Influence of a surfactant and electrolytes on adsorbed polymer layers

Solvent relaxation NMR and small-angle neutron scattering have been used to characterize adsorbed poly(ethylene oxide) (PEO) layers on silica at a range of surfactant and electrolyte concentrations. Below the critical aggregation concentration (cac), the results suggest that sodium dodecyl sulfate (SDS) interacts relatively weakly, perhaps analogously to a simple salt reducing the solvency of PEO. This is evidenced by a decrease in the adsorbed layer thickness combined with an increase in the bound fraction, although the total adsorbed amount is not greatly affected. The layer thickness goes through a minimum at the cac, after which further SDS addition results in the formation of PEO/SDS aggregates that repel each other and, hence, tend to desorb. The adsorbed amount therefore decreases, from 0.7 mg m(-2) initially to 0.2 mg m(-2) with 32 mM SDS. The aggregates that remain adsorbed also repel, and hence, there is an increase in the layer thickness and the persistence length, while the bound fraction is reduced. In comparison, the effects of electrolyte at the ionic strength studied are relatively minimal. There is, however, evidence that the repulsions between adsorbed PEO/SDS aggregates are partially screened, allowing them to approach each other more readily. This leads to a contraction of the adsorbed layer when the SDS concentration is sufficiently high.     

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.     

Effects of electrolytes on adsorbed polymer layers: poly(ethylene oxide)-silica system

The effects of various electrolytes on the adsorption of poly(ethylene oxide) onto silica have been studied. The salts were the chlorides of Na+, Mg2+, Ca2+, and La3+. The methods used were adsorption isotherms, found using a depletion method with phosphomolibdic acid, photon correlation spectroscopy, and solvent relaxation NMR. All the salts increased the particle-polymer affinity and adsorbed amount according to the adsorption isotherms, and a linear relationship was found between the initial slope of the isotherms and the ionic strength of the solution. Final adsorbed amounts were approximately 0.4-0.5 mg m(-2). The polymer layer thicknesses as found by PCS were of the same order as the radius of gyration of the polymer and increased with both the concentration and the valency of the salt due to increased adsorption. Solvent relaxation NMR showed that NaCl is too weak to have a noticeable effect on the polymer train layer, but the divalent salts clearly did increase both the strength of solvent binding close to the silica surface and the amount of PEO required to reach the maximum train density.     

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.     

Interfacial rheological properties of adsorbed protein layers and surfactants: a review

Proteins and low molecular weight (LMW) surfactants are widely used for the physical stabilisation of many emulsions and foam based food products. The formation and stabilisation of these emulsions and foams depend strongly on the interfacial properties of the proteins and the LMW surfactants. Therefore these properties have been studied extensively. In this review an overview is given of interfacial properties of proteins at a mesoscopic scale and the effect of LMW surfactants (emulsifiers) on them. Properties addressed are the adsorbed amount, surface tension (reduction), network formation at interfaces and possible conformational changes during and after adsorption. Special attention is given to interfacial rheological behaviour of proteins at expanding and compressing interfaces, which simulate the behaviour in real emulsions and foams. It will be illustrated that information on interfacial rheological properties, protein conformation and interactions between adsorbed molecules can be obtained by changing experimental conditions. The relation between interfacial rheology and emulsion and foam stabilisation is subsequently addressed. It is concluded that there is a need for new measuring devices that monitor several interfacial properties on a mesoscopic and microscopic scale at the same time, and for physical models describing the various processes of importance for proteins. Real progress will only be possible if both are combined in an innovative way.     

Effect of inorganic electrolytes and nonionogenic surfactants on the stability of soap stock emulsion

Modes in which the fat component of the soap stock emulsion is extracted under heating and introduction of various nonionogenic surfactants and electrolytes were studied. The compression isotherms of the soap stock and its mixtures with OP-7 and NaCl were examined by the Langmuir method, with 2D pressure recorded. Recommendations were developed for separation of the soap stock emulsion for recovery of the fat component from wastes produced in manufacture of vegetable oils.     

