Null Ellipsometry Research Articles

Structural and kinetic properties of laterally stabilized, oligo(ethylene glycol)-containing alkylthiolates on gold: A modular approach

The formation of highly ordered self-assembled monolayers (SAMs) on gold from an unusually long and linear compound HS(CH(2))(15)CONH(CH(2)CH(2)O)(6)CH(2)CONH(CH(2))(15)CH(3) is investigated by contact angle goniometry, ex situ null ellipsometry, cyclic voltammetry and infrared reflection-absorption spectroscopy. The molecules are found to assemble in an upright position as a complete monolayer within 60 min. The overall structure of the SAM reaches equilibrium within 24 h as evidenced by infrared spectroscopy, although a slight improvement in water contact angles is observed over a period of a few weeks. The resulting SAM is 60 A thick and it displays an advancing water contact angle of 112 degrees and excellent electrochemical blocking characteristics with typical current densities about 20 times lower as compared to those observed for HS(CH(2))(15)CH(3) SAMs. The dominating crystalline phases of the supporting HS(CH(2))(15) and terminal (CH(2))(15)CH(3) alkyl portions, as well as the sealed oligo(ethylene glycol) (OEG) "core," appear as unusually sharp features in the infrared spectra at room temperature. For example, the splitting seen for the CH(3) stretching and CH(2) scissoring peaks is normally only observed for conformationally trapped alkylthiolate SAMs at low temperatures and for highly crystalline polymethylenes. Temperature-programmed infrared spectroscopy in ultrahigh vacuum reveals a significantly improved thermal stability of the SAM under investigation, as compared to two analogous OEG derivatives without the extended alkyl chain. Our study points out the advantages of adopting a "modular approach" in designing novel SAM-forming compounds with precisely positioned in plane stabilizing groups. We demonstrate also the potential of using the above set of compounds in the fabrication of "hydrogel-like" arrays with controlled wetting properties for application in the ever-growing fields of protein and cell analysis, as well as for bioanalytical applications.

Detecting a disturbed layer on a silicon surface by means of monochromatic null ellipsometry

This paper discusses the surfaces of silicon wafers cut from crystals and subjected to grinding and optical and chemical-mechanical polishing. An optical model of the surface, including a layer with an altered refractive index and a layer with induced optical anisotropy, was used to determine the presence of a disturbed layer. Four ellipsometric techniques were used to investigate the surface of the wafers. Two of them determine the refractive-index variation, and the other two determine the presence of induced optical anisotropy.

A new route to the formation of biomimetic phosphate assemblies on gold: Synthesis and characterization

A biomimetic model system based on long-chain alkanethiols tailored with serine, threonine and tyrosine side-chain groups is created as a platform for the study of phosphorylated amino acids. The phosphorylated analogues are synthesized with protective tert-butyl groups that after assembly on thin polycrystalline gold films are removed in an acidic deprotection solution to form the corresponding phosphate self-assembled monolayers (SAMs). The SAMs are thoroughly characterized with null ellipsometry, contact angle goniometry, infrared reflection–absorption spectroscopy and X-ray photoelectron spectroscopy. The assembly and the subsequent deprotection process are optimized with respect to molecular orientation and chain conformation by varying the incubation time and the exposure time to the deprotection solution. The high quality of the generated SAMs suggests that the present assembly/deprotection approach is an attractive alternative when traditional synthetic routes become demanding because of solubility problems.

A novel biochip technology for detection of explosives – TNT: Synthesis, characterisation and application

This contribution describes the synthesis, characterisation and evaluation of a novel biochip technology for the detection of the explosive substance 2,4,6-trinitrotoluene (TNT). Two types of thiols are self-assembled to produce the biochip on gold, namely oligo(ethylene glycol) (OEG)-alkyl thiols terminated with a hydroxyl group and a TNT-analogue (2,4-dinitrobenzene), respectively. Three different TNT-analogues are mixed in various proportions with hydroxyl-terminated OEG-thiols to obtain highly selective and sensitive biochips with a low non-specific binding. The produced self-assembled monolayers (SAMs) are thoroughly characterised with null ellipsometry, contact angle goniometry, infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS) and they all meet high standards in terms of molecular conformation, packing and orientation. The biochip is designed to function as a platform for a competitive label-free immunoassay and two real-time transducers – surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) – are used to monitor the dissociation of on-line immobilised monoclonal antibodies produced against TNT. The three TNT-analogues are all potential candidates for the development of a functional biochip, though one of them displayed superior properties in terms of shorter recovery/stabilisation time after antibody immobilisation and a better response/loading capacity ratio. This is particularly evident when using low antigen (TNT-analogue) content in the mixed SAM.

