Wetting characteristics of aqueous rhamnolipids solutions

The wetting properties of surfactants on solid surfaces form the basis of many industrial and biological processes. The preferential adsorption of the surfactants from aqueous solutions onto solid surfaces alter the adhesion tension of the surface and this behavior may cause partial to complete wetting of the surfaces by the aqueous surfactant solutions. However, different types of surfactants show different wetting characteristics. To study the wetting properties of biologically produced rhamnolipids (RL), advancing contact angles of the aqueous solutions of the RL mixture of R1 and R2 in a ratio of R2/R1=1.1 were measured as a function of surfactant concentration. For a comparison of the wetting performance, sodium dodecyl sulfate (SDS) was chosen as the reference surfactant. A hydrophilic glass surface, a hydrophobic polymer, polyethylene terephthalate (PET), and gold surface were used as the solid surfaces to determine the wetting characteristics of rhamnolipids. At low surfactant concentrations (RL concentration <3x10(-5)M, SDS concentration<3x10(-4)M) contact angle (Theta) varied in a certain range depending on the character of the surfactant interactions with the surface. This was followed by a decrease in contact angle. Parallel to this behavior, at low surfactant concentrations the adhesion tension decreased, then remained constant and an increase at higher surfactant concentrations was obtained on hydrophobic surfaces. On hydrophilic surfaces a steady decrease in adhesion tension was observed with both surfactant solutions.     

Dispersion stability of a ceramic glaze achieved through ionic surfactant adsorption

The adsorption of cetylpyridinium chloride (CPC) and sodium dodecylbenzenesulfonate (SDBS) onto a ceramic glaze mixture composed of limestone, feldspar, quartz, and kaolin has been investigated. Both adsorption isotherms and the average particle zeta potential have been studied in order to understand the suspension stability as a function of pH, ionic strength, and surfactant concentration. The adsorption of small amounts of cationic CPC onto the primarily negatively charged surfaces of the particles at pH 7 and 9 results in strong attraction and flocculation due to hydrophobic interactions. At higher surfactant concentrations a zeta potential of more than +60 mV results from the bilayered adsorbed surfactant, providing stability at salt concentrations < or = 0.01 M. At 0.1 M salt poor stability results despite substantial zeta potential values. Three mechanisms for SDBS adsorption have been identified. When anionic SDBS monomers either adsorb by electrostatic interactions with the few positive surface sites at high pH or adsorb onto like charged negative surface sites due to dispersion or hydrophobic interactions, the magnitude of the negative zeta potential increases slightly. At pH 9 this increase is enough to promote stability with an average zeta potential of more than -55 mV, whereas at pH 7 the zeta potential is lower at about -45 mV. The stability of suspensions at pH 7 is additionally due to steric repulsion caused by the adsorption of thick layers of neutrally charged Ca(DBS)2 complexes created when the surfactant interacts with dissolved calcium ions from the calcium carbonate component.     

Effect of SDBS on interfacial electron transfer at the liquid/liquid interface by thin layer method

The effect of an adsorbed anionic surfactant sodium dodecyl benzene sulfonate(SDBS) on electron transfer(ET) reaction between ferricyanide aqueous solution and decamethylferrocene(DMFc) located on the adjacent organic phase was investigated for the first time by thin layer method.The adsorption of SDBS at the interface resulted in a decay in the cathodic plateau current of bimolecular reaction with increasing concentrations of SDBS in aqueous phase.However,the rate constant of electron transfer(k_(et)) i...     

Influence of Light Scattering by Residual Alumina Nanoparticles on the Analysis of Surfactants Adsorption Using Spectroscopy

The adsorption of a surfactant mixture, based on an anionic surfactant, sodium dodecyl benzenesulfonate (SDBS) and a nonionic surfactant (Triton X-100, or TX100), on alumina nanoparticles was determined by solution depletion method combined with spectrometric measurement. It is shown that the light scattering, originated from the residual adsorbent alumina particles in the supernatant after centrifugation separation, interferes with the measurements of absorbance of the surfactant molecules, and therefore constitutes an error source for determination of the surfactant concentration in the supernatant by spectrometric means. The intensity of this light scattering, namely the influence of the residual alumina nanoparticles upon the surfactant adsorption, was related to the surfactant adsorption and its equilibrium concentration and varied among a batch. In this paper we report a Kalman filter method in order to eliminate the variational scattering background caused by non-separated residual alumina nanoparticles in each supernatant. This method is of interest as it is simple, easy to carry out and of high precision.     

