Micellization and adsorption of a series of succinimide dispersants

Characterization of Polyisobutylene Succinic Anhydride (PIBSA) and Its PIBSI Products from the Reaction of PIBSA with Hexamethylene Diamine

The nature of the end-groups of a PIBSA sample, namely a polyisobutylene (PIB) sample, where each chain is supposedly terminated at one end with a single succinic anhydride group, was characterized through a combination of pyrene excimer fluorescence (PEF), gel permeation chromatography, and simulations. The PIBSA sample was reacted with different molar ratios of hexamethylene diamine to generate PIBSI molecules with succinimide (SI) groups in the corresponding reaction mixtures. The molecular weight distribution (MWD) of the different reaction mixtures was determined by fitting the gel permeation chromatography traces with sums of Gaussians. Comparison of the experimental MWD of the reaction mixtures with those simulated by assuming that the reaction between succinic anhydride and amine occurs through stochastic encounters led to the conclusion that 36 wt% of the PIBSA sample constituted unmaleated PIB chains. Based on this analysis, the PIBSA sample was found to be constituted of 0.50, 0.38, and 0.12 molar fractions of PIB chains that were singly maleated, unmaleated, and doubly maleated, respectively.

Design of New Dispersants Using Machine Learning and Visual Analytics

Artificial intelligence (AI) is an emerging technology that is revolutionizing the discovery of new materials. One key application of AI is virtual screening of chemical libraries, which enables the accelerated discovery of materials with desired properties. In this study, we developed computational models to predict the dispersancy efficiency of oil and lubricant additives, a critical property in their design that can be estimated through a quantity named blotter spot. We propose a comprehensive approach that combines machine learning techniques with visual analytics strategies in an interactive tool that supports domain experts' decision-making. We evaluated the proposed models quantitatively and illustrated their benefits through a case study. Specifically, we analyzed a series of virtual polyisobutylene succinimide (PIBSI) molecules derived from a known reference substrate. Our best-performing probabilistic model was Bayesian Additive Regression Trees (BART), which achieved a mean absolute error of 5.50±0.34 and a root mean square error of 7.56±0.47, as estimated through 5-fold cross-validation. To facilitate future research, we have made the dataset, including the potential dispersants used for modeling, publicly available. Our approach can help accelerate the discovery of new oil and lubricant additives, and our interactive tool can aid domain experts in making informed decisions based on blotter spot and other key properties.

A novel approach to improve the oil miscibility and incorporate multifunctionality in ionic liquids as lubricant additives

Recently ionic liquids (ILs) have shown promising tribological properties as additives in base oils; however their lack of miscibility is a problem, with very few ILs being compatible with lubricant oil formulation (non-polar base oils). This work shows the use of a surfactant which can increase the range of available ILs that are stable when added to these base oils. In this study a range of tetraalkylphosphonium based ILs were successfully blended with a PIBSA surfactant and these blends were all shown to be miscible in a non-polar base oil. Without the PIBSA a number of the ILs were immiscible in the base oil. The tribological properties of IL additives that are miscible in the non-polar base oils were compared with and without the surfactant present and showed that the presence of the PIBSA did not affect the IL additives performance. Additionally, two ILs that are immiscible without the surfactant showed the greatest reduction in friction and wear. SEM analysis showed an increase in the amount of phosphorus on the wear surface when the surfactant was present, suggesting that the PIBSA enhances tribo-film formation. NMR, FTIR, DLS and TEM investigations into the interactions between the PIBSA and the ILs showed that the improved stability in the base oil may be due to intermolecular interactions such as hydrophobic, van der Waals, dipole-dipole or ion-dipole that reduce the size distribution of the previously immiscible ILs. The presence of the ILs was also shown to improve the resistance to corrosion. Prior to this study the ILs available for use as lubricant additives was severely limited and compromised, mostly based upon their miscibility. Here the use of PIBSA to increase the range of ILs available as lubricant additives has vastly improved the promise that they represent in this area.