Mimicking the Properties of Antifreeze Glycoproteins: Synthesis and Characterization of a Model System for Ice Nucleation and Antifreeze Studies

Synthesis of beta-D-Gal-(1 --> 3)-beta-D-GalNAc coupled to HOC2H4NHCOC15H30SH is described. This compound was coadsorbed at various proportions with C2H5OC2H4NHCOC15H30SH to form statistically mixed self-assembled monolayers (SAMs) on gold in an attempt to mimic the properties of the active domain in antifreeze glycoproteins (AFGPs). The monolayers were characterized by null ellipsometry, contact angle goniometry, X-ray photoelectron spectroscopy, and infrared reflection-absorption spectroscopy. The disaccharide compound adsorbed preferentially, and SAMs prepared at a solution molar ratio >0.3 displayed total wetting. The mixed SAMs showed well-organized alkyl chains up to a disaccharide surface fraction of 0.8. The amount of gauche conformers in the alkyls increased rapidly above this point, and the monolayers became disordered and less densely packed. Furthermore, the generated mixed SAMs were subjected to water vapor at constant relative humidity and the subsequent ice crystallization on a cooled substrate was monitored via an optical microscope. Interestingly, rapid crystallization occurred within a narrow range of temperatures on mixed SAMs with a high disaccharide content, surface fraction >0.3. The reported crystallization temperatures and the ice layer topography were compared with results obtained for a much simpler reference system composed of -OH/-CH3 terminated n-alkanethiols in order to account for changes in topography of the water/ice layer with surface energy. Although preliminary, the obtained results can be useful in the search for the molecular mechanism behind the antifreeze activity of AFGPs.

Biosensor discrimination of meat juice from various animals using a lectin panel and ellipsometry

In this work, simple microcontact printed gold-wafers were used to make a lectin panel for investigation and discrimination of different meat juices from fresh meat of cattle, chicken, pig, cod, turkey and lamb. Seven different lectins were thus attached to gold surfaces using the streptavidin–biotin method. Lectins recognize and bind specifically to carbohydrate structures present on different proteins. The biorecognition was evaluated with null ellipsometry and the data obtained was related to an internal standard of lactoferrin. The data was evaluated with multivariate data analysis techniques to identify possible discrimination or grouping of data. Scanning ellipsometry was used for visualization of the binding pattern of the lectins and the meat juice proteins. The two-dimensional images obtained could be used to visualize the protein distribution, furthermore, to exclude anomalies. The results showed that the different meat juices from the six different species: cattle, chicken, pig, cod, turkey and lamb could be discriminated from each other. The results showed to be more repetitive for the mammalian meat juices. Using a simple model based on an artificial neuronal net, it was also possible to classify meat juices from the mammals investigated.

The Initial and Photoinduced 3D Orientational Order in Polymethacrylates with Azobenzene Side Groups

ABSTRACT The combination of the transmission null ellipsometry and total absorption method, earlier approved for several polymer classes, is applied to study 3D orientational configurations of azochromophores in polymethacrylates with azobenzene side groups. The transformation of the initial orientation due to the photoexcitation of azochromophores is investigated. The dependence of the orientational configuration on the structure of azochromophore fragments and its concentration in the polymers are also studied. In the non-irradiated films, azochromofores strive to be aligned normally to the film plate. Under irradiation, if reorientation mechanism of the photoinduced ordering prevails, the azochromophores reorient perpendicularly to the polarization direction of the exciting light, E ex, and in the saturation state they are randomly distributed in the plane perpendicular to E ex. This implies that the induced structure is mainly influenced by the photoordering, while selfordering of polymer chains is not effective or overcome, presumably because of high Tg of polymethacrylates. In case when photoselection ordering mechanism dominates, 3D distribution of azochromophores in the saturation state of irradiation is isotropic due to a strong exhaustion of the number of anisotropic trans isomers. The transient photoinduced orientations are biaxial independently on the dominating ordering mechanism. The reduction of the azochromophore's concentration in polymers reduces the anisotropy rate but not influence the trends described above. The observed regularities were earlier described for several other classes of photosensitive polymers and so they may be common rules for photoordering in the photochromic media.