Spectroscopy of adsorbed layers

Recent developments in surface spectroscopic techniques and their applications to adsorbed films are reviewed. Attention is focused on vibrational spectroscopy, including sum-frequency spectroscopy, Fourier transform infrared spectroscopy and Raman scattering. Systems most relevant to colloid and interface science are emphasised, though related systems of interest are also mentioned.     

Structure of adsorbed n-dodecyl-beta-D-maltoside layers on hematite

The composition, structure, and thickness of n-dodecyl-beta-D-maltoside self-assembled layers on hematite have been evaluated using infrared external reflection spectroscopy and spectral simulation techniques. From the qualitative and quantitative analysis of the reflection spectra of the same sample recorded at different specific angles of incidence and two polarizations, the orientation of the sugar ring and hydrocarbon chain were obtained. Both of these molecular groups are positioned parallel to hematite surface, the adsorbed molecules being at low (2.2-nm-thick layer) as well as higher (11-nm) coverages. The maltoside is adsorbed through interaction of sugar ring OH groups with hematite surface hydroxyl groups. The adsorption of maltoside is not very strong and desorption takes place easily from acidic and low-basic solutions but with more difficulty from strong-basic solution.     

Effects of surfactants on the formation and the stability of interfacial nanobubbles

Contamination has previously been invoked to explain the flat shape and the long lifetimes of interfacial nanobubbles (INBs). In this study, the effects of surfactants on the formation and the stability of INBs were investigated when surfactants were added to the system before, during, and after the standard solvent exchange procedure (SSEP) for the formation of INBs. The solutions of sodium dodecyl sulfate (SDS) above critical micelle concentration were found to have little effect on the bubble stability. Likewise, cleaning of the substrate with a surfactant solution had little effect. In contrast, addition of a water-insoluble surfactant during the formation dramatically reduced the INBs. Finally, repeated application of SSEP to surfactant-coated substrates progressively rinsed the surfactant off the system. Thus, we found no evidence to support the hypothesis that (1) INBs are stabilized by a layer of insoluble organic contaminant or that (2) SSEP introduces surface-active materials to the system that could stabilize INBs.     

Activity of lactoperoxidase when adsorbed on protein layers

Lactoperoxidase (LPO) is an enzyme, which is used as an antimicrobial agent in a number of applications, e.g., food technology. In the majority of applications LPO is added to a homogeneous product phase or immobilised on product surface. In the latter case, however, the measurements of LPO activity are seldom reported. In this paper we have assessed LPO enzymatic activity on bare and protein modified gold surfaces by means of electrochemistry. It was found that LPO rapidly adsorbs to bare gold surfaces resulting in an amount of LPO adsorbed of 2.9mg/m(2). A lower amount of adsorbed LPO is obtained if the gold surface is exposed to bovine serum albumin, bovine or human mucin prior to LPO adsorption. The enzymatic activity of the adsorbed enzyme is in general preserved at the experimental conditions and varies only moderately when comparing bare gold and gold surface pretreated with the selected proteins. The measurement of LPO specific activity, however, indicate that it is about 1.5 times higher if LPO is adsorbed on gold surfaces containing a small amount of preadsorbed mucin in comparison to the LPO directly adsorbed on bare gold.     

Polymer thermodynamics of adsorbed protein layers

Since proteins are polymers, their adsorption at interfaces should share some common features with polymers whose adsorption behavior is being rather well understood using current theoretical approaches. Some theoretical developments are highlighted and recent experimental data obtained mostly with β-casein are compared to them. The general conclusions are that the alternating hydrophilic/hydrophobic block theory gives a general frame for the adsorption of proteins. However, the detailed behavior of proteins at interface seems also influenced by non-covalent, say, electrostatic and hydrogen bonds, whose extent and effects are specified by the local conditions (nature of the fluid phases, temperature, pH, ionic strength, etc.).     

Behavior of macromolecules in adsorbed layers

A model for describing the behavior ot macromoiecuies in aosoroea layers is developed by introducing a concept of distribution density of layer thickness U based on stochastic process and probabilistic statistics. The molecular behavior of layers adsorbed on clay particle surfaces is discussed; the random distribution and its statistics of the layer thickness are given by incorporating experimental results with an ionic polyelectrolyte with the molecular weight of 1.08×106and chain charged density of 0.254.     

