Calorimetric Evidence of the Formation of Half-Cylindrical Aggregates of a Cationic Surfactant at the Graphite/Water Interface

Kinetic and thermodynamic investigations of surfactants adsorption from water by carbide-derived carbon

The objective of the study is to investigate the potential of carbide-derived carbon (CDC) for the adsorptive removal of nonionic t-octylphenoxy poly ethoxy ethanol (TX-100), anionic sodium dodecylbenzene sulfonate (SDBS) and cationic 1-hexadecylpyridinium bromide (HDPB) surfactants from water. The CDC was characterized using TEM, SEM, FTIR, BET, EDS, XPS methods and zeta potential measurements. The effects of adsorption parameters included initial surfactant concentration, contact time, temperature, and pH of the feed solution were evaluated. The adsorption capacity and mechanism were determined by modeling the isotherm, kinetic and thermodynamic data. The kinetics results demonstrated that the adsorption of the surfactant by CDC obeys the pseudo 2^nd order model. The thermodynamic results have shown that surfactants adsorption by CDC is an endothermic and spontaneous process. The Sips model agreed with the adsorption isotherm data of SDBS with R² of 0.987, while both Freundlich and Redlich-Peterson models comply well with adsorption data for TX-100 and HDPB. The hydrophobic and electrostatic interactions were found the dominant mechanisms of the adsorption of the surfactant by CDC. The adsorption capacities of CDC were found to be 442.4, 462.0 and 578.4 mg/g for SDBS, HDPB and TX-100, respectively.

Electrodeposition-Assisted Rapid Preparation of Pt Nanocluster/3D Graphene Hybrid Nanozymes with Outstanding Multiple Oxidase-Like Activity for Distinguishing Colorimetric Determination of Dihydroxybenzene Isomers

Here, we demonstrate a facile bottom-up strategy to fabricate Pt nanoclusters (Pt NCs) grafted onto three-dimensional graphene foam (3D GF) assisted by cetyltrimethyl ammonium bromide (CTAB) using the electrodeposition method. The homogeneous grafting of Pt NC onto 3D GF is due to the formation of hemimicelles above some CTAB concentration. With the unique nanocluster structure and the high content of Pt⁰, the Pt NC/3D GF nanohybrid exhibits extremely high activity and shows higher reusability and stability. Apart from the intrinsic oxidase-like activity with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate, the Pt NC/3D GF nanohybrid can act simultaneously as an effective polyphenol oxidase (PPO) mimic, such as tyrosinase, catechol oxidase, and laccase. More importantly, utilizing intrinsic catechol oxidase-like activity and the oxidase-like activity with TMB as the substrate of the nanohybrid, distinguishing colorimetric determination of dihydroxybenzene isomers (catechol and hydroquinone) is performed. Distinguishing colorimetric analysis of dihydroxybenzene isomers was first developed using nanozymes. The present work provides a simple bottom-up approach for the reasonable fabrication of various nanostructured nanozymes with excellent performance using the electrodeposition method assisted with surfactants.

Electrolyte-induced reorganization of SDS self-assembly on graphene: a molecular simulation study

A molecular dynamics simulation was conducted to study the structure and morphology of sodium dodecyl sulfate (SDS) surfactants adsorbed on a nanoscale graphene nanostructure in the presence of an electrolyte. The self-assembly structure can be reorganized by the electrolyte-induced effect. An increase in the ionic strength of the added electrolyte can enhance the stretching of adsorbed surfactants toward the bulk aqueous phase and make headgroups assemble densely, leading to a more compact structure of the SDS/graphene composite. The change in the self-assembly structure is attributed to the accumulation/condensation of electrolyte cations near the surfactant aggregate, consequently screening the electrostatic repulsion between charged headgroups. The role of the electrolyte revealed here provides direct microscopic evidence or an explanation of the reported experiments in the electrolyte tuning of the interfacial structure of a surfactant aggregate on the surface of carbon nanoparticles. Additionally, the buoyant density of the SDS/graphene assembly has been computed. With an increase in the ionic strength of the electrolyte, the buoyant density of the SDS/graphene composite rises. The interfacial accumulation of electrolytes provides an important contribution to the density enhancement. The study will be valuable for the dispersion and application of graphene nanomaterials.