Equilibrium Aspects of Polycation Adsorption on Silica Surface: How the Adsorbed Layer Responds to Changes in Bulk Solution

Adsorption of cationic high molecular weight polyacrylamides (CPAM) (M(w) is about 800 kDa) with different fractions of cationic units tau = 0.09 and tau = 0.018 onto silica surface was studied over a wide range of pH (4-9) and KCl concentration (c(s) = 10(-3)-10(-1) M) by in-situ null ellipsometry. We discuss how the adsorbed layer depends on the bulk conditions as well as kinetically responds to changes in solution conditions. The adsorbed amount Gamma of CPAM increases with pH for all studied electrolyte concentrations until a plateau Gamma is reached at pH > 6. At low pH we observed an increase in adsorbed amount with electrolyte concentration. At high pH there is no remarkable influence of added salt on the values of the adsorbed amount. The thickness of adsorbed polymer layers, obtained by ellipsometry, increases with electrolyte concentration and decreases with pH. At low c(s) and high pH the polyelectrolyte adsorbs in a flat conformation. An overcompensation of the surface charge (charge reversal) by the adsorbed polyelectrolyte is observed at high c(s) and low pH. To reveal the reversibility of the polyelectrolyte adsorption with respect to the adsorbed amount and layer thickness, parameters such as polyelectrolyte concentration (c(p)), c(s), and pH were changed during the experiment. Generally, similar adsorbed layer properties were obtained independent of whether adsorption was obtained directly to initially bare surface or by changing pH, c(s), or the concentration of polyelectrolyte solution in the presence of a preadsorbed layer, provided that the coverage of the preadsorbed layer was low. Once a steady state of the measured parameters (Gamma, d) was reached, experimental conditions were restored to the original values and corresponding changes in Gamma and adsorbed layer thickness were recorded. For initially low surface coverage it was impossible to restore the layer properties, and in this case we always ended up with higher coverage than the initial values. For initial high surface coverage it was usually possible to restore the initial layer properties. Thus, we concluded that polyelectrolyte appears only partially reversible to changes in the solution conditions due the slow rearrangement process within the adsorbed layer.

Swelling of Poly(methacrylic acid) Brushes: Influence of Monovalent Salts in the Environment

The swelling behavior of poly(methacrylic acid) (PMAA) brushes covalently attached to solid planar surfaces in aqueous solutions containing different monovalent salts was investigated with multiangle null ellipsometry. The polymer chains were grown in situ from the surface by surface-initiated polymerization using a surface-attached initiator (“grafting from” method). The swelling of the PMAA brushes was measured as a function of pH value, concentration of added low molecular weight salt, and the grafting density of the surface-attached polymer brushes. At pH values between 4 and 10, the brush thickness initially increases and then decreases with increasing concentration of added salt; i.e., a maximum of brush thickness appears at a medium salt concentration. The influence of the chemical nature of different monovalent co- and counterions on the swelling behavior of the brushes is studied.

Interaction of Poly(methacrylic acid) Brushes with Metal Ions: Swelling Properties

The swelling behavior of poly(methacrylic acid) (PMAA) brushes in contact with aqueous solutions of sodium, silver, alkaline-earth metal, copper, and aluminum nitrate was studied by multiple angle null ellipsometry. Fundamentally different swelling behavior were found for the interaction of the polyacid brush with metal cations of increasing valency or when going from a main group to a transition metal ion at constant valency. The different cations were classified with respect to the nature of their interaction with the surface-attached poly(carboxylic acid), and the findings were compared to theoretical predictions as well as to experimental investigations of related systems, such as free polyacids in solution or polyacid gels interacting with different metal cations.

