Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface

The adsorption behaviour of proteins and systems mixed with surfactants of different nature is described. In the absence of surfactants the proteins mainly adsorb in a diffusion controlled manner. Due to lack of quantitative models the experimental results are discussed partly qualitatively. There are different types of interaction between proteins and surfactant molecules. These interactions lead to protein/surfactant complexes the surface activity and conformation of which are different from those of the pure protein. Complexes formed with ionic surfactants via electrostatic interaction have usually a higher surface activity, which becomes evident from the more than additive surface pressure increase. The presence of only small amounts of ionic surfactants can significantly modify the structure of adsorbed proteins. With increasing amounts of ionic surfactants, however, an opposite effect is reached as due to hydrophobic interaction and the complexes become less surface active and can be displaced from the interface due to competitive adsorption. In the presence of non-ionic surfactants the adsorption layer is mainly formed by competitive adsorption between the compounds and the only interaction is of hydrophobic nature. Such complexes are typically less surface active than the pure protein. From a certain surfactant concentration of the interface is covered almost exclusively by the non-ionic surfactant. Mixed layers of proteins and lipids formed by penetration at the water/air or by competitive adsorption at the water/chloroform interface are formed such that at a certain pressure the components start to separate. Using Brewster angle microscopy in penetration experiments of proteins into lipid monolayers this interfacial separation can be visualised. A brief comparison of the protein adsorption at the water/air and water/n-tetradecane shows that the adsorbed amount at the water/oil interface is much stronger and the change in interfacial tension much larger than at the water/air interface. Also some experimental data on the dilational elasticity of proteins at both interfaces measured by a transient relaxation technique are discussed on the basis of the derived thermodynamic model. As a fast developing field of application the use of surface tensiometry and rheometry of mixed protein/surfactant mixed layers is demonstrated as a new tool in the diagnostics of various diseases and for monitoring the progress of therapies.     

Experimental Study of the Effect of Strong Alkali on Lowering the Interfacial Tension of Oil/Heavy Alkylbenzene Sulfonates System

Experimental studies were conducted to explore the fundamental mechanisms of alkali to lower the interfacial tension of oil/heavy alkylbenzene sulfonates (HABS) system. Sodium hydroxide was used as the strong alkali chemical to investigate the interfacial tension (IFT) of oil/HABS system. The influences of salt and alkali on the interfacial activity were studied by the measurement of interfacial tension and partition coefficient. Moreover, the alkali/surfactant solutions were measured by dynamic laser scattering. The results showed that compared with the salt, the function of alkali to lower the interfacial tension and improve partition coefficient is more significant. The micelles formed by surfactants could be disaggregated because of adding alkali, so the size of micelles decreases and the number of mono‐surfactants increases, then more surfactant molecules move to the interface of oil/surfactant system and the adsorption of surfactants at oil‐water interfaces increases, which can lead to the decrease of IFT.     

有机反离子对C12-s-C12·2Br水溶液表面活性的影响

溶液中添加的苯磺酸钠(SNzS)和萘磺酸钠(SNphS)与C12-s-C12·2Br产生强烈结合, 增大了Gemini表面活性剂分子的疏水性, 明显促进其在气/液界面的吸附和在溶液中的聚集. 这使得体系降低水表面张力的效率和能力大大提高, 并且在表面活性剂浓度很低时就生成了小聚集体. 因而, 此时表面张力法测得的cmc仅具有表观上的意义, 只反映了表面活性剂在气/液界面达到饱和吸附时的临界浓度. SNphS的疏水性强于SNzS, 更有效地促进了C12-s-C12·2Br的吸附和聚集.     

Effect of charged colloidal particles on adsorption of surfactants at oil-water interface

A change of oil/water interfacial tension in the presence of cationic or anionic surfactants in an organic phase was observed due to the addition of charged fine solids in the aqueous phase. The charged fine solids in the aqueous phase adsorb surfactants diffused from the oil phase, thereby causing an increase in the bulk equilibrium surfactant concentration in the aqueous phase, governed by the Stern-Grahame equation. Consequently, surfactant adsorption at the oil-water interface increases, which was demonstrated from the measured reduction of the oil-water interfacial tension. The increased surfactant partition in the aqueous phase in the presence of the charged particles was confirmed by the measured decrease in the surface tension for the collected aqueous solution after solids removal, as compared with the cases without solids addition.     

