Characterization of the adsorbed layer of a mixture of nonionic and cationic hydrocarbon surfactants on silica

The adsorption of hexadecyl trimethylammonium bromide (HDTAB) and nonylphenyl polyglycol ether (NP20) from mixed aqueous solutions on silica has been investigated. The adsorption from mixed solutions is enhanced at low concentration, but is decreased at high concentration compared with those of the single surfactants. Further, measurements of fluorescence spectra of pyrene-3-carboxaldehyde and of ESR spectra of 2,2,6,6-tetramethylpiperidinyl-1-oxy in the surfactant adsorbed layer indicate that the micropolarity of the adsorbed layer is affected by feed mole fraction of HDTAB, in particular over the range of 0.3–0.7, while the microviscosity of the adsorbed layer is almost constant in the same range at a high feed concentration. This suggests that over the same range, the ratio of HDTAB/NP20 at the second layer is almost the same.     

Interactions between nonpolar surfaces in mixed solutions of cationic and nonionic surfactants

The interaction energy between hydrophobic SiO₂ particles in aqueous solutions of a cationic surfactant (dodecylpyridinium bromide, DDPB), a nonionic surfactant (Triton X-100, TX-100), and their mixed solutions was measured as a function of concentration. Synergism has been observed in mixed surfactant solutions: the surfactant concentration required for achieving the set interaction energy in the mixed solutions was lower than in the solutions of the individual surfactants. The molecular interaction parameters in surfactant mixtures were calculated using the Rosen model. Chain-chain interactions between nonionic and cationic surfactants were suggested as the main reason for the synergism.     

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.     

Interactions between adsorbed layers of cationic gemini surfactants

The forces acting between glass and between mica surfaces in the presence of two cationic gemini surfactants, 1,4 diDDAB (1,4-butyl-bis(dimethyldodecylammonium bromide)) and 1,12 diDDAB (1,12-dodecyl-bis(dimethyldodecylammonium bromide)), have been investigated below the critical micelle concentration (cmc) of the surfactants using two different surface force techniques. In both cases, it was found that a recharging of the surfaces occurred at a surfactant concentration of about 0.1 x cmc, and at all surfactant concentrations investigated repulsive double-layer forces dominated the interaction at large separations. At smaller separations, attractive forces, or regions of separation with (close to) constant force, were observed. This was interpreted as being due to desorption and rearrangement in the adsorbed layer induced by the proximity of a second surface. Analysis of the decay length of the repulsive double-layer force showed that the majority of the gemini surfactants were fully dissociated. However, the degree of ion pair formation, between a gemini surfactant and a bromide counterion, increased with increasing surfactant concentration and was larger for the gemini surfactant with a shorter spacer length.     

Forces between hydrophilic surfaces adsorbed with apolipoprotein AII alpha helices

To provide better understanding of how a protein secondary structure affects protein-protein and protein-surface interactions, forces between amphiphilic alpha-helical proteins (human apolipoprotein AII) adsorbed on a hydrophilic surface (mica) were measured using an interferometric surface force apparatus (SFA). Forces between surfaces with adsorbed layers of this protein are mainly composed of electrostatic double layer forces at large surface distances and of steric repulsive forces at small distances. We suggest that the amphiphilicity of the alpha-helix structure facilitates the formation of protein multilayers next to the mica surfaces. We found that protein-surface interaction is stronger than protein-protein interaction, probably due to the high negative charge density of the mica surface and the high positive charge of the protein at our experimental conditions. Ellipsometry was used to follow the adsorption kinetics of this protein on hydrophilic silica, and we observed that the adsorption rate is not only controlled by diffusion, but rather by the protein-surface interaction. Our results for dimeric apolipoprotein AII are similar to those we have reported for the monomeric apolipoprotein CI, which has a similar secondary structure but a different peptide sequence and net charge. Therefore, the observed force curves seem to be a consequence of the particular features of the amphiphilic alpha-helices.     

