Competitive Adsorption Of Surfactants Research Articles

Resistance of Soybean Pectin–Protein Conjugate Pre-Adsorbed to the Air–Water Interface to Displacement by the Competitive Adsorption of Surfactant

The naturally occurring soybean pectin–protein conjugate pre-adsorbed to the air–water interface was shown to be displaced competitively from the interface when a small molecule surfactant was added to the aqueous phase. The soybean pectin containing covalently linked pectin–protein conjugate was prepared by heating defatted soy flour at 121 °C and pH 4.5 for 2 h. The pectin moiety of the pectin–protein conjugate was cleaved using an alkali treatment that breaks the covalent polysaccharide–protein bond. Atomic force microscopy images of the soybean pectin adsorbed to the air–water interface revealed the presence of two-dimensional pectin networks formed within the interfacial film. The soybean pectin adsorbed to the interface was confirmed to be more resistant to displacement by the competitive adsorption of the surfactant than the alkali-treated soybean pectin. The pectin moiety is expected to strengthen interfacial films mechanically, make them more resistant to surfactant-induced competitive displacement, and have beneficial effects on colloidal stability.

Acid-Catalyzed Oligomerization at the Air–Water Interface Modified by Competitive Adsorption of Surfactants

Amphiphilic organic compounds that accumulate naturally on the outermost layers of aqueous aerosols totally change the mechanisms involved in the uptake and reactions of volatile organic compounds adsorbed on the surfaces. By means of mass spectrometry of aqueous microjets, we examined how quaternary alkylammonium cations Me(CH2)n−1N+Me3 (n = 1, 4, 8, 10, 12, and 14) and nonionic octan-1-ol influence acid-catalyzed oligomerization of gaseous isoprene (ISO) at the air–water interface. The oligomerization is initiated by proton-transfer (PT) reaction from an interfacial hydronium ion to isoprene to form a (ISO)H+ and is subsequently followed by chain-propagation (CP) reaction to form (ISO)m≥2H+. We found that although the total mass spectral signals of (ISO)mH+ products were suppressed by the presence of either the alkylammonium cations or octan-1-ol, the suppression by the former surfactants was much larger than that of the latter. The suppression was observed even at aqueous microjets of 5 μM equimolar solution of the alkylammonium cations at which the air–water interface is only sparsely occupied by the alkylammonium cations. We propose that electrostatic repulsions between the alkylammonium cation and the interfacial H3O+ cause the hindrance of the PT reaction. In contrast, the subsequent CP reactions were not suppressed by the presence of either the alkylammonium cations or octan-1-ol. We suggest that the presence of long-chain organic surfactants hinders the initial PT reaction but hardly suppresses the subsequent CP reaction. Our study has revealed hitherto unrecognized interface-specific roles of surfactants that affect multiphase chemistry in the atmosphere.

Using carbonaceous nanoparticles as surfactant carrier in enhanced oil recovery: A laboratory study

Carbonaceous nanoparticles multi-walled carbon nanotubes (MWNTs) and carbon blacks (CBs) exhibit promising properties for potential applications in crude oil production. The combination of large specific surface area and the strong affinity toward surfactants of nanoparticles mark their candidacy for delivering surfactant deep inside the reservoir. This study is aimed to assess the feasibility of surfactant carriers in tertiary oil recovery. Stable dispersions of aqueous-phase MWNTs or CBs that are formulated and able to propagate through the reservoir medium (3 wt% brine and 60 °C) were first examined as a prerequisite for reservoir application. Competitive adsorption of surfactant on nanoparticles was beneficial to decrease adsorptive loss on Ottawa sand at equilibrium concentration below critical micelle concentration (CMC). As a proof of concept, phase behavior of a ternary surfactant microemulsion system confirmed that the chosen nanoparticles (100 mg/L) successfully delivered surfactants and spontaneously released them to the O/W interface. The observed phenomenon is in accordance with calculation of the Gibbs free energy associated with oil/water/surfactant system. Besides, surfactants carried by nanoparticles achieved equilibrium ultralow interfacial tension between excess oil and aqueous phase similar to the value of surfactant-only formulation (0.007–0.009 mN/m). In one-dimensional sand pack tests, injection of MWNT-surfactant blend achieved faster and higher tertiary recovery than surfactant-only formulation, with cumulative tertiary oil recovery of 42.7% versus 38.1%. It has been noticed that once surfactant been released, destabilization of nanoparticle dispersion occurred and thus increased their retention in porous medium. In case of tight formations, further improvements may be addressed by applying functionalized carbonaceous nanoparticles to assure their transport in porous media after release of surfactant.

