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.

     

Vapour adsorption and contact angles on hydrophobic solid surfaces

To test the effects of vapour adsorption on contact angle measurements, contact angles of high-vapour-pressure liquids and low-vapour-pressure liquids on a hydrophobic solid surface (FC721) were measured by using the axisymmetric drop shape analysis-profile (ADSA-P) technique. Details of the surface preparation and the experiments are presented. By plotting the experimental data in terms of _1v cos vs. _1v, this study shows that the vapour adsorption on a fluorocarbon surface, FC721, is negligible.     

Quantifying the hydrophobic effect. 2. A computer simulation-molecular-thermodynamic model for the micellization of nonionic surfactants in aqueous solution

In this article, the validity and accuracy of the CS-MT model is evaluated by using it to model the micellization behavior of seven nonionic surfactants in aqueous solution. Detailed information about the changes in hydration that occur upon the self-assembly of the surfactants into micelles was obtained through molecular dynamics simulation and subsequently used to compute the hydrophobic driving force for micelle formation. This information has also been used to test, for the first time, approximations made in traditional molecular-thermodynamic modeling. In the CS-MT model, two separate free-energy contributions to the hydrophobic driving force are computed. The first contribution, gdehydr, is the free-energy change associated with the dehydration of each surfactant group upon micelle formation. The second contribution, ghydr, is the change in the hydration free energy of each surfactant group upon micelle formation. To enable the straightforward estimation of gdehydr and ghydr in the case of nonionic surfactants, a number of simplifying approximations were made. Although the CS-MT model can be used to predict a variety of micellar solution properties including the micelle shape, size, and composition, the critical micelle concentration (CMC) was selected for prediction and comparison with experimental CMC data because it depends exponentially on the free energy of micelle formation, and as such, it provides a stringent quantitative test with which to evaluate the predictive accuracy of the CS-MT model. Reasonable agreement between the CMCs predicted by the CS-MT model and the experimental CMCs was obtained for octyl glucoside (OG), dodecyl maltoside (DM), octyl sulfinyl ethanol (OSE), decyl methyl sulfoxide (C10SO), decyl dimethyl phosphine oxide (C10PO), and decanoyl-n-methylglucamide (MEGA-10). For five of these surfactants, the CMCs predicted using the CS-MT model were closer to the experimental CMCs than the CMCs predicted using the traditional molecular-thermodynamic (MT) model. In addition, CMCs predicted for mixtures of C10PO and C10SO using the CS-MT model were significantly closer to the experimental CMCs than those predicted using the traditional MT model. For dodecyl octa(ethylene oxide) (C12E8), the CMC predicted by the CS-MT model was not in good agreement with the experimental CMC and with the CMC predicted by the traditional MT model, because the simplifying approximations made to estimate gdehydr and ghydr in this case were not sufficiently accurate. Consequently, we recommend that these simplifying approximations only be used for nonionic surfactants possessing relatively small, non-polymeric heads. For MEGA-10, which is the most structurally complex of the seven nonionic surfactants modeled, the CMC predicted by the CS-MT model (6.55 mM) was found to be in much closer agreement with the experimental CMC (5 mM) than the CMC predicted by the traditional MT model (43.3 mM). Our results suggest that, for complex, small-head nonionic surfactants where it is difficult to accurately quantify the hydrophobic driving force for micelle formation using the traditional MT modeling approach, the CS-MT model is capable of making reasonable predictions of aqueous micellization behavior.     

Computer simulation of adsorption kinetics of surfactants on solid surfaces

Adsorption kinetics of surfactants on solid surfaces has been studied by using computer simulation. Both bulk surfactant concentration and diffusion region are explicitly integrated in our model. Depending on the head-surface interaction, our simulation results indicate that there exist two different kinetic modes in adsorption process of surfactants on solid surfaces. One is the four-regime mode and the other is step-wise mode. For the strongly attractive head-surface interaction, four distinct regimes of surfactant adsorption are found: a diffusion-controlled regime, a self-assembly controlled regime, an intermediate coverage regime and a saturated regime. In particular, the adsorption in second regime displays a power-law time dependence with an exponent unrelated to bulk concentrations and diffusion coefficients. While for the weaker adsorption surfaces, the step-wise mode is found. The mode includes a low-coverage nucleation regime and the saturated regime after a sudden aggregation of surfactants on the substrates which occurs stochastically. Besides the head-surface interaction, in this work, the effects of surfactant diffusivity, bulk concentration, the length of diffusion zone and surfactant architecture on the adsorption kinetics are also considered. The simulated adsorption kinetics is compared qualitatively with experimental results.     