Ion chromatographic determination of hydroxide ion on monolithic reversed-phase silica gel columns coated with nonionic and cationic surfactants

The determination of hydroxide by ion chromatography (IC) is demonstrated using a monolithic octadecylsilyl (ODS)-silica gel column coated first with a nonionic surfactant (polyoxyethylene (POE)) and then with a cationic surfactant (cetyltrimethylammonium bromide (CTAB)). This stationary phase, when used in conjunction with a 10 mmol/l sodium sulfate eluent at pH 8.2, was found to be suitable for the rapid and efficient separation of hydroxide from some other anions, based on a conventional ion-exchange mechanism. The peak directions and detection responses for these ions were in agreement with their known limiting equivalent ionic conductance values. Under these conditions, a linear calibration plot was obtained for hydroxide ion over the range 16 micromol/l to 15 mmol/l, and the detection limit determined at a signal-to-noise ratio of 3 was 6.4 micromol/l. The double-coated stationary phase described above was shown to be superior to a single coating of cetyltrimethylammonium bromide alone, in terms of separation efficiency and stability of the stationary phase. A range of samples comprising solutions of some strong and weak bases was analyzed by the proposed method and the results obtained were in good agreement with those obtained by conventional potentiometric pH measurement.     

Simultaneous determination of cationic surfactants and nonionic surfactants by ion-association titration.

A simultaneous determination of cationic and nonionic surfactants has been developed using ion-association titration. Tetrabromophenolphthalein ethyl ester (TBPE) was used as an indicator. Benzalkonium reacted with TBPE to form a blue ion-associate in the organic phase. When tetrakis(4-fluorophenyl)borate was added dropwise to the solution, the color of the organic phase turned to yellow at the equivalence point. In addition, when a large amount of potassium ion was added to a solution including Triton X-100, Triton X-100 could be determined by the same technique as described above because of formation of the K+-Triton X-100 cation. The proposed method is available for the stepwise determination of cationic and nonionic surfactants in mixtures.     

The displacement of adsorbed polymer from silica surfaces by the addition of a nonionic surfactant

The adsorption of polyvinyl alcohol and Synperonic NP8 (nonyl phenol ethoxylate with an average of eight ethylene oxide groups per molecule) on fumed silica has been studied at various pH values. This was followed by an investigation of the competitive adsorption of NP8 and PVA. It was shown that NP8 can displace the polymer from the silica. This was attributed to a higher adsorption energy for the NP8 molecule compared with the value of the individual adsorbed PVA segments. Sediment volume experiments showed that the addition of NP8 to a colloidally stable silica dispersion with adsorbed PVA can induce flocculation as a result of displacement of some or all of the PVA chains from the surface. Initially the adsorption of the NP8 molecules caused an increase in the hydrophobic interaction (resulting in the flocculation) between the alkyl phenol groups which are oriented towards the bulk solution (since the PEO chains preferentially adsorb on the silica surface). Restabilisation at higher NP8 concentrations occur through the formation of bilayers with the PEO chains now dangling in solution.     

Ion chromatography on reversed-phase materials coated with mixed cationic and nonionic surfactants

Columns suitable for use in anion chromatography can be prepared by coating a packed reversed-phase HPLC column (C18 silica or polystyrene particles) with a cationic surfactant. The efficiency is improved dramatically by first coating the column with a nonionic surfactant and then subsequently with the cationic surfactant. The thickness of the first coated layer as well as the chemical structure of the surfactant have a major effect on the column performance. Actual separations are included to demonstrate the convenience and practical use of the coated columns. Using this approach, columns with 12,900 theoretical plates for the 15-cm column (or 86,000 plates/m) were produced, giving well shaped peaks with an average asymmetry factor of 1.09. The coated layers were found to be stable, giving retention times with an average relative standard deviation of 1.6% for 12 consecutive runs.     