Adsorption and Dilational Rheology of Mixed β-Casein/DoTAB Layers Formed by Sequential and Simultaneous Adsorption at the Water/Hexane Interface

The interfacial behavior of β-casein (βCS) has been investigated in presence of the cationic surfactant dodecyl trimethyl ammonium bromide (DoTAB) at the water/hexane interface and compared to that obtained for the water/air interface. The used experimental technique is a drop profile analysis tensiometer specially equipped with a coaxial double capillary, which allows investigation of sequential adsorption of individual components besides the traditional simultaneous adsorption of two species. This method also provides the dilational rheological measurements based on low frequency harmonic drop oscillations. The tensiometric results show that the equilibrium states of the mixed βCS/DoTAB layers built up on the two different routes do not differ significantly, that is, the general compositions of the mixed layers are similar. However, the results of dilational rheology for the two adsorption strategies are remarkably different indicating different dynamic characteristics of the adsorbed layers. These findings suggest that the respective mixed layers are more proteinlike if they are formed via sequential adsorption and more surfactant-like after simultaneous adsorption. In contrast to the W/A interface, at the W/H interface proteins remain at the interface once adsorbed and cannot be displaced just by competitive adsorption of surfactants.

Competitive adsorption of surfactants and hydrophilic silica particles at the oil–water interface: Interfacial tension and contact angle studies

The effect of surfactants’ type and concentration on the interfacial tension and contact angle in the presence of hydrophilic silica particles was investigated. Silica particles have been shown to have an antagonistic effect on interfacial tension and contact angle in the presence of both W/O and O/W surfactants. Silica particles, combined with W/O surfactant, have no effect on interfacial tension, which is only dictated by the surfactant concentration, while they strongly affect interfacial tension when combined with O/W surfactants. At low O/W surfactant, both particles and surfactant are adsorbed at the interface, modifying the interface structure. At higher concentration, interfacial tension is only dictated by the surfactant.By increasing the surfactant concentration, the contact angle that a drop of aqueous phase assumes on a glass substrate placed in oil media decreases or increases depending on whether the surfactant is of W/O or O/W type, respectively. This is due to the modification of the wettability of the glass by the oil or water induced by the surfactants. Regardless of the surfactant’s type, the contact angle profile was dictated by both particles and surfactant at low surfactant concentration, whereas it is dictated by the surfactant only at high concentration.

Competitive Adsorption of Surfactants and Polymers at the Free Water Surface. A Computer Simulation Study of the Sodium Dodecyl Sulfate−Poly(ethylene oxide) System

Competitive adsorption of a neutral amphiphilic polymer, namely poly(ethylene oxide) (PEO) and an ionic surfactant, i.e., sodium dodecyl sulfate (SDS), is investigated at the free water surface by computer simulation methods at 298 K. The sampled equilibrium configurations are analyzed in terms of the novel identification of the truly interfacial molecules (ITIM) method, by which the intrinsic surface of the aqueous phase (i.e., its real surface corrugated by the capillary waves) instead of an ideally flat surface approximating its macroscopic surface plane, can be taken into account. In the simulations, the surface density of SDS is gradually increased from zero up to saturation, and the structural, dynamical, and energetic aspects of the gradual squeezing out of the PEO chains from the surface are analyzed in detail. The obtained results reveal that this squeezing out occurs in a rather intricate way. Thus, in the presence of a moderate amount of SDS the majority of the PEO monomer units, forming long bulk phase loops in the absence of SDS, are attracted to the surface of the solution. This synergistic effect of SDS of moderate surface density on the adsorption of PEO is explained by two factors, namely by the electrostatic attraction between the ionic groups of the surfactant and the moderately polar monomer units of the polymer, and by the increase of the conformational entropy of the polymer chain in the presence of the surfactant. This latter effect, thought to be the dominant one among the above two factors, also implies the formation of similar polymer/surfactant complexes at the interface than what are known to exist in the bulk phase of the solution. Finally, in the presence of a large amount of SDS the more surface active surfactant molecules gradually replace the PEO monomer units at the interfacial positions, and squeezing out the PEO molecules from the surface in a monomer unit by monomer unit manner.

