Introduction Relying on their advantages of high water reducing rate, low dosage, and strong molecular structure design ability, polycarboxylate superplasticizers (PCEs) have become the most widely used admixture in the field of concrete admixtures. Adsorption of PCEs onto the surfaces of cement particles is the prerequisite for the dispersing performance of PCE molecules. Therefore, the influences of PCE molecules with different structures on the dispersion properties of cement slurry can be understood by revealing the adsorption process of PCE and the conformation characteristics of PCE after adsorption. It should be noted that both the adsorption process and the conformation of the adsorbed PCE involve interfacial interactions between the organic and inorganic phases, which is the dominant factor determining the overall properties of the material. Recently, molecular dynamics (MD) simulations have been applied to the PCEs to elucidate its working mechanism. However, the molecular size of these model PCEs was much smaller than those typically used for industrial applications in order to reduce computation cost. Furthermore, only pure water was used, or only Na+, Ca2+ and Cl− were additionally added to mimic a cement pore solution. The above methods make the results unable to accurately reflect the adsorption mechanism of PCE. Therefore, to better understand and effectively control the performance of PCE, more in-depth studies at the atomic and molecular scales are needed. Our research team innovatively used MD simulations in which the molecular size of the model PCE is consistent with that of PCE commonly used for industrial applications and simulates cement pore solutions. In addition, the adsorption mechanism of sulfonation modification of PCE was studied by combining the structure, dynamics and stability of interfacial connections. Methods The MD simulations used the Gromacs-4.6.7 software package and a conventional oplsaa force field with RESP charges. The UFF force field was used for the substrate. The equilibrium MD simulation was conducted under the NPT ensemble for a total run time of 20 ns at a relaxed liquid configuration (25 ℃). The solution environment simulations were performed using 3000 water molecules. A relaxation system was selected and the energy was minimized by the steepest descent method with a termination gradient of 100 kJ/(mol∙nm) before the relaxation. The system temperature maintained constant through a Nosé–Hoover thermostat, and periodic boundary conditions were applied to all three dimensions. Long-range electrostatics within a relative tolerance of 1×10−6 is calculated by the particle mesh Ewald method and a cut-off distance of 1 nm was used as the Ewald interaction and van der Waals interactions. The bond lengths of hydrogen atoms were constrained by the LINCS algorithm. A leap-frog algorithm was used with a time step of 1 fs. Results and discussion Under cement solution conditions, the side chains of PAA-g-PALS molecules were not entangled with the backbone (present a network-like conformation), while a small part of the side chains of PAA-g-HPEG molecules will curl onto the backbone (most of the side chains curled into a mushroom shape). In addition, the rotation radius of PAA-g-PALS (7.8 nm) was significantly larger than PAA-g-HPEG (7.1 nm) due to the large amount of —SO3– with strong electrostatic repulsion. Part of the backbone of PAA-g-HPEG was adsorbed to the particles (the side chains curl and wrap the backbone), while PAA-g-PALS was adsorbed to the substrate surface by the flat mode of the individual side chains (the backbone and other side chains extend into the solution). Compared with PAA-g-HPEG, the binding point of sulfation modified PCE and the substrate is relatively scattered and increases the binding surface with the substrate, which is consistent with the calculation of binding force PAA-g-PALS (174.3 kJ/mol)> PAA-g-HPEG (134.5 kJ / mol). The vertical heights of the individual PAA-g-HPEG and PAA-g-PALS molecules on the substrate surface were 6.8 nm and 19.7 nm, respectively, and the coverage rate was 26.4% and 21.2%, respectively. When considering the polymer cross-dimensions parallel and perpendicular to the particle surface, the penetration volume formed by PAA-g-PALS on the substrate surface increased by 132.64% relative to PAA-g-HPEG. Conclusions The main conclusions of this paper are summarized as following. PAA-g-HPEG and PAA-g-PALS tend to be comb and network conformations in cement slurry environment, respectively. Sulfonated modification dispersed the distribution of binding points between PCE and the substrate, improved the binding force between PCE and the substrate, extended the polymer far away from the substrate, and significantly increase the permeability volume (by about 132.64%). The present results not only provide new insights into the action mechanism of PCE at the atomic level, but also provide a theoretical basis for promoting the development of high performance cement-based materials. © 2024 Chinese Ceramic Society. All rights reserved.
