Adsorption mechanism in reversed-phase liquid chromatography. Effect of the surface coverage of a monomeric C18-silica stationary phase

Journal Article · Sat Apr 01 00:00:00 EST 2006 · Journal of Chromatography A

DOI:https://doi.org/10.1016/j.chroma.2006.02.095· OSTI ID:989612

Gritti, Fabrice ^[1]; Guiochon, Georges A ^[2]

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1. University of Tennessee, Knoxville (UTK)
2. ORNL

The effect of the bonding density of the octadecyl chains onto the same silica on the adsorption and retention properties of low molecular weight compounds (phenol, caffeine, and sodium 2-naphthalene sulfonate) was investigated. The same mobile phase (methanol:water, 20:80, v/v) and temperature (T = 298 K) were applied and two duplicate columns (A and B) from each batch of packing material (neat silica, simply endcapped or C{sub 1} phase, 0.42, 1.01, 2.03, and 3.15 {micro}mol/m{sup 2} of C{sub 18} alkyl chains) were tested. Adsorption data of the three compounds were acquired by frontal analysis (FA) and the adsorption energy distributions (AEDs) were calculated using the expectation-maximization method. Results confirmed earlier findings in linear chromatography of a retention maximum at an intermediate bonding density. From a general point of view, the saturation capacity of the adsorbent tends to decrease with increasing bonding density, due to the vanishing space intercalated between the C{sub 18} bonded chains and to the decrease of the specific surface area of the stationary phase. The equilibrium constants are maximum for an intermediary bonding density ({approx}2 {micro}mol/m{sup 2}). An enthalpy-entropy compensation was found for the thermodynamic parameters of the isotherm data. Weak equilibrium constants (small {Delta}H) and high saturation capacities (large {Delta}S) were observed at low bonding densities, higher equilibrium constants and lower saturation capacities at high bonding densities, the combinations leading to similar apparent retention in RPLC. The use of a low surface coverage column is recommended for preparative purposes.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: DE-AC05-00OR22725
• OSTI ID: 989612
• Journal Information: Journal of Chromatography A, Vol. 1115, Issue 2006; ISSN 0021-9673
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ADSORBENTS
CAFFEINE
LIQUID COLUMN CHROMATOGRAPHY
ENERGY SPECTRA
ADSORPTION ISOTHERMS
MOLECULAR WEIGHT
PHENOL
SILICA
SPECIFIC SURFACE AREA
THERMODYNAMICS
SULFONATES
NAPHTHALENE
RPLC
C18 bonding density
adsorbent heterogeneity
adsorption energy distribution
frontal analysis
isotherm
retention mechanism
phenol
caffeine
sodium 2-naphthalene sulfonate
silica-C18
endcapping