Structure of adsorbed layers of nitrophenoxy-tailed quaternary ammonium surfactants at the air/water interface studied by neutron reflection

The adsorbed layers of N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (PhiC(10)TAB) and N,N,N('),N(')-tetramethyl-N,N(')-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide [(PhiC(10))(2)C(6)] at the air/water interface have been studied by neutron reflection. The coverage of the surfactants was obtained over the concentration range from critical micelle concentration (CMC) to CMC/100. The area per PhiC(10)TAB molecule changes from 50+/-3 to 390+/-60 A(2) over this concentration range and the area per (PhiC(10))(2)C(6) molecule changes from 139+/-3 to 288+/-10 A(2). The overall thicknesses (single uniform layer) of the surfactant layers at CMC are about 19 and 16 A for PhiC(10)TAB and (PhiC(10))(2)C(6) respectively. The distributions of the C(10) chains show that the chains of both surfactants are tilted away from surface normal, with the tilt increasing in the outer part of the layer. The distribution of C(10) chains in (PhiC(10))(2)C(6) is narrower than that in PhiC(10)TAB, indicating that the alkyl chains of (PhiC(10))(2)C(6) are more tilted. For both surfactants, the broad nitrophenoxy distribution may indicate significant positional disorder of the nitrophenoxy groups along the surface normal direction and their intermixing with alkyl chains in the adsorbed layer.     

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.     

Field emission current fluctuations of adsorbed layers

Studies of adsorbate-induced field emission current fluctuations [field emission flicker noise, (FEFN)] have provided many interesting results on surface diffusion processes and related phenomena. The FEFN is investigated using spectral analysis, auto-correlation (AC) and cross-correlation (CC) measurements.

In this paper a review is given on the FEFN studies carried out in lasting cooperation between the physics groups of Leipzig and Wroc aw Universities. A special emphasis is laid upon the CC measurements (the simultaneous correlation of the FEFN from two probed regions) of potassium submonolayers on a tungsten surface, initiated in 1978 by D browski and Kleint.

The CC studies of potassium submonolayers revealed a new insight into the dynamics of adparticle motions on a crystal surface. Diffusion motion of single adparticles and collective phenomena such as soliton or domain wall propagation within the adlayer have been suggested and discussed as a possible explanation of the observed properties of the CC functions.     

Structure and composition of cationic-nonionic surfactant mixed adsorbed layers on mica

The composition and morphology of mixed adsorbed layers comprising one of several poly(oxyethylene) alkyl ether nonionic surfactants, C(i)E(j), and two cationic surfactants-dodecyltrimethylammonium bromide (DTAB) and tetradecyltriethylammonium bromide (TTeAB)-at the mica/solution interface have been studied using depletion adsorption and atomic force microscopy. The nonionic surfactants do not themselves adsorb onto mica, but can coadsorb with a cationic surfactant. The extent of their hydrophobic association with the adsorbed cationic surfactant depends on alkyl chain length, while the adsorbed layer morphologies are sensitive to the number of ethoxy groups. Nonionic surfactants with headgroups containing less than eight ethylene oxide units decrease the adsorbed aggregate curvature, gradually transforming globular TTeAB or cylindrical DTAB adsorbed aggregates into a rod, mesh, or bilayer structure. Those with larger headgroups favor globular aggregates. The mechanism by which the nonionic surfactant modifies the adsorbed morphology is the formation of defects in the form of cylinder end-caps or branch-points, leading to adsorbed layer compositions that differ from ideal mixing predictions. All mixed adsorbed films become saturated with the nonionic component when the capacity of the aqueous side of the adsorbed layer is reached.     

Surface enhanced Raman scattering of surfactants adsorbed on silver mirror surfaces

The surface enhanced Raman scattering (SERS) spectra of two surfactants: Triton X-100 and n-hexadecylpyridinium bromide adsorbed on the interfaces between chemically reduced silver mirrors and water are obtained. By comparing and analyzing their bulk and SERS spectra, we have studied the adsorption configurations of the two surfactants on the silver/water interfaces.     

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.