Specific Ion Effects on the Self-Assembly of Ionic Surfactants: A Molecular Thermodynamic Theory of Micellization with Dispersion Forces

The self-assembly of amphiphilic molecules is a key process in numerous biological and chemical systems. When salts are present, the formation and properties of molecular aggregates can be altered dramatically by the specific types of ions in the electrolyte solution. We present a molecular thermodynamic model for the micellization of ionic surfactants that incorporates quantum dispersion forces to account for specific ion effects explicitly through ionic polarizabilities and sizes. We assume that counterions are distributed in the diffuse region according to a modified Poisson–Boltzmann equation and can reach all the way to the micelle surface of charge. Stern layers of steric exclusion or distances of closest approach are not imposed externally; these are accounted for through the counterion radial distribution profiles due to the incorporation of dispersion potentials, resulting in a simple and straightforward treatment. There are no adjustable or fitted parameters in the model, which allows for a priori quantitative prediction of surfactant aggregation behavior based only on the initial composition of the system and the surfactant molecular structure. The theory is validated by accurately predicting the critical micelle concentration (CMC) for the well-studied sodium dodecyl sulfate (SDS) surfactant and its alkaline-counterion derivatives in mono- and divalent salts, as well as the molecular structure parameters of SDS micelles such as aggregation numbers and micelle surface potential

Molecular Thermodynamic Modeling of Reverse Micelles and Water-in-Oil Microemulsions

Surfactant aggregation plays an important role in a variety of chemical and biological nanoscale processes. On a larger scale, using small amounts of amphiphiles compared to large volumes of bulk-phase modifiers can improve the efficiency and reduce the environmental impact of many chemical and industrial processes. To model ternary mixtures of polar, nonpolar, and amphiphilic molecules, we develop a molecular thermodynamic theory for polydisperse water-in-oil (W/O) droplet-type microemulsions and reverse micelles based on global minimization of the Gibbs free energy of the system. The incorporation of size polydispersity into the theoretical formulation has a significant effect on the Gibbs free energy landscape and allows us to accurately predict micelle size distributions and micelle size variation with composition. Results are presented for two sample ionic surfactant/water/oil systems and compared with experimental data. By predicting the structural and compositional characteristics of w/o microemulsions, the molecular thermodynamic approach provides an important bridge between the modeling of ternary systems at the molecular and the macroscopic level

Induced Charge Density and Thin Liquid Film at Hydrate/Methane Gas Interfaces

The hydrate/methane gas interface is studied by molecular dynamics simulations. Below the hydrate melting temperature a thin liquid film forms with an associated surface charge density and electrostatic potential. The thickness of the thin liquid film, the charge density, and electrostatic potential at the hydrate/gas interface are established at different subcooling temperatures for the first time. The hydrate interface has mixed polarity, being predominantly positive. A comparison is made with the ice/methane interface, which reveals similarities and differences in the induced charge density. The thin liquid film and the induced charge density have important implications for the interfacial properties of methane hydrates

Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates

Adsorption of functional molecules on the surface of hydrates is key in the understanding of hydrate inhibitors. We investigate the adsorption of a hydrocarbon chain, nonionic and ionic surfactants, and ions at the hydrate–aqueous interface. Our results suggest a strong connection between the water ordering around solutes in bulk and the affinity for the hydrates surface. We distinguish two types of water ordering around solutes: (i) hydrophobic hydration where water molecules form a hydrogen bond network similar to clathrate hydrates, and (ii) ionic hydration where water molecules align according to the polarity of an ionic group. The nonionic surfactant and the hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed on the hydrate surface. Adsorption of ions and the ionic headgroups on the hydrate surface is not favorable because ionic hydration and the hydrogen bond structure of hydrates are incompatible. The nonionic surfactant is adsorbed by the headgroup and tail while adsorption of the ionic surfactants is not favorable through the head. Water ordering is analyzed using the hydrogen bond and tetrahedral density profiles as a function of the distance to the chemical groups. The adsorption of solutes is studied through the free energy profiles as a function of the distance to the hydrate surface. Salt lowers the melting temperature of hydrates, disrupts hydrophobic hydration, reduces the solubility of solutes in the aqueous solution, and increases the propensity of solutes to be adsorbed on hydrate surfaces. Our studies are performed by the unbiased and steered molecular dynamics simulations. The results are in line with experiments on the effect of salt and alkanes in hydrate antiagglomeration

