Wettability Of Mineral Surfaces Research Articles

Effect of dissolved metal ions from mineral surfaces on the surface wettability of phosphate ore by flotation

The dissolved metal ions have a significant effect on the wettability of mineral surface in flotation slurry. In this work, the composition and characteristic of phosphate ore raw (R), flotation concentrate (FC), and flotation tailing (FT), the dissolution of metal ions from mineral surface and its effect on mineral surface wettability were investigated. The results of the dissolution test indicated that the concentrations of Ca2+ and Mg2+ are high (FT > R > FC), while Fe3+ and Al3+ were low in the slurry. H3PO4, as the slurry pH regulator, has the weakest dissolved ability of metal ions from mineral surface compared with HCl, HNO3 and H2SO4. The surface tension tests revealed that metal ions will combine with NaOL in the slurry, causing the consumption of NaOL. The contact Angle tests performed that some of NaOL adsorption sites on the FC and FT surface were covered the hydrolysates of Fe3+ and Al3+, which inhibited the hydrophobicity of the FC and FT surfaces. Ca2+ and Mg2+ consumed part of NaOL in the slurry, inhibiting the hydrophobicity of the FT surface. However, the adsorption of Ca2+ and Mg2+ on the FC surface increased the adsorption of NaOL, which enhanced the hydrophobicity of the FC surface.

Exploring wettability variations on minerals surfaces: Insights from spreading coefficient and interaction energy analysis

The wettability of minerals plays a pivotal role in determining the distribution of residual oil and devising effective flooding strategies for enhanced oil recovery. Utilizing molecular dynamics simulations, we investigated the surface wettability of various typical minerals. The simulation results illustrate that the modeled crude oil spreads completely on most mineral surfaces. We proposed an efficient approach to calculate the spreading coefficient of the modeled crude oil on mineral surfaces, yielding accurate predictions of the spreading state — whether it is completely spread or not. Compared with the conventional spreading simulation method, our approach offers significant time savings in the calculations. Additionally, our findings underscore the influence of various crude oil components on mineral surface wettability, emphasizing the crucial role of microscopic interactions between these components and the mineral. These insights contribute to a refined understanding of mineral surface wettability and its correlation with crude oil composition, offering valuable guidance for optimizing reservoir management and production strategies.

Molecular dynamics simulation of the surface wettability of typical minerals in shale

Nanoscale pores are an important storage space in shale petroleum reservoirs. Mineral surface wettability significantly affects the adsorption ability of a fluid, and it is of great significance for the efficient development of shale petroleum. Within a nanoscale pore, however, we cannot measure the contact angle through a conventional measuring method, so we used a molecular dynamics (MD) simulation method. A nanoscale ‘oil–water–rock’ wettability model was used to study the wettability of reservoir minerals with micropores. In this study, by means of MD simulation, the wettability models of different minerals (calcite, quartz and illite) and different liquid hydrocarbons (n‐hexane, toluene and acetic acid) were established, and then the temperature and pressure were changed to study the influence of mineral type, liquid hydrocarbon type, temperature and pressure on wettability. The results show that calcite has the strongest wettability of water in n‐hexane, followed by illite and quartz, and toluene has the strongest wettability of water on the surface of calcite minerals, followed by n‐hexane and acetic acid. In the temperature range of 313–378 K, the higher the temperature, the stronger the water wettability. In the pressure range of 5–20 MPa, the higher the pressure, the water wettability is stronger. By comparing the MD simulation results with the previous experimental results, similar wetting characteristics were obtained, which verified the reliability of the simulation results. This MD simulation method provides a new idea for the study of shale mineral wettability, which is of great significance for shale petroleum exploration.

