29-Silicon NMR evidence for the improved chromatographic siloxane bond stability of bulky alkylsilane ligands on a silica surface

Abstract

29-Silicon Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication Citation for published version (APA):Scholten, A. B., Haan, de, J. W., Claessens, H. A., Ven, van de, L. J. M., & Cramers, C. A. (1994). 29-Silicon NMR evidence for the improved chromatographic siloxane bond stability of bulky alkylsilane ligands on a silica surface. Journal of Chromatography, A, 688(1-2), 25-29. DOI: 10.1016/0021-9673%2894%2900928-7, 10.1016/0021-9673(94)00928-7 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. AbstractA stable bond stationary phase for reversed-phase high-performance liquid chromatography, with a diisobutyl-noctadecylsilane derivatized surface. was studied using "Si cross-polarization magic angle spinning (CP MAS) NMR. Fumed silica surfaces (Aerosil), trimethylsilylated to different extents, were used to illustrate the effect of ligand surface loading on the hydrogen bonding contribution IO the ligand silane CP MAS NMR signal. Spectral comparison of the diisobutyl-n-octadecylsilane derivatized silica with the conventional dimethyl-n-octadecylsilane derivatized silica revealed significantly decreased hydrogen bonding of residual silanols to the ligand siloxane bond in the diisobutyl-n-octadecyl phase. This illustrates the increased steric protection of the ligand siloxane bond by the bulky alkyl substituents, which is assumed to be the reason for the improved hydrolytic stability at low pH of this phase.

Review on the chemical and thermal stability of stationary phases for reversed-phase liquid chromatography

“…Applying NMR techniques, Scholten et al [104,105] could explain and provide evidence for the high chemical stability of these bulky phases in acidic eluents. Fig.…”

Chromatographic activity of residual silanols of alkylsilane derivatized silica surfaces

“…Indeed, Kirkland et al have shown that phases with isobutyl substituted C18 silane groups exhibit a longer lifetime in low-pH aqueous organic mobile phases than their conventional methyl substituted analogues [7]. In an earlier paper we provided 29Si solid-state NMR evidence for a decreased hydrogen bonding contribution of residual silanols to the ligand sil0xane bond of di-isobutyl-n-octadecylsilane 0021-9673/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0021-9673(96)00765-0 ligands [8]. This was considered a result of the sterically protecting properties of the bulky side groups, hampering the siloxane bond hydrolysis by acidic mobile phases.…”

“…It was surmised that these silanols will be analyte accessible during chromatography. From Table 1 it can be inferred that 60% of all silanols of the phase Rx-C, 8 and 45% of all silanols of the phase SB-C~8 are not directly accessible for analytes during the chromatographic separation process. These percentages are probably minimum values, since larger analytes will have more difficulty penetrating the alkylsilane layer before possibly encountering a residual silanol.…”

Fundamental Study of Residual Silanol Populations on Alkylsilane-Derivatized Silica Surfaces

“…The other is based on the same substrate but has been derivatized with bulky di-isobutyloctadecylsilane structures, which has proven to result in an increased stability under low pH conditions. − Moreover, it was argued that the bulky side groups would increase the steric shielding of the underlying residual silanols by the silane ligand. Details on the relation between the 29 Si NMR signals of these ligand silanes and the observed increased low-pH stability have been published elsewhere . In order to obtain information on residual silanol concentration, mobility and solute accessibility, a combination of deuterium exchange in acetonitrile/D 2 O and 29 Si CP MAS NMR experiments was performed.…”