Assessment of the Surface Heterogeneity of Talc Materials

The hydrophobic and hydrophilic components of the surface of talc materials in aqueous solution were determined using ionic surfactants and their polar headgroup adsorption isotherms. The hydrophilic and hydrophobic surface areas are inferred from the amount of probe molecule adsorbed and the structure of the adsorbed layer. Natural dispersion of talc shows at 298 K a pH of 9.4 and the electrophoretic measurements indicate that the particles are negatively charged. The hydrophilic surface area is estimated from the adsorption of benzyltrimethylammonium ions (BTMA(+)) through electrostatic interactions as supported by the increase of divalent ions in the bulk phase and the decrease in the exothermic displacement enthalpy. It was also observed from the adsorption isotherm of benzene sulfonate anions that the density of positive surface sites is very low and is thus neglected. The adsorption of an anionic surfactant essentially occurs through dispersive interactions between the nonpolar organic tail of the molecule and the hydrophobic surface. Furthermore, some assumptions on the structure of dodecyl sulfate surfactant aggregates at the interface allow the hydrophobic part of the talc particles to be estimated. The cationic surfactant adsorption has been investigated and found to corroborate the hydrophilic and hydrophobic area values first obtained. Copyright 2001 Academic Press.     

Monte Carlo study of surfactant adsorption on heterogeneous solid surfaces

The equilibrium between free surfactant molecules in aqueous solution and adsorbed layers on structured solid surfaces is investigated by lattice Monte Carlo simulation. The solid surfaces are composed of hydrophilic and hydrophobic surface regions. The structures of the surfactant adsorbate above isolated surface domains and domains arranged in a checkerboard-like pattern are characterized. At the domain boundary, the adsorption layers display a different behavior for hydrophilic and hydrophobic surface domains. For the checkerboard-like surfaces, additional adsorption takes place at the boundaries between surface domains.     

表面活性剂和聚乙烯吡咯烷酮在ZrO2上的吸附对其悬浮体稳定性的影响

研究了水溶液中十二烷基苯磺酸钠（ＳＤＢＳ）、溴代十四烷基吡啶（ＴＰＢ）和聚乙烯吡咯烷酮（ＰＶＰ）在ＺｒＯ２上的吸附及对ＺｒＯ２水悬浮体稳定性的影响。结果表明：ｐＨ＝２．４时ＳＤＢＳ在ＺｒＯ２上的吸附等湿线为ＬＳ型，ｐＨ＝７．０时ＴＰＢ的吸附等温线为Ｓ型，吸附等温线与ＺｒＯ２粒子ζ电势变化同线有大体一致的关系。在最大吸附量时ＺｒＯ２粒子ζ电势较大，且吸附的表面活性剂离子形成大部分亲水基朝向水相的表面     

Effect of sodium dodecyl benzene sulfonate to the displacement performance of hyperbranched polymer

Experimental studies were conducted to realize displacement performance effect of anionic surfactant sodium dodecyl benzene sulfonate (SDBS) on hyperbranched poly(AM/AA/AMPS/GA), which was successfully synthesized via free radical polymerization using modified dendritic functional monomer (GA), acrylamide (AM), acrylate (AA), and 2-acrylamido-2-methyl propane sulfonic acid (AMPS). Compared with individual polymer, SP (surfactant polymer) binary systems showed lower apparent viscosity, interfacial tension, and hydrodynamic radius as the result of the electrostatic repulsion between the tail end of hydrophilic polymer branched chain and the head of the surfactant. It was found from abundant static adsorption and dynamic retention tests that the values of static adsorption and dynamics retention of SDBS which is mixed with hyperbranched polymer decrease due to the competitive interaction. However, unlike this phenomenon, SDBS would heighten the dynamic retention value of poly(AM/AA/AMPS/GA), resulting in addition of residual resistance factor. Oil displacement experiment indicated that SP solutions have greater capability of enhance oil recovery than individual polymer under same conditions.     