30th Anniversary of the Expert Group Detergent Chemistry: From an Isolated Problem Solving to Sustainable Responsibility

The 30th anniversary of the GDCh-Fachgruppe Waschmittelchemie (Expert Group Detergent Chemistry) was celebrated at this year's Conference in Königswinter. Unfortunately, only some 80 people attended this fine meeting that was focused on Ecology/Toxicology but also encompassed stimulating papers in Basics/Mechanisms, Raw Materials/Syntheses, and Applications. 19 lectures and 13 posters were given. The anniversary marked the end of the conference in this form because the annual meeting of the Expert Group will be developed to a European Detergents Conference (EDC) that will be embedded in the next SEPAWA Kongress 2005 for the first time.

At the anniversary's celebratory convention D. Schermer, Chairman of the Expert Group, reviewed the activities of the Expert Group, which was founded in 1974 in Bamberg to meet the need of scientifically based open discussions on ecological issues of detergent chemistry. In the meantime, the Group has been developed into an active forum for scientific exchange in all fields of detergent chemistry. Furthermore, it has contributed to the practicable solution of many environmental problems and to a more serious public discussion about surfactants and detergents. Many problems remain to be solved, and therefore an active cooperation also with other expert groups of the German Chemical Society (GDCh) is crucial.

In his celebrating address W. Koch, General Manager of the GDCh, stressed that in its early days the Group had implemented consumer and environmental protection in its catalogue of topics. In so far, the today's motto “From an isolated problem solving to a sustainable responsibility” is just a consequent extension of its activities. He also asked all members to strive for new members of the GDCh to strengthen this scientific society as one of the largest in Europe.

In his celebratory lecture D. Ruchay, Monheim, Germany, summarized “30 Years of Water Protection in Germany – Through Societal Cooperation to Success”.

After World War Two there were no municipal and industrial sewage plants in Germany able to work. The infrastructure of waste water management was built up very slowly in the first 20 years after the war. Correspondingly the quality of the inshore waters impaired dramatically.

Since 1970 the water quality improved significantly. At that time the water management act had been tightened considerably and the waste water fee depending on the quality and pollution of the waste water was established.

Today the water quality in the German inshore waters is fair up to good, but the ecological situation has still to be improved. For some years the water regulations are laid down decisively by the European Union (The Water Framework Directive, The Urban Waste-water Treatment Directive, The Integrated Pollution Prevention Control Directive).

Adsorption of the cationic surfactant benzyldimethylhexadecylammonium chloride at the silica-water interface and metal salt effects on the adsorption kinetics

The adsorption of the cationic surfactant benzyldimethylhexadecylammonium (BDMHA(+)) chloride has been studied at the hydrophilic silica-water interface by Raman spectroscopy in total internal reflection geometry (TIR Raman). This Raman spectroscopic technique takes advantage of an evanescent electric field that is generated at the silica-water interface in TIR mode with specific probing depth. The present study demonstrates the capabilities of the TIR Raman sampling configuration to provide structural information and simultaneously serve as an experimental platform for studying thermodynamic and kinetic properties of BDMHA(+)Cl(-) at the silica-water interface at neutral pH and compare its adsorption behavior with the modified adsorption properties in the presence of four different concentrations of a divalent metal salt. Spectral analysis of the Raman scattering intensities as a function of time and concentration provided the input data for evaluating adsorption properties of the surfactant in the absence and presence of the metal salt additive. Addition of the magnesium metal salt lowered the cmc, altered the surface excess of the surfactant, and increased the Langmuir adsorption constants, as well as the magnitude of the free energy of adsorption, and adsorption kinetics, proportional to the concentrations of the metal salt. Adsorption isotherms based on a modified Langmuir adsorption model were established for five systems: the pure surfactant in aqueous solution, and the surfactant in the presence of 5, 10, 50, and 100 mM of magnesium chloride. The metal salt did not enhance surfactant adsorption at very low surfactant concentrations below 5 μM, where adsorption occurs by electrostatic attraction; the divalent metal salt, however, favorably influenced the adsorption behavior in the aggregate formation region by reducing the electrostatic repulsion between the polar surfactant head groups, and enhancing the hydrophobic effect between the hydrophobic surfactant alkyl chains and the polar water molecules.