Study of the co-adsorption of hexane from the gas phase at the surface of aqueous C10EO8 drops

Alkanes, although not amphiphilic molecules, adsorb at the surface of water drops from the vapour phase. At the drop surface of the surfactant solution formed in hexane saturated air, a competitive adsorption of surfactant and hexane is observed. Experiments with drop profile analysis tensiometry are performed to quantify the adsorption of the non-ionic surfactant C10EO8 at the water/hexane vapour interface. The equilibrium surface tension isotherm shows a remarkable shift towards much lower tensions, even much below those values reached by micellar C10EO8 solutions. A thermodynamic model allows the description of the mixed hexane/C10EO8 adsorption layer at the water/vapour interface quite well. The incorporation of hexane molecules also significantly increases the dilation elasticity values, although the layers of hexane alone show only very low dilational elasticities and high viscosities. While a diffusional exchange of matter theory describes the visco-elasticity of the surfactant very well, it fails for the mixed adsorption layer.

Microcapsules with Macroholes Prepared by the Competitive Adsorption of Surfactants on Emulsion Droplet Surfaces

We demonstrate a simple, unique method for preparing microcapsules with holes in their shells. Cross-linked polymelamine microcapsules are prepared by the phase-separation method. The holey shell of each microcapsule is synthesized on the surface of an oil-in-water (O/W) emulsion droplet where a water-soluble polymeric surfactant and an oil-soluble surfactant are competitively adsorbed. The water-soluble polymeric surfactant provides a reaction site for shell formation. The oil-soluble surfactant molecules seem to self-assemble while the shells are being formed, so holes appear where they assemble. The critical degree of surface coverage of an emulsion droplet by the water-soluble polymeric surfactant needed to form the holey shells is determined to be 0.90 from theoretical calculations in which competitive adsorption is considered. Theoretical consideration suggests that the size and quantity of the holes in the microcapsule shells are controlled by the composition of the surfactants adsorbed on the surface of an emulsion droplet. This theoretical consideration is confirmed by experiments. The prepared microcapsule with controllable macroholes in its shell has the potential to be used for controlled release applications and can be used to fabricate a microcapsule that encapsulates hydrophilic compounds.

Competitive adsorption of surfactants at air/water interfaces

The zeta potential of an air bubble suspended in an aqueous solution of mixed fluorocarbon and hydrocarbon anionic surfactants is studied over a wide range of concentrations and mixture compositions. The zeta potential is related to surfactant ion adsorption. The two surfactants, which exhibit an antipathy manifested by micellar demixing, compete for surface sites. The total surfactant adsorption is reduced when both surfactants are present. Adsorption phenomena are closely correlated to the micellar phase diagram.

Indirect determination of surfactants by adsorptive stripping voltammetry

The effect of some anionic, cationic and nonionic surface-active substances on the suppression of adsorptive accumulation of the bis(dimethylglyoximato)nickel(II) complex, Ni(DMG) 2, is described. Competitive adsorption of surfactants can be used to determine surfactants commonly used in commercial detergents. Triton X-100 shows the most pronounced effect on the peak height. The shape of the calibration curve depends on the concentration and on the adsorption potential. Highest sensitivity is obtained when equilibrium between Ni(DMG) 2 in solution and on the electrode surface is attained rapidly. Under these conditins, the detection limit is 1 μ 1 −1 Triton X-100. Calibrations are linear over 1–2 orders of magnitude.