Interactions between Friction Modifiers and Dispersants in Lubricants: The Case of Glycerol Monooleate and Polyisobutylsuccinimide-Polyamine

The structural and frictional properties of 10 wt % solutions of the amphiphilic molecules glycerol monooleate (GMO) and polyisobutylsuccinimide-polyamine (PIBSA-PAM) in squalane are studied using molecular dynamics simulations in bulk and under confinement between iron oxide surfaces. GMO is a friction modifier, PIBSA-PAM is a dispersant, and squalane is a good model for typical base oils. A range of liquid compositions and applied pressures is explored, and the formation and stability of reverse micelles are determined under quiescent and shear conditions. Micellization is observed mainly in systems with a high GMO content, but PIBSA-PAM may also form small aggregates on its own. In the confined systems under both static and shear conditions, some surfactant molecules adsorb onto the surfaces, with the rest of the molecules forming micelles or aggregates. Shearing the liquid layer under high pressure causes almost all of the micelles and aggregates to break, except in systems with around 7.5 wt % GMO and 2.5 wt % PIBSA-PAM. The presence of micelles and adsorbed surfactants is found to be correlated with a low kinetic friction coefficient, and hence, there is an optimum composition range for friction reduction. This work highlights the importance of cooperative interactions between lubricant additives.

Evaluating the Dispersant Stabilization of Colloidal Suspensions from the Scaling Behavior of Gel Rheology and Adsorption Measurements

Maintaining suspension stability by effective particle dispersion in systems with attractive interactions can be accomplished by the addition of dispersants that modify the interparticle potential to provide steric or electrostatic barriers against aggregation. The efficacy of such dispersants is typically considered simply by the modification of suspension rheological properties as a function of the overall added dispersant concentration. However, such considerations do little to reveal the molecular origin of differences in dispersant efficacy because they do not consider differences in surface activity. We combine measured adsorption isotherms with the rheological characterization of the elasticity of colloidal gels formed by particle aggregation to provide a more meaningful assessment of dispersant efficacy. The rheological data show that the dispersants are effective at reducing particle aggregation, whereas, from the adsorption isotherms, they differ considerably in their surface coverage at constant overall concentration. When compared at constant dispersant particle surface coverage, the gel rheology shows marked differences across the different dispersants, as opposed to comparisons at constant overall dispersant concentration in the suspensions. In particular, the power-law volume fraction scaling of gel elasticity at constant coverage reveals clear differences in the critical volume fraction for gel formation for the different dispersants. The most efficacious dispersant is that associated with the largest critical volume fraction for gel formation at a given surface coverage. This work demonstrates the utility of rheological investigations coupled with accurate determinations of surface coverage to better differentiate dispersant performance, which may improve efforts to engineer new dispersant molecules.

Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality

Surface charging phenomena in nonpolar dispersions are exploited in a wide range of industrial applications, but their mechanistic understanding lags far behind. We investigate the surface charging of a variety of polymer particles with different surface functionality in alkane solutions of a custom-synthesized and purified polyisobutylene succinimide (PIBS) polyamine surfactant and a related commercial surfactant mixture commonly used to control particle charge. We find that the observed electrophoretic particle mobility cannot be explained exclusively by donor-acceptor interactions between surface functional groups and surfactant polar moieties. Our results instead suggest an interplay of multiple charging pathways, which likely include the competitive adsorption of ions generated among inverse micelles in the solution bulk. We discuss possible factors affecting the competitive adsorption of micellar ions, such as the chemical nature of the particle bulk material and the size asymmetry between inverse micelles of opposite charge.

Mechanisms of Particle Charging by Surfactants in Nonpolar Dispersions

Electric charging of colloidal particles in nonpolar solvents plays a crucial role for many industrial applications and products, including rubbers, engine oils, toners, or electronic displays. Although disfavored by the low solvent permittivity, particle charging can be induced by added surfactants, even nonionic ones, but the underlying mechanism is poorly understood, and neither the magnitude nor the sign of charge can generally be predicted from the particle and surfactant properties. The conclusiveness of scientific studies has been limited partly by a traditional focus on few surfactant types with many differences in their chemical structure and often poorly defined composition. Here we investigate the surface charging of poly(methyl methacrylate) particles dispersed in hexane-based solutions of three purified polyisobutylene succinimide polyamine surfactants with "subtle" structural variations. We precisely vary the surfactant chemistry by replacing only a single electronegative atom located at a fixed position within the polar headgroup. Electrophoresis reveals that these small differences between the surfactants lead to qualitatively different particle charging. In the respective particle-free surfactant solutions we also find potentially telling differences in the size of the surfactant aggregates (inverse micelles), the residual water content, and the electric solution conductivity as well as indications for a significant size difference between oppositely charged inverse micelles of the most hygroscopic surfactant. An analysis that accounts for the acid/base properties of all constituents suggests that the observed particle charging is better described by asymmetric adsorption of charged inverse micelles from the liquid bulk than by charge creation at the particle surface. Intramicellar acid-base interaction and intermicellar surfactant exchange help rationalize the formation of micellar ions pairs with size asymmetry.