Adsorption of nonionic surfactants on cellulose surfaces: adsorbed amounts and kinetics

Kinetic and equilibrium aspects of three different poly(ethylene oxide) alkylethers (C12E5, C12E7, C14E7) near a flat cellulose surface are studied. The equilibrium adsorption isotherms look very similar for these surfactants, each showing three different regions with increasing surfactant concentration. At low surfactant content both the headgroup and the tail contribute to the adsorption. At higher surface concentrations, lateral attraction becomes prominent and leads to the formation of aggregates on the surface. The general shape of the isotherms and the magnitude of the adsorption resemble mostly those for hydrophilic surfaces, but both the ethylene oxide and the aliphatic segments determine affinity for the surface. The adsorption and desorption kinetics are strongly dependent on surfactant composition. At bulk concentrations below the CMC, the initial adsorption rate is attachment-controlled. Above the CMC, the micellar diffusion coefficient and the micellar dissociation rate play a crucial role. For the most hydrophilic surfactant, C12E7, both parameters are relatively large. In this case, the initial adsorption rate increases with increasing surfactant concentration, also above the CMC. For C12E5 and C14E7 there is no micellar contribution to the initial adsorption rate. The initial desorption kinetics are governed by monomer detachment from the surface aggregates. The desorption rate constants scale with the CMC, indicating an analogy between the surface aggregates and those formed in solution.     

Surfactant fouling of nanofiltration membranes: measurements and mechanisms.

Fouling of nanofiltration membranes is studied during filtration of aqueous surfactant solutions under different conditions. To this purpose, four typical nanofiltration membranes (Desal51HL, NF270, NTR7450 and NFPES10) and three typical surfactants (nonionic neodol, anionic SDBS and cationic cetrimide) are selected. Fouling is studied as a function of the surfactant concentration, with and without addition of an electrolyte (NaCl), at different pH and when filtering a mixture of surfactants. Adsorption experiments and hydrophobicity measurements (to study the orientation of the surfactants on the membrane surface) are also performed under the different conditions. The least membrane fouling is found for the anionic surfactant SDBS, while for the cationic surfactant cetrimide very low relative fluxes are observed. Neodol shows an intermediate degree of fouling. Both hydrophobic and electrostatic interactions (in the case of ionic surfactants) between the membrane surface and the surfactant explain the degree of adsorption and hence fouling, as membrane fouling is correlated with the amount of adsorbed surfactant. The difference between cetrimide and SDBS becomes especially visible when changing the pH: increasing the pH leads not only to an opposite orientation of the adsorbed surfactants, but also to an opposite trend in adsorbed amount and membrane fouling. This study permits selection of an optimal nanofiltration membrane to recycle wastewater containing surfactants in the carwash industry. The optimal choice would be a hydrophilic membrane with a low molecular weight cut-off and a small negative surface charge at neutral pH. Cationic surfactants in the wastewater should also be avoided as much as possible.     

Mixed adsorption of poly(vinylpyrrolidone) and sodium dodecylbenzenesulfonate on kaolinite

The mixed adsorption of the nonionic polymer poly(vinylpyrrolidone) (PVP) and the anionic surfactant sodium dodecylbenzenesulfonate (SDBS) on kaolinite has been studied. Both components adsorb from their mixture onto the clay mineral. The overall adsorption process is sensitive to the pH, the electrolyte concentration, and the amounts of polymer and surfactant. Interpretation of the experimental data addresses also the patchwise heterogeneous nature of the clay surface. In the absence of PVP, SDBS adsorbs on kaolinite by electrostatic and hydrophobic interactions. However, when PVP is present, surfactant adsorption at 10(-2) M NaCl is mainly driven by charge compensation of the edges. The adsorption of PVP from the mixture shows similar behavior under different conditions. Three regions can be distinguished based on the changing charge of polymer-surfactant complexes in solutions with increasing SDBS concentration. At low surfactant content, PVP adsorbs by hydrogen bonding and hydrophobic interactions, whereas electrostatic interactions dominate at higher surfactant concentrations. Over the entire surfactant concentration range, polymer-surfactant aggregates are present at the edges. The composition of these surface complexes differs from that in solution and is controlled by the surface charge.     

