Adsorption Of Ionic Surfactants Research Articles

Colloid thermophoresis in surfactant solutions: Probing colloid-solvent interactions through microscale experiments.

Thermophoresis has emerged as a powerful tool for characterizing and manipulating colloids at the nano- and micro-scales due to its sensitivity to colloid-solvent interactions. The use of surfactants enables the tailoring of surface chemistry on colloidal particles and the tuning of interfacial interactions. However, the microscopic mechanisms underlying thermophoresis in surfactant solutions remain poorly understood due to the complexity of multiscale interaction coupling. To achieve a more fundamental understanding of the roles of surfactants, we investigated the thermophoretic behavior of silica beads in both ionic and nonionic surfactant solutions at various background temperatures. We provide a complete mechanistic picture of the effects of surfactants on interfacial interactions through mode-coupling analysis of both electrophoretic and thermophoretic experiments. Our results demonstrate that silica thermophoresis is predominantly governed by the dissociation of silanol functional groups at silica-water interfaces in nonionic surfactant solutions, while in ionic surfactant solutions, the primary mechanism driving silica thermophoresis is the adsorption of ionic surfactants onto the silica surface.

Ionic structure of cationic surfactant at the air/water interface

The adsorption of ionic surfactants and counter-ions at the air/water interface is important for many industrial processes. In the literature, the surface tension and surface potential data showed a complicated relationship that cannot be explained by the conventional models, either fully associated or fully dissociated models. Alternatively, ionic surfactants can have a partial association at the air/water interface. The degree of association varies with surface concentration and generates a distribution of counter-ions near the surface. This study reconciles the partial association and ionic distribution by a new quantification model. The model successfully described the surface tension and surface potential data for CTAB (cetyltrimethylammonium bromide). The model shows that the counter-ion association degree plays a deterministic role in limiting the adsorption capacity of ionic surfactants. The proposed structure is also consistent with X-ray surface reflectivity, in which CTAB demonstrated a condensed layer of counter-ion. The molecular insights can be extended to other ionic surfactants and oil/water interfaces.

Effects of surfactant adsorption on the wettability and friction of biomimetic surfaces.

The properties of solid-liquid interfaces can be markedly altered by surfactant adsorption. Here, we use molecular dynamics (MD) simulations to study the adsorption of ionic surfactants at the interface between water and heterogeneous solid surfaces with randomly arranged hydrophilic and hydrophobic regions, which mimic the surface properties of human hair. We use the coarse-grained MARTINI model to describe both the hair surfaces and surfactant solutions. We consider negatively-charged virgin and bleached hair surface models with different grafting densities of neutral octadecyl and anionic sulfonate groups. The adsorption of cationic cetrimonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) surfactants from water are studied above the critical micelle concentration. The simulated adsorption isotherms suggest that cationic surfactants adsorb to the surfaces via a two-stage process, initially forming monolayers and then bilayers at high concentrations, which is consistent with previous experiments. Anionic surfactants weakly adsorb via hydrophobic interactions, forming only monolayers on both virgin and medium bleached hair surfaces. We also conduct non-equilibrium molecular dynamics simulations, which show that applying cationic surfactant solutions to bleached hair successfully restores the low friction seen with virgin hair. Friction is controlled by the combined surface coverage of the grafted lipids and the adsorbed CTAB molecules. Treated surfaces containing monolayers and bilayers both show similar friction, since the latter are easily removed by compression and shear. Further wetting MD simulations show that bleached hair treated with CTAB increases the hydrophobicity to similar levels seen for virgin hair. Treated surfaces containing CTAB monolayers with the tailgroups pointing predominantly away from the surface are more hydrophobic than bilayers due to the electrostatic interactions between water molecules and the exposed cationic headgroups.

Counterions under a Surface-Adsorbed Cationic Surfactant Monolayer: Structure and Thermodynamics.

