Journal of Colloid and Interface Science 327 (2008) 370–376 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis A 31 P CP/MAS NMR study of PbS surface O,O ′ -dialkyldithiophosphate lead(II) complexes ✩ Anna-Carin Larsson a,∗ , Alexander V. Ivanov b , Oleg N. Antzutkin a , Willis Forsling a a Division of Chemistry, Luleå University of Technology, S-97187, Luleå, Sweden b Institute of Geology and Nature Management, Far Eastern Branch of the Russian Academy of Sciences, 675000, Blagoveschensk, Amur Region, Russia article info abstract Article history: Received 18 April 2008 Accepted 14 August 2008 Available online 22 August 2008 Keywords: 31 P CP/MAS NMR spectroscopy Dithiophosphate Lead(II) complexes Surface complexation Adsorption Synthetic galena 31 P CP/MAS NMR spectroscopy was used to study the adsorption of six different O,O ′ -dialkyldithiophos- phate ions on the surface of synthetic galena (PbS). The 31 P CP/MAS NMR spectra of the surface lead(II) dithiophosphates were compared with the 31 P CP/MAS NMR spectra of polycrystalline lead(II) dithiophosphate complexes of the same ligands. Surface complexation of the dialkyldithiophosphate ions was established on the surface of PbS. A terminal S , S ′ -chelating coordination is suggested for the surface complexes. The bulkier alkyl groups lead to surface precipitation in addition to the surface adsorption. Derivatives of monothiophosphoric and phosphoric acids were displayed as hydrolysis products of dialkyldithiophosphates on the synthetic PbS, the amount of which depends on the type of alkyl group.  2008 Elsevier Inc. All rights reserved. 1. Introduction Surface processes at the solid/liquid interface are very impor- tant in many different chemical contexts, e.g. flotation, lubrication, catalytic reactions, etc., and a long list of publications describe dif- ferent techniques used for probing the surfaces. NMR spectroscopy is a very sensitive technique for probing the electronic environ- ment around nuclei under investigation and, therefore, has the po- tential to be a suitable method for studying surface complexation. We wanted to determine whether it is possible to use 31 P CP/MAS NMR to distinguish between different dialkyldithiophosphate (Dtp) species adsorbed on a synthetic and heterogeneous lead sulfide surface, whether these species are physisorbed or chemisorbed, and finally, if it is possible to determine the coordination of the species that are chemisorbed. A similar study has been performed earlier on Dtp adsorption on ZnS [1]. Dialkyl derivatives of phosphorus(V) dithioacids are commonly used as starting materials for the synthesis of insecticides, fungi- cides and pesticides, and for vulcanization of rubber, but they are also used as collectors in froth flotation enrichment of metal sulfide ores and, therefore, adsorption studies of different O,O ′ - dialkyldithiophosphates can be helpful for understanding the flota- ✩ Part of the results has been presented at the Centenary of Flotation Symposium held in Brisbane, Australia, June 6–9, 2005. * Corresponding author. Fax: +46 920 491199. E-mail addresses: anna-carin.v.larsson@ltu.se (A.-C. Larsson), alexander.v.ivanov@chemist.com (A.V. Ivanov), oleg.antzutkin@ltu.se (O.N. Antzutkin), willis.forsling@ltu.se (W. Forsling). tion mechanism. Knowledge of the coordination mode can be of importance, e.g. in improving flotation selectivity and recovery, and designing highly selective collectors for froth flotation. Dtps have not been as frequently studied as the xanthates, but they are pre- sumed to function in much the same way as the latter, changing the hydrophobic properties of the mineral surfaces and facilitating froth flotation of mineral particles. Different metal sulfides require different alkyl groups for optimized flotation. From this point of view, three different sorption processes can be hypothesized (see Fig. 1): (i) chemisorption on the mineral surface by covalent bond- ing of the collector-molecules to the metal ions (from left to right: bidentate bridging, bidentate terminal and monodentate coordina- tion), (ii) formation of disulfide through a red-ox reaction with highly oxidizing ions on the mineral surface and (iii) formation of metal–collector complexes with low solubility by reactions be- tween ions of collectors and non-lattice metal-ions. According to a review by Finkelstein and Poling [2], Dtp ions form precipitated lead(II) complexes upon contact with galena, whereby the central metal atom has a lone pair of stereochemi- cally active electrons and this pair can be donated to the mineral surface. In a DRIFT (Diffuse Reflectance Infrared Fourier Transform) study by Valli et al. [3], precipitated lead(II) diethyldithiophosphate was found to be the only species on the surface of galena. In another study, Leppinen et al. [4–6] used ATR (Attenuated Total Re- flection) IR spectroscopy, a method known to be more surface sen- sitive than DRIFT, and thermodynamic calculations on the system galena–diethyldithiophosphate; both chemisorbed Dtp ions, form- ing surface compounds, and precipitated bulk Dtp lead(II) com- plexes on the lead sulfide surface were identified [4–6]. 0021-9797/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.08.031