Physicochemical characterization of an anatase TiO2 surface and the adsorption of a nonionic surfactant: an atomic force microscopy study

Atomic force microscopy was used to characterize an anatase TiO2 surface, prepared by the helical vapor preparation method. The forces between two bare TiO2 surfaces were measured in the presence of water at various pH values. This TiO2 isoelectric point (iep) was characterized by the presence of only a van der Waals attraction and was measured at pH 5.8; this value is similar to that for a rutile TiO2 surface. The adsorption mechanism of a nonionic surfactant molecule to this anatase TiO2 surface was investigated by measuring the forces between two such TiO2 surfaces at their iep pH in the presence of linear dodecanol tetraethoxylate (C12E4), a poly(ethoxylene oxide) n-alkyl ether. C12E4 was seen by the presence of steric forces to adsorb to the uncharged TiO2 surface. For low surfactant concentrations, C12E4 adsorbed with its hydrophobic tail facing the TiO2 substrate, to reduce its entropically unfavorable contacts with water. Additional surfactant adsorption occurred at higher surfactant concentrations by the hydrophobic and hydrophilic interactions between the surfactant tails and heads, respectively, and gave sub-bilayers. A two-step adsorption isotherm was subsequently proposed with four regions: (1) submonolayer, (2) complete monolayer, (3) sub-bilayer, and (4) bilayer. The absence of a long-range repulsive force between the two TiO2 surfaces in the presence of the C12E4 surface aggregates indicated that a C12E4 nonionic surfactant aggregate did not possess charge.     

Measurement of the kinetic rate constants for the adsorption of superspreading trisiloxanes to an air/aqueous interface and the relevance of these measurements to the mechanism of superspreading

Super-spreading trisiloxane surfactants are a class of amphiphiles which consist of nonpolar trisiloxane headgroups ((CH3)3-Si-O)2-Si(CH3)(CH2)3-) and polar parts composed of between four and eight ethylene oxides (ethoxylates, -OCH2CH2-). Millimeter-sized aqueous drops of trisiloxane solutions at concentrations well above the critical aggregate concentration spread rapidly on very hydrophobic surfaces, completely wetting out at equilibrium. The wetting out can be understood as a consequence of the ability of the trisiloxanes at the advancing perimeter of the drop to adsorb at the air/aqueous and aqueous/hydrophobic solid interfaces and to reduce considerably the tensions of these interfaces, creating a positive spreading coefficient. The rapid spreading can be due to maintaining a positive spreading coefficient at the perimeter as the drop spreads. However, the air/aqueous and solid/aqueous interfaces at the perimeter are depleted of surfactant by interfacial expansion as the drop spreads. The spreading coefficient can remain positive if the rate of surfactant adsorption onto the solid and fluid surfaces from the spreading aqueous film at the perimeter exceeds the diluting effect due to the area expansion. This task is made more difficult by the fact that the reservoir of surfactant in the film is continually depleted by adsorption to the expanding interfaces. If the adsorption cannot keep pace with the area expansion at the perimeter, and the surface concentrations become reduced at the contact line, a negative spreading coefficient which retards the drop movement can develop. In this case, however, a Marangoni mechanism can account for the rapid spreading if the surface concentrations at the drop apex are assumed to remain high compared to the perimeter so that the drop is pulled out by the higher tension at the perimeter than at the apex. To maintain a high apex concentration, surfactant adsorption must exceed the rate of interfacial dilation at the apex due to the outward flow. This is conceivable because, unlike that at the contact line, the surfactant reservoir in the liquid at the drop center is not continually depleted by adsorption onto an expanding solid surface. In an effort to understand the rapid spreading, we measure the kinetic rate constants for adsorption of unaggregated trisiloxane surfactant from the sublayer to the air/aqueous surface. The kinetic rate of adsorption, computed assuming the bulk concentration of monomer to be uniform and undepleted, represents the fastest that surfactant monomer can adsorb onto the air/aqueous surface in the absence of direct adsorption of aggregates. The kinetic constants are obtained by measuring the dynamic tension relaxation as trisiloxanes adsorb onto a clean pendant bubble interface. We find that the rate of kinetic adsorption is only of the same order as the area expansion rates observed in superspreading, and therefore the unaggregated flux cannot maintain very high surface concentrations at the air/aqueous interface, either at the apex or at the perimeter. Hence in order to maintain either a positive spreading coefficient or a Marangoni gradient, the surfactant adsorptive flux needs to be augmented, and the direct adsorption of aggregates (which in the case of the trisiloxanes are bilayers and vesicles) is suggested as one possibility.     