X型与线型嵌段聚醚在空气/水和正庚烷/水界面上聚集行为的比较研究

通过阴离子聚合方法合成了环氧乙烷(EO)含量和分子量均相同的线型聚氧丙烯(PEO)-聚氧乙烯(PPO) (LPE)和X型聚氧丙烯-聚氧乙烯(TPE)嵌段聚醚,考察了它们在空气/水及正庚烷/水界面上聚集行为的差异. 界面活性的研究结果表明,TPE降低水、正庚烷界面张力的效率和效能均低于LPE的. 聚醚分子在正庚烷/水界面达到吸附平衡的时间比在空气/水表面短. 由于正庚烷分子插入到聚醚吸附层中,聚醚分子可以在正庚烷/水界面上采取更为直立的状态,因此聚醚分子在正庚烷/水界面扩散较快. 聚醚在正庚烷/水界面的扩张弹性高于空气/水表面的.     

Interfacial dilational properties of anionic gemini surfactants with polymethylene spacers

The dilational properties of anionic gemini surfactants alkanediyl-α,ω-bis(m-octylphenoxy sulfonate) (C₈C_mC₈) with polymethylene spacers at the water–air and water–decane interfaces were investigated by oscillating barriers and interfacial tension relaxation methods. The influences of oscillating frequency and bulk concentration on the dilational properties were explored. The experimental results show that the linking spacer plays an important role in the interfacial dilational properties. The moduli pass through one maximum for all three gemini surfactants at both water–air and water–decane interfaces. However, the values of moduli at the water–air interface are obviously higher than those at the water–decane interface because the sublayer formed by spacer chains will be destroyed by the insertion of oil molecules. Moreover, with the increase of spacer length, the surface adsorption film becomes more viscous at high concentration, which can be attributed to the process involving the formation of the sublayer. On the other hand, the spacers of the adsorbed C₈C₆C₈ molecules will extend into the oil phase when the interface is compressed. As a result, the interfacial film becomes more elastic with the increase of spacer length at high concentration. The experimental results obtained by the interfacial tension relaxation measurements are in accord with those obtained by the oscillating barriers method.     

Effect of liquid paraffin on the stability of aqueous foam in the presence and absence of electrolytes

The foam stability of dodecyl diphenyl ether disulfonate solution with liquid paraffin droplets in the presence and absence of electrolytes was evaluated, and the stabilization mechanisms were deduced. The foam film is stabilized when the monovalent and divalent counterion concentration is lower than a critical value. However, the foam stability declined with the addition of trivalent counterions. There are two mechanisms we have speculated. Firstly, the counterions reduce the repulsive interaction between the polar groups of surfactants adsorbed at the air/water and oil/water interfaces in the pseudo-emulsion film. Secondly, comparing with the monovalent counterions, the multivalent counterions are not only able to interconnect head groups of adjacent ionic surfactant molecules which are adsorbed at the air/water or oil/water interface, but also to attract the couples of neighboring surfactant head groups adsorbed at the air/water and oil/water interfaces. The attractive interaction between both the interfaces promotes the emulsified droplets piercing the air/water interface.     

Synthesis and micellar properties of surface-active ionic liquids: 1-alkyl-3-methylimidazolium chlorides

A series of surface-active ionic liquids, RMeImCl, has been synthesized by the reaction of purified 1-methylimidazole and 1-chloroalkanes, RCl, R=C(10),C(12),C(14), and C(16), respectively. Adsorption and aggregation of these surfactants in water have been studied by surface tension measurement. Additionally, solution conductivity, electromotive force, fluorescence quenching of micelle-solubilized pyrene, and static light scattering have been employed to investigate micelle formation. The following changes resulted from an increase in the length of R: an increase of micelle aggregation number; a decrease of: minimum area/surfactant molecule at solution/air interface; critical micelle concentration, and degree of counter-ion dissociation. Theoretically-calculated aggregation numbers and those based on quenching of pyrene are in good agreement. Gibbs free energies of adsorption at solution/air interface, DeltaG(ads)(0), and micelle formation in water, DeltaG(mic)(0), were calculated, and compared to those of three surfactant series, alkylpyridinium chlorides, RPyCl, alkylbenzyldimethylammonium chlorides, RBzMe(2)Cl, and benzyl(3-acylaminoethyl)dimethylammonium chlorides, R(')AEtBzMe(2)Cl, respectively. Contributions to the above-mentioned Gibbs free energies from surfactant methylene groups (in the hydrophobic tail) and the head-group were calculated. For RMeImCl, the former energy is similar to that of other cationic surfactants. The corresponding free energy contribution of the head-group to DeltaG(mic)(0) showed the following order: RPyCl approximately RBzMe(2)Cl>RMeImCl>R(')AEtBzMe(2)Cl. The head-groups of the first two surfactant series are more hydrophobic than the imidazolium ring of RMeImCl, this should favor their aggregation. Micellization of RMeImCl, however, is driven by a relatively strong hydrogen-bonding between the chloride ion and the hydrogens in the imidazolium ring, in particular the relatively acidic H2. This interaction more than compensates for the relative hydrophilic character of the diazolium ring. As indicated by the corresponding DeltaG(mic)(0), micellization of R(')AEtBzMe(2)Cl is more favorable than that of RMeImCl because the CONH group of the former surfactant series forms hydrogen bonds to both the counter-ion and the neighboring molecules in the micelle.     