Adsorption and Aggregation of Cationic Amphiphilic Polyelectrolytes on Silica

The adsorption of two cationic amphiphilic polyelectrolytes, which are copolymers of two charged monomers, triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride (which is the amphiphilic one) with different contents of amphiphilic groups (40% (40DT) and 80% (80DT)), onto the hydrophilic silica-aqueous solution interface has been studied by in situ null ellipsometry and tapping mode atomic force microscopy (AFM). Adsorption isotherms for both polyelectrolytes were obtained at 25 degrees C and at different ionic strengths, and the adsorption kinetics was also investigated. At low ionic strength, thin adsorbed layers were observed for both polyelectrolytes. The adsorption increases with polymer concentration and reaches, in most cases, a plateau at a concentration below 50 ppm. For the 80DT polymer, at higher ionic strength, an association into aggregates occurs at concentrations at and above 50 ppm. The aggregates were observed directly by AFM at the surface, and by dynamic light scattering in the solution. The adsorption data for this case demonstrated multilayer formation, which correlates well with the increase in viscosity with the ionic strength observed for 80DT.

Highly ordered monodomain ionic self-assembled liquid-crystalline materials

Liquid-crystalline properties of the ionic self assembled complex benzenehexacarboxylic- (didodecyltrimethylammonium)6 [BHC- (C12D)6] were investigated by polarizing microscopy, differential scanning calorimetry (DSC), x-ray analysis, null ellipsometry, UV and IR spectroscopy. The complex exhibits a bilayer smectic Sm- A2 liquid-crystalline phase and aligns spontaneously. Alignment properties do not depend on the hydrophobic or hydrophilic treatment of the surfaces. The aligned complex possesses a negative (delta n=-0.02) homeotropically oriented optical axis, with layers aligned parallel to the surface. X-ray analysis of the aligned sample revealed a lamellar structure with a d spacing of 3.15 nm, consisting of sublayers of thicknesses d1 = 1.41 and d2 = 1.74 nm . This was confirmed by simple geometrical calculations and detailed temperature-dependent investigations, revealing that the first layer contains the BHC molecules and oppositely charged groups of the surfactants, and the second the alkyl tails of the surfactant. Changes in the order parameters (as calculated from the IR investigations) are correlated with the phase transitions as found by DSC. The properties of the complex are strongly influenced by the ionic interactions within the complex. The presence of these groups slows down the dynamics within the material sufficiently to allow for crystallization of the complex from an aligned LC phase into a single crystal domain, as well as restricting the transition to the isotropic phase.

Direct Determination of Effective Interfacial Optical Constants by Nonlinear Optical Null Ellipsometry of Chiral Films

Nonlinear optical null ellipsometry (NONE) measurements of chiral interfaces allowed direct experimental measurement of the linear interfacial optical constants in surface second harmonic generation (SHG) measurements. Since phase information is retained in NONE measurements, the real and imaginary components of the interfacial refractive index (n and k, respectively) were uniquely obtained from the measured chiral chi((2)) tensor elements of a fluorescein-labeled bovine serum albumin film. The sensitivity of the calculated chi((2)) tensor elements on the assumed values of the interfacial optical constants allowed measurements of n and k to four significant figures with no additional adjustable parameters and independent of molecular symmetry. The optical constants measured by SHG agreed within a relative error of 0.8% with values predicted independently using a simple effective medium approximation, also with no adjustable parameters. Additionally, those same optical constants produced relationships between the achiral chi((2)) tensor elements in excellent agreement with predictions for systems exhibiting weak orientational order. This study suggests that the far-field intensity and polarization state of the nonlinear optical beam may be largely independent of the near-field optical constants within the interfacial layer in the limit of a film thickness much less than the wavelength of light.

Adsorption of cationic cellulose derivative/anionic surfactant complexes onto solid surfaces. II. Hydrophobized silica surfaces.