Electrokinetic sonic amplitude method as applied to studying adsorption of high-molecular-mass compounds at interfaces

It is shown that the regularities of the adsorption of high-molecular-mass compounds on particle surfaces in disperse systems can be studied by the electrokinetic sonic amplitude method. Procedures are developed for calculating parameters of polymer adsorption layers and the quantitative analysis of their structure. The parameters of the adsorption layers are calculated by the example of a hydrophilic polymer, ethyl-hydroxyethylcellulose, and hydrophilic particles of titanium dioxide and iron oxide dispersed in aqueous disperse systems subjected and nonsubjected to an intense mechanical treatment. The treatment of the disperse systems in the field of intense wave vibrations is found to enhance the adsorption interaction.     

薄层法研究十二烷基苯磺酸钠对硝基苯/水界面电子转移的影响

用薄层法研究了阴离子表面活性剂十二烷基苯磺酸钠(SDBS)对硝基苯/水界面电子转移的影响. 实验结果表明, 随着水相中十二烷基苯磺酸钠浓度的增加, 有机相中十甲基二茂铁(DMFc)和水相中Fe(CN)₆^3-发生的界面双分子反应的阴极平台电流呈现递减趋势, 但是界面双分子反应速率常数却呈递增趋势. 这是由于阴离子表面活性剂十二烷基苯磺酸钠在硝基苯/水界面形成了修饰层, 影响了界面双电层结构. SDBS在液/液界面的吸附为Langmuir吸附.     

Morphology and mechanical properties of surfactant aggregates at water-silica interfaces: molecular dynamics simulations

Dilute and concentrated surfactant systems at the solid-liquid interface are examined using classical molecular dynamics simulations. Particular emphasis is placed on understanding how surfactants aggregate and form the micellar structure, how micelles change shape at high concentrations in aqueous media and in the presence of hydrophilic surfaces, and at what force this micellar structure breaks apart during indentation of micelle-covered surfaces with a proximal probe microscope tip. The specific system of interest is C12TAB (n-dodecyltrimethylammonium bromide) surfactant in an aqueous medium that is modeled with empirical potentials. The simulations predict that the micelle structure in water is compact and either spherical or elliptical in shape. In the presence of a hydrophilic surface of silica, the structure evolves into a flat elliptical shape, in agreement with experimental findings. The simulated indentation of the micelle/silica system causes the micelle to break apart at an indentation force of about 1 nN and form a surfactant monolayer. The predicted force curve is in excellent agreement with experimental measurements.     

Calorimetric studies of cationic surfactant adsorption at low surface coverages on different silica samples

Calorimetric measurements of adsorption for the surfactant (benzyldimethyldodecylammonium bromide) and its polar head-group (benzyltrimethylammonium bromide) from aqueous solutions on two different silica surfaces (hydrophilic and hydrophobic one) allow a more detailed picture of the subsequent stages of the adsorption process to be drawn. It is possible to determine more precisely a boundary between the adsorption of individual molecules and the formation of surface aggregates. The local disruption of the structure of the interfacial water molecules by surfactant cations gives an endothermic contribution to the total enthalpy of displacement. This contribution depends on the length of alkyl chain as well as on the type and the origin of solid surface.     