Effect of Water on Deposition, Aggregate Size, and Viscosity of Asphaltenes

The aggregation and structure of polar molecules in nonpolar media may have a profound effect on bulk phase properties and transport. In this study, we investigate the aggregation and deposition of water and asphaltenes, the most polar fraction in petroleum fluids. In flow-line experiments, we vary the concentration of water from 500 up to 175 000 ppm and provide the evidence for clear changes in asphaltene deposition. Differential interference contrast (DIC) microscopy and dynamic light scattering (DLS) are used to measure the size of the aggregates. Rheological measurements are performed to get fixed ideas on the structural changes that water induces at different concentrations. This study demonstrates the significant effect of water on asphaltene aggregation and deposition and explores the molecular basis of water–asphaltene interaction. Our aggregate size measurements show that while asphaltene molecules increase the solubilization of water, there is no increase in the aggregate size. Our aggregation size measurements are different from the reports in the literature

Controlling Nonpolar Colloidal Asphaltene Aggregation by Electrostatic Repulsion

While aromatic chemicals are applied to petroleum oil systems to thermodynamically prevent asphaltene precipitation, amphiphilic dispersants can truncate the precipitation process and create stable suspensions of asphaltene colloids in the submicrometer size range. Bulk sedimentation and dynamic light scattering have shown that stabilizing dispersants inhibit colloidal asphaltene aggregation at approximately the same concentration as is needed to effectively slow bulk sedimentation. At the same time, these same types of dispersants can alter the electrostatic properties of colloidal asphaltenes in nonpolar suspensions. While electrostatic stabilization has been linked to aggregation dynamics in several types of colloidal systems, both aqueous and nonpolar, the complete linkage between electrostatic interactions and aggregation inhibition has yet to be shown in colloidal asphaltene suspensions. In this work, we present dynamic light scattering and electrophoresis measurements in colloidal asphaltene suspensions, using three different petroleum fluids and a dispersant which truncates asphaltene precipitation and colloidal aggregation by enabling uniform electrostatic charging at the colloidal asphaltene surface

Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates

Adsorption of functional molecules on the surface of hydrates is key in the understanding of hydrate inhibitors. We investigate the adsorption of a hydrocarbon chain, nonionic and ionic surfactants, and ions at the hydrate–aqueous interface. Our results suggest a strong connection between the water ordering around solutes in bulk and the affinity for the hydrates surface. We distinguish two types of water ordering around solutes: (i) hydrophobic hydration where water molecules form a hydrogen bond network similar to clathrate hydrates, and (ii) ionic hydration where water molecules align according to the polarity of an ionic group. The nonionic surfactant and the hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed on the hydrate surface. Adsorption of ions and the ionic headgroups on the hydrate surface is not favorable because ionic hydration and the hydrogen bond structure of hydrates are incompatible. The nonionic surfactant is adsorbed by the headgroup and tail while adsorption of the ionic surfactants is not favorable through the head. Water ordering is analyzed using the hydrogen bond and tetrahedral density profiles as a function of the distance to the chemical groups. The adsorption of solutes is studied through the free energy profiles as a function of the distance to the hydrate surface. Salt lowers the melting temperature of hydrates, disrupts hydrophobic hydration, reduces the solubility of solutes in the aqueous solution, and increases the propensity of solutes to be adsorbed on hydrate surfaces. Our studies are performed by the unbiased and steered molecular dynamics simulations. The results are in line with experiments on the effect of salt and alkanes in hydrate antiagglomeration

