Molecular dynamics study of the influence of surfactant structure on surfactant-facilitated spreading of droplets on solid surfaces

The spreading of a partially wetting aqueous drop in air on a hydrophobic surface can be facilitated by the adsorption of surfactants from the drop phase onto the air/aqueous and aqueous/hydrophobic solid interfaces of the drop. At the contact line at which these interfaces meet, conventional surfactants with a linear alkyl hydrophobic chain attached to a polar group adsorb onto the surfaces, forming monolayers which remain distinct as they merge at the contact juncture. The adsorption causes a decrease in the interfacial tensions and reduction in the contact angle but the angle remains above zero so the drop is still nonwetting. Trisiloxane surfactants with a T-shaped geometry in which the hydrophobic group is composed of a trisiloxane oligomer with a polar group attached at the center of the chain can give rise to a zero contact angle at the contact line and complete wetting (superspreading). Experimental evidence suggests the adsorption of the T-shaped molecule, in addition to significantly decreasing the tensions of the interfaces (relative to the conventional surfactants), promotes the formation of a precursor film consisting of a surfactant bilayer at the contact line which facilitates the spreading. The aim of this study is to use molecular dynamics to examine if the T-shaped structure can promote spreading by the formation of a bilayer and to contrast this case with that of the linear chain surfactant where complex assembly does not occur. The simulation models the solvent as a monatomic liquid, the substrate as a particle lattice, and the surfactants as united atom structures, with all interactions given by Lennard-Jones potentials. We start with a base case in which the solvent partially wets a substrate comprised of a lattice of particles. We demonstrate that adsorbed T-shaped surfactant monolayers can, when the interaction between the solvent and the hydrophile particles is strong enough, assemble into a bilayer, allowing the drop to extend to a thin planar film. In the case of the flexible linear chain surfactant, there is no interaction between the monolayers on the two interfaces in the case of a strong hydrophile-solvent interaction and less coordination for a weaker interaction. In either case, the monolayers remain distinct, as the surfactant only marginally improves wetting.     

Wetting characteristics of aqueous rhamnolipids solutions

The wetting properties of surfactants on solid surfaces form the basis of many industrial and biological processes. The preferential adsorption of the surfactants from aqueous solutions onto solid surfaces alter the adhesion tension of the surface and this behavior may cause partial to complete wetting of the surfaces by the aqueous surfactant solutions. However, different types of surfactants show different wetting characteristics. To study the wetting properties of biologically produced rhamnolipids (RL), advancing contact angles of the aqueous solutions of the RL mixture of R1 and R2 in a ratio of R2/R1=1.1 were measured as a function of surfactant concentration. For a comparison of the wetting performance, sodium dodecyl sulfate (SDS) was chosen as the reference surfactant. A hydrophilic glass surface, a hydrophobic polymer, polyethylene terephthalate (PET), and gold surface were used as the solid surfaces to determine the wetting characteristics of rhamnolipids. At low surfactant concentrations (RL concentration <3x10(-5)M, SDS concentration<3x10(-4)M) contact angle (Theta) varied in a certain range depending on the character of the surfactant interactions with the surface. This was followed by a decrease in contact angle. Parallel to this behavior, at low surfactant concentrations the adhesion tension decreased, then remained constant and an increase at higher surfactant concentrations was obtained on hydrophobic surfaces. On hydrophilic surfaces a steady decrease in adhesion tension was observed with both surfactant solutions.     

Single-chain surfactant monolayer on carbon paste electrode and its application for the studies on the direct electron transfer of hemoglobin

A variety of single-chain surfactants with different charge properties and tail lengths can spontaneously adsorb on the hydrophobic surface of carbon paste electrode and form stable monolayers on the electrode surface. Hemoglobin (Hb) was successfully immobilized on these surfactant monolayers to form stable protein-surfactant composite films regardless of the charge and the tail length of surfactants. The resulting surface-confined Hb exhibited well-defined direct electron-transfer behaviors in all positively, neutrally and negatively charged surfactant films, suggesting the important role of hydrophobic interactions in the adsorption of Hb on surfactant films. When the density of surfactant monolayers was controlled to be the same, Hb was found to possess a better direct electron-transfer behavior on monolayers of cationic surfactants with a longer tail length. This, in combination with the tunneling effect in the direct electron transfer of Hb on surfactant films, demonstrated that the adsorption of Hb on surfactant monolayers may be mainly achieved by the partial intercalation of Hb in the loose structures of surfactant films through hydrophobic interactions between the alkane chains of surfactants and the hydrophobic regions of Hb. The native conformation of Hb adsorbed on these surfactant films was proved to be unchanged, reflected by the unaltered ultraviolet-visible (UV-vis) and reflection-absorption infrared (RAIR) spectra, and by the catalytic activity toward hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) in comparison with the free Hb molecules.     