Synthesis, characterization and comparative evaluation of phenoxy ring containing long chain gemini imidazolium and pyridinium amphiphiles

The effect of Triton X-100 on the colloidal dispersion stability of CuPc-U (unsulfonated and hydrophobic) and CuPc-S (surface sulfonated and hydrophilic) particles in aqueous solutions (water and NaNO(3)) was investigated at 25 °C. Its adsorption density was determined from surfactant concentrations analyzed by an HPLC method with a UV detector. The experimental dispersion stability ratios of the particles were determined from dynamic light scattering (DLS) data, with the Rayleigh-Debye-Gans (RDG) light scattering theory. The adsorption densities of Triton X-100 on both the CuPc-U and CuPc-S increase with increasing concentration of surfactant up to the critical micelle concentration (cmc), and then reach a plateau. The maximum adsorption density Γ(m) is higher for the CuPc-U (d(h)=160 nm) than that for the CuPc-S (d(h)=90 nm). The hydrophobic chains are inferred to be adsorbed onto the surfaces, and the hydrophilic ethylene oxide chains are in a coil conformation. The W(app)-values for the CuPc-U dispersions are affected mainly by the surfactant fractional surface coverage θ. Adding NaNO(3) has no significant effect on the dispersion stability. The stabilization mechanism for the CuPc-U is inferred to be primarily steric, as expected. The stability ratios for the CuPc-S in solutions with NaNO(3) are higher than those for CuPc-U, and decrease with increasing concentration of NaNO(3), indicating that the stabilization is affected by the screening of electrostatic repulsive forces. The zeta potential is not a good predictor of the electrostatic stabilization, pointing to the need for new and improved theories.     

The array and interfacial activity of sodium dodecyl benzene sulfonate and sodium oleate at the oil/water interface

There is a close correlation between the interfacial activity and the adsorption of the surfactant at the interface, but the detailed molecular standard information was scarce. The interfacial activity of two traditional anionic surfactants sodium dodecyl benzene sulfonate (SDBS) and sodium oleate (OAS) were studied by experimental and computer simulation methods. With the spinning drop method and the suspension drop method, the interfacial tension of oil/aqueous surfactant systems was measured, and the influence of surfactant concentration and salinity on the interfacial tension was investigated. The dissipative particle dynamics (DPD) method was used to simulate the adsorption of SDBS and OAS at the oil/water interface. It was shown that it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. The accession of inorganic salts causes surfactant molecules to form more compact and ordered arrangements and helps to decrease the interfacial tension. There is an osculation relation between interfacial density and interfacial activity. The interfacial density calculated by molecular simulation is an effective parameter to exhibit the interfacial activity.     

Heat of adsorption of surfactants and its role on nanoparticle stabilization

In this work, the binding between sodium oleate (SO), sodium laurate (SL), sodium dodecyl sulfate (SDS), and sodium dodecylphosphonate (SDP) and iron oxide nanoparticles was systematically investigated using isothermal titration calorimetry (ITC). Comparing the heat exchanged during the isothermal titration with the corresponding surfactant adsorption isotherm, in the cases of SO and SDP, a strong binding takes place at low surfactant concentrations. The binding enthalpy at this low surfactant concentrations depends on the type of surfactant anionic head group. For C12 surfactants, the phosphonate group produced the strongest endothermic binding, followed by the exothermic binding with the carboxylate group, followed by weak exothermic interaction with the sulfate group. For carboxylate surfactants, longer surfactant tails result in larger exothermic binding. Surfactants that exhibited large binding enthalpies also produced more stable suspensions. The Langmuir (L), Freundlich (F), and Langmuir–Freundlich (L–F) adsorption models were used to interpret the adsorption isotherms during the titration with sodium oleate. The L–F adsorption isotherm model was selected to calculate the heat of the formation of the SO monolayer and bilayer on the iron oxide nanoparticles. The L–F model reflects the finite or limited adsorption of the Langmuir model, but accounts for non-homogeneous adsorption of the Freundlich model that help account for surfactant self-assembly before and after adsorption. Coupling the adsorption model with the titration data is possible to calculate the real heat of adsorption of the surfactants on the metal oxide.