Effects of Wettability and Minerals on Residual Oil Distributions Based on Digital Rock and Machine Learning

Abstract The wettability of mineral surfaces has significant impacts on transport mechanisms of two-phase flow, distribution characteristics of fluids, and the formation mechanisms of residual oil during water flooding. However, few studies have investigated such effects of mineral type and its surface wettability on rock properties in the literature. To unravel the dependence of hydrodynamics on wettability and minerals distribution, we designed a new experimental procedure that combined the multiphase flow experiments with a CT scan and QEMSCAN to obtain 3D digital models with multiple minerals and fluids. With the aid of QEMSCAN, six mineral components and two fluids in sandstones were segmented from the CT data based on the histogram threshold and watershed methods. Then, a mineral surface analysis algorithm was proposed to extract the mineral surface and classify its mineral categories. The in situ contact angle and pore occupancy were calculated to reveal the wettability variation of mineral surface and distribution characteristics of fluids. According to the shape features of the oil phase, the self-organizing map (SOM) method, one of the machine learning methods, was used to classify the residual oil into five types, namely, network, cluster, film, isolated, and droplet oil. The results indicate that each mineral’s contribution to the mineral surface is not proportional to its relative content. Feldspar, quartz, and clay are the main minerals in the studied sandstones and play a controlling role in the wettability variation. Different wettability samples show various characteristics of pore occupancy. The water flooding front of the weakly water-wet to intermediate-wet sample is uniform, and oil is effectively displaced in all pores with a long oil production period. The water-wet sample demonstrates severe fingering, with a high pore occupancy change rate in large pores and a short oil production period. The residual oil patterns gradually evolve from networks to clusters, isolated, and films due to the effects of snap-off and wettability inversion. This paper reveals the effects of wettability of mineral surface on the distribution characteristics and formation mechanisms of residual oil, which offers us an in-deep understanding of the impacts of wettability and minerals on multiphase flow and helps us make good schemes to improve oil recovery.

Experimental study on the interaction forces between water droplets and mineral surfaces

Research on the interaction between droplets and solids is of great significance for fundamental research and practical applications. This work comprehensively investigated the interaction process between millimeter-scale water droplets and three minerals (kaolinite, SiO 2 and Al 2 O 3 ) and directly measured their interaction forces, including wetting force, maximum adhesion force and detachment force, using the macroscopic force measurement technology and camera technology. In addition, the influence of surface wettability and impact distance on the interaction forces was examined, and the relationships between surface wettability, impact distance and interaction forces were also discussed. Results showed that at the same impact distance, the wetting force, maximum adhesion force and detachment force between water droplets and mineral substrates increased with increasing the hydrophilicity of mineral surface. Regardless of the mineral type, due to the enlargement of TPC line length or the expansion of contact area, both wetting force and maximum adhesion force increased as an increase in impact distance, while the detachment force exhibited an opposite trend. Moreover, irrespective of impact distance, the interaction forces of water-kaolinite (60.3°) were always between those of water-SiO 2 (54.5°) and those of water-Al 2 O 3 (94.5°), indicating that surface wettability of minerals was the dominant factor determining the interaction forces between water droplet-mineral surface. These results can provide guidance for studying the interaction between liquid droplets and solid surfaces from the perspective of macroscopic forces.

Electrochemically deposited molybdenum disulfide surfaces enable polymer adsorption studies using quartz crystal microbalance with dissipation monitoring (QCM-D)

Polymer and small molecules are often used to modify the wettability of mineral surfaces which facilitates the separation of valuable minerals such as molybdenum disulfide (MoS2) from gangue material through the process of froth flotation. By design, traditional methods used in the field for evaluating the separation efficacy of these additives fail to give proper access to adsorption kinetics and molecule conformation, crucial aspects of flotation where contact times may not allow for full thermodynamic equilibrium. Thus, there is a need for alternative methods for evaluating additives that accurately capture these features during the adsorption of additives at the solid/liquid interface. Here, we present a novel method for preparing MoS2 films on quartz crystals used for Quartz Crystal Microbalance with Dissipation (QCM-D) measurements through an electrochemical deposition process. The resulting films exhibit well-controlled structure, composition, and thickness and therefore are ideal for quantifying polymer adsorption. After deposition, the sensors can be annealed without damaging the quartz crystal, resulting in a phase transition of the MoS2 from the as-deposited, amorphous phase to the 2H semiconducting phase. Furthermore, we demonstrate the application of these sensors to study the interactions of additives at the solid/liquid interface by investigating the adsorption of a model polymer, dextran, onto both the amorphous and crystalline MoS2 surfaces. We find that the adsorption rate of dextran onto the amorphous surface is approximately twice as fast as the adsorption onto the annealed surface. These studies demonstrate the ability to gain insight into the short-term kinetics of interaction between molecules and mineral surface, behavior that is key to designing additives with superior separation efficiency.