Adsorption of comb copolymers on weakly attractive solid surfaces

In this work continuum and lattice Monte Carlo simulation methods are used to study the adsorption of linear and comb polymers on flat surfaces. Selected polymer segments, located at the tips of the side chains in comb polymers or equally spaced along the linear polymers, are attracted to each other and to the surface via square-well potentials. The rest of the polymer segments are modeled as tangent hard spheres in the continuum model and as self-avoiding random walks in the lattice model. Results are presented in terms of segment-density profiles, distribution functions, and radii of gyration of the adsorbed polymers. At infinite dilution the presence of short side chains promotes the adsorption of polymers favoring both a decrease in the depletion-layer thickness and a spreading of the polymer molecule on the surface. The presence of long side chains favors the adsorption of polymers on the surface, but does not permit the spreading of the polymers. At finite concentration linear polymers and comb polymers with long side chains readily adsorb on the solid surface, while comb polymers with short side chains are unlikely to adsorb. The simple models of comb copolymers with short side chains used here show properties similar to those of associating polymers and of globular proteins in aqueous solutions, and can be used as a first approximation to investigate the mechanism of adsorption of proteins onto hydrophobic surfaces.     

Measurement of the kinetic rate constants for the adsorption of superspreading trisiloxanes to an air/aqueous interface and the relevance of these measurements to the mechanism of superspreading

Super-spreading trisiloxane surfactants are a class of amphiphiles which consist of nonpolar trisiloxane headgroups ((CH3)3-Si-O)2-Si(CH3)(CH2)3-) and polar parts composed of between four and eight ethylene oxides (ethoxylates, -OCH2CH2-). Millimeter-sized aqueous drops of trisiloxane solutions at concentrations well above the critical aggregate concentration spread rapidly on very hydrophobic surfaces, completely wetting out at equilibrium. The wetting out can be understood as a consequence of the ability of the trisiloxanes at the advancing perimeter of the drop to adsorb at the air/aqueous and aqueous/hydrophobic solid interfaces and to reduce considerably the tensions of these interfaces, creating a positive spreading coefficient. The rapid spreading can be due to maintaining a positive spreading coefficient at the perimeter as the drop spreads. However, the air/aqueous and solid/aqueous interfaces at the perimeter are depleted of surfactant by interfacial expansion as the drop spreads. The spreading coefficient can remain positive if the rate of surfactant adsorption onto the solid and fluid surfaces from the spreading aqueous film at the perimeter exceeds the diluting effect due to the area expansion. This task is made more difficult by the fact that the reservoir of surfactant in the film is continually depleted by adsorption to the expanding interfaces. If the adsorption cannot keep pace with the area expansion at the perimeter, and the surface concentrations become reduced at the contact line, a negative spreading coefficient which retards the drop movement can develop. In this case, however, a Marangoni mechanism can account for the rapid spreading if the surface concentrations at the drop apex are assumed to remain high compared to the perimeter so that the drop is pulled out by the higher tension at the perimeter than at the apex. To maintain a high apex concentration, surfactant adsorption must exceed the rate of interfacial dilation at the apex due to the outward flow. This is conceivable because, unlike that at the contact line, the surfactant reservoir in the liquid at the drop center is not continually depleted by adsorption onto an expanding solid surface. In an effort to understand the rapid spreading, we measure the kinetic rate constants for adsorption of unaggregated trisiloxane surfactant from the sublayer to the air/aqueous surface. The kinetic rate of adsorption, computed assuming the bulk concentration of monomer to be uniform and undepleted, represents the fastest that surfactant monomer can adsorb onto the air/aqueous surface in the absence of direct adsorption of aggregates. The kinetic constants are obtained by measuring the dynamic tension relaxation as trisiloxanes adsorb onto a clean pendant bubble interface. We find that the rate of kinetic adsorption is only of the same order as the area expansion rates observed in superspreading, and therefore the unaggregated flux cannot maintain very high surface concentrations at the air/aqueous interface, either at the apex or at the perimeter. Hence in order to maintain either a positive spreading coefficient or a Marangoni gradient, the surfactant adsorptive flux needs to be augmented, and the direct adsorption of aggregates (which in the case of the trisiloxanes are bilayers and vesicles) is suggested as one possibility.     