The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behavior of cationic hydroxyethyl cellulose (Polymer JR-400) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at hydrophobized silica has been investigated by null ellipsometry and compared with the previous data for adsorption onto hydrophilic silica surfaces. The adsorbed amount of LM-200 is found to be considerably larger than the adsorbed amount of JR-400 at both surfaces. Both polymers had higher affinity toward hydrophobized silica than to silica. The effect of SDS on polymer adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and addition of SDS to preadsorbed polymer layers. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. For the JR-400/SDS complex, the adsorbed amount on hydrophobized silica started to increase progressively from much lower SDS concentrations, while the adsorbed amount on silica increased sharply only slightly below the phase separation region. For the LM-200/SDS complex, the adsorbed amounts increased progressively from very low SDS concentrations at both surfaces, and no large difference in the adsorption behavior was observed between two surfaces below the phase separation region. The complex desorbed from the surface at high SDS concentrations above the critical micelle concentration. The reversibility of the adsorption of polymer/SDS complexes upon rinsing was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by adding water, the adsorbed amount increased due to the precipitation of the complex. The effect of the rinsing process on the adsorbed layer was determined by the hydrophobicity of the polymer and the surface.

Structural characterization of microcontact printed arrays of hexa(ethylene glycol)-terminated alkanethiols on gold.

This paper reports on the structural characteristics of microcontact printed oligo(ethylene glycol)-terminated alkanethiol layers, HS(CH2)15CONH-(CH2CH2O)6-H (hereafter EG6), on gold. Microwetting, contact angle goniometry, imaging null ellipsometry, and infrared reflection-absorption spectroscopy (IRAS) are used to characterize the printed EG6 layers, and the quality of these layers in terms of layer thickness and the crystallinity of the alkyl and ethylene glycol portions is compared with data obtained from analogous layers prepared by solution self-assembly. The outcome of the printing process is critically dependent on the experimental parameters used to prepare the patterns. It is found that high quality layers, consisting of densely packed all-trans alkyl chains terminated with relatively helical hexa(ethylene glycol) tails, are formed by inking the poly(dimethylsiloxane) (PDMS) stamp with a 1 mM EG6 solution and contacting it with gold for 15 min. The homogeneity of printed layers is not as good as the homogeneity of those prepared from solution under similar conditions, most likely because of simultaneous transfer of low molecular weight residues from the PDMS stamp. These residues, however, can be easily removed upon ultrasonication in ethanol without affecting the quality of the printed layer. Further on, the microscopic square-shaped bare gold patterns formed after microcontact printing (muCP) are subsequently filled with 16-hexadecanoic acid (hereafter THA) or HS(CH2)15CONH-(CH2CH2O)6-COOH (hereafter EG6COOH) to provide a microarray platform for further covalent attachment of biomolecules. Well-defined structures in terms of wettability contrast, sharpness, and height differences between the printed and back-filled areas are confirmed by imaging null ellipsometry and microscopic wetting.

Reversible Hydrophobic Barriers Introduced by Microcontact Printing: Application to Protein Microarrays

Microcontact printing (µCP) has been used to introduce temporary hydrophobic barriers on carboxymethylated dextran (CMD) hydrogels on gold. Among the investigated types of inks, tetraoctadecylammonium bromide (TOAB), electrostatically bound to the CMD layer, provided the most well-defined features both with respect to pattern-definition and reversibility upon exposure to a regeneration solution. The printed patterns were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), microscopic wetting and imaging null ellipsometry to explore the influence of concentration of ink solution and contact time on the appearance of the printed layer. AFM revealed that the printed TOAB molecules aggregated into clusters rather than into a homogeneous mono- or multilayer on the CMD hydrogel. It was also observed that printed areas of TOAB that are larger than 25?µm are inhomogeneous most likely because of an edge transfer lithography (ETL) mechanism. A protein model system based on Protein A-rabbit antimouse Fc ? was used to evaluate the potential of the patterned surface as a protein microarray chip by means of surface plasmon microscopy (SPM). Moreover, non-specific adsorption of several proteins onto TOAB barriers was also studied using surface plasmon resonance (SPR), and it is evident that undesired adsorption can be eliminated by removing barriers after ligand immobilization, but prior to analyte exposure, by treating the patterned surface with a simple salt regeneration solution. © Springer-Verlag/Wien 2004.

Electrolyte-induced collapse of a polyelectrolyte brush.