Nucleation of Methane Hydrates at Moderate Subcooling by Molecular Dynamics Simulations

Methane hydrates are crystalline structures composed of cages of hydrogen-bonded water molecules in which methane molecules are trapped. The nucleation mechanisms of crystallization are not fully resolved, as they cannot be accessed experimentally. For methane hydrates most of the reported simulations on the phenomena capture some of the basic elements of the full structure. In few reports, formation of crystalline structures is reached by imposing very high pressure, or dynamic changes of temperature, or a pre-existing hydrate structure. In a series of nanoscale molecular dynamics simulations of supersaturated water–methane mixtures, we find that the order of the crystalline structure increases by decreasing subcooling. Crystalline structures I and II form and coexist at moderate temperatures. Crystallization initiates from the spontaneous formation of an amorphous cluster wherein structures I, II, and other ordered defects emerge. We observe the transient coexistence of sI and sII in agreement with experiments. Our simulations are carried out at high methane supersaturation. This condition dramatically reduces the nucleation time and allows simulating nucleation at moderate subcooling. Moderate temperatures drive hydrates to more ordered structures

Contact Angle, Liquid Film, and Liquid–Liquid and Liquid–Solid Interfaces in Model Oil–Brine–Substrate Systems

Oil–water–substrate wettability is of prime importance in most branches of science and technology, from biology and nanomaterials to geology and petroleum science. Wetting is a three-phase interaction phenomenon as expressed in Young’s equation. Microscopically wetting is from the fluid–substrate interactions and surfaces are designated as lyophilic and lyophobic (fluid-wet and nonwet). Here we investigate the microscopic mechanisms of wettability changes by salt concentration in oil–water–mineral substrate systems. A model oil droplet (<i>n</i>-decane) placed in an aqueous electrolytic solution next to a solid substrate surface (muscovite mica) is simulated. A thin water layer between oil and the substrate regulates the oil–substrate interaction. We find that at zero and low salt concentrations, the oil adsorption on the hydrophilic mineral substrate is stabilized by a thin layer of water giving rise to a nonzero contact angle (partial oil wetting). As the salt concentration increases ionic adsorption and the water layer thickness increase reducing the oil–substrate wettability. Ions adsorb unsymmetrically on the substrate and promote water adsorption into the water layer. Ionic adsorption is higher away from the droplet than under the droplet. Our contact angles by molecular dynamics simulations are in agreement with experimental measurements

Tunable Substrate Wettability by Thin Water Layer

In oil–water–mineral substrate systems, we show that the contact angle can be tuned by ionic structures in the water layer confined between an oil droplet and the substrate. We perform molecular dynamics simulations of a complex oil droplet in a NaCl aqueous solution on a mica surface; the oil is a mixture of <i>n</i>-decane and surfactant molecules. The surfactant head contains an OH group and an aromatic ring. A thin water layer between the oil droplet and the substrate and ionic stratification regulate the wetting behavior. The concentration of salt ions in the thin film is nonmonotonic; it first increases, then decreases, and starts increasing again as the salt concentration in the bulk increases. On the other hand, the surfactant head adsorption in the thin film first increases as the bulk salt concentration increases. Then, it decreases with further increase in the bulk salt concentration. The change of contact angle with salt concentration also shows a nonmonotonic behavior; the contact angle is first nearly constant to a low salt concentration of 0.1 wt % NaCl. Then, it decreases sharply as the salt concentration increases from 0.1 to 1.1 wt % NaCl. A reverse trend in contact angle follows with further salt concentration increase. The nonmonotonic trend unlike the monotonic trend of interfacial tension with salt concentration is in line with recent measurements of contact angle of oil–brine–substrate systems. A sharp increase of surfactant head adsorption in the thin film, the decrease of ion adsorption, and the minimum of contact angle are all related. This is the first report of such correlations with change of wetting in the brine–complex oil–mineral substrate predicted from molecular simulations