Interaction of sodium polyacrylate adsorbed on TiO2 with cationic and anionic surfactants

Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was used to identify the structures formed during the adsorption of sodium polyacrylate (NaPA) on charged TiO2 particles and to determine the subsequent interaction of the adsorbed polymer structure with cationic and anionic surfactants. The nature of the polymer structure was deduced from the adsorbed amount in tandem with the information obtained from monitoring the change in the relative intensity of the COO- and COOH infrared bands. In particular, it is found that the relative number of COO- and COOH groups on the polymer backbone for the adsorbed state differs from that of the same polymer in solution. This difference is due to a shift in the population of COO-/COOH groups on the polymer backbone that arises when the COO- groups bind to positively charged sites on the surface. A change in the number COO-/COOH groups on the polymer is thus related to a change in the bound fraction of polymer. It is shown that the initial NaPA approaching the bare surface adopts a flat conformation with high bound fraction. Once the bare sites on the surface are covered, the accommodation of additional polymer on the surface requires the existing adsorbed layer to adopt a conformation with a lower bound fraction. When the adsorbed NaPA is probed with a solution containing the anionic surfactant sodium dodecyl sulfate (SDS), the SDS competes for surface sites and displaces some of the bound NaPA segments from the surface, giving rise to an polymer layer adsorbed with an even lower bound fraction. In contrast, addition of a solution containing the cationic surfactant cetyltrimethylammonium bromide (CTAB) results in the binding of the surfactant directly to the free COO- sites on the adsorbed polymer backbone. Confirmation of a direct interaction of the CTAB headgroup with the free COO- groups of the polymer is provided by intensity changes in the headgroup IR bands of the CTAB.     

Wetting effect of aqueous binary mixed solutions of cationic and nonionic surfactants

Wetting of low-energy solid surfaces (polymers, hydrophobized glass) with aqueous solutions of binary mixtures of cationic and nonionic surfactants was investigated at molar fractions of the cationic surfactant of 0.2, 0.5, and 0.8. In a narrow concentration range, the non-additive effect of wetting was observed: wetting of the solid surfaces with solutions of the mixtures is better than that would be expected from the additive behavior of the components. The magnitude of the effect depends on the surface energy of the solid substrate, total surfactant concentration in a mixture, and molar fraction of the cationic component. The wetting effect of surfactant mixtures with respect to low-energy solid surfaces can be predicted using the surface tension isotherms.     

Multiple-stimulus-responsive hydrogels of cationic surfactants and azoic salt mixtures

New hydrogels having high water content, ～96 wt%, composed of cationic surfactants, alkyltrimethylammonium bromides (C _n TAB, n?=?12, 14, 16, and 18), and a small dye molecule, sodium azobzenzene 4,4′-dicarboxylic acid (AzoNa₂), was firstly obtained. The three-dimensional network structures of hydrogels were determined by transmission electron microscopy images, scanning electron microscopy images, ¹H nuclear magnetic resonance, and small-angle X-ray scattering measurements. The mechanism of hydrogel formation was also illustrated. The rheological data were obtained to investigate the mechanical strength of hydrogels, which were turned out to be strong mechanical strength (～10⁴ Pa) materials. We found that the strength of the hydrogel depends on the fiber density, which can be controlled by changing the proportion of the two compounds, concentration of surfactants, temperature, and the chain length of the surfactant. Interestingly, the hydrogels were found to have a multiple-stimulus response property. A reversible thermal, UV–vis, or a chemical response was investigated in the mixtures of cationic surfactants and azoic salt for the first time. These findings may find potential applications such as sensors, actuators, shape memories, and drug delivery systems, etc.

Role of cationic and nonionic surfactants on biocidal efficiency in diesel-water interface

Biodegradation occurs at the interface between diesel and water. The microbial contamination can result in inhibitor/fuel degradation that leads to the unacceptable level of turbidity, filter plugging, corrosion of storage tanks, pipeline and souring of stored products. Hence, selection of biocides/inhibitors is an important aspect in petroleum product transporting pipeline. Three biocides (cationic and nonionic) were employed to study the biodegradation of diesel in diesel–water interface. The biocidal efficiency on biodegradation of diesel was examined using Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and gas chromatography mass spectrometry (GC–MS). Polyoxyethyleneglycol dodecyl ether [BRIJ-35] and polyethylene glycol-p-isooctylphenyl ether [TRITON-X-100] had higher bactericidal efficiency than Dodecyl ethyl dimethyl ammonium bromide [DDAB]. But the cationic biocide (DDAB) gave good biocidal efficiency at the interface. The data are explained in terms of a model that postulates the formation of “micelle” at the diesel–water interface.     