New insights into the depressive mechanism of citric acid in the selective flotation of scheelite from fluorite

The effects of citric acid (CA) as a depressant in the separation of scheelite and fluorite were evaluated by microflotation test. The underlying selective separation mechanism was revealed via wetting analysis, Fourier transform infrared (FTIR) spectroscopy, thermodynamic calculations, adsorption amount tests, inductively coupled plasma–optical emission spectroscopy (ICP–OES) and atomic force microscopy (AFM). Microflotation tests showed that CA strongly depressed the flotation of fluorite but not of scheelite. Artificial mixed mineral separation tests showed that scheelite and fluorite achieved a good flotation separation effect in the presence of CA as the depressant. Mineral surface wettability analyses and adsorption amount tests indicated that CA was more strongly adsorbed onto the surface of fluorite than onto the surface of scheelite. FTIR spectroscopy proved that CA had a strong chemical adsorption on fluorite surface. Thermodynamic calculations indicated that CA could spontaneously form the organic complex CaL− on the surface of fluorite but not on the surface of scheelite. ICP–OES revealed that a strong dissolution behaviour occurred on the surface of fluorite that caused more calcium ions to dissolve into the slurry, thereby decreasing the number of adsorption sites of calcium ions on the surface of fluorite. AFM intuitively confirmed the intense adsorption and the strong dissolution behaviour of CA on the surface of fluorite.

Differential adsorption of a high-performance collector at solid–liquid interface for the selective flotation of hematite from quartz

Different adsorption behaviors of the collector on the mineral surface play a crucial part in the flotation separation of different minerals by selectively modifying the wettability of mineral surfaces. In this investigation, to effectively remove quartz from hematite via reverse flotation, decaethoxylated stearylamine (DEOSA) was first introduced as a high-performance collector for quartz. The selective collecting performance and differential adsorption mechanisms of DEOSA for both hematite and quartz were studied by flotation experiments and surface analysis. Flotation results suggested that DEOSA displayed excellent collecting capacity for quartz against hematite and the effective removal of quartz from hematite was achieved. Adsorption capacity measurements and surface wettability analysis demonstrated that much larger amounts of DEOSA adsorbed onto quartz than hematite significantly enhanced the quartz surface hydrophobic whether the depressant starch was present or not, which resulted in the excellent flotation performance of quartz. Surface electrical characterization and Fourier transform infrared (FTIR) analysis further proved that the combination of hydrogen bonding and electrostatic attracting force enhanced the strong adsorption of DEOSA onto the quartz, achieving the efficient collection performance of DEOSA for quartz. Based on these findings, DEOSA can be used potentially as a strong-collection and high-performance quartz collector for recovering hematite from quartz via reverse flotation.

Molecular Dynamics Simulations of Mineral Surface Wettability by Water Versus CO2: Thin Films, Contact Angles, and Capillary Pressure in a Silica Nanopore

The wettability of mineral surfaces is an important property influencing multiphase flow in soils and sedimentary rocks. In particular, for CO2 abatement technologies that rely on trapping supercri...

