Adsorption Of Dodecyltrimethylammonium Bromide Research Articles

Investigation of synergistic mechanism of magnesium ions and cationic surfactant in low-rank coal flotation

The effect of magnesium ions on the adsorption of DTAB (Dodecyltrimethylammonium Bromide) surfactants by low-rank coal using adsorption experiments and microscopic mechanism analysis. In addition, the flotation recovery of coal was summarized for application by micro flotation test. The results of adsorption experiments showed that 0.1 mM magnesium ions concentration is optimal, with 12.55 mg/g adsorption capacity. Besides, correlation analysis reveals that the adsorption capacity is positively correlated with temperature, surfactant concentration, reaction time and the magnesium ions density. Adsorption thermodynamics and kinetics experiments indicate that magnesium ions promote adsorption by increasing the adsorption active sites of coal and accelerating the adsorption rate of DTAB on coal. Furthermore, the results of contact angle, zeta potential, model calculations and FTIR (Fourier transform infrared spectroscopy) analyses reveal that magnesium ions and its hydrolysates can adsorb on the coal as a bridge and destroy the stable hydrogen bond on the coal to activate the adsorption, which enhance the negatively charged coal to electrostatically adsorb positively charged DTAB. Moreover, the recovery rate of low-rank coal flotation using DTAB combined with magnesium ions was tested by a micro flotation apparatus, which provides a guidance for the flotation application by saline surfactant solution on coal.

Surface and Interfacial Interactions in Dodecane/Brine Pickering Emulsions Stabilized by the Combination of Cellulose Nanocrystals and Emulsifiers.

Interfacial properties of cellulose nanocrystals (CNC) and surfactants were studied in high ionic strength (I) brines and correlated to the stability of dodecane/brine Pickering emulsions. Bis-(2-hydroxyethyl) cocoalkylamine (CAA), dodecyltrimethylammonium bromide (DTAB), and octyl-β-d-glucopyranoside (OGP) were adsorbed onto CNC in American Petroleum Institute (API) brine (I = 1.9 M) and synthetic seawater (SSW), with I = 0.65 M. Raman spectroscopy indicated that hydroxyl groups on the CNC surface interact with all three surfactants in high ionic strength media. Ionic interactions still play a role at the very large ionic strengths studied herein. Despite all surfactants adsorbing onto CNC, only the surface tension of CAA solutions in both brines was increased by the addition of 0.5 wt % CNC. The effect was much more prominent in API than in SSW. Contact angle measurements indicated that CAA increased the wettability of CNC by both brines in dodecane; DTAB, on the other hand, decreased wettability. Emulsion stability studies revealed that ionic strength, wettability, adsorption energy, and oil content strongly affect emulsion stability, more so than surfactant adsorption. In API, CNC aggregates alone stabilized the emulsions better compared to samples with additional emulsifiers; the same was true in SSW for oil contents below 50% v/v. For oil contents above 50% v/v in SSW, CAA was either detrimental or failed to improve emulsion stability. On the other hand, DTAB increased the stability of dodecane in SSW emulsions. Emulsions stable for over 21 months were prepared with oil contents of 75% v/v. The adsorption of CAA onto CNC limits the migration of both CNC and CAA to the dodecane/brine interface, while DTAB adsorption has the opposite effect.

Enhancement of the surface hydrophobicity of low-rank coal by adsorbing DTAB: An experimental and molecular dynamics simulation study

This study investigated the mechanism of improving the surface hydrophobicity of low-rank coal by adsorbing dodecyltrimethylammonium bromide (DTAB). Experimental tests were conducted to analyze the adsorption characteristic of DTAB on the coal surface including the adsorption amount, Zeta potential, wetting heat, and X-ray photoelectron spectroscopy measurement. Furthermore, the adsorption configuration, spatial location of the simulation systems, and the coal–water interaction were investigated by molecular dynamics (MD) simulation at the atomic scale. Experimental results indicate that the hydrophobicity of the low-rank coal first increased, then decreased as the DTAB concentration increased. The electrostatic force plays a dominant role in the adsorption of DTAB on low-rank coal surface. The adsorption of DTAB reduced the wetting heat between water and low-rank coal because the oxygen-containing groups were covered. The decrease in hydrophobicity at high DTAB concentration was due to the formation of a bilayer or micelle adsorption. MD simulation results show that the adsorption of DTAB reduced the thickness of the water adsorption layer. The nitrogen atoms of DTAB were oriented toward the coal surface, such that the alkyl chains were oriented to the water phase, thereby producing an additional repulsive effect and subsequently inhibiting the adsorption of water molecules. As a result, the number of hydrogen bonds and the interaction energy between water molecules and low-rank coal decreased, indicating that the hydrophobicity of low-rank coal was enhanced.