The self-assembly of lipophilic guanosine derivatives in solution and on solid surfaces

The self-assembly of lipophilic deoxyguanosine derivatives 1 and 2 has been studied in solution by NMR spectroscopy and ESI-MS (electrospray ionization mass spectrometry). NMR data show the existence of two types of self-assembled, ribbonlike structures (A and B), which are connected at the guanine moieties through two different H-bonded networks. The first species (A), which is stable in the solid state and characterised by cyclic NH(2)-O(6) and NH(1)-N(7) hydrogen bonds, is detected soon after dissolving the polycrystalline powder in rigorously anhydrous CDCl3. In solution it slowly undergoes a structural transition towards a thermodynamically stable ribbon characterised by NH(1)-O(6) and NH(2)-N(3) cyclic hydrogen bonds (B). On the other hand, at surfaces, self-assembled ribbon nanostructures have been grown from solutions of derivative 1 both on mica and at the graphite-solution interface. They have been investigated by means of tapping mode scanning force microscopy (SFM) and scanning tunnelling microscopy (STM), respectively. SFM revealed dry, micrometer-long nanoribbons with a molecular cross-section. while STM imaging at submolecular resolution indicates a molecular packing of type A, like the one detected in the solid state. This indicates that, upon adsorption at the solid-liquid interface, the guanosine moieties undergo a structural rearrangement from a B-type to an A-type ribbon.     

Preparation and dilute solution properties of model gemini nonionic surfactants

Dimeric poly(ethylene oxide) surfactants (or nonionic gemini surfactants) with the structure (Cn-2H2n-3CHCH2O(CH2CH2O)mH)2(CH2)6 (or GemnEm), where n is the alkyl length and m is the average number of ethylene oxides per head group, were synthesized. Surfactants were synthesized with alkyl chain lengths n = 12, 14, and 20 and m = 5, 10, 15, 20, and 30. Water solubilities and cloud temperatures at 1 wt% were determined by measuring turbidity as a function of temperature. Cloud temperatures increase with m and decrease with n, as observed for conventional surfactants. For large m the cloud temperatures were all above 100 degrees C. Surfactants with small m (i.e., n = 12, 14, m = 5 and n = 20, m = 10) were insoluble at room temperature, forming two-phase mixtures. Critical micelle concentrations (CMCs) were measured using a pyrene fluorescence method and are all in the range of 10(-7) to 10(-6) M, with the lowest values from the surfactants with large n and small m. CMCs of mixtures with both anionic and nonionic conventional (monomeric) surfactants were well described by an ideal mixing model.     

Experimental observations on the scaling of adsorption isotherms for nonionic surfactants at a hydrophobic solid-water interface

The self-assembly of nonionic surfactants in bulk solution and on hydrophobic surfaces is driven by the same intermolecular interactions, yet their relationship is not clear. While there are abundant experimental and theoretical studies for self-assembly in bulk solution and at the air-water interface, there are only few systematic studies for hydrophobic solid-water interfaces. In this work, we have used optical reflectometry to measure adsorption isotherms of seven different nonionic alkyl polyethoxylate surfactants (CH3(CH2)I-1(OCH2CH2)JOH, referred to as CIEJ surfactants, with I = 10-14 and J = 3-8), on hydrophobic, chemically homogeneous self-assembled monolayers of octadecyltrichlorosilane. Systematic changes in the adsorption isotherms are observed for variations in the surfactant molecular structure. The maximum surface excess concentration decreases (and minimum area/molecule increases) with the square root of the number of ethoxylate units in the surfactant (J). The adsorption isotherms of all surfactants collapse onto the same curve when the bulk and surface excess concentrations are rescaled by the bulk critical aggregation concentration (CAC) and the maximum surface excess concentration. In an accompanying paper we compare these experimental results with the predictions of a unified model developed for self-assembly of nonionic surfactants in bulk solution and on interfaces.     

Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

The nanotribological responses of a series of nonionic polyoxyethylene surfactants (Tween 20, Tween 40, Tween 60, and Tween 80) were investigated after they were adsorbed from aqueous solution onto atomically smooth hydrophobic substrates. The hydrophobic surfaces were composed of a condensed monolayer of octadecyltriethoxysilane (OTE; contact angle theta>110 degrees ). The nanorheological measurements were performed using a modified surface forces apparatus after coating atomically smooth mica with these OTE monolayers, while adsorption measurements were performed using phase-modulated ellipsometry on silicon wafers coated with these same monolayers. The minimum surface-surface separation observed under high load in friction studies agreed quantitatively with the thickness obtained from ellipsometry. For Tweens 20, 40, and 60, the thickness of the adsorbed film increases with increasing alkyl chain length. Systematic investigations of the nanorheological response showed that there is a "solid-like" elastic response from confined surfactant layers, which is the case for the smallest separations to separations up to slightly larger than twice the adsorbed film thickness. In kinetic friction, these confined layers are characterized by a shear stress of approximately 3 MPa with minimal dependence on shear rate. The magnitude of the sliding shear stress is the same as the apparent yield stress at approximately 3 MPa; it is independent of alkyl chain length within the Tween family of surfactants and corresponds to a nominal friction coefficient of mu approximately 1. A similar friction coefficient is observed for boundary lubrication on the macroscopic scale in a tribometer utilizing hydrophobic surfaces and mu approximately 1.1 for Tweens 20, 40, and 60. These results suggest that while Tween molecules adsorb onto hydrophobic surfaces to form a robust separating layer, the lubricating properties of these layers are dominated by a highly dissipative slip plane, the same for all alkyl chain lengths.     

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.     

Mixed adsorption of ionic and nonionic surfactants on active carbon

Summary Adsorption depends mainly on the relative amounts of anionic and nonionic surfactants present, the equilibrium concentration and the duration of exposure. In the case of similar hydrophobic chain lengths nonionic surfactants will be adsorbed more strongly than anionic compounds, thus displacing the latter from the carbon surface.The difference in the attraction to the carbon surface can be such, that significant adsorption of anionics is only observed where anionics are present in considerable excess.Under such conditions, anionics will diffuse more rapidly into the pore system of the adsorbant than nonionics. Therefore, the surface coverage with anionics will be higher after short exposure than after a longer period of time, when replacement by nonionics has started.At very low equilibrium concentrations (corresponding to low surface coverage), adsorption of anionics will be even increased by the presence of nonionics. This is due to the formation of mixed layers and the fact that in such layers the repulsion between the charged hydrophilic groups of the anionic surfactants will decrease.

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Zusammenfassung Die Adsorption hängt entscheidend von dem Mischungsverhältnis Aniontensid/ nichtionogenes Tensid, der Gleichgewichtskonzentration und der Adsorptionszeit ab. Bei annähernd gleicher hydrophober Kette werden nichtionogene Tenside stärker adsorbiert als Aniontenside und verdrängen diese von der Kohlenstoffoberfläche. Der Unterschied in der Attraktion zur Kohlenstoffoberfläche ist so groß, daß eine signifikante Adsorption von Aniontensiden erst bei hohem Überschuß in der Mischung im Vergleich zum nichtionogenen Tensid beobachtet werden kann. Unter diesen Verhältnissen diffundieren Aniontenside schneller in das Porensystem des Adsorbens, so daß im Bereich kurzer Zeiten, bevor die Verdrängung durch das nichtionogene Tensid einsetzt, an der Oberfläche Aniontenside stärker adsorbiert sind. Im Bereich sehr geringer Gleichgewichtskonzentrationen und dementsprechend geringen Oberflächenbelegungen wird jedoch wegen der Bildung von Mischfilmen beider Tensidarten und Verminderung der gegenseitigen Abstoßung der gleichsinnig geladenen hydrophilen Gruppen des Aniontensides durch das nichtionogene Tensid die Adsorption des Aniontensids sogar gesteigert.

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With 7 figures

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Presented at IUPAC-International Conference on Colloid and Surface Science, Budapest 15–20 September 1975.     

Microcalorimetric study on micellization of nonionic surfactants with a benzene ring or adamantane in their hydrophobic chains

The micellization of a novel family of nonionic surfactants poly(oxyethylene) glycol alkyl ethers has been studied by microcalorimetry. One of the surfactants has adamantane, and the other nonionic surfactants have a benzene ring in their hydrophobic chains, which moves from the terminal of the hydrophobic chain toward the headgroup. Moreover, the alkyl chain of the nonionic surfactants is straight or branched. Both the critical micelle concentration (cmc) and the thermodynamic parameters associated with the micelle formation have been obtained. The cmc decreases and the enthalpy of micelle formation (deltaH(mic)) becomes less positive gradually as the length of hydrophobic chain increases, whereas the values of cmc and deltaH(mic) tend to increase for the surfactants with a longer ethylene oxide chain. However, the deltaH(mic) value of the surfactant with seven carbon atoms in a hydrophobic chain is more positive than that of the surfactant with six carbon atoms in a hydrophobic chain. Comparing with the nonionic surfactant with a methylene hydrophobic chain, the surfactants with benzene rings and adamantane groups have larger cmc values and the cmc values increase with the size of the groups. Furthermore, moving the phenyl group from the terminal of the hydrophobic chain to the neighbor of the hydrophilic headgroup leads to the decreased cmc. Both the variation of hydrophobic interaction from the movement of phenyl group and pi-pi interaction among adjacent phenyl groups affect deltaH(mic) values.     

