THEORETICAL MODELING OF IONIC SURFACTANT ADSORPTION ON MINERAL OXIDE SURFACES

A model for the adsorption of ionic surfactants on oppositely charged solid surfaces of uniform charge density is developed. The model is based on the assumption that, on the solid surface, adsorbed surfactant monomers, monolayered and bilayered surfactant aggregates of different sizes and specifically adsorbing ions of added electrolyte constitute a mixture of hard discs. It means that only excluded area interactions between the surface discs are taken into account. To avoid a rapid two-dimensional condensation of the adsorbed surfactant the potential energy per molecule in the surface aggregates, which is a sum of chemical and electrostatic interactions, is assumed to decrease linearly with the increasing aggregate size. The electrostatic interactions of ionic species with the charged solid surface are described in terms of the Guy-Chapman theory of the double layer formation. The appropriate equations for adsorption isotherms of surfactant and electrolyte ions are derived and used to predict the experimental adsorption isotherms of DTAB on the precipitated silica at two different salt concentrations in the aqueous solution, On the basis of the obtained results the evolution of the adsorbed phase structure and the charge of silica particles with an increasing surface coverage is discussed.     

Effect of surfactant structure on the adsorption of carboxybetaines at the air–water interface

In order to study the effect of charge on the adsorption of surfactants at the air–water interface, two carboxybetaines have been synthesized with different number of separation methylenes between their charged groups. After purification and structure confirmation, the equilibrium and dynamic surface tensions were measured as a function of surfactant concentration for both the cationic and neutral forms of the surfactant molecules. The effect of ionic strength on the adsorption process was also studied. The equilibrium surface tension values were interpreted according to the Langmuir model and the dynamic surface tension data, converted to surface concentration by the Langmuir parameters, are consistent with the assumption of diffusion control over the range of surfactant concentrations studied. The diffusion coefficients show a progressive decrease in the rate of adsorption when the number of methylene units between the betaine charged groups increase.     

十二烷基三甲基溴化铵在聚苯乙烯胶乳粒子上的吸附

实验测得C~1~2TAB在PS胶乳粒子表面的吸附等温线呈L型的二阶段吸附特征,这表明初始的C~1~2TA^+离子是将其季铵正电性头基吸引在PS链的负电性硫酸根端基上,并将碳氢链通过疏水相互作用吸附在PS链上。结合光子相关谱测得胶乳粒子流体力学半径R~H的变化,表明第I阶段围绕着这些初始吸附位的聚集吸附,产生平均聚集数为4.0的松散小聚集体,此时对应的浓度c/cmc=0.32是文献通常所指的临界表面胶团浓度csmc。其后的进一步聚集吸附最终生成了附着在PS链端基处且平均聚集数为19.5的球形吸附胶团。这一饱和吸附的结果增加了胶乳粒子在水溶液中的分散稳定性。     

Equilibrium and Dynamic Surface Tension of Carboxymethylchitosan and Alkyl Trimethylammonium Bromide Mixtures

A water-soluble derivative of chitosan, carboxymethylchitosan (CMCH), was mixed with alkyltrimethylammoniumbromides (C_mTAB) and was studied on the adsorption at air/water interface using equilibrium and dynamic surface tension method. The effects of surfactant and polymer concentrations, surfactant chain length, as well as pH of solution were investigated. Addition of the surfactants remarkably promotes the polymer adsorption. Increasing any one of surfactant concentration, surfactant chain length, and pH will facilitate the adsorption of the mixture whereas little effects of polymer concentration were observed. The results are explained in terms of the interaction between CMCH and C_mTAB under different conditions.     

Study on the Surface Property of Surfactant Ionic Liquids Solutions

The surfactant TX-100 can be dissolved in ionic liquid bmimPF6 and decrease the surface tension of 1-buty1-3-methylimidazolium hexafluorophosphate (bmimPF6) solutions. Here, we confirmed that in this new system, the pure solvents need rearrangement at the air-wate rinterface at the initial stage. The dynamic surface tension (DST) study shows that at the initial adsorption stage, the adsorption model of surfactant accords with the diffusion-controlled adsorption mechanism, and the dilute ionic liquids solutions is further close to the diffusion-controlled adsorption.     