Determining the Boundaries of the Concentration Area of Action of the Rehbinder Effect for an S-shaped Adsorption Isotherm

This paper continues a study of the magnitude of thermodynamic estimation of the Rehbinder effect as a function of volume concentration in a liquid solution of a surface-active substance (surfactant) in the case of an S-shaped adsorption isotherm on an undeformed and uncharged surface. A generally piecewise linear approximation of the isotherm is used. The conditions of existence of a bounded concentration action area of the Rehbinder effect, such that the area does not fall outside the bounds of defining data from the adsorption isotherm, are derived analytically in terms of theory parameters. The width of this area is compared to the data in the literature on the Rehbinder effect for quartz and granite obtained during dodecyltrimethylammonium bromide adsorption.

Effects of dodecyltrimethylammonium bromide surfactant on both corrosion and passivation behaviors of zinc electrodes in alkaline solution

The corrosion and passivation behaviors of zinc electrodes in 7.0 M KOH saturated with ZnO in the presence of dodecyltrimethylammonium bromide (DTAB) surfactant were evaluated by Tafel, linear sweep voltammetry, electrochemical impedance spectroscopy (EIS) and field emission scanning electron microscopy (FE-SEM) tests. It was found that DTAB had a considerable effect on inhibiting zinc corrosion by the adsorption of DTAB. The adsorption of DTAB obeyed the Freundlich isotherm. Tafel polarization curves revealed that DTAB worked as an anodic-type inhibitor. The highest corrosion inhibition efficiency of DTAB was up to 80.2% in the presence of 0.07 wt% DTAB. In addition, potentials of both passivation and anodic dissolution current peak positively shifted in the presence of DTAB, indicating that the passivation was delayed. Based on the EIS tests and FE-SEM analysis in the presence and absence of DTAB, it was shown that the adsorption and favorable moisture retention of DTAB were beneficial to the formation of uniform and fluffy protective layer on the surface of zinc electrode. Such a morphological modification of zinc surface greatly improved the diffusion rate of ions and the utilization of zinc electrode.

Infrared external reflection spectra of Janus gold nanoparticle monolayers prepared at the air–water interface

Abstract. Nanoparticle monolayers at the air–water interface have received broad attention because of their potential applications owing to the tunability of their surface properties. We prepared gold nanoparticle monolayers by adsorption of hydrophilized nanoparticles from solution onto Langmuir monolayers of amphiphilic organic molecules (polyion complexes of deuterated stearic acid and polyallylamine) at the air–water interface, followed by adsorption of dodecyltrimethylammonium bromide onto the underside of these particles. The infrared external reflection spectra of the monolayers recorded both during and after preparation showed that the adsorption of gold nanoparticles on the Langmuir monolayers and that of dodecyltrimethylammonium bromide on the nanoparticles occurred in several tens of minutes. The infrared spectra also showed that both stearic acid and dodecyltrimethylammonium bromide molecules were highly ordered on the gold nanoparticles at the air–water interface. These results suggested that this preparation procedure is a useful fabrication method for Janus particles.

Effect of surfactants on temperature-dependent self-assembling property of copolyethylenimine/cinnamic acid aqueous mixture

ABSTRACTThe transmittance of polyethylenimine (PEI)/cinnamic acid (CA) aqueous mixture was close to zero at 20–40°C, and it began to increase around 40°C due to the disassembling of the self-assembly of the PEI/CA conjugate. As the concentration of sodium dodecyl sulfate (SDS) increased, the increasing rate of the transmittance decreased and the onset temperature increased, indicating that the self-assembly of the PEI/CA conjugate became more stable against heat with the aid of SDS. Tween 20 could also suppress the thermally induced disassembling of the self-assembly, possibly because poly(ethylene oxide) chains of the surfactant could be entangled with the PEI chains. Dodecyltrimethyl ammonium bromide (DTAB) did not have an effect on the temperature-dependent self-assembling phenomena as much as SDS and Tween 20 did. The interfacial tension of the PEI/CA/SDS aqueous mixture and that of the PEI/CA/Tween 20 aqueous mixture at 70°C were lower than the respective tensions observed at 25°C. On the contrary, the interfacial tension of the PEI/CA/DTAB aqueous mixture at 70°C was higher than that observed at 25°C, possibly because the PEI/CA conjugate could lose its surface activity at the higher temperature due to the adsorption of DTAB on CA molecules.