Self-association and self-assembly of molecular clips in solution and in the solid state

Clip molecules based on diphenylglycoluril form well-defined dimeric structures in chloroform solution and in the solid state. In solution the dimerization process is based on favourable π-π interactions and cavity filling effects. A combination of favourable π-π interactions and crystal packing forces determine the self-assembly of clips in the solid state. The geometry that the clip molecules adopt in solution and in a series of X-ray crystal structures is compared with favourable geometries predicted by molecular modelling calculations.     

Polyelectrolyte modified solid surfaces: the consequences for ionic and mixed ionic/nonionic surfactant adsorption

This paper describes how the cationic polyelectrolyte, polyDMDAAC (poly(dimethyl diallylammonium chloride)), is used to manipulate the adsorption of the anionic surfactant SDS and the mixed ionic/nonionic surfactant mixture of SDS (sodium dodecyl sulfate)/C(12)E(6) (monododecyl hexaethylene glycol) onto the surface of hydrophilic silica. The deposition of a thin robust polymer layer from a dilute polymer/surfactant solution promotes SDS adsorption and substantially modifies the adsorption of SDS/C(12)E(6) mixtures in favor of a surface relatively rich in SDS compared to the solution composition. Different deposition conditions for the polyDMDAAC layer are discussed. In particular, at higher solution polymer concentrations and in the presence of 1 M NaCl, a thicker polymer layer is deposited and the reversibility of the surfactant adsorption is significantly altered.     

Quantifying the hydrophobic effect. 1. A computer simulation-molecular-thermodynamic model for the self-assembly of hydrophobic and amphiphilic solutes in aqueous solution

Surfactant micellization and micellar solubilization in aqueous solution can be modeled using a molecular-thermodynamic (MT) theoretical approach; however, the implementation of MT theory requires an accurate identification of the portions of solutes (surfactants and solubilizates) that are hydrated and unhydrated in the micellar state. For simple solutes, such identification is comparatively straightforward using simple rules of thumb or group-contribution methods, but for more complex solutes, the hydration states in the micellar environment are unclear. Recently, a hybrid method was reported by these authors in which hydrated and unhydrated states are identified by atomistic simulation, with the resulting information being used to make MT predictions of micellization and micellar solubilization behavior. Although this hybrid method improves the accuracy of the MT approach for complex solutes with a minimum of computational expense, the limitation remains that individual atoms are modeled as being in only one of two states-head or tail-whereas in reality, there is a continuous spectrum of hydration states between these two limits. In the case of hydrophobic or amphiphilic solutes possessing more complex chemical structures, a new modeling approach is needed to (i) obtain quantitative information about changes in hydration that occur upon aggregate formation, (ii) quantify the hydrophobic driving force for self-assembly, and (iii) make predictions of micellization and micellar solubilization behavior. This article is the first in a series of articles introducing a new computer simulation-molecular thermodynamic (CS-MT) model that accomplishes objectives (i)-(iii) and enables prediction of micellization and micellar solubilization behaviors, which are infeasible to model directly using atomistic simulation. In this article (article 1 of the series), the CS-MT model is introduced and implemented to model simple oil aggregates of various shapes and sizes, and its predictions are compared to those of the traditional MT model. The CS-MT model is formulated to allow the prediction of the free-energy change associated with aggregate formation (gform) of solute aggregates of any shape and size by performing only two computer simulations-one of the solute in bulk water and the other of the solute in an aggregate of arbitrary shape and size. For the 15 oil systems modeled in this article, the average discrepancy between the predictions of the CS-MT model and those of the traditional MT model for gform is only 1.04%. In article 2, the CS-MT modeling approach is implemented to predict the micellization behavior of nonionic surfactants; in article 3, it is used to predict the micellization behavior of ionic and zwitterionic surfactants.     

The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C(36)H(74) or n-alcohol C(35)H(71)OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.     