Adsorption Behavior and Wettability of Rhodochrosite Surface: Effect of C18 Fatty Acid Unsaturation

Mineral surface wettability and its regulation by the adsorption of collectors have an important influence on the flotation performance. The adsorption behavior of C18 fatty acid with different unsaturation and its effect on rhodochrosite wettability was investigated with surface tension, contact angle, and atomic force microscopy (AFM) measurements. The results indicated that rhodochrosite hydrophobicity increased with the increasing concentration of fatty acid, along with the maximum contact angle (θmax) between hemimicelle concentration (HMC) and critical micelle concentration (CMC). Oleic acid (OA), linoleic acid (LA), and α-linolenic acid (ALA) had a higher θmax than stearic acid (SA), but the value decreased with the increase of C=C bond number. Besides, preferential adsorption of unsaturated fatty acids on the liquid-air interface can be attributed to the molecule’s steric hindrance resulting from C=C double bond, and the θ kept almost invariant with a higher value of ΓLG than ΓSL until HMC. The oriented monolayer and bilayer structure of fatty acids formed gradually on rhodochrosite surface with increasing concentration. However, the θmax may not necessarily correspond to the beginning of bilayer formation. Cylindrical monolayer and bilayer micelles of SA molecules were observed on rhodochrosite surface at HMC and CMC, respectively. While bilayer structures of unsaturated fatty acids formed before complete coverage of monolayer on rhodochrosite surface because of surface heterogeneity. This work provided a good understanding on the adsorption mechanism of fatty acid on rhodochrosite for flotation.

Differential adsorption of hydrolytic polymaleic anhydride as an eco-friendly depressant for the selective flotation of apatite from dolomite

Differential adsorption of surfactants (e.g., collectors and depressants) on the minerals plays a key role in mineral flotation through selectively altering the surface wettability of minerals. In this study, to improve the removal efficiency of dolomite from apatite by direct flotation, hydrolytic polymaleic anhydride (HPMA) was introduced as an eco-friendly depressant for dolomite. The depressing performance and adsorption mechanism of HPMA on the dolomite and apatite surfaces were studied through flotation tests and surface analyses. Flotation results demonstrated that HPMA displayed an intense depressing impact on the dolomite flotation while affected slightly the apatite flotation. Surface wettability analysis indicated that the selective adsorption of HPMA at the dolomite/water interface declined significantly the dolomite hydrophobicity even in the case of sodium oleate (NaOl). Surface adsorption detection and zeta-potential results demonstrated that HPMA had a much stronger interaction with dolomite than apatite, resulting in the intense adsorption of HPMA onto dolomite. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) further evidenced that compared with apatite, the strong interaction between HPMA and magnesium sites of dolomite enhanced the chemisorption of HPMA onto dolomite surface, which prevented strongly NaOl adsorption onto dolomite. As a result, dolomite was strongly depressed by HPMA to achieve the effective recovery of apatite. Based on these findings, HPMA can be potentially utilized as an eco-friendly and high-selectivity depressant for removing dolomite from apatite.

Surface interaction of crude oil, maltenes, and asphaltenes with calcite: An atomic force microscopy perspective of incipient wettability change