Core Flooding of Complex Nanoscale Colloidal Dispersions for Enhanced Oil Recovery by in Situ Formation of Stable Oil-in-Water Pickering Emulsions

This study reports a Pickering emulsion flooding system, in which the oil–water interface is structurally stabilized by a complex colloidal layer consisting of silica nanoparticles, dodecyltrimethylammonium bromide (DTAB), and poly(4-styrenesulfonic acid-co-maleic acid) sodium salt (PSS-co-MA). The colloidal layer was generated by adsorption of PSS-co-MA on the silica nanoparticles as a result of the van der Waals attraction and by adsorption of DTAB onto the PSS-co-MA layer as a result of the electrostatic attraction, thus providing the mechanically robust, stable interface. To demonstrate a practical applicability to the enhanced oil recovery, the complex colloidal dispersion fluid was injected into the Berea sandstone for a core flooding experiment. The result revealed that the colloidal dispersion significantly increased the oil recovery by ∼4% compared to the case of flooding water. This means that the emulsion drops in situ produced in the core could readily flow through the rock pores. We attribute...

Mechanism of calcite wettability alteration by alkyl polyglucoside

Surfactant injection into oil-wet carbonate reservoirs offers a large potential for increased oil recovery due to reservoir wettability alterations. Here, the mechanism of wettability alteration by non-ionic surfactant alkyl polyglucoside (APG) is investigated and compared to that of the cationic surfactant dodecyltrimethylammonium bromide (DTAB). Techniques used include 1H and 13C magic angle spinning NMR, zeta potential, adsorption and contact angle (CA) measurements. APG monomers effectively desorb stearic acid (SA) from an SA treated calcite surface by replacing it at cationic sites on the surface, both above and below the APG critical micelle concentration (CMC). Increased polarization of the APG hydroxyl groups upon interaction with Ca+ sites is a driving force for APG adsorption. CAs show that APG treatment changes wettability to water-wet both below (CA = 66.9°) and above (CA = 77.2°) CMC. An observed sphere-to-rod transition occurring in APG solution above 1 mM is accompanied by a structural transition from half-micelles to patches of partial bilayers at the calcite surface, which further increases APG adsorption and CA (CA = 88.1°). In the presence of SA, adsorption of APG onto calcite is similar to that of DTAB below CMC, while above CMC APG adsorption is 2–3 times higher than DTAB. In absence of SA, DTAB adsorption is negligible. CAs are comparable after APG and DTAB treatment, making APG an interesting candidate for such purposes due to its effectiveness at very low concentrations.

Adsorption kinetics investigation for alkyltrimethylammonium bromides on ITO-aqueous interface

The adsorption of dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide onto indium-tin oxide (ITO) was studied by in-situ monitoring the capacitance and resistance of adsorbed layers for a series of concentrations of aqueous solutions. The experimental results suggested that the adsorption of both surfactants showed a three-region adsorption process and that the adsorption was driven by electrostatic attraction and hydrophobic interactions in two regions below critical micelle concentration (cmc). Above the cmc, the changes in electrical properties could be explained by the assumption that entire micelles were directly adsorbed onto the ITO surface and then underwent an adjustment process. The variations of water contact angle against surfactant concentration showed two distinct regions. The effect of high electrolyte concentration on the adsorbed layer structure was also discussed.

