Surfactant adsorption at solid-liquid interfaces

High-Yield and Sustainable Production of Phosphatidylserine in Purely Aqueous Solutions via Adsorption of Phosphatidylcholine on Triton-X-100-Modified Silica

Triton X-100 was covalently bound to a surface of silica and acted as an anchor molecule to facilitate the adsorption of phosphatidylcholine (PC) in a purely aqueous solution. The silica-adsorbed PC obtained was successfully used for phospholipase D (PLD)-mediated transphosphatidylation in the production of phosphatidylserine (PS). Organic solvents were completely avoided in the whole production process. The PC loading and PS yield reached 98.9 and 99.0%, respectively. Two adsorption models were studied, and the relevant parameters were calculated to help us understand the adsorption and reaction processes deeply. In addition, the silica-adsorbed PC provides a promising way to continuously biosynthesize PS. A packed-bed reactor was employed to demonstrate the process flow of the continuous production of PS. The recyclability and stability of the Triton-X-100-modified silica were excellent, as demonstrated by its use 30 times during continuous operation without any loss of the productivity.

Adsorptive Removal of Copper by Using Surfactant Modified Laterite Soil

Removal of copper ion (Cu2+) by using surfactant modified laterite (SML) was investigated in the present study. Characterizations of laterite were examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), inductively coupled plasma mass spectrometry (ICP-MS), and total carbon analysis. The optimum conditions for removal of Cu2+ by adsorption using SML were systematically studied and found as pH 6, contact time 90 min, adsorbent dosage 5 mg/mL, and ionic strength 10 mM NaCl. The equilibrium concentration of copper ions was measured by flame atomic absorption spectrometry (F-AAS). Surface modification of laterite by anionic surfactant sodium dodecyl sulfate (SDS) induced a significant increase of the removal efficiency of Cu2+. The surface modifications of laterite by preadsorption of SDS and sequential adsorption of Cu2+ were also evaluated by XRD and FT-IR. The adsorption of Cu2+ onto SML increases with increasing NaCl concentration from 1 to 10 mM, but at high salt concentration this trend is reversed because desorption of SDS from laterite surface was enhanced by increasing salt concentration. Experimental results of Cu2+/SML adsorption isotherms at different ionic strengths can be represented well by a two-step adsorption model. Based on adsorption isotherms, surface charge effects, and surface modification, we suggest that the adsorption mechanism of Cu2+ onto SML was induced by electrostatic attraction between Cu2+ and the negatively charged SML surface and nonelectrostatic interactions between Cu2+ and organic substances in the laterite.

Using Neutron Reflectometry to Discern the Structure of Fibrinogen Adsorption at the Stainless Steel/Aqueous Interface

Neutron reflectometry has been successfully used to study adsorption on a stainless steel surface by means of depositing a thin steel film on silicon. The film was characterized using XPS (X-ray photoelectron spectroscopy), TOF-SIMS (time-of-flight secondary ion mass spectrometry), and GIXRD (grazing incidence X-ray diffraction), demonstrating the retention both of the austenitic phase and of the required composition for 316L stainless steel. The adsorption of fibrinogen from a physiologically-relevant solution onto the steel surface was studied using neutron reflectometry and QCM (quartz crystal microbalance) and compared to that on a deposited chromium oxide surface. It was found that the protein forms an irreversibly bound layer at low concentrations, with maximum protein concentration a distance of around 20 Å from the surface. Evidence for a further diffuse reversibly-bound layer forming at higher concentrations was also observed. Both the structure of the layer revealed by the neutron reflectometry data and the high water retention predicted by the QCM data suggest that there is a significant extent of protein unfolding upon adsorption. A lower extent of adsorption was seen on the chromium surfaces, although the adsorbed layer structures were similar, suggesting comparable adsorption mechanisms.