Close examination of surface interactions between calcite and crude oil is relevant to better understand the mechanisms that lead to wettability changes in petroleum reservoirs. Mineral surface wettability is a determinant factor in petroleum production and recovery, and the surface-active compounds in crude oil, typically concentrated in its asphaltene fraction, are mainly responsible for these wettability changes. Here, we use atomic force microscopy (AFM) to examine the incipient interactions between calcite {101‾4} surfaces and the Kuwait UG8 crude oil, and its asphaltene and maltene (i.e., crude oil after asphaltene has been extracted) fractions. The resulting adsorbates are interpreted on the basis of coverage, size, shape, and distribution. Adsorbates from crude oil are equally-spaced hemispheroidal droplets occurring predominantly at [4‾41] and [481‾] surface steps with a separation of about 550 nm between centers of adjacent adsorbates. Adsorbates from asphaltenes are irregularly spaced droplets or continuously cover the steps along the edges of dissolution pits. The average and standard deviation of diameter and height of 10–16 representative adsorbates from the selected images highlight the contrast among samples. Crude oil adsorbates average about (240 ± 120) nm in diameter and (60 ± 30) nm in height. Diameters of isolated adsorbates and lateral dimension of elongated adsorbates of asphaltene average (100 ± 20) nm and (150 ± 40) nm, respectively, and about (30 ± 10) nm in height overall. Isolated non-periodic smaller adsorbates (average diameter (20 ± 5) nm and height (7 ± 3) nm) occur from the maltene fraction and are mostly restricted to steps surrounding dissolution pits. Adsorbates from maltenes are most likely produced by resins that remain in this fraction after extracting the asphaltenes. Adsorption from crude oil involves the greatest surface area compared to asphaltenes and maltenes. Based on the sizes and heights of adsorbates reported here, we infer that, unlike resins, asphaltenes in crude oils act as anchors for increased oil adsorption and help enhance the change of an original water-wet surface to an oil-wet one. Differences in average height and architecture between adsorbates from oil and asphaltenes (i.e., isolated vs. continuous) may in part be associated to increased aggregation in toluene, which will intensify colloidal interactions before adsorption. We argue that asphaltenes in crude oil help stabilize larger, almost periodically spaced adsorbates all throughout the calcite surface and not only along the deeper steps that border large dissolution pits. The computational model of a chosen specific asphaltene molecule attached to a calcite surface shows strong interactions between hydroxyl groups and Ca2+ cations on the calcite surface in addition to weaker interactions of the negatively charged π orbitals of the polycyclic aromatic hydrocarbon (PAH) center and the surface cations, which is in agreement with the notion of establishing asphaltene anchors on calcite. Along with basic chemical information, systematic AFM imaging of a variety of crudes may become part of a practical strategy to evaluate surface-active compounds in oil and to predict their likely molecular structure and participating functional groups.

A Study on the Surface Wettability of Clastic Rocks with Potential Application for CO2 Storage Sites

There have been many studies carried out in the past decades attempting to develop strategies for a safe injection of CO2 into storage sites without leakage and contamination of valuable resources. Leakage from these storage sites through capillary seals is one of the long-standing issues, which require a comprehensive assessment prior to CO2 injection. However, surface wettability, which is one of the major parameters controlling the capillary pressure of seals, is a poorly understood parameter, changing as a function of temperature, pressure, surface roughness, etc. The aim of this paper is to provide a deeper insight into the surface wettability of minerals in rocks, especially sandstone and shales, under subsurface conditions for a better assessment of the structural integrity in CO2 storage sites. The results obtained from a series of contact angle measurements under different pressure and temperature conditions indicated that quartz- and feldspar-dominated sandstones are strongly water wet, while kaolinite-dominated shales have a weakly water to intermediate wet system. It is also apparent that the interfacial tension is a function of the pressure and temperature conditions. It appears that the size of pore throats in rocks is the major contributor to capillary threshold pressure and it must be determined very accurately. It is recommended that surface wettability and interfacial tension under reservoir conditions are determined a priori to ensure that the seal integrity analysis is as concise as possible.

The relationship between enthalpy of immersion and flotation response

The enthalpy of immersion is the heat change arising from the replacement of the solid-gas interface with the solid-liquid interface when a solid surface is immersed in a liquid. Although immersion calorimetry has been established as a reliable means of determining wettability of solid surfaces, it has found only limited applications in flotation research where wettability of mineral ores is a key variable. In this study, precision solution calorimetry was employed to measure the enthalpies of immersion of different minerals in water. These values were then related to the first-order flotation rate constants as determined by microflotation. The same measurements were also made on sulphide minerals whose surfaces were modified using different percentage coverages of potassium amyl xanthate collector.It was found that there was a strong inverse relationship between the enthalpy of immersion of the minerals studied and their wettability as indicated by their rates of flotation in a microflotation cell. In addition, a critical enthalpy of immersion (CEI) value was observed, above which no flotation occurred. The variance in the inverse relationship between enthalpy of immersion and rate of flotation decreased when the data was normalized with respect to particle density which was the only variable in the flotation studies with respect to bubble-particle encounter efficiency. These results have shown that the enthalpy of immersion is an excellent indicator of both the natural mineral hydrophobicity and of the extent to which collectors render a mineral hydrophobic. This technique has considerable potential to be used as a highly sensitive measure of mineral surface wettability in predicting flotation performance.