The surface adsorption of ionic surfactants is fundamental for many widespread phenomena in life sciences and for a wide range of technological applications. However, direct atomic-resolution structural experimental studies of noncrystalline surface-adsorbed films are scarce. Thus, even the most central physical aspects of these films, such as their charge density, remain uncertain. Consequently, theoretical models based on contradicting assumptions as for the surface films' ionization are widely used for the description and prediction of surface thermodynamics. We employ X-ray reflectivity to obtain the Ångström-scale surface-normal structure of surface-adsorbed films of the cationic surfactant cetyltrimethylammonium bromide (CTAB) in aqueous solutions at several different temperatures and concentrations. In conjunction with published neutron reflectivity data, we determine the surface-normal charge distribution due to the dissociated surfactants' headgroups. The distribution appears to be inconsistent with the Gouy-Chapman model yet consistent with a compact Stern layer model of condensed counterions. The experimental surfactant adsorption thermodynamics conforms well to classical, Langmuir and Kralchevsky, adsorption models. Furthermore, the Kralchevsky model correctly reproduces the observed condensation of counterions, allowing the values of the adsorption parameters to be resolved, based on the combination of the present data and the published surface tension measurements.

Adsorption of Hydrophobic Ions on Environmentally Relevant Sorbents.

Environmental monitoring and remediation often requires the collection of harmful substances from aqueous solutions. Absorption with solids is a useful technique for binding such substances even at very low concentration levels. Many of these contaminants are weak acids or bases. Some novel, nonionic polymeric sorbents, such as hypercrosslinked polymers or polymers with balanced hydrophilic-lipophilic properties (HLB) have been found to bind weak acids and bases with high distribution coefficients even at pH values where these compounds are almost completely ionized (typically near pH 7). To understand this phenomenon and its practical consequences, we have experimentally studied the adsorption of ionizable weak acids and bases as a function of pH and ionic strength on a the OASIS® HLB sorbent. Not surprisingly, the ionic forms of the weak acids and bases were found to be much less bound in the aqueous solution than their neutral forms. In spite of this, OASIS® HLB binds weak acids and bases around pH 7 considerably better than typical hydrophobic sorbents. The high overall distribution coefficients around pH 7 could be explained by two factors. One is that on OASIS® HLB, and on some other novel polymeric sorbents, the binding constant of the moderately hydrophobic neutral form is on the order of 100,000, i.e., much higher than on typical hydrophobic sorbents. Thus, even if the proportion of the neutral form in solution is only around 1% near pH 7, the adsorption of the neutral form is still significant. On the other hand, the binding of the apparently hydrophilic ionized forms occurs with distribution coefficients well above 100. The distribution coefficient of the ionic form appears to depend on ionic strength and the presence of competing ions. Adsorption of the ionic forms is found to be very similar to the adsorption of ionic surfactants. The pH dependence of the total adsorption of neutral and ionic forms together, is found to be steep around pH 7, and therefore the varying pH of natural waters may strongly influence the binding efficiency in practical applications, such as the collection (concentration) of contaminants or their passive sampling.

Effect of salt on the adsorption of ionic surfactants at the air-water interface

• NaCl has significant effect on the equilibrium and dynamic surface tension of ionic surfactant solutions. • The SRT model predicts the adsorption dynamics more accurately than the DKC model. • Salt bridging leads to the compact packing of ionic surfactant at the air-water interface. The combined effect of ionic surfactants and inorganic salts on the interfacial properties is of great importance to various industrial processes. In this work, the effect of ionic surfactants (SDS and CTAB), in the presence of inorganic salt (NaCl), on the surface tension is systematically investigated using experiments, theoretical models, and all-atom molecular dynamics simulations. The equilibrium and dynamic surface tension of surfactant-salt-water systems in a salinity range of 0.01–0.1 M are measured using the pendant drop method. The theoretical models viz., statistical rate theory and diffusion kinetic controlled, suggest that adsorption drives the migration of surfactant molecules to the interface. Finally, the molecular dynamics simulations illustrate increased surfactant packing at the air-liquid interface in the presence of salt due to counterions bridging between the surfactant molecules, which results in a lower value of surface tension for both the surfactants.

Determination of Time-Evolving interfacial tension and ionic surfactant adsorption kinetics in microfluidic droplet formation process

Surfactant adsorption plays an important role in microfluidics, which can be investigated by the dynamic evolution of interfacial tension. A differential pressure-based method is proposed to understand the basic laws of time-evolving interfacial tension and adsorption kinetics of ionic surfactants during the microfluidic droplet formation processes. Instantaneous flow rates and flow resistances are precisely analyzed based on auto-recognized microscopic images and the waveform of differential pressure, and the interfacial tension is determined from the Young-Laplace equation with a time resolution of 1/60 s. The concentration of surfactant at the liquid–liquid interface is obtained according to the Frumkin adsorption model, leading to an in-depth understanding of the adsorption rates and the apparent mass transfer rates of surfactants. The surfactant adsorption is demonstrated to obey the kinetic-controlled adsorption mechanism and therefore a measurement method for the adsorption rate constant is created to understand the kinetic characteristics of small-molecule ionic surfactants.