Forces and friction between hydrophilic and hydrophobic surfaces: influence of oleate species

The atomic force microscope has been used to investigate normal surface forces and lateral friction forces at different concentrations of sodium oleate, a frequently used fatty acid in the deinking process. The measurements have been performed using the colloidal probe technique with bead materials consisting of cellulose and silica. Cellulose was used together with a printing ink alkyd resin and mica, whereas silica was used with a hydrophobized silica wafer. The cellulose-alkyd resin system showed stronger double layer repulsion and the friction was reduced with increasing surfactant concentration. The adhesive interaction disappeared immediately on addition of sodium oleate. The normal surface forces for cellulose-mica indicated no apparent adsorption of the sodium oleate however, the friction coefficient increased on addition of sodium oleate, which we ascribe to some limited adsorption increasing the effective surface roughness. The silica-hydrophobic silica system showed a completely different surface force behavior at the different concentrations. An attractive hydrophobic interaction was evident since the surfaces jumped into adhesive contact at a longer distance than the van der Waals forces would predict. The strong adhesion was reflected in the friction forces as a nonlinear relationship between load and friction and a large friction response at zero applied load. Indirect evidence of adsorption to the hydrophilic silica surface was also observed in this case, and QCM studies were performed to confirm the adsorption of material to both surfaces.     

Surface and solution behavior of the mixed dialkyl chain cationic and nonionic surfactants

The surface and solution behavior of the mixed dialkyl chain cationic and nonionic surfactant mixture of dihexadecyldimethylammonium bromide, DHDAB, and hexaethylene monododecyl ether, C12E6, has been investigated, using primarily the scattering techniques of small-angle neutron scattering and neutron reflectivity. Within the time scale of the measurements, the adsorption of the pure component C12E6 at the air-solution interface shows no time dependence. In contrast, the adsorption of the DHDAB/C12E6 mixture and pure DHDAB has a pronounced time dependence. The characteristic time for adsorption varies with surfactant concentration, composition, and temperature. It is approximately 2-3 h for the DHDAB/C12E6 mixture, dependent upon concentration and composition, and approximately 50 min for DHDAB. At the air-solution interface, the equilibrium composition of the adsorbed layer shows a marked departure from ideal mixing, which is dependent upon both the solution concentration and the concentration of added electrolyte. In contrast, the composition of the aggregates in the bulk solution that are in equilibrium with the surface is close to ideal mixing, as expected for solution concentrations well in excess of the critical micellar concentration. The structure of the mixed adsorbed layer has been measured and compared with the structure of the equivalent pure surfactant monolayer, and no substantial changes in structure or conformation are observed. The extreme departure from ideal mixing in the adsorption behavior of the DHDAB/C12E6 mixture is discussed in the context of the structure of the adsorbed layer, changes in the underlying solution structures, and the failure of regular solution theory to predict such behavior.     

The effect of surfactants on adsorbed layers of a cationic polyelectrolyte

A comparative study of the influence of anionic (sodium dodecyl sulfate, SDS), cationic (tetradecyltrimethylammonium bromide, TTAB) and non-ionic (penta-ethyleneglycol mono n-dodecyl ether, C₁₂E₅) surfactants on the structure and composition of adsorbed layers of cationic hydrophobically modified hydroxyethylcellulose (Quatrisoft LM 200) on hydrophilic surfaces (mica and silica) was carried out using surface force apparatus andin situ null ellipsometry. It is shown that a complex interplay of electrostatic, hydrophobic, and steric effect govern polymer/surfactant/surface interactions and that the effect of surfactant addition strongly depends on its nature and concentration.Both anionic and non-ionic surfactants exhibit aggregation on the polymer hydrophobes. SDS has the most profound influence on Quatrisoft interfacial behavior due to the changes in electrostatics accompanying formation of the polymer/surfactant complex. In the case of C₁₂E₅, large surfactant clusters bound to the polymer affect the macromolecules' conformation in the adsorbed layer via steric effects. In contrast to SDS and C₁₂E₅, no evidence of interaction between the polycation and a like-charged surfactant, TTAB, was obtained. At the same time, TTAB adsorbs on the surface in competition with the polyelectrolyte. This results in partial displacement of the latter and its looser attachment to the surface.     