Influence of Clay Surface Modification on Morphology and Rheology of Polyethylene Glycol/Montmorillonite Nanocomposites

The effects of surface modification of Na-montmorillonite (Na-MMT) on morphology and the rheological properties of polyethylene glycol/ organoclay nanocomposites are investigated. Dodecyltrimethylammonium bromide (DTABr) is used to modify the clay surface. Adsorption of DTABr on Na-MMT is monitored by zeta potential measurements. From zeta potential measurements, three DTABr concentrations, 1 × 10—3, 7.5 × 10— 3, and 5 × 10—2 mol/L, are chosen to fabricate the polyethylene glycol/organoclay composites. The morphology of three sets of organoclays (OMMT1, OMMT2, and OMMT3) is studied by X-ray diffraction (XRD) technique. The increase of basal spacing is found to be 0.2 nm as the DTABr concentration is increased to 7.5 × 10— 3 mol/L. The rheological properties of polyethylene glycol (PEG)/ organoclay nanocomposites are studied for three organoclays at different PEG concentrations. All nanocomposites show Bingham plastic flow behavior. The OMMT3 shows maximum viscoelastic properties in the polymer matrix. The morphology and electrokinetic properties of the PEG/organoclay nanocomposites are also investigated.

Adsorption of Dodecyltrimethylammonium Bromide and Sodium Bromide on Gold Studied by Liquid Chromatography and Flow Adsorption Microcalorimetry

Here, we report on a new aspect of the adsorption of Br- on the surface of gold. The adsorption of dodecyltrimethylammonium bromide (C12TABr) from aqueous solutions onto macroporous gold particles was studied by continuous flow frontal analysis solid/liquid chromatography and flow adsorption microcalorimetry. The material balance and enthalpy balance of adsorption and the change in the solution pH were measured simultaneously. Initially, Br- is irreversibly bound to high-affinity surface sites counterbalanced by the adsorption of H+ from the aqueous phase. The surface speciation is accompanied by the formation of C12TAOH, which in turn results in a significant pH increase in the bulk solution. The net process was found to be strongly exothermic (-280 kJ.mol(-1)), which is indicative of the occurrence of chemisorption. The specific adsorption of Br- is followed by the reversible adsorption of C12TABr to produce a firmly bound monolayer in a head-to-surface arrangement (-53 kJ.mol(-1)). In a relatively narrow range of the surface coverage, various composite structures may develop on the top layer and eventually transform to full-cylindrical surface aggregates. The surface aggregation was found to be reversible, with an enthalpy change of -11 kJ.mol(-1). The importance of the specific binding of Br- to the surface of gold was confirmed by measurement of the initial adsorption of NaBr on the microparticles. The initial adsorption was found to be irreversible, with an enthalpy change of approximately -240 kJ.mol(-1). This process involved the formation of an AuBr-/H+ electric double layer at the gold/water interface, accompanied by a dramatic increase in the solution pH due to the release of a copious amount of OH- in the bulk liquid phase.

Coal Reverse Flotation. Part I. Adsorption of Dodecyltrimethyl Ammonium Bromide and Humic Acids onto Coal and Silica

Adsorption of dodecyltrimethyl ammonium bromide (DTAB) and humic acids (HA) on bituminous, oxidized bituminous, and subbituminous coals, as well as on silica, was studied through direct adsorption, electrokinetic, and contact angle measurements. It was concluded that the adsorption of DTAB and HA on a hydrophobic coal surface takes place through hydrophobic interactions between the hydrocarbon chains of these compounds and the hydrophobic coal surface. The adsorbed DTAB molecules cause a gradual decrease of coal hydrophobicity due to their tail-to-surface orientation. The adsorption density of DTAB on a hydrophilic/oxidized coal is much higher than on a bituminous coal. The results indicate that the adsorption mechanism involves some strong interactions between the cationic head group of the surfactant and the negatively charged oxygen groups on the coal surface. Despite the apparent head-to-surface orientation, the adsorbed DTAB molecules do not render the surfaces hydrophobic; this probably results from the fairly chaotic orientation of DTAB molecules on the surface of oxidized coal. The well-ordered conformation of the DTAB ions adsorbed onto the negatively charged silica surface, combined with significant adsorption of this surfactant at the solution/air interface makes the silica surface very hydrophobic.

Influence of the Chemical Surface State of Silica on Adsorption Process of Conventional Cationic and Gemini Surfactants: Thermodynamic Investigations

The aim of this investigation was to study the effect of molecular structure on the adsorption of cationic surfactants onto silica. Thus, the thermodynamics of adsorption of DTAB (dodecyltrimethylammonium bromide) and of the gemini surfactant 12–2–12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) on raw silica (SiNa) and HCl-washed silica (SiH) has been investigated mainly by the microcalorimetric technique. The determination of differential molar enthalpies of adsorption first requires accurate knowledge of the dilution enthalpy of each surfactant under the same experimental conditions as those used during adsorption. The values of the adsorption enthalpies, corrected by the dilution effect, give information on adsorption processes. Moreover, the amount of surfactant adsorbed, the sodium ion concentration, the pH in equilibrated supernatants, and the electrophoretic mobility data complete the calorimetric results. Analysis of the experimental results allows estimation of the number of the surface sites and building up, in both cases, a picture of the adsorption process. The different steps of the adsorption process (by electrostatic and hydrophobic interactions) are described, as are also the different shapes of aggregates at the end of the adsorption. These depend on the state of the surface and the surfactant. A high density of SiNa sites leads to a dense bilayer. A spread density of SiH sites gives rise to pinned micelles.