We have investigated the electrolyte-induced collapse of a polyelectrolyte brush covalently attached to a planar solid surface. Positively charged poly-4-vinyl [N-methyl-pyridinium] (MePVP) brushes were prepared in situ at the surface by free radical chain polymerization using a surface-immobilized initiator monolayer ("grafting from" technique) and 4-vinylpyridine as the monomer, followed by a polymer-analogous quaternization reaction. The height of the brushes was measured as a function of the external salt concentration via multiple-angle null ellipsometry. As predicted by mean-field theory, the height of the MePVP brushes remains unaffected by the addition of low amounts of external salt. At higher salt concentrations the brush height decreases. The extent to which the brush shrinks strongly depends on the nature of the salt present in the environment. MePVP brushes collapse to almost the dry layer thickness upon the addition of potassium iodide to a contacting aqueous medium. In contrast, the collapse of MePVP brushes having bromide or chloride counterions is much less pronounced. These brushes remain in a highly swollen state even after large amounts of salt have been added to the solution.

Molecular Structure of Azopolymers and Photoinduced 3D Orientational Order. 1. Azobenzene Polyesters

The combination of transmission null ellipsometry (TNE) and attenuated total reflection (ATR) methods supported by absorption measurements is shown to be an effective tool to study spontaneous and photoinduced 3D order in azopolymers. We investigated a series of azobenzene containing side-chain polyesters differing by the length of the main-chain spacer (CH2)m (m = 2, 8, 9, 10, 12, 13, 14, 16) and the tail substitutes (NO2, OCH3, and OC4H9) in azochromophore. The 3D order was induced by monochromatic polarized light of several wavelengths strictly distinguished by absorption efficiency of azochromophores. The orientational order under irradiation and after irradiation is studied. A big variety of 3D orientations (biaxial, uniaxial, and isotropic) is realized. The uniaxial and isotropic configurations correspond to the initial state and the saturation state of irradiation, while biaxiality is an attribute of the transient orientations. It is shown that both initial and photoinduced 3D orders are strongly d...

Tailoring the Charge Density of Surface-Attached Polyelectrolyte Brushes

The preparation and swelling behavior of surface-attached charged statistical copolymer brushes are described. The brushes are prepared by free radical chain polymerization of monomer mixtures of 4-vinylpyridine (VP) and dimethylacrylamide (DMAA) using a surface bound azo-initiator (surface-initiated polymerization). So-prepared neutral PVP-stat-PDMAA copolymer brushes are then transferred into polyelectrolyte brushes through a polymer-analogous quaternization reaction of the respective PVP segments. By adjusting the ratio of the monomers added to the polymerization feed, the concentration of the corresponding polymer segments generated in the final surface-attached layer can be controlled, and thus, the charge density of the resulting polyelectrolyte copolymer brush can be determined. Finally, the swelling behavior of such polyelectrolyte brushes in aqueous solutions containing low molecular weight salts is investigated by using multiple-angle null ellipsometry. Depending on the charge density of the cop...

Adsorption of Cationic Cellulose Derivatives/Anionic Surfactant Complexes onto Solid Surfaces. I. Silica Surfaces

The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behaviors of cationic hydroxyethyl celluloses (Polymer JR-125, JR-400, and JR-30M) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at silica surfaces in the presence of a 10 mM NaCl solution has been investigated by null ellipsometry. The adsorbed amount of LM-200 is found to be considerably larger than adsorbed amounts of other polymers. The rate of adsorption for the LM-200 is also lower than that for the Polymer JR series under comparable conditions. Electrostatic interaction is found to be the major driving force for the adsorption. The effect of SDS on adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and additions of SDS to the preadsorbed polymer layers. In all cases, associative binding of the surfactant to the polymer seems to control interfacial behavior, which depends on the surfactant concentration. Maximum adsorption was obtained at the surfactant concentration below the phase separation region, and the complex desorbed from the surface at a high SDS concentration above the critical micelle concentration. The reversibility of the polymer/SDS complexes adsorption upon rinsing with NaCl solutions was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by salt solutions, the adsorbed amount increased sharply with precipitation of the complex. A larger amount of the JR-400/SDS complex precipitated during the rinsing process seemed to adsorb to a larger extent to silica than the corresponding LM-200/SDS complex.