Adsorption and conformation of a cationic surfactant on single-walled carbon nanotubes and their influence on naphthalene sorption

Surfactants are used in synthesis and dispersion of carbon nanotubes (CNTs) and can thus be released into the environment with CNTs. In this study, it was observed that the coupled release of surfactants and CNTs altered the sorption of organic contaminants on the CNTs. The cationic surfactant, cetylpyridinium chloride (CPC), decreased naphthalene sorption on single-walled carbon nanotubes (SWCNT). In the most dramatic example, the adsorption capacity of naphthalene on SWCNT was reduced from 240 to 61.1 mg/g. The decrease of naphthalene sorption could be largely attributed to the competition of adsorbed CPC cations (i.e., C(21)H(38)N(+)) with naphthalene by occupying the SWCNT surface as surfaces decreased from 737 to 88.9 m(2)/g after the coating of CPC. However, the adsorbed CPC may form hemimicelles and result in a favorable media for naphthalene partition to counteract the decrease in naphthalene sorption. Configuration changes of adsorbed CPC hemimicelles might occur because the naphthalene partition into the adsorbed CPC decreased with the increase of adsorbed CPC. A partition-adsorption model was introduced to describe the partition fraction of naphthalene into adsorbed CPC hemimicelles as well as the adsorption fraction of naphthalene on unoccupied surfaces of SWCNT.

Adsorption of the cationic surfactant cetyltrimethylammonium bromide to silica in the presence of sodium salicylate: surface excess and kinetics

The adsorption of cetyltrimethylammonium bromide (CTAB) to silica in the presence of sodium salicylate has been investigated using atomic force microscopy, optical reflectometry, and a quartz crystal microbalance. Salicylate is found to have a dramatic influence on surface adsorption in terms of the kinetics, surface excess, structure of adsorbed aggregates and the mechanical rigidity of the adsorbed film. This is consistent with the bulk solution behavior of more concentrated CTAB-salicylate solutions and reflects the higher local concentration induced by adsorption to the silica surface. Slow adsorption kinetics are found over a wide range of concentrations below the critical micelle concentration.

Structure of the sodium dodecyl sulfate surfactant on a solid surface in different NaCl solutions

Studies of molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules adsorbed on a graphite surface in different salt (NaCl)/water solutions were conducted. The results showed the formation of hemicylindrical aggregates, at different salt concentrations, in agreement with atomic force microscopy (AFM) results. However, the hemicylinders exhibited different structures as the salt concentration was increased. At low concentrations, the internal structure of the hemicylinder formed well-defined SDS layers, parallel to the surface. However, when the amount of salt was increased, the top layer became less pronounced until it disappeared at the highest concentration. Density profiles of the SDS headgroups were also analyzed, and those profiles were found to become sharper as the NaCl concentration increased. The phenomenon was investigated in terms of how the aggregates wet the solid surface.

Colloidal stability of hydrophobic nanoparticles in ionic surfactant solutions: definition of the critical dispersion concentration