Enhancement of soil retention for phenanthrene in binary cationic gemini and nonionic surfactant mixtures: characterizing two-step adsorption and partition processes through experimental and modeling approaches

The enhancement of soil retention for phenanthrene (PHE) through the addition of a binary mixture of cationic gemini (12-2-12) and nonionic surfactants (C12E10) was investigated. The maximum apparent sorption coefficient Kd(*) reached 4247.8 mL/g through the addition of mixed 12-2-12 gemini and C12E10 surfactants, which was markedly higher than the summed individual results in the presence of individual 12-2-12 gemini (1148.6 mL/g) or C12E10 (210.0 mL/g) surfactant. However, the sorption of 12-2-12 gemini was inhibited by the increasing C12E10 dose; and a higher initial 12-2-12 gemini dose showed a higher "desorption" rate. The present study also addressed the sorption behavior of the single 12-2-12 gemini surfactant at the soil/aqueous interface. The sorption isotherm was divided into two steps to elucidate the sorption process; and the sorption schematics were proposed to elaborate the growth of surfactant aggregates corresponding to the various steps of the sorption isotherm. Finally, a two-step adsorption and partition model (TAPM) was developed to simulate the sorption process. Analysis of the equilibrium data indicated that the sorption isotherms of 12-2-12 gemini fitted the TAPM model better. Thermodynamic calculations confirmed that the 12-2-12 gemini sorption at the soil/aqueous interface was spontaneous and exothermic from 288 to 308K.

Adsorption of Polyanion onto Large Alpha Alumina Beads with Variably Charged Surface

Adsorption of strong polyelectrolyte, poly(styrenesulfonate), PSS, of different molecular weights onto large α-Al2O3 beads was systematically investigated as functions of pH and NaCl concentrations. The ultraviolet (UV) absorption spectra of PSS at different pH and salt concentrations confirmed that the structure of PSS is independent of pH. With the change of molecular weight from 70 kg/mol (PSS 70) to 1000 kg/mol (PSS 1000), adsorption amount of PSS increases and proton coadsorption on the surface of α-Al2O3 decreases at given pH and salt concentration. It suggests that higher molecular weight of PSS was less flat conformation than lower one. The adsorption density of PSS 70 and PSS 1000 decreases with decreasing salt concentrations, indicating that both electrostatic and nonelectrostatic interactions are involved. Experimental results of both PSS 70 and PSS 1000 adsorption isotherms onto α-Al2O3 at different pH and salt concentrations can be represented well by two-step adsorption model. The effects of molecular weight and salt concentration are explained by structure of adsorbed PSS onto α-Al2O3. The influence of added SDS on the isotherms is evaluated from the sequential adsorption. The SDS uptake onto α-Al2O3 in the presence of hemimicelles can prevent the adsorption of PSS at low concentration so that adsorption of PSS reduces with preadsorbed SDS.

Adsorption behavior and mechanism of perfluorinated compounds on various adsorbents--a review

Perfluorinated compounds (PFCs) have drawn great attention recently due to their wide distribution in aquatic environments and potential toxic to animals and human beings. Adsorption not only is an effective technology to remove PFCs from water or wastewater, but also affects PFC distribution at solid-liquid interfaces and their fate in aquatic environments. This article reviews the adsorption behavior of different PFCs (mainly perfluorooctane sulfonate and perfluorooctanoate) on various adsorptive materials. Some effective adsorbents are introduced in detail in terms of their preparation, characteristics, effects of solution chemistry and PFC properties on adsorption. Adsorption mechanisms of PFCs on different adsorbents are summarized, and various interactions including electrostatic interaction, hydrophobic interaction, ligand exchange, and hydrogen bond are fully reviewed. The adsorbents with amine groups generally have high adsorption capacity for PFCs, and formation of micelles/hemi-micelles plays an important role in achieving high adsorption capacity of perfluorinated surfactants on some porous adsorbents. Hydrophobic interaction is mainly responsible for PFC adsorption, but the difference between PFCs and traditional hydrocarbons has not clearly clarified. This review paper would be helpful for the preparation of effective adsorbents for PFC removal and understanding interfacial process of PFCs during their transport and fate in aquatic environments.