The Adsorption of Dodecyltrimethylammonium Bromide on Mica in Aqueous Solution Studied by X-Ray Diffraction and Atomic Force Microscopy

The adsorption of dodecyltrimethylammonium bromide (DTAB) onto natural muscovite mica and a synthetic expandable mica (EM) in aqueous solution has been investigated using both microscopic and macroscopic surface characterization techniques. The electrokinetic properties of the surfaces were monitored as a function of the concentration of DTAB using atomic force microscopy and microelectrophoresis. The adsorption isotherm of DTAB on EM was measured up to a solution concentration just below the critical micelle concentration of the surfactant. The thickness of the adsorbed layer on EM was determined using X-ray diffraction. Results indicate that the adsorbed layer consists of molecules lying quite flat on the mica surface at low concentrations and adsorbed in interleaved aggregate structures at concentrations approaching the critical micelle concentration of the surfactant in solution.

Π-A isotherms for anionic, cationic and mixed anionic–cationic surfactants

Adsorption of dodecyltrimethylammonium bromide (DTMABr) and sodium octyl sulfonate (NaOS) from single component solutions and from solutions of their mixtures has been investigated. Π-A isotherms of adsorption at the air/water and the decane/water interfaces are presented. The mean molecular area of surfactants in the monomolecular films is shown, versus the bulk concentration of the particular surfactant. It has been discovered that the adsorption films at the air/water surface are more condensed than at the decane/water interface. The molecular area larger for DTMABr than for NaOS is a result of the additional Van der Waals attractive forces between decane molecules and methyl groups in DTMABr molecule. When mixtures of surfactants are concerned, it has been found that the higher the content of NaOS in the mixture, the more condensed are the adsorption films at the air/water interface.

Adsorption Mechanism of Conventional and Dimeric Cationic Surfactants on Silica Surface: Effect of the State of the Surface

The aim of this study was to investigate the effect of the state of the silica surface and of the surfactant molecular structure on the adsorption of cationic surfactants onto silica. Thus, the adsorption of DTAB (dodecyltrimethylammonium bromide) and of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) on raw silica (SiNa) and on HCl-washed silica (SiH) has been investigated under “free” system conditions. The amount of surfactant adsorbed (adsorption isotherm), the sodium ion and bromide ion concentrations and the pH in the equilibrated supernatant, and the silica particle electrophoretic mobility have been measured along the isotherms. The adsorption mechanisms of the two surfactants on the raw and washed silica are qualitatively similar. Nevertheless, important quantitative differences are observed which are all due to (i) the larger number of surface sites present at the surface of SiNa with respect to SiH and (ii) the larger ionic strength of the supernatant in SiNa/surfactant systems with respect to SiH/surfactant systems, due to the much larger amount of sodium ions released by SiNa upon surfactant binding. Thus, the amounts of surfactant adsorbed at the point of zero charge and at saturation of the silica particles, of sodium ions released by the surface and the decrease of critical micelle concentration (cmc) in the supernatant with respect to pure water are all larger for the raw silica than for the treated silica. For the four silica/surfactant systems investigated, the first adsorption step corresponds to the adsorption of individual surfactant ions on the negative sites of the silica surface. It is driven by electrostatic interactions and strongly dependent on the number of surface sites and ionic strength associated to the released ions. At the end of the first adsorption step, which is clearly seen with SiH/surfactant systems, the second adsorption step starts. This step is driven by hydrophobic interaction between surfactant alkyl chains and results in the formation of surface aggregates. The surfactant adsorption on the surface is shown to continue even after the cmc in the supernatant is reached.