The dispersion stability diagrams of hydrophobic boehmite nanoparticles in aqueous n-alkyltrimethylammonium bromide solutions (alkyl chain lengths 10-16) were studied over a wide range of particle and surfactant concentrations. The surfactant molecules adsorb tail-on on the particle surface, which provides the colloidal stability through electrostatic repulsion. In the stable region of each diagram, bimodal particle size distributions (50 and 500 nm) are found at lower surfactant concentration, which give way to monomodal distributions (50 nm) at higher concentration. This deagglomeration is connected with the cmc of the surfactants and can be explained by a desorption of counterions from the self-assembled surfactant layer. The desorption is caused by changes in the counterion concentration upon micellization. At low particle concentrations, the transition from the intermediate to the stable region, that is, the disappearance of the precipitate, occurs at a constant surfactant concentration. This concentration is introduced as the "critical dispersion concentration" (cdc), this being the lowest required concentration of a surfactant that is necessary to disperse the hydrophobic particles. The logarithm of the cdc shows a linear dependence on the surfactant chain length, thus a cmc-analogous behavior. The ratio cdc/cmc decreases with increasing surfactant chain length, indicating that long-chain surfactants are more efficient in dispersing nanoparticles than are their lower homologues. The existence of a system-specific critical cdc/cmc ratio, beyond which stable dispersions cannot be obtained, is proposed, which explains the disability of short-chain surfactants to disperse colloids.

Biosorption of chromate anions from aqueous solution by a cationic surfactant-modified lichen (Cladonia rangiformis (L.))

Biosorption has been appearing as a useful alternative to conventional treatment systems for the removal of toxic metals from aqueous stream. The batch removal of chromate anions (CrO(4)(2-)) from wastewater under different experimental conditions using a cationic surfactant-modified lichen (Cladonia rangiformis (L.)) was investigated in this study. Cetyl trimethyl ammonium bromide (CTAB) was used for biomass modification. The results of the experiments showed that biomass modification substantially improved the biosorption efficiency. Effects of pH, biosorption time, initial CrO(4)(2-) concentration, biosorbent dosage, and the existence of the surfactant on the biosorption of CrO(4)(2-) anions were studied. Studies up to date have shown that the biosorption efficiency of chromium increased as the pH of the solution decreased. In the present study, the removal of chromate anions from aqueous solutions at high pH values with surfactant-modified lichen was investigated. From the results of the experiments it was seen that the removal of chromate anions by modified lichen was 61% at the solution natural pH (pH 5.11) but at the same pH value the removal of chromate anions by unmodified lichen was 6%. Also concentrations ranging from 30 to 150 mg/L Cr(IV) were tested and the biosorptive removal efficiency of the metal ions from aqueous solution at high pH was achieved more than 98%.

Self-organization of surfactant-metal-ion complex nanofibers on graphite surfaces and their application to fibrously concentrated platinum nanoparticle formation

We report the fabrication of self-organized surfactant nanofibers containing platinum ions on a highly oriented pyrolytic graphite (HOPG) surface from mixed solutions of hexadecyltrimethylammonium hydroxide (C16TAOH) and hydrogen hexachloroplatinate (IV) (H2PtCl6). The fibrous surfactant self-assembly was stable in air, even after being soaked in water, in contrast to surfactant hemicylindrical micelles, which are stable only at graphite/solution interfaces. The results show that the graphite surface served as an essential template for the specific formation of fibrous surfactant self-assemblies. In addition, when surfactant nanofibers containing metal ions were treated with hydrazine, platinum nanoparticles concentrated in the nanofibers formed on the HOPG surface. We also prepared surfactant nanofibers containing gold ions on HOPG surfaces and formed gold nanoparticles in the nanofibers.

Surfactant Aggregates at Rough Solid−Liquid Interfaces

We demonstrate improved atomic force microscopic imaging of surfactant surface aggregates, featuring an increase in the topography contrast by several hundred percent with respect to all previous studies. Surfactant aggregates on rough gold surfaces, which could not be imaged previously because of low resolution, display substantially different morphologies when compared with atomically smooth materials.