Wetting in oil/water/surfactant systems

The behaviour of oils at aqueous interfaces is ubiquitous to many industrially and biologically relevant processes. In this review we consider modifications to the wetting properties of oils at the air/water, oil/water and solid/liquid interfaces in the presence of surfactants. First-order wetting transitions can be induced in a wide range of oils by varying the aqueous surfactant concentration, leading to the formation of mixed monolayers at the interface. In certain cases, these mixed monolayers display novel surface freezing behaviour, including the formation of unusual bilayer structures, which further modifies the properties of the interface. The effects of surfactant on line tension at the three-phase contact line and differences between the air/liquid and liquid/liquid interfaces are discussed.     

界面扩张流变法研究C_(12)mimBr对Gemini12-2-12在空气/水界面聚集行为的影响

采用界面扩张流变技术研究了季铵盐偶联表面活性剂C12-(CH2)2-C12·2Br(Gemini12-2-12)及其与离子液体表面活性剂溴化1-十二烷基-3-甲基咪唑(C12mim Br)复配体系的动态界面张力、扩张流变性质和界面弛豫过程等,探讨了C12mim Br对C12mim Br/Gemini12-2-12混合体系界面性质的影响及C12mim Br对Gemini12-2-12界面聚集行为影响的机制.结果表明,随着离子液体表面活性剂的不断引入,体系界面吸附达到平衡所需的时间逐渐缩短,扩张模量和相角明显降低,界面吸附膜由粘弹性膜转变为近似纯弹性膜;同时,界面及其附近的弛豫过程也发生显著变化,慢弛豫过程消失,快弛豫过程占主导地位,且离子液体浓度越高,快弛豫的贡献越大.这些界面性质的变化主要归因于离子液体表面活性剂C12mim Br参与界面形成及两表面活性剂在界面竞争吸附的结果.少量离子液体表面活性剂C12mim Br的加入可以填补疏松的Gemini12-2-12界面上的空位,形成混合界面吸附膜.随着C12mim Br含量的增加,嵌入界面的C12mim Br分子数不断增多,导致界面上相互缠绕的Gemini12-2-12烷基链"解缠",在体相和界面分子扩散交换的过程中"解缠"的Gemini12-2-12分子从界面上解吸回到体相,与此同时,C12mim Br分子相对较小的空间位阻及较强的疏水作用促使其优先扩散至界面进而取代Gemini12-2-12分子,最终界面几乎完全被C12mim Br分子所占据.     

Co-adsorption of carboxymethyl-cellulose and cationic surfactants at the air-water interface

We investigated the interaction between an anionic polyelectrolyte (carboxymethylcellulose) and cationic surfactants (DTAB, TTAB, and CTAB) at the air/water interface, using surface tension, ellipsometry, and Brewster angle microscopy techniques. At low surfactant concentration, a synergistic phenomenon is observed due to the co-adsorption of polyelectrolyte/surfactant complexes at the interface, which decreases the surface tension. When the surfactant critical aggregation concentration (cac) is reached, the adsorption saturates and the thickness of the adsorbed monolayer remains constant until another characteristic surfactant concentration, C0, is reached, at which all the polymer charges are bound to surfactant in bulk. Above C0, the absorbed monolayer becomes much thicker, suggesting adsorption of bulk aggregates, which have become more hydrophobic due to charge neutralization.     

Adsorption of Alkyltrimethylammonium Bromides at the Various Interfaces

The adsorption of alkyltrimethylammonium bromide homologues has been studied at oil/water and air/water interfaces. Dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammomium bromide were used. As an oil phase the aliphatic hydrocarbons of the same chain length as that of surfactants molecules were used. The interfacial tension was measured by the drop volume method and the equilibrium values were received. The surface tension was measured by the drop weight method. The values of surface excess Gamma were calculated from the Gibbs equation. The experimental results were tested by the Langmuir isotherm as well as by the isotherm developed by E. Blomgren and J. O'M. Bockris (J. Phys. Chem. 63, 1475 (1959)). Copyright 2000 Academic Press.     