Alkyltrimethylammonium Bromide Adsorption on Polystyrene Latex Particles Studied by Dynamic Light Scattering and Adsorption Isotherms:  Effects of the Surface Polymer Layer and Modified Aromatic Amino Groups

The adsorption of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) on polystyrene latex particles modified with aromatic amino groups has been examined as a function of surfactant concentration by dynamic light scattering. Adsorption isotherms were determined by a surface tension method. At pH 6 without added salt, DTA + ions are initially adsorbed at the ends of the polymer chains through electrostatic attraction between the quarternary ammonium head-groups of DTA + ions and the sulfate groups terminating the polymer chains as well as through hydrophobic interaction between the hydrocarbon tails of the surfactant and the polymer backbone. Combination of these forces leads to an L-type isotherm. An extended layer of polymer chains at the latex surface favors further adsorption of DTA + ions which bridge neighboring polymer chains and lead to their aggregation. Further adsorption around the initially occupied sites results in formation of micelles along the polymer chains. Premature aggregation of the polymer chains during adsorption leads to a smaller adsorbed amount at saturation compared with DTAB adsorption to polystyrene latex particles having a flat surface layer of polymer chains. The bridging role of the adsorbed surfactant is enhanced on increasing the surfactant hydrocarbon chain length. The adsorbed amount at saturation is thus reduced within the series DTAB → TTAB → CTAB owing to premature aggregation of the polymer chains. Hydrophobic interaction between the polymer chains and the alkyl tails of DTAB, and also between the alkyl tails themselves, is enhanced in the presence of NaBr. Reduced electrostatic repulsion between the head-groups of the adsorbed DTA + ions results in close-packing along the polymer chains. Semiglobular micelles grow on further adsorption ultimately leading to coalescence of the aggregates into a continuous monolayer in 32 mM NaBr. At pH 2, the polymer chains are anchored at the bare latex surface by electrostatic interaction between the positive amino groups and the negative sulfate groups. Conformational changes of the polymer chains provide a suitable steric geometry for DTA + ion adsorption through hydrophobic interaction of the alkyl tails with the polymer chains and also of the alkyl tails themselves with each other. The adsorption isotherm is thus of the S-type. Semiglobular micelles are formed on the polymer chains at saturation. At pH 12, the negatively charged amino and sulfate groups directly associate with the positively charged quarternary ammonium groups of DTAB. This leads to substantial reduction of the surface charge density and a more hydrophobic surface owing to extension of the alkyl tails of the adsorbed DTA + ions into the solution. Precipitation of the latex particles thus occurs when the DTAB concentration is close to its critical micelle concentration. DTAB adsorption at pH 12 corresponds to a C-type isotherm.

Adsorption of dodecyltrimethylammonium bromide on aqueous surfaces of sodium bromide solutions

Surface tension of aqueous NaBr solutions of dodecyltrimethylammonium bromide has been measured by the drop weight method at different NaBr concentrations from 0 to 6.00 M at 25°C. Surface tension decreases sharply with increasing surfactant concentration below the critical micelle concentration (CMC) at a given NaBr concentration, but it has a constant value above the CMC. By applying the Gibbs adsorption isotherm for the surface tension at surfactant concentrations lower than the CMC, surface excess densities of surfactant cation, Na + and Br − have been derived as functions of surfactant concentration at different NaBr concentrations. Generally, surface excess densities of surfactant cation and Br − increase with increasing surfactant concentration, and they approach saturated values at the CMC. The adsorption of Na + is very low in magnitude and negative. The Corrin—Harkins plot of the CMC consists of two straight lines intersecting at 1.30 M NaBr, which can be assigned to the threshold NaBr concentration for the sphere—rod transition of the micelle.

Mixed adsorption of cationic and anionic surfactants from aqueous solution on silica gel

The adsorption of dodecyltrimethylammonium bromide (DTAB), dodecylpyridinium bromide (DPB), sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) from their single aqueous solutions and DTAB—SDBS and DPB—SDS binary mixed solutions on silica gel at 25°C has been investigated. The results show that the individual cationic surfactants can be adsorbed strongly onto the silica gel, but no significant adsorption of anionic surfactants can be detected. However, in the mixed systems, the adsorption amounts of both the cationic and anionic surfactant ions are enhanced, and the excess adsorption of cationic surface-active ions is exactly equal to the adsorption of anionic surface-active ions. Thus, it is reasonable to suggest that the excess adsorption of cationic and anionic surface-active ions is in the form of ion-pairs.