ARTICLE pubs.acs.org/JPCC
Evaluation of Steric Repulsive Force in the Aqueous Dispersion System of Dimethomorph Powder with Lignosulfonates via X-ray Photoelectron Spectroscopy Zhili Li,*,†,‡ Yuxia Pang,‡ Yuanyuan Ge,† and Xueqing Qiu*,‡ † ‡
School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China ABSTRACT: X-ray photoelectron spectroscopy (XPS) was adopted to evaluate steric repulsive force in the aqueous dispersion system of dimethomorph powder with lignosulfonate (LS). The calculation of adsorption layer thickness (ALT) of LS on dimethomorph based on the relative intensity of the N 1s peak from XPS was presented. The ALT was correlated with the volume-based average particle size (VAPS) of dimethomorph suspensions that indicated the dispersion efficiency of LS. As expected, VAPS decreased linearly with growing ALT; namely, the dispersion efficiency of LS increased with growing ALT except Kinsperse. Zeta potential experiments demonstrated that the dominant electrostatic repulsive forces controlled the particle dispersing mechanism when Kinsperse was used as dispersant. The positive-going relationship of dispersion efficiency of LS with ALT confirmed that it would be feasible to use ALT to evaluate steric repulsive forces in the dispersion system of dimethomorph with LS, and that would lead to a potential utilization in other dispersion systems.
1. INTRODUCTION Lignosulfonate (LS), a macromolecule ionic surfactant derived from lignin, is three-dimensional cross-linked by phenyl propane with sulfonic, carboxylic, and phenolic OH groups, etc.1,2 It is water-soluble that makes it prior to lignin and can be used in many industrial applications, such as cements,3,4 coal water slurry,5 textile dyeing,6,7 and agrochemicals,8 etc. Surfactants can stimulate proper repulsive force formation between the droplets or particles in a dispersion system by being adsorbed on their surfaces.9 That includes electrostatic repulsive forces and steric repulsive forces.10,11 Nowadays, lignosulfonate is receiving increased attention because it will be a benefit to forming both electrostatic repulsive forces and steric repulsive forces in dispersion systems.12 As is known, when ionic surfactants are adsorbed on the surface of substances, the electrostatic repulsive forces can be formed, and the magnitudes are well indicated by the particle’s zeta potentials.13 Anderson14 has reported that the adsorption of ionic surfactants on the cement particle surface enhanced the particle’s zeta potentials which led to an increase of electrostatic repulsive forces. However, when macromolecular surfactants are adsorbed onto the surface of substances, the formed steric repulsive force is not well determined by some traditional electrochemical and microscopic techniques. In some reported literature, it is simply indicated by adsorption amount.15 It is therefore important to better understand the steric repulsive forces formed by the macromolecular surfactant being adsorbed on the surface of substances via some sort of modern characterization techniques. r 2011 American Chemical Society
X-ray photoelectron spectroscopy (XPS) is one of the widely used techniques to analyze surface layers of materials,16,17 such as adsorption18,19 and coating.20 This technique depends on the liberation of electrons from the surface of substances using an X-ray photon source. The binding energies of these liberated photoelectrons are recorded by an electron spectrometer and provide the most complete information on the elementary composition of the surface layers including the layer thickness. Our interest in this study lies in the search for information about the adsorption layer of lignosulfonate on the dimethomorph surface via XPS and the calculation of the adsorption layer thickness (ALT) to explore the possibility of evaluating the formed steric repulsive forces during the adsorption process.
2. EXPERIMENTAL SECTION 2.1. Dimethomorph. Dimethomorph (C21H22ClNO4),21 a
fungicide, was used as a solid adsorbent being provided as a light white powder by Jiangsu Frey Agrochemicals Ltd., China. It was washed twice in distilled water for 10 min and then filtered to remove organic contaminants before utilization. The BET surface area and total porosity of dimethomorph powder were measured by means of a nitrogen adsorption technique via an ASAP2010 M surface analyzer (Micrometritics Ltd., USA), and the results were Received: August 28, 2011 Revised: November 7, 2011 Published: November 07, 2011 24865
dx.doi.org/10.1021/jp2083117 | J. Phys. Chem. C 2011, 115, 24865–24870
The Journal of Physical Chemistry C
ARTICLE
Table 1. Physical Properties of Dimethomorph Powder properties
dimethomorph
appearance
light white
BET surface area/m2 3 g1
0.6175
total porosity/cm3 3 g1
0.0050
average pore diameter/nm
77.80
purity/wt%
95.0
density/kg 3 m3
1.318 103 1
solubility in 20 °C water/mg 3 L