支链化阳离子和甜菜碱表面活性剂对聚四氟乙烯表面润湿性的影响

利用座滴法研究了支链化阳离子表面活性剂十六烷基羟丙基氯化铵(C₁₆GPC)和两性离子表面活性剂十六烷基羧酸甜菜碱(C₁₆GPB)在聚四氟乙烯(PTFE)表面上的吸附机制和润湿性质, 考察了表面活性剂浓度对表面张力、接触角、粘附张力、固液界面张力和粘附功的影响趋势. 研究发现, 低浓度条件下, 表面活性剂疏水支链的多个亚甲基基团与PTFE表面发生相互作用, 分子以平躺的方式吸附到固体界面; 支链化表面活性剂形成胶束的阻碍较大, 浓度大于临界胶束浓度(cmc)时, C₁₆GPC和C₁₆GPB分子在固液界面上继续吸附, 与PTFE作用的亚甲基基团减少, 分子逐渐直立, 固液界面自由能(γ_sl)明显降低. 对于支链化的阳离子和甜菜碱分子, 接触角均在浓度高于cmc后大幅度降低.     

Adsorption Behavior of Lysozyme and Tween 80 at Hydrophilic and Hydrophobic Silica–Water Interfaces

Nonionic surfactants such as Tween 80 are used commercially to minimize protein loss through adsorption and aggregation and preserve native structure and activity. However, the specific mechanisms underlying Tween action in this context are not well understood. Here, we describe the interaction of the well-characterized, globular protein lysozyme with Tween 80 at solid–water interfaces. Hydrophilic and silanized, hydrophobic silica surfaces were used as substrates for protein and surfactant adsorption, which was monitored in situ, with ellipsometry. The method of lysozyme and Tween introduction to the surfaces was varied in order to identify the separate roles of protein, surfactant, and the protein–surfactant complex in the observed interfacial behavior. At the hydrophobic surface, the presence of Tween in the protein solution resulted in a reduction in amount of protein adsorbed, while lysozyme adsorption at the hydrophilic surface was entirely unaffected by the presence of Tween. In addition, while a Tween pre-coat prevented lysozyme adsorption on the hydrophobic surface, such a pre-coat was completely ineffective in reducing adsorption on the hydrophilic surface. These observations were attributed to surface-dependent differences in Tween binding strength and emphasize the importance of the direct interaction between surfactant and solid surface relative to surfactant–protein association in solution in the modulation of protein adsorption by Tween 80.     

Adsorption of different amphiphilic molecules onto polystyrene latices

In order to know the influence of the surface characteristics and the chain properties on the adsorption of amphiphilic molecules onto polystyrene latex, a set of experiments to study the adsorption of ionic surfactants, nonionic surfactants and an amphiphilic synthetic peptide on different latex dispersions was performed. The adsorbed amount versus the equilibrium surfactant concentration was determined. The main adsorption mechanism was the hydrophobic attraction between the nonpolar tail of the molecule and the hydrophobic regions of the latex surface. This attraction overcame the electrostatic repulsion between chains and latex surface with identical charge sign. However, the electrostatic interactions chain-surface and chain-chain also played a role. General patterns for the adsorption of ionic chains on charged latex surfaces could be established. Regarding the shape, the isotherms presented different plateaus corresponding to electrostatic effects and conformational changes. The surfactant size also affects the adsorption results: the higher the hydrophilic moiety in the surfactant molecule the lower the adsorbed amount.     