Selective flotation of rare earth oxides from hematite and quartz mixtures using oleic acid as a collector

Flotation, which exploits the differences in the surface wettability of minerals to effect separation, has been crucial in rare earth elements (REE) beneficiation. Monazite, a phosphate mineral commonly containing REE (typically lanthanum, cerium, and neodymium), occurs in association with hematite and quartz gangue minerals in some low grade deposits. In this study, the physicochemical properties including contact angle, zeta potential, and floatability of monazite, hematite, and quartz were determined in the presence of oleic acid as a collector. Contact angle measurements indicated adsorption of oleic acid onto the minerals' surfaces. Zeta potential measurements were used to elucidate oleic acid adsorption mechanism onto the mineral particle surfaces. Results from zeta potential measurements indicated that depressants are required to achieve selective flotation recovery of monazite from hematite and quartz. The flotation test results confirmed poor selectivity between monazite; and hematite and quartz, respectively. However, rare earth oxides (REO) in monazite floated better than both hematite and quartz at all the oleic acid dosages investigated. The use of sodium silicate and starch as depressants enhanced the selective flotation recovery of REO from hematite and quartz mixtures.

Calcite Wettability in the Presence of Dissolved Mg2+ and SO42–

The wettability of mineral surfaces controls a range of phenomena in natural and industrial processes. In reservoirs, rock wettability determines the effectiveness of oil production; thus, modification of mineral surface properties can lead to enhanced oil recovery. Recent work reports that potential determining ions in seawater, Mg2+, Ca2+, and SO42–, are responsible for altering the wettability of calcite surfaces. In favorable conditions, e.g., elevated temperature, calcium at the calcite surface can be replaced by magnesium, making organic molecules bind more weakly and water molecules bind more strongly, rendering the surface more hydrophilic. We used atomic force microscopy in chemical force mapping mode to probe the adhesion forces between a hydrophobic CH3-terminated AFM tip and a freshly cleaved calcite {10.4} surface to investigate wettability change in the presence of Mg2+ and SO42– at 75 and 80 °C. We made submicrometer scale maps of adhesion force and contact angle and demonstrated that the a...

The Nanoscale Basis of CO2 Trapping for Geologic Storage.

Carbon capture and storage (CCS) is likely to be a critical technology to achieve large reductions in global carbon emissions over the next century. Research on the subsurface storage of CO2 is aimed at reducing uncertainties in the efficacy of CO2 storage in sedimentary rock formations. Three key parameters that have a nanoscale basis and that contribute uncertainty to predictions of CO2 trapping are the vertical permeability kv of seals, the residual CO2 saturation Sg,r in reservoir rocks, and the reactive surface area ar of silicate minerals. This review summarizes recent progress and identifies outstanding research needs in these areas. Available data suggest that the permeability of shale and mudstone seals is heavily dependent on clay fraction and can be extremely low even in the presence of fractures. Investigations of residual CO2 trapping indicate that CO2-induced alteration in the wettability of mineral surfaces may significantly influence Sg,r. Ultimately, the rate and extent of CO2 conversion to mineral phases are uncertain due to a poor understanding of the kinetics of slow reactions between minerals and fluids. Rapidly improving characterization techniques using X-rays and neutrons, and computing capability for simulating chemical interactions, provide promise for important advances.