Role of Counterions in the Adsorption and Micellization Behavior of 1:1 Ionic Surfactants at Fluid Interfaces─Demonstrated by the Standard Amphiphile System of Alkali Perfluoro-n-octanoates.

In our latest communication, we proved experimentally that the ionic surfactant’s surface excess is exclusively determined by the size of the hydrated counterion.[Lunkenheimer, Langmuir,2017, 33, 10216−1022410.1021/acs.langmuir.7b00786]28925711. However, at this stage of research, we were unable to decide whether this does only hold for the two or three lightest ions of lithium, sodium, and potassium, respectively. Alternatively, we could also consider the surface excess of the heavier hydrated alkali ions of potassium, rubidium, and cesium, having practically identical ion size, as being determined by the cross-sectional area of the related anionic extended chain residue. The latter assumption has represented state of art. Searching for reliable experimental results on the effect of the heavier counterions on the boundary layer, we have extended investigations to the amphiphiles’ solutions of concentrations above the critical concentration of micelle formation (cmc).We provided evidence that the super-micellar solutions’ equilibrium surface tension will remain constant provided the required conditions are followed. The related σcmc-value represents a parameter characteristic of the ionic surfactant’s adsorption and micellization behavior. Evaluating the amphiphile’s surface excess obtained from adsorption as a function of the related amphiphile’s σcmc-value enables you to calculate the radius of the hydrated counterion valid in sub- and super-micellar solution likewise. The σcmc-value is directly proportional to the counterion’s diameter concerned. Taking additionally into account the radii of naked ions known from crystal research, we succeeded in exactly discriminating the hydrated alkali ions’ size from each other. There is a distinct sequence of hydration radii in absolute scale following the inequality, Li+ > Na+ > K+ > (NH4)+ > Rb+ > Cs+. Therefore, we have to extend our model of counterion effectiveness put forward in our previous communication. It represents a general principle of the counterion effect.

Effects of Salt-Induced Charge Screening on AOT Adsorption to the Planar and Nanoemulsion Oil-Water Interfaces.

Nanoemulsions, nanosized droplets of oil, are easily stabilized by interfacial electric fields from the adsorption of ionic surfactants. While mean-field theories can be used to describe the impact of these interfacial fields on droplet stability, the influence of these fields on the adsorption properties of ionic surfactants is not well-understood. In this work, we study the adsorption of the surfactant sodium dioctyl sulfosuccinate (AOT) at the nanoemulsion and planar oil-water interfaces and investigate how salt-induced charge-screening affects AOT adsorption. In the absence of salt, vibrational sum-frequency scattering spectroscopy measurements reveal the ΔGads and the maximum surface density is the same for AOT at the hexadecane nanoemulsion surface as at the planar hexadecane-H2O interface. Upon the addition of NaCl, an increase in AOT surface density is detected at both interfaces, indicating that repulsive electrostatic interactions between AOT monomers are the dominant force limiting surfactant adsorption at both interfaces. The bulky alkyl chains of AOT molecules cause our observations in this study to differ from those found in previous studies investigating the adsorption of linear-chain ionic surfactants to the nanoemulsion surface. These results provide necessary information for understanding factors limiting the adsorption of ionic surfactants to nanodroplet surfaces and highlight the need for further studies into the adsorption properties of more complex macromolecules at nanoemulsion surfaces.

Light-Responsive, Reversible Emulsification and Demulsification of Oil-in-Water Pickering Emulsions for Catalysis.

Pickering emulsions are an excellent platform for interfacial catalysis. However, developing simple and efficient strategies to achieve product separation and catalyst and emulsifier recovery is still a challenge. Herein, we report the reversible transition between emulsification and demulsification of a light-responsive Pickering emulsion, triggered by alternating between UV and visible light irradiation. The Pickering emulsion is fabricated from Pd-supported silica nanoparticles, azobenzene ionic liquid surfactant, n-octane, and water. This phase behavior is attributed to the adsorption of azobenzene ionic liquid surfactant on the surface of the nanoparticles and the light-responsive activity of ionic liquid surfactant. The Pickering emulsion can be used as a microreactor that enables catalytic reaction, product separation as well as emulsifier and catalyst recycling. Catalytic hydrogenation of unsaturated hydrocarbons at room temperature and atmospheric pressure has been performed in this system to demonstrate product separation and emulsifier and catalyst re-use.