Adsorption of fluoranthene in surfactant solution on activated carbon: equilibrium, thermodynamic, kinetic studies

Adsorption of fluoranthene (FLA) in surfactant solution on activated carbon (AC) was investigated. Isotherm, thermodynamic, and kinetic attributes of FLA adsorption in the presence of the surfactant on AC were studied. Effects of AC dosage, initial concentration of TX100, initial concentration of FLA, and addition of fulvic acid on adsorption were studied. The experimental data of both TX100 and FLA fitted the Langmuir isotherm model and the pseudo-second-order kinetic model well. Positive enthalpy showed that adsorption of FLA on AC was endothermic. The efficiency of selective FLA removal generally increased with increasing initial surfactant concentration and decreasing fulvic acid concentration. The surface chemistry of AC may determine the removal of polycyclic aromatic hydrocarbons. The adsorption process may be controlled by the hydrophobic interaction between AC and the adsorbate. The microwave irradiation of AC may be a feasible method to reduce the cost of AC through its regeneration.

Effect of temperature on high shear-induced gelation of charge-stabilized colloids without adding electrolytes

We demonstrated previously (Wu, H.; Zaccone, A.; Tsoutsoura, A.; Lattuada, M.; Morbidelli, M. Langmuir 2009, 25, 4715) that, for a colloid stabilized by charges from both polymer chain-end groups and adsorbed sulfonate surfactants, when the surfactant surface density reaches a certain critical value, the shear-induced gelation becomes unachievable at room temperature, even at an extremely large Peclet number, Pe = 4.6 x 10(4). This is due to the presence of the short-range, repulsive hydration force generated by the adsorbed surfactant. In this work, we investigate how such hydration force affects the shear-induced gelation at higher temperatures, in the range between 303 and 338 K. It is found that a colloidal system, which does not gel at room temperature in a microchannel at a fixed Pe = 3.7 x 10(4), does gel when temperature increases to a certain value. The critical initial particle volume fraction for the gelation to occur decreases as temperature increases. These results indicate that the effect of the hydration force on the gelation decreases as temperature increases. Moreover, we have observed that at the criticality only part of the primary particles is converted to the gel network and the effective particle volume fraction forming the gel network does not change significantly with temperature. The effective particle volume fraction is also independent of the surfactant surface coverage. Since the effective particle volume fraction corresponds to space filling requirement of a standing gel network, which is mainly related to the clusters structure, this result indicates that at a given shear rate the cluster structure does not change significantly with the surfactant surface coverage. On the other hand, since the cluster morphology is a strong function of the shear rate, we have observed that when the Peclet number is lowered from Pe = 3.7 x 10(4) to 1.7 x 10(4), the effective particle volume fraction reduces from 0.19 to 0.12 at 313 K.

High shear-induced gelation of charge-stabilized colloids in a microchannel without adding electrolytes

We study shear-induced gelation in a microchannel without adding any salt, for a polymer colloid that is fully stable under stagnant conditions. The initial stability is achieved by negative charges from the polymer chain end groups. Then, sulfonate surfactants are added to the system. The surfactant adsorption is characterized by coexistence of domains of gaseous-like noninteracting molecules (G) and condensed patches of interacting molecules (K). It is found that, for a fixed, substantially high shear rate of 1x10(6) 1/s (Peclet number=4.5x10(4)), in the absence of surfactants, the shear-induced gelation of the system does occur, and when the surfactants are added, as the surfactant surface coverage on the particles increases to a certain value, the shear-induced gelation or even small extent of aggregation becomes impossible. We have estimated the Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy barrier from the measured zeta-potentials and found that in all the cases the corresponding shear-induced collision energy is orders of magnitude larger than the required energy to overcome the barrier. Thus, one would expect occurrence of gelation for all the systems. This clearly indicates presence of additional non-DLVO interactions, which under very low ionic strength are related to the adsorbed surfactant patches that generate strongly repulsive, short-range hydration force. Moreover, when no gelation but only aggregation occurs after passing through the microchannel, the cluster size distribution exhibits distinct bimodality. The structure of the obtained gels also depends on the surfactant surface coverage.