Capillary Pressure and Mineral Wettability Influences on Reservoir CO2 Capacity

The movement of injected CO2 through permeable pore networks determines its distribution and stability within reservoirs used for carbon sequestration (Fig. 1), and this process is dependent on capillary interactions with the displaced brine (IPCC 2005; Benson and Cole 2008). Capillary phenomena controlling the distribution of CO2 and reservoir brine depend on pore size (from mm to nm), wetting and interfacial properties. The pressure of the usually nonwetting CO2 phase relative to that of the native brine is the capillary pressure, P c , which in combination with pore size, wettability, and interfacial properties determines the saturation of each fluid phase. In typically reservoirs used for geologic carbon sequestration, CO2 exists as a supercritical (sc) fluid because pressures and temperatures associated with storage depths exceed the critical point values for CO2 (7.38 MPa, 31.1 °C). Thus, predicting carbon sequestration in reservoirs require understanding of interfacial tension, mineral surface wettability, and the capillary pressure dependence on saturation, for interactions between scCO2, brine, and reservoir mineral surfaces. Investigations into these aspects of scCO2 behavior, especially at elevated pressures and temperatures characteristic of geologic carbon sequestration, have largely only recently begun. Given the fairly early stage of research, many models for CO2 transport in reservoirs rely heavily on the better understood capillary characteristics of other immiscible fluid pairs such as air-water, oil-water, and oil-gas, supplemented with properties of scCO2. However, a growing number of experimental studies are being published on the behavior of scCO2 under reservoir conditions, leading to better understanding of C sequestration while also giving rise to new questions. In this chapter, we review aspects of the current (mostly laboratory-based) understanding of equilibrium capillary interactions between scCO2 and brines, and their implications for scCO2 behavior in …

Wetting alteration of mica surfaces with polyethoxylated amine surfactants

Characterization of reservoir wettability is an important part of assessment of potential oil recovery. Oil-based drilling fluids include surfactants, which can alter the wettability of mineral surfaces. Cores exposed to these fluids may not reflect the true wettability of the reservoir materials. The focus of this study was to observe wettability changes induced by adsorption and removal of surfactants of known structure on mica surfaces using tools that are applicable to studies of wetting alteration by crude oil components. The surfactants used were polyethoxylated coconut and tallow amines with chain lengths of 12 and 18 carbons and head groups consisting of two to five ethoxy groups. Mica was exposed to decane solutions of the surfactants. The treated mica was characterized macroscopically using contact angle measurements and microscopically using atomic force microscopy (AFM). Upon exposure to the surfactant solutions, the mica became oil-wet (∼ 170° for both advancing and receding conditions). AFM examination of similarly treated surfaces imaged in air revealed surfactant layers that were easily disrupted or surfaces that showed no surfactant at all. Contact angles were in the intermediate to water-wet range if the mica samples were removed from the surfactant solution, rinsed with non-aqueous solvents, and submerged in decane for measurements of water/decane contact angles. These results suggest only weak surfactant adsorption occurred from non-aqueous solutions. Differences among the structures tested were greater for increased levels of ethoxylation; differences due to hydrocarbon chain length were negligible. Stronger adsorption, higher contact angles, and more stable surfactant layers could be demonstrated when mica was exposed to aqueous solutions after surfactant sorption, depending on the pH of the aqueous phase. Low pH conditions that promote protonation of the surfactants' amine head group produced the greatest wetting alteration. Above a pH of 8 or 9, no surfactant remained adsorbed on mica surfaces.

A computational chemical study of penetration and displacement of water films near mineral surfaces

A series of molecular dynamics simulations have been performed on organic–water mixtures near mineral surfaces. These simulations show that, in contrast to apolar compounds, small polar organic compounds such as phenols can penetrate through thin water films to adsorb on these mineral surfaces. Furthermore, additional simulations involving demixing of an organic–water mixture near a surfactant-covered mineral surface demonstrate that even low concentrations of adsorbed polar compounds can induce major changes in mineral surface wettability, allowing sorption of apolar molecules. This strongly supports a two-stage adsorption mechanism for organic solutes, involving initial migration of small polar organic molecules to the mineral surface followed by water film displacement due to co-adsorption of the more apolar organic compounds, thus converting an initial water-wet mineral system to an organic-covered surface. This has profound implications for studies of petroleum reservoir diagenesis and wettability changes.