A review on experimental studies of surfactant adsorption at the hydrophilic solid-water interface

The progresses of understanding of the surfactant adsorption at the hydrophilic solid-liquid interface from extensive experimental studies are reviewed here. In this respect the kinetic and equilibrium studies involves anionic, cationic, non-ionic and mixed surfactants at the solid surface from the solution. Kinetics and equilibrium adsorption of surfactants at the solid-liquid interface depend on the nature of surfactants and the nature of the solid surface. Studies have been reported on adsorption kinetics at the solid-liquid interface primarily on the adsorption of non-ionic surfactant on silica and limited studies on cationic surfactant on silica and anionic surfactant on cotton and cellulose. The typical isotherm of surfactants in general, can be subdivided into four regions. Four-regime isotherm was mainly observed for adsorption of ionic surfactant on oppositely charged solid surface and adsorption of non-ionic surfactant on silica surface. Region IV of the adsorption isotherm is commonly a plateau region above the CMC, it may also show a maximum above the CMC. Isotherms of four different regions are discussed in detail. Influences of different parameters such as molecular structure, temperature, salt concentration that are very important in surfactant adsorption are reviewed here. Atomic force microscopy study of different surfactants show the self-assembly and mechanism of adsorption at the solid-liquid interface. Adsorption behaviour and mechanism of different mixed surfactant systems such as anionic-cationic, anionic-non-ionic and cationic-non-ionic are reviewed. Mixture of surface-active materials can show synergistic interactions, which can be manifested as enhanced surface activity, spreading, foaming, detergency and many other phenomena.     

全氟丁基磺酸钠与辛基三乙基溴化铵的相互作用

通过测定辛基三乙基溴化铵(C8H17N(CH2CH3)3Br,C8NE)与全氟丁基磺酸钠(C4F9SO3Na,C4F)组成的不同混合比的碳氢-碳氟正负离子表面活性剂混合体系的表面张力,得到不同摩尔比时C8NEC4F体系的临界胶束浓度(cmc)、cmc处的表面张力(γcmc)、总饱和吸附量、不同表面张力时表面吸附层的组成,利用Gibbs-Duhem方程求得cmc处的胶团组成。 采用规则溶液理论计算了胶团中分子间相互作用参数(βm),并求得cmc以上的胶团组成。 实验表明,C8NEC4F复配体系的cmc远远小于单体系的cmc,这也体现在该体系的βm负值很大,胶团内分子相互作用很强。 但是C4F与C8NE复配后γcmc较C4F单体系的变化幅度不是特别大(γcmc降低2~4 mN/m),这是由于C8NEC4F碳链的不对称性导致部分C8NE的碳链在溶液表面弯曲而覆盖了C4F端基CF3基团。 表面吸附层中氟表面活性剂相对于本体溶液是富集的,即使对于C8NE大大过量的体系,表面吸附层组成也在等摩尔附近;对于C4F过量的体系,C4F在表面吸附层中的比例比溶液中的略高。 随着表面张力的降低,表面吸附层的组成相对更偏向于氟表面活性剂。 cmc处的胶团组成随着体系中C4F含量的增大偏向于形成显著富含C4F的胶团,对于C8NE大大过量的体系,胶团组成接近等摩尔。 cmc之后的胶团组成接近等摩尔,主要归因于此时静电相互作用占主导,这和溶液配制过程中发现复配体系超过cmc一定浓度后就易生成沉淀的现象是相符的。     

Interaction of Plasma Proteins with Tri‐quaternary Ammonium Salt Cationic Surfactant Studied by QCM‐D

A tri‐quaternary ammonium salt cationic surfactant was synthesized. Its structure was confirmed by using Fourier‐transform infrared spectroscopy, ¹H nuclear magnetic resonance spectroscopy, and X‐ray photoelectron spectroscopy analyses. Three model surfaces, including Au‐CH₃, Au‐OH and Au‐COOH, were fabricated. Adsorptions of surfactant on the three model surfaces and subsequent plasma proteins adsorption were investigated by quartz crystal microbalance with dissipation (QCM‐D). The mass of surfactant on the Au‐COOH surface was the largest, followed by that on the Au‐CH₃ surface, and that on the Au‐OH surface. These results suggested that the main driving force of surfactant immobilization was electrostatic interaction followed by hydrophobic interaction. Based on the results obtained, we concluded that the protein mass adsorbed on Au‐CH₃‐ S , Au‐OH‐ S , and Au‐COOH‐ S surfaces depended on the protein size and orientation. The mass and thickness of S on the Au‐COOH surface is the largest and the protein adsorption capacity of Au‐COOH‐ S surface is inferior to that of Au‐CH₃‐ S . The Au‐COOH‐ S surface could inhibit lysozyme adsorption, maintain the adsorption balance of bovine serum albumin, and induce fibrinogen‐binding protein adsorption.     