Extension to the charge fluctuation model for the prediction of the conductivity of apolar, reverse micellar systems

This paper presents an extension to current theory regarding charging behavior in apolar, micellar systems. Electrical conductivity in such systems accompanying the formation of neutral reverse micelles is commonly explained by the possibility of intermicellar collisions resulting in a pair of oppositely charged micelles. The sequestration of the resulting charges within the micelles prevents their immediate recombination. The current theory underlying the charging process has thus far been applied in only approximate form, and is only used to validate experimental trends and to abstract values for the fraction of charged micelles. The extended theory proposed here uses knowledge of the solvent and surfactant characteristics, together with water content, to predict solution conductivity in absolute terms. It is verified in experiments with the solvent Isopar-L and surfactants Aerosol OT, OLOA 11000, and Span 80, in which significant differences from the approximate theory are observed.

Effects of trace water on charging of silica particles dispersed in a nonpolar medium

This paper presents an investigation of the effects of trace water on the charging of silica (SiO(2)) particles dispersed in a nonpolar medium. There are a growing number of applications that seek to use electrostatic effects in apolar media to control particle movement and aggregation stability in such systems. One factor that is often overlooked in the preparation of nonpolar colloidal dispersions is the amount of water that is introduced to the system by hygroscopic particles and surfactants. The amount and location of this water can have significant effects on the electrical properties of these systems. For nonpolar surfactant solutions it has been shown that water can affect the conductivity, and it has been speculated that this is due to swelling of the polar cores of inverse micelles, increasing the fraction of them that are charged. Some studies have suggested that particle surface charging may also be sensitive to water content, but a clear mechanism for the process has not been fully developed. The situation with particles is further complicated by the fact that it is often unclear whether the water resides on the particle surfaces or in the polar cores of inverse micelles. The current work explores not only the effect of water content on reverse micelle and particle charging but seeks to differentiate between water bound to the particles and water located in the micelles. This is accomplished by measuring the solution conductivity and the electrophoretic mobility of silicon dioxide particles dispersed in solutions of Isopar-L and OLOA 11000. The water content is determined for both the dispersion and the supernatant after centrifuging the particles out. It is found that at equilibrium the majority of the water in the system adsorbs to the surface of the hygroscopic silica particles. In addition, the effect of water on particle electrophoretic mobility is found to be dependent on surfactant concentration. At small OLOA concentrations, additional water results in an increase in particle mobility due to increased particle charging. However, at large OLOA concentrations, additional water leads to a decrease in particle mobility, presumably as a result of increased electrostatic screening or neutralization. Thus, particle charging and electrophoretic mobility in an apolar surfactant solution are found to be highly sensitive to both the total water content in the system and to its concentration relative to the amount of surfactant present.

Influence of surfactant structure on reverse micelle size and charge for nonpolar electrophoretic inks

Electrophoretic inks, which are suspensions of colorant particles that are controllably concentrated and dispersed by applied electric fields, are the leading commercial technology for high-quality reflective displays. Extending the state of the art for high-fidelity color in these displays requires improved understanding and control of the colloidal systems. In these inks, reverse micelles in nonpolar media play key roles in media and particle charging. Here we investigate the effect of surfactant structure on reverse micelle size and charging properties by synthesizing different surfactants with variations in polyamine polar head groups. Small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) were used to determine the micelle core plus shell size and micelle hydrodynamic radius, respectively. The results from SAXS agreed with DLS and showed that increasing polyamines in the surfactant head increased the micelle size. The hydrodynamic radius was also calculated on the basis of transient current measurements and agreed well with the DLS results. The transient current technique further determined that increasing polyamines increased the charge stabilization capability of the micelles and that an analogous commercial surfactant OLOA 11000 made for a lower concentration of charge-generating ions in solution. Formulating magenta inks with the various surfactants showed that the absence of amine in the surfactant head was detrimental to particle stabilization and device performance.