Monte Carlo simulation of surfactant adsorption on hydrophilic surfaces

Monte Carlo simulations have been carried out to study the adsorption behavior of small flexible amphiphilic molecules on solid surfaces from aqueous solutions. A simple coarse-grained solvent-free off-lattice model, with a square-well pair potential and hard core excluded volume effect, has been used. Adsorption isotherms for weakly and strongly hydrophilic homogeneous surfaces have been determined. The adsorbed layer displays a coexistence region with an upper critical point. Below the critical temperature a densely packed patch coexists with a two-dimensional gas-analogous phase. Above the critical temperature, a percolating network forms at higher surfactant concentrations. Depending on the ratio between the strength of the hydrophobic effect and the adsorption energy, a large variety of associates has been observed. Monolayers, bilayers, admicelles, small clusters, and percolating networks as typical associate structures have been found. In the four-region model, which is extended by the coexistence region, a characteristic adsorbed layer structure for each region can be detected. Intermediate structure types have been produced by variation of the adsorption energy.

A model for the cooperative adsorption of surfactant mixtures on solid surfaces

Adsorption of surfactant mixtures on solids is of considerable theoretical and practical importance. In this study, cooperative adsorption of surfactant mixtures of nonyl phenol ethoxylated decyl ether (NP-10) and n-dodecyl-beta-D-maltoside (DM) on silica and alumina has been investigated as a function of the distribution of individual surfactants between solution and solid surface. In the mixed adsorption process, DM is identified to be the "active" adsorbing component and NP is the "passive" co-adsorbing one in the process of adsorption on alumina, while their roles are reversed on silica. A modified model has been proposed to quantify the adsorption behavior of surfactant mixtures and to obtain information in terms of aggregation number and standard free energy for surface aggregation. This model is the first model applied to the aggregation of the surfactant mixture at the solid/solution interfaces.

Effect of clays, metal oxides, and organic matter on rhamnolipid biosurfactant sorption by soil

Rhamnolipids produced by Pseudomonas aeruginosa have been proposed as soil washing agents for enhanced removal of metal and organic contaminants from soil. A potential limitation for the application of rhamnolipids is sorption by soil matrix components. The objective of this study is to empirically determine the contribution of representative soil constituents (clays, metal oxides, and organic matter) to sorption of the rhamnolipid form most efficient at metal complexation (monorhamnolipid). Sorption studies show that monorhamnolipid (R1) sorption is concentration dependent. At low R1 concentrations that are relevant for enhancing organic contaminant biodegradation, R1 sorption followed the order: hematite (Fe(2)O(3))>kaolinite>MnO(2) approximately illite approximately Ca-montmorillonite>gibbsite (Al(OH)(3))>humic acid-coated silica. At high R1 concentrations, relevant for use in complexation/removal of metals or organics, R1 sorption followed the order: illite>>humic acid-coated silica>Ca-montmorillonite>hematite>MnO(2)>gibbsite approximately kaolinite. These results allowed prediction of R1 sorption by a series of six soils. Finally, a comparison of R1 and R2 (dirhamnolipid) shows that the R1 form sorbs more strongly alone than when in a mixture of both the R1 and R2 forms. The information presented can be used to estimate, on an individual soil basis, the extent of rhamnolipid sorption. This is important for determining: (1) whether rhamnolipid addition is a feasible remediation option and (2) the amount of rhamnolipid required to efficiently remove the contaminant.