Modeling of confinement-induced phase transitions for surfactant layers on amphiphilic surfaces

A self-consistent field model is used to consider a solution of positively charged surfactants up to its critical micellization concentration adsorbing onto two surfaces in close proximity. Each surface mimics a polystyrene sulfonate interface; that is, hydrophobic properties are combined with a (fixed) negative charge. We observe large and sudden changes in adsorption as a function of separation, which are not normally considered when interpreting surface force measurements. The parameters are chosen such that the adsorbed surfactant layer is of a monolayer type when the surfaces are far apart. A typical interaction curve is presented for a fixed surfactant chemical potential, which is extracted from the set of adsorption isotherms each with a fixed slit width. When the slit width approaches the thickness of the two surfactant layers, a first-order phase transition takes place, which is driven by the unfavorable hydrophobic-water contacts. At the transition, the average orientation of the surfactants switches from a high concentration of tails at the surface to a bilayer configuration where tail profiles from both sides merge in the center. The headgroups are pulled slightly away from the surface. The interaction force jumps from a weak electrostatic repulsion at large distances (two effectively positively charged surface layers repel each other) to a strong electrostatic attraction at short distances (the central surfactant bilayer is attracted to the oppositely charged surfaces). The amount of adsorbed surfactants tend to decrease with decreasing distance between the surfaces but suddenly increases at the transition. Because of this, we anticipate that in surface force experiments, for example, there is a hysteresis associated with this transition: the forces and also the adsorbed amounts depend not only on the distance between the surfaces but also on the history if nonsufficient equilibration times are implemented.     

Model and experimental studies for contact angles of surfactant solutions on rough and smooth hydrophobic surfaces

Despite the practical need, no models exist to predict contact angles or wetting mode of surfactant solutions on rough hydrophobic or superhydrophobic surfaces. Using Gibbs' adsorption equation and a literature isotherm, a new model is constructed based on the Wenzel and Cassie equations. Experimental data for aqueous solutions of sodium dodecyl sulfate (SDS) contact angles on smooth Teflon surfaces are fit to estimate values for the adsorption coefficients in the model. Using these coefficients, model predictions for contact angles as a function of topological f (Cassie) and r (Wenzel) factors and SDS concentration are made for different intrinsic contact angles. The model is also used to design/tune surface responses. It is found that: (1) predictions compare favorably to data for SDS solutions on five superhydrophobic surfaces. Further, the model predictions can determine which wetting mode (Wenzel or Cassie) occurred in each experiment. The unpenetrated or partially penetrated Cassie mode was the most common, suggesting that surfactants inhibit the penetration of liquids into rough hydrophobic surfaces. (2) The Wenzel roughness factor, r, amplifies the effect of surfactant adsorption, leading to larger changes in contact angles and promoting total wetting. (3) The Cassie solid area fraction, f, attenuates the lowering of contact angles on rough surfaces. (4) The amplification/attenuation is understood to be due to increased/decreased solid-liquid contact-area.     

高分子表面活性剂在固/液界面上的吸附形态

采用紫外光谱、XPS研究了羧甲基纤维素型高分子表面活性剂在硅胶 /水界面上的吸附形态 ,结果表明随着高分子表面活性剂溶液浓度增大 ,分子在硅胶表面的吸附由单层逐渐变为多层 ,生成半胶束结构 .     

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.     

Forces between glass surfaces in mixed cationic-zwitterionic surfactant systems

We report atomic force microscopy (AFM) measurements of the forces between borosilicate glass solids in aqueous mixtures of cationic and zwitterionic surfactants. These forces are used to determine the adsorption of the surfactant as a function of the separation between the interfaces (proximal adsorption) through the application of a Maxwell relation. In the absence of cationic surfactant, the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS) undergoes little adsorption to glass at concentrations up to about 2/3 critical micelle concentration (cmc). In addition, DDAPS does not have much effect on the forces over the same concentration range. In contrast, the cationic surfactant dodecylpyridinium chloride (DPC) does adsorb to glass and does affect the force between glass surfaces at concentrations much lower than the cmc. In the presence of a small amount of DPC (0.05 mM = cmc/300), the net force between the glass surfaces is quite sensitive to the solution concentration of DDAPS. A model-independent thermodynamic argument is used to show that the surface excess of DDAPS depends on the separation between the glass interfaces when the cationic surfactant is present and that the surface excess of the cationic surfactant is more sensitive to interfacial separation in the presence of the zwitterionic surfactant. The change in adsorption of the zwitterionic surfactant is explained in terms of an intermolecular coupling between the long-range electrostatic force acting on the cationic surfactant and the short-range hydrophobic interaction between the alkyl chains on the cationic and zwitterionic surfactants. The adsorptions of cationic and zwitterionic surfactants in mixtures were measured independently and simultaneously by attenuated total internal reflection infrared spectroscopy (ATR-IR). The adsorption of the zwitterionic surfactant is enhanced by the presence of a small amount of cationic surfactant.     

