilicity or hydrophobicity of the polymer. However, the interfacial phenomena generally cannot be interpreted by molecular level parameters that describe the bulk phase of a polymer such as hydrophilicity or hydrophobicity. 

A highly hydrophobic polymer such as Teflon PFA has weak x interaction with water. The y interaction is strong in liquid water but very weak in contact with water vapor. When the hydrophilicity of polymer increases the contribution of the x interaction increases, and the relative contribution of the y interaction changes according to the value of A(yg). 

The choice of eluent system depends on the polymer type. For most non-ionic hydrophilic polymers, water can be used. However much more complex eluent systems are needed, for anionic and cationic polymers where interactions with the column based on ion exclusion, inclusion and exchange, adsorption by hydrogen bonding or hydrophobic interactions and intramolecular electrostatic effects, are possible. This can often make method development in aqueous SEC extremely difficult and time-consuming. 

In this presentation, two examples of the use of vibrational spectroscopy to probe water-solid interactions in materials of interest to the food and pharmaceutical sciences are described. First, the interaction of water vapor with hydrophilic amorphous polymers has been investigated. Second, water accessibility in hydrated crystalline versus amorphous sugars has been probed using deuterium exchange. In both of these studies, Raman spectroscopy was used as the method of choice. Raman spectroscopy is especially useful of these types of studies as it is possible to control the environment of the sample more easily than with infrared spectroscopy. 

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