Orientation of nonionic surfactants on solid surfaces: n-alkyl polyglycol ethers on montmorillonite

Summary Complexes of nonionic surfactantsR-(OCH₂CH₂)x OH with montmorillonite have been studied (R =n-hexadecyl,n-octadecyl and oleyl ;x=2, 10 and 20).On internal surfaces the surfactant molecules are arranged in bilayers. Withx=2 the alkyl chain and about one half of the polar group, (-R OCH₂CH₂O-) stand perpendicularly whereas the terminal —CH₂CH₂OH group is attached to the silicate surface (₁-phase). The bilayer thickness decreases stepwise with rising temperature due to the formation of kinks (i-phases). At higher temperature (52 °C withR=C₁₈H₃₇-, 43°C withR=n-C₁₆H₃₃-, and 12 °C withR = oleyl-) surfactant molecules are squeezed out of the interlayer space reversibly, the packing density decreases, the whole polar head group gets attached to the silicate surface and the alkyl chains rearrange into a gauche-block structure. This structure undergoes further structural changes at still higher temperatures ( _i -phases).The complexes withx =10 and 20 form -phases even at room temperature. These -phases take up long chain alkanol molecules under formation of -structures which rearrange at higher temperatures into -phases. Long-chain impurities of the surfactants can be intercalated in a similar way.Previous data indicating mono- or bilayers of flatly lying surfactant molecules refer to metastable phases due to steric hindrances of lattice expansion.It is proposed that the surfactant molecules build up similar films on the external surfaces, which can adopt - or -structures depending upon number of ethylene oxide groups and temperature. The films of hexadecyl polyglycol ethers for instance are about 27 Å thick in the -phases and about 17 Å in the a-phases.

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Zusammenfassung Es wurden Montmorillonitkomplexe mit nichtionogenen TensidenR(-OCH₂CH₂) _x ,OH (R =n-hexadecyl-,n-octadecyl- und oleyl-;x = 2, 10 and 20) hergestellt. Die Tensidmoleküle bilden zwischen den Silicatschichten bimolekulare Filme.Mitx=2 sind die hydrophoben Reste und die anschließende -OCH₂CH₂-Äthergruppe bei niedriger Temperatur gestreckt und senkrecht zu den Silicatschichten orientiert ( _i -Phase); nur die endständige HOCH₂CH₂-Gruppe sitzt direkt auf der Silicatschicht auf. Beim Erwärmen erniedrigt sich die Dicke der bimolekularen Tensidschicht stufenweise durch den Einbau von Kinken ( _i -Phasen). Bei höheren Temperaturen (52°C mitR =C₁₈H₃₇-, 43 °C mitR =C₁₆H_33- und 12 °C mitR = oleyl) werden Tensidmoleküle reversibel aus den Schichtzwischenräumen verdrängt, die Packungsdichte erniedrigt sich, die gesamte polare Gruppe kommt in direkten Kontakt mit der Silicatschicht und die Alkylketten ordnen sich in eine Gaucheblockstruktur ( _i -Phase). Diese kann bei noch höheren Temperaturen weitere Phasenumwandlungen erleiden.Mitx =10 undx = 20 werden auch bei Zimmertemperatur nur -Phasen gebildet. Diese -Phasen können zusätzlich langkettige Alkanolmoleküle aufnehmen und ternäre Komplexe mit -Struktur bilden, die sich beim Erhitzen reversibel in -Formen umwandeln. Langkettige polare Verunreinigungen in den Tensiden wirken ähnlich wie die Alkanolmoleküle.An den äußeren Oberflächen werden die Tensidmoleküle gleichartige Filme mit - oder -Struktur bilden, je nach der Zahl der -CH₂CH₂O-Gruppen und der Temperatur. Ein Film aus Hexadecylpolyglykoläthern wird etwa 27 Å dick sein in der -Phase und etwa 17 Å in der -Form.

