Polar Interactions Hydrogen Bonding

Other selectivity characteristics of a stationary phase can be measured using the same technique [11]. Some of the phases with an incorporated polar group form hydrogen bonds with either the mobile phase or with analytes. These properties can be measured with suitable probes. For this purpose, we have selected the compound pair dipropylphthalate and butylparaben. Dipropylphthalate represents the polar reference compound. Butylparaben interacts via its phenol function with the polar group of the stationary phase. We have termed this interaction polar selectivity or phenolic activity . Snyder [9] has determined the same or at 

A few other classes of compounds are subject to similar interactions with the stationary phase as phenols. Examples are sulfonamides, and carboxylic acids in their uncharged form, i.e. at acidic pH, with acetonitrile as the mobile phase. This special interaction increases the retention of the compoimds affected. 

Purospher RPjg that is endcapped with an aminosilane. In the left upper part of the chart, one finds Fluofix 120N, 5, and at the lower fringe of the chart there is Fluophase RP, 11. Both are fluorinated phases, but other details of the structures of these stationary phases are not known to us. Another phase found in the upper left-hand comer is Discovery HS PEG, 121, which is formed using polyethylene glycol. Packing 122 is the Discovery Cyano packing. 

One can see that the use of the polar selectivity creates a grouping of the columns that is distinctly different from the classification by silanol activity. This demonstrates that column selectivity is truly multidimensional. Here, we have discussed three dimensions, while Snyder [4—9] deals with five dimensions. In our treatment, other dimensions are possible as well, for example the extended polar selectivity discussed in Ref. [13]. However, we believe that the three dimensions discussed here represent the strongest influences on column selectivity. 

How well these selectivity groupings work, and how these similarities translate into the everyday separations of the chromatographer s work is left to the judgement of the reader. There are many separations in many laboratories that can be carried out on more than one stationary phase. The column selection used for the creation of the charts described here is rather large, and the reader will surely find at least some of the columns that he or she is commonly using. With the information provided here, the interested user can then test the described classification scheme and see how well it applies to his or her separation problem. 

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Fig. 9.16. Simplified schematics illustrating two different molecular recognition mechanisms exemplified for native (i-CD and propranolol. Case A is the polar-organic phase mode where the solvent molecules occupy the cavity and the SA is bound to the outer surface of the CD via polar interactions (hydrogen bonding and/or dipole-dipole interactions) which contribute to chiral recognition in combination with steric interactions. In the reversed-phase mode, the primary binding mechanism is similar to case B SO-SA association may be driven by inclusion type complexation into the hydrophobic cavity of the CD macrocycle (reprinted with permission from Ref. [27. ]).

In a more general approach, attempts are made to build up a complete picture of all the actual attractions and repulsions between atoms in terms of van der Waals forces, polar interactions, hydrogen bonding, and torsional contributions (75JCS(P2)830 77JCS(P2)654). 

Inorganic particles can be selectively wetted by the liquid phase if the particles and the liquid undergo favorable energetic interactions, such as polar-polar interactions, hydrogen bonding, or acid-base interactions. Thermodynamic equilibrium and the highest attainable entropy are easily reached in the cases of suspensions in liquids of low molecular weight due to very low liquid viscosity and fast molecular diffusion. 

There exist no specific interactions (e.g., polar interactions, hydrogen bonding) between the molecules, which leads to a statistical distribution of the molecules in the mixture. 

The values of % and 8 are much less widely available for aqueous systems than for nonaqueous systems, however. This reflects the relative lack of success of the solution thermodynamic theory for aqueous systems. The concept of the solubility parameter has been modified to improve predictive capabilities by splitting the solubility parameter into several parameters which account for different contributions, e.g., nonpolar, polar, and hydrogen bonding interactions [89,90],... 

In this investigation, you will examine the differences between molecules that contain different functional groups. As you have learned, the polarity and hydrogen bonding abilities of each functional group affect how these molecules interact among themselves and with other molecules. You will examine the shape of each molecule and the effects of intermolecular forces in detail to make predictions about properties. 

Equation (21) reduces to Eq. (19) if polar and hydrogen bonding interactions are neglected. Hence, not the absolute value of the solubility parameter, as suggested by the simplified Hildebrand concept, but its individual contributions must be considered in order to express the solution and phase separation behavior. Thus, even though the norm of the solubility parameter of various solvents is constant, the partial contributions can be very different, resulting in a totally different solution and phase separation behavior respectively. 

Consequentely, we have chosen solvents in order to change separately either the norm of the solubility parameter or its direction (see Fig. 4). These solvents are listed in Table 1. It can be clearly seen that the polar and hydrogen bonding interactions are zero for all of the aliphatic and cycloaliphatic alkanes. This allows one to change only the value of without changing its direction. For a second series of experiments, we compare 2,6-dimethyl-4-heptanone, dib-utylether and methyl-cyclohexane which have nearly identical lengths, but different vector directions. 

The attractive forces that can hold molecules together include van der Waals interaction, electrostatic attraction (when molecules are charged or polar), and hydrogen bonding. Since there is no clear border between a very weak hydrogen bond and van der Waals interaction, the latter requires some explanation. 

A so-called bare nuclear potential descriptors that probably describe interactions involving polar and hydrogen bonding. 

Compared to hydrocarbonaceous silica RPC sorbents, not as much commitment has been made to the development of bonded, polar-phase sorbents suitable for the high-performance chromatographic separation of peptides. Due to polar, notably hydrogen bonding, interactions between the peptide and the hydrophilic surface of the sorbent useful selectivity effects can, however, be achieved. In fact, at least two types of separation mechanisms can be identified with bonded polar-phase sorbents. In the first mode, the peptides do not interact per se with the bonded polar-phase sorbent but, rather, are separated on the basis of their ability to permeate into the pores and elute in order of their hydrodynamic volume. In this mode, peptides are separated by steric exclusion effects, with the retention (in terms of elution volume, Ve) of a partial retained peptide, Pb described by the following relationships ... 

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