Wettability of a Polymethylmethacrylate Surface by Extended Anionic Surfactants: Effect of Branched Chains

The adsorption behaviors of extended anionic surfactants linear sodium dodecyl(polyoxyisopropene)₄ sulfate (L-C₁₂PO₄S), branched sodium dodecyl(polyoxyisopropene)₄ sulfate (G-C₁₂PO₄S), and branched sodium hexadecyl(polyoxyisopropene)₄ sulfate (G-C₁₆PO₄S) on polymethylmethacrylate (PMMA) surface have been studied. The effect of branched alkyl chain on the wettability of the PMMA surface has been explored. To obtain the adsorption parameters such as the adhesional tension and PMMA-solution interfacial tension, the surface tension and contact angles were measured. The experimental results demonstrate that the special properties of polyoxypropene (PO) groups improve the polar interactions and allow the extended surfactant molecules to gradually adsorb on the PMMA surface by polar heads. Therefore, the hydrophobic chains will point to water and the solid surface is modified to be hydrophobic. Besides, the adsorption amounts of the three extended anionic surfactants at the PMMA–liquid interface are all about 1/3 of those at the air–liquid interface before the critical micelle concentration (CMC). However, these extended surfactants will transform their original adsorption behavior after CMC. The surfactant molecules will interact with the PMMA surface with the hydrophilic heads towards water and are prone to form aggregations at the PMMA–liquid interface. Therefore, the PMMA surface will be more hydrophilic after CMC. In the three surfactants, the branched G-C₁₆PO₄S with two long alkyl chains exhibits the strongest hydrophobic modification capacity. The linear L-C₁₂PO₄S is more likely to densely adsorb at the PMMA–liquid interface than the branched surfactants, thus L-C₁₂PO₄S possesses the strongest hydrophilic modification ability and shows smaller contact angles on PMMA surface at high concentrations.     

Adsorption and morphology transition of surfactants on hydrophobic surfaces: a lattice Monte Carlo study

In this work, we first show that there are only five independent interchange parameters in the surfactant-solvent-interface system in Larson's model, and then adsorption and morphology transition of surfactants on hydrophobic surfaces are studied by extensive lattice Monte Carlo simulations. In our simulations, we found that there exist six adsorbed morphologies: (1) premature admicelle, (2) hemisphere, (3) hemisphere-hemicylinder mixture, (4) wormlike hemicylinder, (5) perforated monolayer, and (6) monolayer. The surface morphologies and the amount of adsorption on hydrophobic surfaces are found to be affected obviously by two interchange parameters. One is the attractive interaction between tail groups and surface (chiTS), and the other is the solubility of head groups in bulk (chiHW). Phase diagrams in chiHW versus chiTS planes for surfactants with different hydrophobicities (chiTW) and for surfactants with different molecular structures are determined in this work, from which the transitions of surface morphologies and adsorption behaviors are discussed.     

苄基取代甜菜碱对聚四氟乙烯表面润湿性的影响

利用座滴法研究了两性离子表面活性剂苄基取代烷基羧基甜菜碱(BCB)和苄基取代烷基磺基甜菜碱(BSB)在聚四氟乙烯(PTFE)表面上的润湿性质,考察了表面活性剂浓度对接触角的影响趋势,并讨论了粘附张力、固-液界面张力和粘附功的变化规律.研究发现,在低浓度时,表面活性剂通过疏水作用吸附到PTFE表面,疏水链苄基取代支链化使其在固-液界面上的吸附明显低于气-液界面,接触角在很大的范围内保持不变.当体相浓度增加到大于临界胶束浓度(cmc)时, BCB和BSB分子在固-液界面上继续吸附,分子逐渐直立,造成PTFE-液体之间的界面张力(γ_SL)进一步降低,表面亲水性增加,接触角随浓度增加明显降低;另一方面, BSB由于具有较大的极性头,在高浓度时空间阻碍作用明显,导致其对PTFE表面润湿性改变程度小于BCB.     