十八烷基二甲基氯化铵水溶液动态表面张力及吸附动力学研究

使用最大气泡法测定了十八烷基二甲基氯化铵（C_(18)DAC）水溶液的动态表 面张力,考察了浓度、温度等对其DST的影响,详细表征了DST随时间的变化过程, 计算了动态表面张力的各种参数（n,t_i,t~*,t_m,R_(1/2)）。结合Word- Tordai方程计算了表观扩散系数（D_a）和吸附势垒（E_a）,对其吸附动力学模式 进行了研究,探讨了DST参数的物理意义。结果表明,t~*值越小,吸附势垒E_a越 大,宏观扩散系数D_a越小,表面活性剂分子越不易吸附在溶液表面;C_(18)DAC低 浓度时吸附属于扩散控制模式,高浓度时属于混合控制模式;高浓度时,在吸附初 期（t → 0）为扩散控制模式,吸附后期（t → ∞）为混合控制模式。     

Comparison of solvation-effect methods for the simulation of peptide interactions with a hydrophobic surface

In this study we investigated the interaction behavior between thirteen different small peptides and a hydrophobic surface using three progressively more complex methods of representing solvation effects: a united-atom implicit solvation method [CHARMM 19 force field (C19) with Analytical Continuum Electrostatics (ACE)], an all-atom implicit solvation method (C22 with GBMV), and an all-atom explicit solvation method (C22 with TIP3P). The adsorption behavior of each peptide was characterized by the calculation of the potential of mean force as a function of peptide-surface separation distance. The results from the C22/TIP3P model suggest that hydrophobic peptides exhibit relatively strong adsorption behavior, polar and positively-charged peptides exhibit negligible to relatively weak favorable interactions with the surface, and negatively-charged peptides strongly resist adsorption. Compared to the TIP3P model, the ACE and GBMV implicit solvent models predict much stronger attractions for the hydrophobic peptides as well as stronger repulsions for the negatively-charged peptides on the CH(3)-SAM surface. These comparisons provide a basis from which each of these implicit solvation methods may be reparameterized to provide closer agreement with explicitly represented solvation in simulations of peptide and protein adsorption to functionalized surfaces.     

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.     

Calorimetric studies of cationic surfactant adsorption at low surface coverages on different silica samples

Calorimetric measurements of adsorption for the surfactant (benzyldimethyldodecylammonium bromide) and its polar head-group (benzyltrimethylammonium bromide) from aqueous solutions on two different silica surfaces (hydrophilic and hydrophobic one) allow a more detailed picture of the subsequent stages of the adsorption process to be drawn. It is possible to determine more precisely a boundary between the adsorption of individual molecules and the formation of surface aggregates. The local disruption of the structure of the interfacial water molecules by surfactant cations gives an endothermic contribution to the total enthalpy of displacement. This contribution depends on the length of alkyl chain as well as on the type and the origin of solid surface.     

Single and binary adsorption of proteins on ion-exchange adsorbent: The effectiveness of isothermal models

Simultaneous and sequential adsorption equilibria of single and binary adsorption of bovine serum albumin and bovine hemoglobin on Q Sepharose FF were investigated in different buffer constituents and initial conditions. The results in simultaneous adsorption showed that both proteins underwent competitive adsorption onto the adsorbent following greatly by protein-surface interaction. Preferentially adsorbed albumin complied with the universal rule of ion-exchange adsorption whereas buffer had no marked influence on hemoglobin adsorption. Moreover, an increase in initial ratios of proteins was benefit to a growth of adsorption density. In sequential adsorption, hemoglobin had the same adsorption densities as single-component adsorption. It was attributed to the displacement of preadsorbed albumin and multiple layer adsorption of hemoglobin. Three isothermal models (i.e. extended Langmuir, steric mass-action, and statistical thermodynamic (ST) models) were introduced to describe the ion-exchange adsorption of albumin and hemoglobin mixtures. The results suggested that extended Langmuir model gave the lowest deviation in describing preferential adsorption of albumin at a given salt concentration while steric mass-action model could very well describe the salt effect in albumin adsorption. For weaker adsorbed hemoglobin, ST model was the preferred choice. In concert with breakthrough data, the research further revealed the complexity in ion-exchange adsorption of proteins.     

表面活性剂溶液动态表面张力及吸附动力学研究

本文简介了动态表面张力的定义、测定方法及吸附动力学,对非离子、阴离子、阳离子及两性表面活性剂溶液动态表面张力的研究情况进行了总结,重点讨论了浓度、温度、添加剂及化学结构因素对动态表面张力的影响.     

Conformational reorientation of immunoglobulin G during nonspecific interaction with surfaces.

To achieve a better understanding of the nonspecific adsorption process of proteins on solid surfaces, the mechanism of this interaction was investigated by a model system comprising the structurally flexible ("soft") protein goat anti-rabbit immunoglobulin G and a set of chemically defined surfaces. The thermodynamic properties of both protein and surfaces were derived from contact angle measurements by applying the Lifshitz-van der Waals acid-base approach, and the Gibbs free enthalpy of interaction was calculated. The protein shows two conformational states, one hydrophobic and the other hydrophilic. The interaction energy indicates that the hydrophobic conformation favorably adsorbs onto the surfaces. With real-time binding kinetics, measured by a supercritical angle fluorescence biosensor, we show that during the nonspecific adsorption the protein performs a reorientation in its three-dimensional amino acid structure from a hydrophilic to a hydrophobic molecular structure. Unlike the rates of adsorption and desorption, the transition rate is independent of the type of surface and only influenced by the structural reorganization of the protein.     