     

Spectrophotometric determination of cationic and anionic surfactants with anionic dyes in the presence of nonionic surfactants,part I: A general aspect

The behavior of color development of anionic azo dyes, methyl orange and its analogues, was examined in aqueous media by changing the microenvironment of the dyes. The addition of alcohols, organic onium ions, anionic surfactants and nonionic surfactants brought about a decrease of the band at wavelengths near 480 nm and an increase of the band at wavelengths near 420 nm. Such a shift toward the shorter wavelengths in spectra was attributed to the change of the micro-environment around the dyes from a polar field to a less polar field; that is, the shift is caused by the change of the contribution of the following resonance forms, On the basis of the color change phenomena, the spectrophotometric methods for the determination of organic onium ions and anionic surfactants were proposed.     

Aqueous films on silica in the presence of cationic surfactants

Summary The existence of thick, stable water films on silica as described byDerjaguin and Kussakov (1939) has been confirmed in several laboratories. There is much evidence showing that their thicknesses can be substantially accounted for by electrical double layer theory. Such wetting films can be influenced by a third component. For example, they are thinned in the presence of inorganic electrolytes or rendered unstable (or metastable) by prior methylation of the silica surface.Here, we consider the influence of alkyltrimethyl ammonium bromides on the stability of aqueous films on polished fused silica. In particular, the thickness and stability of aqueous films of C₅, C₈, C₁₀ and C₁₆ trimethyl ammonium bromides were studied over a range of surfactant concentrations. Contact angles of the TAB solutions on silica were also measured.The concentration of each cationic surfactant proved important in determining film stability. In dilute solution (well below the bulk CMC) uniform films could be formed only below a critical concentration which varied with the chainlength of the surfactant. The longer the chainlength, the lower was the critical concentration. In this regime, the film thickness decreased significantly with increasing surfactant concentration. This decrease was greater than that found for a simple 1:1 electrolyte such as KCl at the same concentration. As the surfactant concentration increased towards the critical value, the uniform films became increasingly more prone to rupture.Once the critical concentration of each surfactant was exceeded, uniform wetting films on silica could not be formed. The films ruptured immediately regardless of their initial diameter. In this regime, the contact angles, measured through the aqueous phase, increased with TAB concentration up to a maximum value which occurred near the CMC.For C₈, C₁₀ and C₁₆ trimethyl ammonium bromides, wetting films were again formed at concentrations above the CMC. At these concentrations, films were studied in detail only for the C_l6 surfactant. In this case, the wetting films were found to be somewhat thicker than free aqueous films of anionic surfactants formed at similar concentrations and hydrostatic pressures.

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Zusammenfassung Die Existenz dicker stabiler Wasserfrlme auf SiO₂ (Derjaguin undKussakov (1939)) ist in verschiedenen Laboratorien bestätigt worden. Es gibt zahlreiche Hinweise, nach denen die Dicke durch die elektrische Doppelschichttheorie gedeutet werden kann. Solche benetzenden Filme können durch eine dritte Komponente beeinflußt werden; in Gegenwart anorganischer Elektrolyte werden sie dünner, durch vorhergehende Methylierung der SiO₂-Oberfläche instabil oder metastabil.In der Arbeit wird der Einfluß von Alkyltrimethylammoniumbromiden (C₅, C₈, C₁₀ und C₁₆ auf die Stabilität von Wasserfilmen auf polierten Quarzglasoberflächen untersucht. Die Konzentration der kationischen Tenside auf die Filmstabilität erwies sich als wichtig. In verdünnten Lösungen bilden sich einheitliche Filme nur unter einer kritischen Konzentration, die von der Kettenlänge des Tensids abhängt. Über der kritischen Konzentration bilden sich keine einheitlich benetzenden Filme; sie reißen sofort unabhängig von ihrem ursprünglichen Durchmesser. Es wird versucht, die Ergebnisse auf der Basis einer Doppelschichttheorie zu deuten.

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With 7 figures and 2 tables