Light‐Responsive, Reversible Emulsification and Demulsification of Oil‐in‐Water Pickering Emulsions for Catalysis

AbstractPickering emulsions are an excellent platform for interfacial catalysis. However, developing simple and efficient strategies to achieve product separation and catalyst and emulsifier recovery is still a challenge. Herein, we report the reversible transition between emulsification and demulsification of a light‐responsive Pickering emulsion, triggered by alternating between UV and visible light irradiation. The Pickering emulsion is fabricated from Pd‐supported silica nanoparticles, azobenzene ionic liquid surfactant, n‐octane, and water. This phase behavior is attributed to the adsorption of azobenzene ionic liquid surfactant on the surface of the nanoparticles and the light‐responsive activity of ionic liquid surfactant. The Pickering emulsion can be used as a microreactor that enables catalytic reaction, product separation as well as emulsifier and catalyst recycling. Catalytic hydrogenation of unsaturated hydrocarbons at room temperature and atmospheric pressure has been performed in this system to demonstrate product separation and emulsifier and catalyst re‐use.

ADSORPTION BEHAVIOUR OF IONIC AND NON-IONIC SURFACTANTS ONTO TALC A NATURALLY HYDROPHOBIC MINERAL-A COMPARATIVE STUDY

In present study, the adsorption behaviour of surfactants which are cationic (hexadecyltrimethylammonium bromide, HTAB), anionic (sodium dodecyl sulphate, SDS) and non-ionic (Triton X-100, TX-100) onto naturally hydrophobic talc were investigated. In this scope, a series of batch adsorption tests at natural pH were performed to determine adsorption isotherms and zeta potential (ZP) measurements of the ionic and non-ionic surfactants onto talc surfaces were measured using electrophoresis technique. To understand the mechanism of the adsorption process, the adsorption of ionic and non-ionic surfactants was studied as a function equilibrium concentration (mol/L). The amount of maximum adsorption of the surfactants onto talc are ordered as in the following: TX-100 (9.0x10-5 mol/m2)>HTAB (8.5x10-5 mol/m2) > SDS (5.32x10-5 mol/m2). Even though both the SDS and talc have negative surface charge, SDS can adsorb onto talc. Moreover, a good correlation was seen between the adsorption isotherms and the zeta potential curves. Considering their adsorption isotherms, the ionic surfactants were showed different adsorption behaviour concerning the non-ionic surfactant molecules. The adsorption isotherms of TX-100 and SDS increase rapidly in a narrow concentration range until the plateau region, while such a sharp increase does not appear for HTAB. The maximum adsorption amount of TX-100 and HTAB is greater than SDS. The results indicate that hydrophobic interaction and hydrogen bonding play a decisive role on the adsorption of non-ionic and anionic surfactants onto talc a naturally hydrophobic mineral, whereas electrostatic interaction becomes more important in the adsorption of cationic surfactant.

Ion-specific effects on equilibrium adsorption layers of ionic surfactants

This review article reports the effect of the counter-ions on the ionic surfactant adsorption layer and its relation to the stability of foams and emulsions. The adsorption theory of Davies about the ionic surfactant monolayer was revisited and it is shown how to account for the type of the counter-ions. The experimental validation of this theory on thin liquid films was shown as well, thus explaining the effect of Hofmeister. However, their effect on foams and emulsions is more complex. Furthermore, it is shown how the counter-ions affect in complex way the stability of foams and emulsions via the surfactant adsorption layer in the light of the newest theory. To elucidate the nature of this effect, further investigation is called for.