Electrochemical behavior of hemoglobin in neutral surfactants with different poly(ethylene oxide) unit lengths adsorbed on an electrode

The direct electron transfer and adsorption behavior of hemoglobin(Hb) in a series of surfactants with different poly(ethylene oxide)(PEO) unit lengths on a glassy carbon electrode have been studied.With a surfactant of appropriate PEO unit length,the surfactant film-modified electrode exhibited a more stable adsorption state with a larger surface coverage of Hb and a more positive formal potential,which can be attributed to the effect of hydrogen bonding between proteins and surfactants.The electrochemical behavior of surfactants with different PEO unit lengths is discussed in detail.Moreover,UV-visible spectroscopy demonstrated that the structure of Hb was not destroyed in the surfactant films.The electrocatalytic activity of hydrogen peroxide on three neutral surfactant-modified electrodes has also been investigated.     

Physicochemical Properties and Surface Tension Prediction of Mixed Surfactant Systems: Triton X‐100 with Dodecylpyridinium Bromide and Triton X‐100 with Sodium Dodecylsulfonate

Dependences of the surface tension of aqueous solutions of ionic (dodecylpyridinium bromide, sodium dodecylsulfonate) and nonionic (Triton X‐100) surfactants and their mixtures on total surfactant concentration and solution composition were studied, and the surface tension of the mixed systems were predicted using different Miller's model. It was found that how to select the model for calculation of ω is corresponding to the degree of the deviation from the ideality during the adsorption of mixed surfactants. The compositions of micelles and adsorption layers at air‐solution interface as well as parameters (β^m, β^ads) of headgroup‐headgroup interaction between the molecules of ionic and nonionic surfactants were calculated based on Rubingh model. The parameters (B₁) of chain‐chain interaction between the molecules of ionic and nonionic surfactants were calculated based on Maeda model. The free energy of micellization calculated from the phase separation model (ΔG ₂ ^m ), and by Maeda's method (ΔG ₁ ^m ) agree reasonably well at high content of nonionic surfactant. The excess free energy ΔG _ads ^E and ΔG _m ^E (except α=0.4) for TX‐100/SDSn system are more negative than that TX‐100/DDPB system. These can be probably explained with the EO groups of TX‐100 surfactant carrying partial positive charge.     

Binding of salivary proteins and oral bacteria to hydrophobic and hydrophilic surfaces in vivo and in vitro

Chemical modifications of mineral surfaces were performed in order to gain insight into what surface properties are decisive of the accumulation of dental plaque. A non-charged, hydrophilic surface was made by two consecutive plasma polymerizations, firstly with allyl alcohol, secondly with acrylic acid, followed by adsorption of a poly(ethylene glycol)-poly(ethylene imine) adduct. A strongly hydrophobic surface was obtained by plasma polymerization of hexamethyldisiloxane. Ellipsometry was used to monitor protein interaction with the surfaces. The hydrophilic surface gave very little adsorption of both a model protein, IgG, and of saliva proteins. The hydrophobic surface, on the other hand, adsorbed high amounts of both types of proteins. In vitro adhesion of an oral bacterium,S. mutans, as well as in vivo studies, gave the opposite result, the hydrophobic surface giving less adhesion and less plaque accumulation than the hydrophilic surface. A tentative explanation of this behavior is that the saliva proteins that bind to the hydrophobic surface adsorb in an unnatural conformation which does not favor bacteria adherence.     

基于密度泛函理论计算的CO2在SrTiO3(100)表面的吸附

通过密度泛函理论的第一性原理,模拟了CO2分子在SrTiO3(100)表面TiO2-和SrO-位点上的吸附行为,获得了CO2在几种不同吸附模型下的结构参数及表面吸附能,进而研究了吸附机理和结构稳定性.计算结果表明,当CO2的C原子吸附在SrTiO3(100)表面SrO-及TiO2-位点的氧原子上时,吸附结构较稳定,尤其是C、O原子共吸附在TiO2-位点时最稳定,而其余吸附模型则不稳定.对吸附稳定模型的Mulliken布局数及态密度分析显示:CO2分子在SrTiO3(100)表面吸附主要是由于SrTiO3(100)面的电子跃迁至CO2分子,CO2分子得到电子形成弯曲的CO2-阴离子结构,并伴随着C-O键的伸长,从而达到吸附活化CO2的目的.