Adsorption Kinetics of Some Polyethylene Glycol Octylphenyl Ethers Studied by the Fast Formed Drop Technique

The adsorption kinetics of Triton X-100 and Triton X-405 at solution/air and solution/hexane interfaces is studied by the recently developed fast formed drop technique. The dynamic interfacial tension of Triton X-100 and Triton X-405 solutions against hexane has been measured without preequilibration of the water and oil phases. It is found that the dynamic interfacial tension of Triton X-100 solutions passes through a minimum. This strange behavior is attributed to partial solubility of the surfactant in hexane. Such minima of the dynamic interfacial tension of Triton X-405 solutions have not been observed, which correlates well with the solubilities of both surfactants in hexane reported in the literature. The dynamic surface tension of solutions of both surfactants and the dynamic interfacial tension of Triton X-405 solutions are interpreted by the Ward and Tordai model for diffusion controlled adsorption. It is shown that proper interpretation of the experimental data depends on the type of isotherm used. More consistent results are obtained when the Temkin isotherm is used instead of the Langmuir isotherm. The results obtained with Triton X-100 at the solution/air interface confirm that the adsorption of this surfactant occurs under diffusion control. The adsorption of Triton X-405 at solution/air and at solution/hexane interfaces seems to occur under diffusion control at short periods of time, but under mixed (diffusion-kinetic) control at long periods of time. A hypothesis is drawn to explain this phenomenon by changes in the shape of the large hydrophilic heads of Triton X-405 molecules. Copyright 2000 Academic Press.     

Monte Carlo simulations of Lennard-Jones nonionic surfactant adsorption at the liquid/vapor interface

We present Monte Carlo simulations of nonionic surfactant adsorption at the liquid/vapor interface of a monatomic solvent. All molecules in the system, solvent and surfactant, are characterized by the Lennard-Jones (LJ) potential using differing interaction parameters. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and a solvophobic tail. Adjacent atoms along the surfactant chain are connected by finitely extensible harmonic springs. Solvent molecules move via the Metropolis random-walk algorithm, whereas surfactant molecules move according to the continuum configurational bias Monte Carlo (CBMC) method. We generate quantitative thermodynamic adsorption and surface tension isotherms in addition to surfactant radius of gyration, tilt angles, and potentials of mean force. Surface tension simulations compared to those calculated from the simulated adsorbed amounts and the Gibbs adsorption isotherm agree confirming equilibrium in our simulations. We find that the classical Langmuir isotherm is obeyed for our LJ surfactants over the range of head and tail lengths studied. Although simulated surfactant chains in the bulk solution exhibit random orientations, surfactant chains at the interface orient roughly perpendicular and the tails elongate compared to bulk chains even in the submonolayer adsorption regime. At a critical surfactant concentration, designated as the critical aggregation concentration (CAC), we find aggregates in the solution away from the interface. At higher concentrations, simulated surface tensions remain practically constant. Using the simulated potential of mean force in the submonolayer regime and an estimate of the surfactant footprint at the CAC, we predict a priori the Langmuir adsorption constant, KL, and the maximum monolayer adsorption, Gammam. Adsorption is driven not by proclivity of the surfactant for the interface, but by the dislike of the surfactant tails for the solvent, that is by a "solvophobic" effect. Accordingly, we establish that a coarse-grained LJ surfactant system mimics well the expected equilibrium behavior of aqueous nonionic surfactants adsorbing at the air/water interface.     

Ion-selective Marangoni instability coupled with the nonlinear adsorption/desorption rate

An oil/water interface containing bis(2-ethylhexyl)phosphate and Ca(2+) or Fe(3+) exhibits spontaneous Marangoni instability associated with the fluctuation in interfacial tension. This instability rarely appears for oil/water systems with Mg(2+), Sr(2+), Ba(2+), Cu(2+), or Co(2+). The same ion selectivity is observed for n-heptane and nitrobenzene despite their significant differences in density, viscosity, and the dielectric constant of oil. We studied this instability under acidic pH conditions to avoid the neutralization reaction effects. The result of the equilibrium interfacial tension and the extraction ratio of cations indicates that a large number of oil-soluble complexes form at the interfaces of Ca(2+)-containing systems and probably for Fe(3+)-containing systems. The results obtained by oscillating drop tensiometry and Brewster angle microscopy indicate that desorption, rather than adsorption, is more significant to the onset of instability and that the resulting complex tends to form aggregates in the interface. This aggregation gives the nonlinear desorption rate of the oil-soluble complex. Then, exfoliation of the aggregating matter occurs, which triggers the Marangoni instability. The induced convection removes the oil-soluble complex accumulated at the interface, creating a renewed interface, which is necessary for the successive occurrence of the Marangoni instability. For the other cations, the oil-soluble compounds are insignificant, and they rarely form aggregates. In such cases, adsorption/desorption proceeds without instability.     