PH-Dependent surface charge at the interfaces between aluminum gallium nitride (AlGaN) and aqueous solution revealed by surfactant adsorption

HypothesisThe net surface charge of AlGaN/GaN structures, where AlGaN is in contact with the solution, is controlled by the pH-dependent protonation and deprotonation of the surface hydroxyl groups and possibly the electron-deficient surface electronic states. We hypothesize that atomic force microscopy (AFM) force measurements of ionic surfactant adsorption can reveal how the AlGaN surface properties vary with pH. ExperimentsAFM force curves and images were used to probe the AlGaN/solution interface in water as a function of pH, and with added cationic surfactant cetyltrimethylammonium bromide (CTAB) or anionic surfactant sodium dodecylsulfate (SDS). FindingsThe AlGaN/solution interface is negatively charged at pH 12, has an isoelectric point near pH 5.5, and is positively charged at pH values less than 5.5. Surfactant adsorption data suggests AlGaN surface is somewhat hydrophobic at acidic pH. Compared to gallium nitride (GaN), at pH 2, AlGaN has a lower charge density and hydrophobicity, but at other values of pH, the surface properties of AlGaN and GaN are similar.

Dynamics of Competitive Adsorption of Lipase and Ionic Surfactants at the Water-Air Interface.

Lipase is one of the most important enzymes playing a key role in many biological and chemical processes, in particular for fat hydrolysis in living systems and technological applications such as food production, medicine, and biodiesel production. As lipase is soluble in water, the major hydrolysis process occurs at the water-oil interface, where lipase can get in contact with the oil. To provide optimum conditions, the emulsification of the oil is essential to provide a large interfacial area which is generally done by adding surfactants. However, the presence of surfactants can influence the lipase activity and also cause competitive adsorption, resulting in a removal of lipase from the interface or its conformational changes in the solution bulk. Here we have studied the dynamics of competitive adsorption and interfacial elasticity of mixed solutions containing lipase and the anionic surfactant sodium dodecyl sulfate (SDS) or the cationic surfactant cetyltrimethylammonium bromide (CTAB), respectively, at the water-air interface. The experiments were performed with a special coaxial double capillary setup for drop bulk-interface exchange developed for the drop profile analysis tensiometer PAT with two protocols: sequential and simultaneous adsorption of single components and mixed systems. The results in terms of dynamic surface tension and dilational viscoelasticity illustrate fast and complete desorption of a preadsorbed CTAB and SDS layers via subphase exchange with a buffer solution. In contrast, the preadsorbed lipase layer cannot be removed either by SDS or CTAB from the interface during drop bulk exchange with a buffer solution due to the unfolding process and conformation evolution of the protein molecules at the interface. In the opposite case, lipase can remove preadsorbed SDS and CTAB. The dynamic surface tension and viscoelasticity data measured before and after subphase exchange show joint adsorption of lipase and CTAB in the form of complexes, while SDS is adsorbed in competition with lipase. The results are in good correlation with the determined surface charges of the lipase gained by computational simulations which show a dominant negatively charged surface for lipase that can interact with the cationic CTAB while partial positively charged regions are observed for the interaction with the anionic SDS.

Effect of Triton X-100 surfactant on the interfacial activity of ionic surfactants SDS, CTAB and SDBS at the air/water interface: A study using molecular dynamic simulations

Molecular dynamics simulations were carried out to investigate the effect of TX-100 surfactant on the interfacial activity of ionic surfactants SDS, CTAB and SDBS at the air/water interface. Here, interfacial properties, molecular orientation of hydrocarbon chains of ionic surfactant and radial distribution functions between hydrophilic headgroups and water molecules at the air/water interface was evaluated by means of MD simulations. Particularly, TX-100 surfactant produces a reduction of the interfacial film thickness obtained using the water layer and increases the hydrophobic film thickness of the systems. In this investigation, we found that TX-100 surfactant present in mixed monolayers and distributed randomly on the water surface can modify the hydrophilic-lipophilic balance of the monolayer increasing the hydrophobic film thickness and reducing the adsorption of ionic surfactants at the air/water interface. Furthermore, the molecular array of these mixed monolayers obtained by the TX-100 surfactant improve the elasticity of the liquid film, which avoid the rupture of foams reducing the liquid drainage velocity of these systems. Finally, the results suggest that hydrophobic film thickness of the mixed monolayers produce by the TX-100 surfactant can reduce the diffusion of gas molecules from air phase toward the water phase, which increase the stability of foams.

Direct Quantification of Water Surface Charge by Phase-Sensitive Second Harmonic Spectroscopy.