The influence of surface active molecules on the crystallization of biominerals in solution

In the following article studies pertaining to "in situ" interactions of growing biogenic crystals (calcium phosphates, carbonates and oxalates) with, soluble, surface active molecules, including small, highly charged organic molecules, natural and synthetic polymers and synthetic surfactants, are discussed. Such interactions are at the roots of crystallization processes occurring in nature (biological mineralization) and in the controlled production of materials with well defined crystal structure, morphology and phase composition. The main characteristics of the crystals, including crystallographic data, and of the organic molecules, including their molecular structures, are briefly described. Most of the model crystals are crystal hydrates, whose dominant crystal planes are covered with continuous layers of structural water molecules (hydrated layer). The experimental methods reviewed include kinetic experiments determining induction times and/or the rates and rate controlling mechanisms of seeded and unseeded crystallization, techniques for the characterization of the nascent solid phase(s), and techniques, suitable for the assessment of interactions on the molecular level. Numerous examples show that the dominant mechanism underlying host crystal/additive interactions is selective adsorption of the additive at the crystal/solution interface, with the main driving forces ranging from purely electrostatic to highly specific recognition of crystal faces by the additive. Selective electrostatic interactions take place between growing crystals and flexible, highly charged small and macromolecules and/or surfactants because of differing ionic structures and charges of the crystal planes, some of them being shielded by hydrated layers. As in solution, surfactant molecules at high concentrations self-assemble into various superstructures (hemimicelles, bilayers) at the crystal/solution interface. Recognition of crystal planes by rigid small molecules and macromolecules with partial beta-sheet conformation (such as proteins or polyelectrolytes) is highly specific. It requires a dimensional fit between the distances of constituent ions protruding from the affected crystal plane(s) and the distances between functional groups that are part of the additive molecules. The consequences of selective additive/crystal interactions range from changes in crystal growth morphology to changes in the composition of the crystallizing phase. Examples showing the dual role of macromolecules as initiators and retarders of crystallization are discussed.

Advances in adsorption of surfactants and their mixtures at solid/solution interfaces

Surfactants and their mixtures can drastically change the interfacial properties and hence are used in many industrial processes such as dispersion/flocculation, flotation, emulsification, corrosion inhibition, cosmetics, drug delivery, chemical mechanical polishing, enhanced oil recovery, and nanolithography. A review of studies on adsorption of single surfactant as well as mixtures of various types (anionic-cationic, anionic-nonionic, cationic-nonionic, cationic-zwitterionic and nonionic-nonionic) is presented here along with mechanisms involved. Results obtained using techniques such as zeta potential, flotation, AFM, specular neutron reflectivity, small angle neutron scattering, fluorescence, ESR, Raman spectroscopy, ellipsometry, HPLC and ATR-IR are reviewed along with those from traditional techniques to elucidate the mechanisms of adsorption and particularly to understand synergistic/antagonistic interactions at solution/liquid interfaces and nanostructures of surface aggregates. In addition, adsorption of several mixed surfactant systems is considered due to their industrial relevance. Finally an attempt is made to derive structure-property relationships to provide a solid foundation for the design and use of surfactant formulations for industrial applications.

Calcium lignosulfonate adsorption and desorption on Berea sandstone

This paper describes adsorption and desorption studies carried out with calcium lignosulfonate (CLS) on Berea sandstone. Circulation experiments were performed to determine CLS adsorption isotherms and the effects of CLS concentration, temperature, salinity, brine hardness, and injection rate on adsorption density. Flow-through experiments were performed to assess the reversibility of CLS adsorption and the influence of postflush rate, brine concentration, brine hardness, brine pH, and temperature on the desorption process. Results indicate that CLS adsorption isotherms on Berea sandstone follow the Freundlich isotherm law. The results presented in this paper on the effects of CLS adsorption and desorption on Berea sandstone show that: (1) increasing CLS concentration and salinity increases CLS adsorption density; (2) increasing temperature will decrease adsorption density; (3) increasing injection rate of CLS solution will slightly decrease CLS adsorption density; (4) postflush rate and salinity of brine have a large impact on the CLS desorption process; (5) the adsorption and desorption process are not completely reversible; and (5) temperature and pH of the postflush brine have little effect on desorption.