Effect of temperature on adsorption and chromatography of polystyrene macromolecules

Study of the effect of temperature on the parameter s of adsorption and liquid chromatography of molecules and macromolecules constantly attracts th e a tention of specialists. The potential benefits of elevated temperature column, particularly enhanced kin etic and transport properties, which are based on t he reduction of mobile phase viscosity and increase th e diffusion of analyte at high temperature, began t o be actively used for the rapid analysis of molecules a nd macromolecules by chromatographic methods in rec ent years. It is now recognized that temperature is an important tool to optimize chromatographic paramete rs, such as retention, selectivity and efficiency, part icularly for macromolecules The effect of temperature on adsorption and liquid chromatography of polystyrene macromolecules on silicas has been investigated by static and .chr omatographic methods. The sign and value of the tem perature adsorption coefficient depend on many factors, namely, the chemical nature of the solvent, the ch emical structure of macromolecules and adsorbent surface c hemistry. Thus temperature elevation in the chromat graphic system alters the macromolecular coil dimen sions and total energy of elution of solvent molecu s from the adsorption space of the porous adsorbents. Keeping these factors in view we can find the sign of the temperature adsorption coefficient. Diffusion coeff icients increase in proportion to temperature, thus efficiency is usually enhanced, too, as peaks become mo r narrow. Prospects for optimization of the chroma tographic process and, above all, the efficiency of s eparation of components of mixtures, include the ef f ct of temperature on the adsorption of the components and their retention in the chromatographic column.


Introduction
Study of the effect of temperature on the parameters of adsorption and liquid chromatography of molecules and macromolecules constantly attracts the attention of specialists [1][2][3].The potential benefits of elevated temperature column, particularly enhanced kinetic and transport properties, which are based on the reduction of mobile phase viscosity and increase the diffusion of analyte at high temperature, began to be actively used for the rapid analysis of molecules and macromolecules by chromatographic methods in recent years [4][5][6][7][8].
Meanwhile, the interest in the liquid chromatography of macromolecules at elevated temperatures is due to the fact that some polymers, e.g.polyolefines, are dissolved at elevated temperatures only, therefore the chromatographic analysis of such polymers can be performed only with the help of a high-temperature chromatography [3,9,10].Thus, it is now recognized that temperature is an important tool to optimize chromatographic parameters, such as retention, selectivity and efficiency, particularly for macromolecules [2,6,[10][11][12].This contribution describes the results of the study of adsorption and liquid chromatography of polystyrene macromolecules from several solutions on silica stationary phases at different temperatures.
Under static conditions, adsorption from diluted solutions was studied with the help of an interferometer by changes in the polymer concentration [13][14][15].Chromatographic experiments were run using liquid chromatograph Waters-200 (column 1.2 m, particle size 85+5 µm) and liquid chromatograph LC-1309 (NTO, S-Peterburg, Russia) (column 300x0.5 mm, particles size 5+2 µm).The eluent flow rate was 1 ml/min.The overall solvent volume in the column was found by the time of the peak maximum elution on the chromatogram after introduction of a n-nonene solution sample into the column.
Increase in temperature brings down the PS adsorption from solutions in cyclohexane due to greater mobility of macromolecule segments and coil sizes resulting in weaker adsorption bond energy and smaller pore surface accessible for PS.In this case, the competitive ability of solvent molecules to occupy the silica surface is small and does not change.
With PS dissolved in tetrachloromethane whose molecules are little competitive adsorption of PS grows slightly with temperature.Interaction of solvent molecules with polymer and adsorbent in this adsorption system seems to be weakened.Lability of more compacted macromolecular coil increases encouraging greater adsorption of PS under these conditions [3,[13][14][15].
Adsorption of PS from solutions in ethylbenzene with hydroxylated silica is negative at 293 and 343 K.It should be stressed here that dehydroxylation of the silica surface results in small, but positive PS adsorption values.Ethylbenzene and other aromatic hydrocarbons and their chlorine derivatives compete greatly with PS macromolecules for silane groups on the silica surface and, at moderate temperatures, expel them from the surface almost completely.The molecular mass of PS also influences the temperature adsorption coefficient.
Table 1 contains kitenic and equilibrium parameters of PS1 adsorption from tetrachlormethane solutions on silicagels SG-20 and SG-40 and silochrom S-80.The diffusion coefficient D was found from the formula 5 .0

/πτ
where K is the particle shape factor equal to 0.3; r is the particle radius and 5 .0 τ is the time required for the relative adsorption value (0.5) to be reached.
The activation energy a E was found through the temperature diffusion coefficient (2) Kinetic characteristics shown in Table 1 point to activated character of adsorption of PS macromolecules by porous sorbents.Diffusion coefficients increase in proportion to temperature, thus efficiency is usually enhanced, too, as peaks become more narrow [3,6,[9][10][11].
Table 2 contains distribution coefficients d K of macromolecules of polystyrene standards between the mobile and stationary phases in the chromatographic column packed by silochrom C-80 with hydroxylated and silylated surfaces, and paraxylene served as the mobile phase.The distribution coefficient d K of polymer represents the concentration ratio [14,15 ] where s c and c are the concentrations of polymer in surface and bulk phases, correspondingly.In accordance with the definition where m Γ is the excess value, A is the total surface area of the sorbent in chromatographic column and a v is the adsorption volume (cm 3 ) of sorbent in chromatographic column.
Then for d K we get The ratio A v a / gives the average statistical thickness of the adsorption layer, Therefore we can write the expressions for d K (6) and ( 7) where '  k -retention coefficient, calculated as  The initial decrease of d K values is due to the diminution of adsoption and intermolecular energy in chromatographic system and the increase of macromolecular coils.The increase of d K values with the increase of temperature is due to the action of adsorption effect.

Conclusion
The sign and value of the temperature adsorption coefficient depend on many factors, namely, the chemical nature of the solvent, the chemical structure of macromolecules and adsorbent surface chemistry.
Thus temperature elevation in the chromatographic system alters the macromolecular coil dimensions and total energy of elution of solvent molecules from the adsorption space of the porous adsorbents.Keeping these factors in view we can find the sign of the temperature adsorption coefficient.
Diffusion coefficients increase in proportion to temperature, thus efficiency is usually enhanced, too, as peaks become more narrow.Prospects for optimization of the chromatographic process and, above all, the efficiency of separation of components of mixtures, include the effect of temperature on the adsorption of the components and their retention in the chromatographic column.

Fig. 2 .
Fig. 2. Dependences of d K values on a temperature for PS standards with

Table 1 .
Values of m values for a trimethylsilylated sample at 293 and 388 K also shows that with the increasing temperature the macromolecular coil is compacted due to weaker interaction with molecules of solvent -aromatic hydrocarbon which results in a greater fraction of pore volume accessible for these macromolecules and, consequently, increased retention volume.Fig.2shows the dependences of d K values on different temperature for two polys-