Adsorption behavior of phospholipid vesicles at oil/water interfaces

By measuring the change in interfacial tension after adding phospholipid vesicles to an aqueous solution of electrolyte, we studied the adsorption behavior of phospholipid vesicles at oil/water interfaces. The effects of concentration of three kinds of electrolyte (NaCl, MgCl₂, LaCl₃) and of the mixing ratio of two kinds of phospholipids (phosphatidylcholine and phosphatidylserine), on the adsorption behavior at an oil/water interface were examined. The results were interpreted using the DLVO theory.     

Interfacial dilational behaviour of adsorbed β-lactoglobulin layers at the different fluid interfaces

β-Lactoglobulin adsorption layers at the interfaces solution/air, /tetradecan and /sunflower oil were characterised by dynamic interfacial tension measurements and harmonic drop oscillation experiments in a time scale of some seconds. Axialsymmetric drop shape analysis (ADSA) was used to calculate drop volume, area and interfacial tension. Within a definite range of drop volume amplitude, the oscillation of the surface tension is harmonic and interfacial dilation parameters can be determined. Dependence of the dilational parameters on the amplitude and frequency of drop volume oscillation were determined and methodical demands are given for this special kind of ADSA application. The concentration of interfacial saturation is minimal at the interface with sunflower oil. Interfacial dilational elasticities, and viscosities are maximal at the saturation concentration of all systems investigated. The dilational viscosities are maximal in the frequency range 0.007–0.011 Hz and characterise molecular rearrangement processes in the adsorption layer. Interfacial dilational elasticity and viscosity are the largest at the interface with air. They are the smallest at the interface with sunflower oil. Similarities and differences of the systems investigated are discussed by taking into account the adsorption behaviour and the solvatation of different apolar and polar parts of the protein molecules in the neighbouring phase.     

Comparison between inhomogeneous adsorption of charged surfactants on air-water and on solid-water interfaces by self-consistent field theory

We use a realistic molecular model to study the interfacial behavior of hydrocarbon sulfate surfactants within a self-consistent field model and consider the adsorption both at the air-water interface and at a hydrophobic solid-water interface. We focus on the structural properties of the hemimicelles at the critical interface aggregation concentration (CIAC) for the air-water system and the critical surface aggregation concentration (CSAC) for the solid-water system. The major difference between the two systems is that the liquid interface is penetrable but the solid surface is intrinsically impenetrable for the molecular species. At the LG interface the hemimicelles have a lens shape with their centers of mass positioned slightly toward the aqueous side and feature an aspect ratio of approximately 2, with the long dimension parallel to the interface. Hemimicelle formation occurs below a critical (interfacial) area per molecule and above a critical surface pressure depending on tail length and ionic strength. Hemimicelles are not expected at air-water interfaces for a surfactant with a tail length ( t) lower than 15 CH2 units. In contrast, at a hydrophobic solid the formation of laterally inhomogeneous micelles even takes place for surfactants with the tail length as short as t = 12. This difference is attributed to the screening of the lateral interactions in the vapor phase. The shape of surface hemimicelles is caplike (or half-lens) with an aspect ratio lower than 2 and the long dimension parallel to the solid surface. The tail length, the ionic strength, the adsorption energies, and the surfactant concentration have an effect on the surface micelle properties such as the aggregation number and size and shape.     

Effects of electrolyte and nonelectrolyte additives on adsorption and surface rheological characteristics of humic acid salt solutions

The du Noüy and oscillating droplet shape methods are employed to study the effects of the ionic strength and pH of a medium, as well as the addition of nonelectrolytes (lower alcohols and acetone), on the adsorption and surface rheological characteristics of aqueous solutions of humic acid salts (sodium humates) at the liquid-air interface. When added in concentrations at which the aggregation of humic substances is not yet observed, strong electrolytes (NaCl and HCl) decrease the equilibrium surface tension and increase the dilatational viscoelastic modulus of aqueous sodium humate solutions. The aggregation of humic substances enhances the surface tension and reduces the viscoelastic modulus of surface layers. Nonelectrolyte additives decrease the surface tension and dilatational modulus of aqueous humic acid salt solutions. The equilibrium surface tension of sodium humate-nonelectrolyte mixed solutions is described in terms of two different models, namely, a relatively exact model of polyelectrolyte-nonionic surfactant adsorption and a simple additive model. It is shown that the additive model may be used to predict the equilibrium surface tension for the mixtures of high- and low-molecular-mass surfactants.