We developed and verified a phase-sensitive second harmonic generation spectroscopic scheme that allows for direct determination of the absolute surface charge density and surface potential of a water interface without the need for prior interfacial information. The method relies on selective probing of surface-field-induced reorientation order of water molecules in the electrical double layer and is, hence, independent of the interfacial molecular bonding structure. Application of this technique to a mixed surfactant monolayer on water suggests the manifest effect of the chain-chain interactions among the monolayer on adsorption of soluble ionic surfactants. We also deduce the third-order nonlinear susceptibility of bulk water and prove its applicability to analysis of charges of various water interfaces. In addition, we show that Debye-Hückle theory should be avoided in the spectroscopic analysis for its potential significant error, as evidenced experimentally and theoretically.

Surface charging and points of zero charge of less common oxides: Beryllium oxide

HypothesisThe surface charging and adsorption properties of beryllium oxide follow the same principles as the properties of other metal oxides. ExperimentsThe point of zero charge PZC was determined by potentiometric titration, and by salt addition method at 15, 25 and 35 °C. The ζ potential of beryllium oxide was measured at different concentrations of various 1-1 salts. The effect of ionic surfactants and of multivalent inorganic ions on the isoelectric point IEP was studied. FindingsThe PZC of beryllium oxide in the presence of NaCl is at pH 9.1 at 25 °C, and it decreases by 0.04 pH unit per 1 °C. The IEP of beryllium oxide in the presence of NaCl, KCl and KNO3 is at pH about 9, and it is independent of the electrolyte concentration and of aging. The presence of anionic surfactant (SDS) and of phosphate results in a shift in the IEP to low pH, and the presence of cationic surfactant (CTAB) results in a shift in the IEP to high pH. The IEP roughly matches the maximum in the particle size. The surface charging behavior of beryllium oxide confirms specific adsorption of ionic surfactants and of multivalent inorganic ions, and it is qualitatively similar to that of more common metal oxides (titania, alumina, hematite).

Adsorption and viscoelastic behaviour of ionic liquid surfactants on gold surfaces

Adsorption isotherms of five ionic liquid surfactants belonging to different families and diverse structures, namely: 1-dodecyl-3-methylimidazolium chloride, [C12C1im]Cl; 1-dodecyl methylpyrrolidinium bromide, [C12C1Pyrr]Br; 1-dodecyl methylpyridinium chloride, [C12C1Py]Cl; 1-dodecyl methylpiperidiniumm bromide, [C12C1Pip]Br; and 1-octyl-3-methylimidazolium octylsulfonate, [C8C1im][C8SO3] on a gold surface have been studied using a quartz crystal microbalance with dissipation (QCM-D). All surfactants show critical micelles concentrations (CMCs) in 10–20 mM range. Simultaneous adsorption and dissipation measurements were made in pre- and post-micellar concentration regions at 298.15 K. Peculiar results were obtained for all the IL surfactants. Frequency measurements show that the adsorbed amounts in the pre-micellar region are small and, as soon as the aggregates formation takes place, the adsorbed amount increases significantly. Dissipation measurements indicate the formation of rigid layers with very low values in the pre-micellar region. However, in the post-micellar region, with the increase of concentration (up to 75 mM~5xCMC) the dissipation significantly increases demonstrating the presence of loosely bound structures/multilayers on the gold surface.

Particle adsorption at the oil-water interface studied with second harmonic generation.

In this work, energetics of the adsorption of polystyrene nanoparticles at the hexadecane-water interface was studied with second harmonic generation. The adsorption of positively and negatively charged nanoparticles at the oil-water interface induced a decrease and an increase in the SHG emission from the interface, respectively. This change in the SHG emission, which is similar to that upon the adsorption of ionic surfactants at the hexadecane-water interface, which we reported previously, was then used as an indicator of particle adsorption at the interface. The adsorption free energies of the particles with a diameter of 20 nm at the hexadecane-water interface were found to be -14.7 ± 0.5 kcal mol-1, -14.4 ± 0.4 kcal mol-1 and -15.1 ± 0.3 kcal mol-1 for the amidine, carboxyl and sulfate latex beads, respectively. This result implied that the van der Waals interaction between the oil phase and the polystyrene particles is capable of driving negatively charged particles to the negatively charged hexadecane-water interface. The principle of like dissolves like played a major role in the adsorption of polystyrene particles from the aqueous phase to the oil-water interface. The origin of the SHG emission from the oil-water interface was also discussed.