Kinetics of PL quenching during single-walled carbon nanotube rebundling and diameter-dependent surfactant interactions

The kinetics of single-walled carbon nanotube rebundling have been investigated by photoluminescence (PL) spectroscopy. The rate of loss of PL intensity was measured for 12 different nanotubes in three common aqueous surfactants (sodium dodecyl sulfate, SDS; sodium dodecylbenzene sulfonate, SDBS; and sodium cholate, SC) as the surfactant suspensions were diluted to promote nanotube rebundling, quenching of semiconductor nanotube PL, and precipitation. The rate of PL decay was first-order in the concentration of isolated nanotubes, as expected if surfactant desorption is rate-limiting in the rebundling process. Temperature-dependent measurements permitted an Arrhenius analysis from which diameter-dependent activation energies were determined. SDS was found to have very strong diameter dependence for activation energy, with stronger binding to smaller-diameter nanotubes, whereas SDBS displayed a weaker diameter dependence. SC was found to bind strongly to certain nanotubes and weakly to the (10,2) nanotube. The PL emission red shifted with time after dilution as surfactant desorption proceeded. This effect is attributed to an increase in the micropolarity at the nanotube surface.

Adsorption and morphology transition of surfactants on hydrophobic surfaces: a lattice Monte Carlo study

In this work, we first show that there are only five independent interchange parameters in the surfactant-solvent-interface system in Larson's model, and then adsorption and morphology transition of surfactants on hydrophobic surfaces are studied by extensive lattice Monte Carlo simulations. In our simulations, we found that there exist six adsorbed morphologies: (1) premature admicelle, (2) hemisphere, (3) hemisphere-hemicylinder mixture, (4) wormlike hemicylinder, (5) perforated monolayer, and (6) monolayer. The surface morphologies and the amount of adsorption on hydrophobic surfaces are found to be affected obviously by two interchange parameters. One is the attractive interaction between tail groups and surface (chiTS), and the other is the solubility of head groups in bulk (chiHW). Phase diagrams in chiHW versus chiTS planes for surfactants with different hydrophobicities (chiTW) and for surfactants with different molecular structures are determined in this work, from which the transitions of surface morphologies and adsorption behaviors are discussed.

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.

Morphology transitions in nonionic surfactant adsorbed layers near their cloud points

The structure of adsorbed layers of several polyoxyethylene alkyl ether (C(n)E(m)) nonionic surfactants on silica and graphite surfaces has been imaged using atomic force microscopy as a function of temperature up to their cloud points. For all surfactants with a cloud point within the experimentally accessible range, the adsorbed layer morphology on silica evolved from globules at low temperatures first into rods and then a mesh with increasing temperature. This mesh structure was retained even when the solutions were heated above their cloud points into the two-phase coexistence region. Only C(12)E(3) was observed to form a laterally unstructured bilayer. On graphite, all surfactants formed straight, parallel hemicylinders at all temperatures examined.

Self-Assembly of a Nonionic Surfactant at the Graphite/Ionic Liquid Interface

In this study, we demonstrate by AFM imaging that nonionic surfactants self-assemble into hemicylindrical aggregates at the interface between graphite and the room temperature ionic liquid ethylammonium nitrate. Like aqueous systems, surfactant first adsorbs in a tail-to-tail monolayer arrangement along one of the three symmetry axes of graphite, templating subsequent self-assembly into adsorbed hemicylinders. Longer surfactant tails and higher concentrations are required to produce hemicylindrical aggregates in the ionic liquid than in aqueous solutions.

Pulsed-flow microcalorimetric study of the template-monolayer region of nonionic surfactants adsorbed at the graphite/water interface

The formation of half-cylindrical surfactant aggregates at the graphite/aqueous solution interface is templated by an ordered monolayer of molecules disposed parallel to the graphite basal plane. Beyond a critical alkyl chain length, monolayer formation is effectively irreversible. Since enthalpic interactions in this template-monolayer region cannot be resolved with adequate accuracy by the traditional adsorption calorimetric methods, we applied a novel method, pulsed-flow calorimetry, for simultaneous measurement of the material balance and the enthalpy balance in this high-affinity region. For the three nonionic surfactants studied, n-octyl beta-D-glucoside (C(8)G(1)), dimethyl-n-decylamine oxide (C(10)DAO), and n-octyl tetraethylene glycol monoether (C(8)E(4)), the adsorption was found to be strongly exothermic and effectively irreversible at low adsorbate densities, and the differential heat of adsorption markedly decreased with increasing surface coverage in this region. This deviation from the ideal adsorption behavior was attributed to intermolecular interactions within the adsorption layer rather than to surface heterogeneity of the graphite basal planes. A thermodynamic consistency test clearly demonstrated that pulsed-flow calorimetry is a unique experimental method for the study of nonreversible adsorption phenomena at solid/solution interfaces, representing an excellent tool to complement traditional methods, e.g., frontal-flow and titration adsorption calorimetry. Studies by the frontal-flow method revealed that aggregation on top of the surfactant monolayer was endothermic and reversible.