Self-assembly of mixed anionic and nonionic surfactants in aqueous solution

We present the phase diagram and the microstructure of the binary surfactant mixture of AOT and C(12)E(4) in D(2)O as characterized by surface tension and small angle neutron scattering. The micellar region is considerably extended in composition and concentration compared to that observed for the pure surfactant systems, and two types of aggregates are formed. Spherical micelles are present for AOT-rich composition, whereas cylindrical micelles with a mean length between 80 and 300 ? are present in the nonionic-rich region. The size of the micelles depends on both concentration and molar ratio of the surfactant mixtures. At higher concentration, a swollen lamellar phase is formed, where electrostatic repulsions dominate over the Helfrich interaction in the mixed bilayers. At intermediate concentrations, a mixed micellar/lamellar phase exists.     

Forces between silica surfaces with adsorbed cationic surfactants: influence of salt and added nonionic surfactants

Forces have been measured between silica surfaces with adsorbed surfactants by means of a bimorph surface force apparatus. The surfactants used are the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the nonionic surfactant hexakis(ethylene glycol) mono-n-tetradecyl ether (C(14)E(6)) as well as mixtures of these two surfactants. The measurements were made at elevated pH, and the effect of salt was studied. At high pH the glass surface is highly charged, which increases the adsorption of TTAB. Despite the low adsorption generally seen for nonionic surfactants on silica at high pH, addition of C(14)E(6) has a considerable effect on the surface forces between two glass surfaces in a TTAB solution. The barrier force is hardly affected, but the adhesion is reduced remarkably. Also, addition of salt decreases the adhesion, but increases the barrier force. In the presence of salt, addition of C(14)E(6) also increases the thickness of the adsorbed layer. The force barrier height is also shown to be related to literature values for surface pressure data in these systems.     

Influence of the molecular architecture on the adsorption onto solid surfaces: comb-like polymers

The processes of adsorption of grafted copolymers onto negatively charged surfaces were studied using a dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The control parameters in the study of the adsorption are the existence or absence on the molecular architecture of grafted polyethyleneglycol (PEG) chains with different lengths and the chemical nature of the main chain, poly(allylamine) (PAH) or poly(L-lysine) (PLL). It was found out that the adsorption kinetics of the polymers showed a complex behavior. The total adsorbed amount depends on the architecture of the polymer chains (length of the PEG chains), on the polymer concentration and on the chemical nature of the main chain. The comparison of the thicknesses of the adsorbed layers obtained from D-QCM and from ellipsometry allowed calculation of the water content of the layers that is intimately related to the grafting length. The analysis of D-QCM results also provides information about the shear modulus of the layers, whose values have been found to be typical of a rubber-like polymer system. It is shown that the adsorption of polymers with a charged backbone is not driven exclusively by the electrostatic interactions, but the entropic contributions as a result of the trapping of water in the layer structure are of fundamental importance.     

Influence of nonionic surfactants on the chromatographic behaviour of proteins in hydrophobic interaction chromatography

The chromatographic characteristics of proteins in the presence of additives of nonionic surfactants Brij-35 and Tween-80 in the conditions of descending gradient of ammonium sulfate and phenyl-coated polymeric stationary phase were investigated. It was revealed that retention factors of proteins may be regulated by use of mentioned additives. The improvement of resolution is achieved for some hardly separated pairs of proteins, viz. albumin egg/albumin bovine, aldolase/tripsin. A reversion of the elution order is observed for tripsin/chymotrypsinogen A.     

Factors controlling adsorption equilibria from solution onto solid surfaces: the uptake of cinchona alkaloids on platinum surfaces

The room-temperature adsorption of four closely related cinchona alkaloids and three reference quinoline-based compounds from CCl4 solutions onto a polycrystalline platinum surface was characterized by in situ reflection-absorption infrared spectroscopy (RAIRS). The adsorption equilibrium constants (Kads) were found to follow the sequence cinchonine > quinidine > cinchonidine > quinine > 6-methoxyquinoline > lepidine > quinoline. Some of this ordering can be explained by differences in solubility, but quinidine displays a much larger Kads than expected on the basis of its large relative solubility; bonding to the surface must also play a role in determining its behavior. It was determined that each alkaloid binds differently on Pt at saturation coverages. While the quinoline ring of cinchonidine tilts along its long axis to optimize pi-pi intermolecular interactions, in cinchonine it tilts along the short axis and bonds through the lone electron pair of the nitrogen atom instead, and both quinine and quinidine exhibit additional bonding via the methoxy oxygen atom at intermediate concentrations. Perhaps a more surprising result from this work is the fact that cinchonine displays a higher Kads than cinchonidine, quinine, or quinidine even though, according to previous work, it can be easily displaced from the surface by any of those other cinchona alkaloids. A full explanation of these observations requires consideration of the solvent above the adsorbed species.