Solvation processes in liquid chromatography - the importance and measurement

Solvation processes in liquid chromatography

- the importance and measurement

Szymon Bocian, Magdalena Skoczylas, Katarzyna Krzemińska, Bogusław Buszewski

Chair of Environmental Chemistry & Bioanalytics, Faculty of Chemistry, Nicolaus Copernicus University, Toruń, Poland

Solvation is an indispensable part of the elution in liquid chromatography. The adsorption of solvents allows the elution of a solute from the adsorbent surface. The tendency for the adsorption of solvent molecules on the stationary phase surface results in the elution strength of a solvent. Therefore, the knowledge and the understanding of solvation process are crucial for the understanding of the retention mechanism of liquid chromatography separation.

Summarizing, the solvent from binary mobile phase interacts preferentially with the functional groups on the surface of the stationary phase. For understanding the distribution of the solvent between the stationary phase and a bulk binary solution the excess adsorption of the organic modifier has to be described. The presence of polar functional groups incorporated into the structure of bonded ligands changes the solvation process due to polar interactions and hydrogen bond creation. The measurement of solvation process allows to calculate the number of silanols on silica surface.

Acknowledgements

This work was supported by Ministry of Science and Higher Education, Grant no. NCN 2013/09/D/ST4/03807 for period 2014–2017.

l I x1,

INTRODUCTION

SELECTIVE SOLVATION

SILANOLS CALCULATION

INFLUENCE ON RETENTION

CONCLUSIONS

0.0 0.2 0.4 0.6 0.8 1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Phenyl-amine Phenyl-amide Cogent-phenyl Phenoxy-propyl Phenyl-propyl Phenyl-hexyl Ace to n itri le e xce ss a mo u n t [  mo l/m 2 ]

ACN volume fraction (

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Me th an ol e xce ss amo un t [  mo l/m 2 ]

MeOH volume fraction ()

Phenyl-amine Phenyl-amide Cogent phenyl Phenoxy-propyl Phenyl-propyl Phenyl-hexyl 0 2 4 6 8 10 12 14 16 0 2000 4000 6000 8000 10000 12000 14000 Ab so rb a n ce (µ AU )

Retention time (min)

50% ACN 60% MeOH 0 1 2 3 4 5 6 7 8 9 0 2000 4000 6000 8000 10000 12000 14000 16000 Ab so rb a n ce (µ AU )

Retention time (min)

50% ACN 60% MeOH 0 1 2 3 4 5 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Ab so rb a n ce (µ AU )

Retention time (min)

50% ACN 60% MeOH 0 1 2 3 4 5 6 7 8 0 2000 4000 6000 8000 10000 12000 14000 16000 Ab so rb a n ce (µ AU )

Retention time (min)

50% ACN 60% MeOH 0 2 4 6 8 10 12 14 16 18 20 22 0 2000 4000 6000 8000 10000 12000 14000 16000 Ab so rb a n ce (µ AU )

Retention time (min)

50% ACN 60% MeOH 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 5000 10000 15000 20000 25000 30000 Ab so rb a n ce (µ AU )

Retention time (min)

50% ACN 60% MeOH A B

C D

E F

Changes of solute retention causes by solvation process; benzene, anthracene, phenanthrene in isoeluotropic mobile phases conditions ACN/H₂O (50/50_v/v) and MeOH/H₂O (60/40_v/v) on a series of phenyl-bonded stationary phases: Phenyl-amine (A), Phenyl-amide (B), Phenoxy-propyl (C), Phenyl Hydride (D), Phenyl-hexyl (E), Phenyl-propyl (F)

0 1 2 3 4 5 6 7 8 9 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 re ten tion f act or (k)

maximum excess of adsorbed water [mmol/column]

vitamin B₆

tryptophan

Column Functional group Maximum excess of

water [mmol/column] Amino-C3 aminopropyl 3.103 AP-C2 aminopropyl, n-acylamide, alkyl chains (C₂) 0.989 AP-C12 aminopropyl, n-acylamide, alkyl chains (C₁₂) 1.704 Amino-Cholesterol aminopropyl, n-acylamide, cholesterol 0.640

Amino-P-C10 aminopropyl, phosphate,

alkyl chains (C₁₀) 3.351

Kinetex HILIC unbonded silica 2.288

Luna HILIC cross-linked DIOL 5.164

Venusil amide acylamide 6.546

TSK-gel

Amide carbamoyl 8.301

SOLVATION vs. RETENTION IN HILC

0,0 0,2 0,4 0,6 0,8 1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 MeOH Exce ss a mo u n t [  mo l/m 2 ]

methanol volume fraction []

NH₂ CN Phenyl CH₃ 0,0 0,2 0,4 0,6 0,8 1,0 -1 0 1 2 3 4 5 6 7 8 9 10 11 ACN Exce ss a m o u n t [  m o l/m 2 ]

acetonitrile volume fraction []

NH₂ CN Phenyl CH₃

Stationary

phase MeOH ACN

ACN / MeOH ratio NH₂ 0.58 1.80 3.08 AP 1.36 3.30 2.42 Phenyl 0.32 1.94 5.99 CN 0.48 2.49 5.23 Chol 0.82 3.56 4.33 C18 0.41 1.88 4.59 C4 0.98 3.79 3.86

Theoretical solvent layers

Silica gel

Silanol groups concentration

[μmol/m2_] measured literature Kromasil 100 7.73 7.1 [1] 7.6 [2] 7.6-7.8 [3] 8±0.1 [4] 8.44±0.1 [5,6] 6.21 [7] Kromasil 300 8.06 Microsolv Technology 7.81 LiChrosorb Si 60 7.50 LiChrospher Si 100 8.55 Silpearl 6.13 SG 7/G 6.26