Unusual wetting dynamics of aqueous surfactant solutions on polymer surfaces

Static and dynamic contact angles of aqueous solutions of three surfactants--anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C(12)E(5))--were measured in the pre- and micellar concentration ranges on polymer surfaces of different surface free energy. The influence of the degree of substrate hydrophobicity, concentration of the solution, and ionic/nonionic character of surfactant on the drop spreading was investigated. Evaporation losses due to relatively low humidity during measurements were taken into account as well. It was shown that, in contrast to the highly hydrophobic surfaces, contact angles for ionic surfactant solutions on the moderately hydrophobic surfaces strongly depend on time. As far as the nonionic surfactant is considered, it spreads well over all the hydrophobic polymer surfaces used. Moreover, the results obtained indicate that spreading (if it occurs) in the long-time regime is controlled not only by the diffusive transport of surfactant to the expanding liquid-vapor interface. Obviously, another process involving adsorption at the expanding solid-liquid interface (near the three-phase contact line), which goes more slowly than diffusion, has to be active.

Removal of surfactants by powdered activated carbon and microfiltration

Direct and indirect releases of large quantities of surfactants to the environment may result in serious health and environmental problems. Therefore, surfactants should be removed from water before release to the environment or delivery for public use. Using powdered activated carbon (PAC) as adsorbent and separating particles with a membrane may be an effective technique to remove surfactants. In this study, the removal of surfactants by microfiltration and PAC was investigated and the influences of the operating parameters on the effectiveness on microfiltration were determined. An anionic (LABS) and a cationic surfactant (CTAB) were selected for the experiments. A series of batch experiments were performed to determine the sorption isotherms of surfactants to PAC. Then microfiltration experiments were carried out. The results showed that formation of secondary membrane on the surface and, within the pores of the membrane, increased the retention of surfactants significantly. Increase in transmembrane pressure and pore size of the membrane decreased the rejection rates, but increase in cross-flow velocity increased the rejection rate. Temperature had no apparent affect on the efficiency of surfactant removal. Presence of electrolyte had different effects on CTAB and LABS. The rejection rates of CTAB significantly increased when the concentration of NaCl increased; however, a slight decrease was observed in the rejection rate of LABS at the same conditions.

Mechanism of cationic surfactant adsorption at the solid-aqueous interface

Until recently, the rapid time scales associated with the formation of an adsorbed surfactant layer at the solid-aqueous interface has prevented accurate investigation of adsorption kinetics. This has led to the mechanism of surfactant adsorption being inferred from thermodynamic data. These explanations have been further hampered by a poor knowledge of the equilibrium adsorbed surfactant morphology, with the structure often misinterpreted as simple monolayers or bilayers, rather than the discrete surface aggregates that are present in many surfactant-substrate systems. This review aims to link accepted equilibrium data with more recent kinetic and structural information in order to describe the adsorption process for ionic surfactants. Traditional equilibrium data, such as adsorption isotherms obtained from depletion approaches, and the most popular methods by which these data are interpreted are examined. This is followed by a description of the evidence for discrete aggregation on the substrate, and the morphology of these aggregates. Information gained using techniques such as atomic force microscopy, fluorescence quenching and neutron reflectivity is then reviewed. With this knowledge, the kinetic data obtained from relatively new techniques with high temporal resolution, such as ellipsometry and optical reflectometry, are examined. On this basis the likely mechanisms of adsorption are proposed.