Coarse-graining MARTINI model for molecular-dynamics simulations of the wetting properties of graphitic surfaces with non-ionic, long-chain, and T-shaped surfactants

We report on a molecular dynamics investigation of the wetting properties of graphitic surfaces by various solutions at concentrations 1-8 wt. % of commercially available non-ionic surfactants with long hydrophilic chains, linear or T-shaped. These are surfactants of length up to 160 A?. It turns out that molecular dynamics simulations of such systems ask for a number of solvent particles that can be reached without seriously compromising computational efficiency only by employing a coarse-grained model. The MARTINI force field with polarizable water offers a framework particularly suited for the parameterization of our systems. In general, its advantages over other coarse-grained models are the possibility to explore faster long time scales and the wider range of applicability. Although the accuracy is sometimes put under question, the results for the wetting properties by pure water are in good agreement with those for the corresponding atomistic systems and theoretical predictions. On the other hand, the bulk properties of various aqueous surfactant solutions indicate that the micellar formation process is too strong. For this reason, a typical experimental configuration is better approached by preparing the droplets with the surfactants arranged in the initial state in the vicinity of contact line. Cross-comparisons are possible and illuminating, but equilibrium contact angles as obtained from simulations overestimate the experimental results. Nevertheless, our findings can provide guidelines for the preliminary assessment and screening of surfactants. Most importantly, it is found that the wetting properties mainly depend on the length and apolarity of the hydrophobic tail, for linear surfactants, and the length of the hydrophilic headgroup for T-shaped surfactants. Moreover, the T-shaped topology appears to favor the adsorption of surfactants onto the graphitic surface and faster spreading.     

The effect of polymer adsorption on the wetting properties of partially hydrophobized magnetite

Upon reverse flotation of iron ore, the surface of the iron ore concentrate may become partially hydrophobized due to adsorption of flotation collector, which is facilitated by the calcium ions present in the process water. Hydrophobic areas on the concentrate surface may introduce problems in subsequent pelletization of the concentrate. A possible way to restore the wettability of the surface could be by modifying the surface with a hydrophilic polymer. The effect of hydrophilic polymers of different types, viz. cationic, anionic, and non-ionic, on the wettability of the magnetite surface after adsorption of a surfactant was investigated. Although all the polymers could adsorb on magnetite at pH 8.5, the contact angle measurements revealed that only anionic ammonium polyacrylate could decrease the contact angle of synthetic magnetite after surfactant adsorption to a level close to that of as-synthesized magnetite. Such effect was probably achieved due to shielding of the hydrophobic surfactant chains from the aqueous phase by hydrophilic polyacrylate molecules. The fact that polyacrylate adsorption on magnetite occurred via calcium ions makes polyacrylate suitable for application in calcium-rich process water. The results presented in this work illustrate that ammonium polyacrylate could be successfully used to improve the wettability of magnetite after adsorption of surfactants.     

Monte Carlo simulation of mixed lennard-jones nonionic surfactant adsorption at the liquid/vapor interface

New Monte Carlo simulations are presented for nonionic surfactant adsorption at the liquid/vapor interface of a monatomic solvent specifically investigating the roles of tail attraction and binary mixtures of different tail lengths. Surfactant molecules consist of an amphiphilic chain with a solvophilic head and a solvophobic tail. All molecules in the system, solvent and surfactant, are characterized by the Lennard-Jones (LJ) potential. 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 thermodynamic adsorption and surface-tension isotherms and compare results quantitatively to single-surfactant adsorption (Langmuir, 2007, 23, 1835). Surfactant tail groups with attractive interaction lead to cooperative adsorption at high surface coverage and higher maximum adsorption at the interface than those without. Moreover, adsorption and surface-tension isotherms with and without tail attraction are identical at low concentrations, deviating only near maximum coverage. Simulated binary mixtures of surfactants with differing lengths give intermediate behavior between that of the corresponding single-surfactant adsorption and surface-tension isotherms both with and without tail attraction. We successfully predict simulated mixture results with the thermodynamically consistent ideal adsorbed solution (IAS) theory for binary mixtures of unequal-sized surfactants using only the simulations from the single surfactants. Ultimately, we establish that a coarse-grained LJ surfactant system is useful for understanding actual surfactant systems when tail attraction is important and for unequal-sized mixtures of amphiphiles.     

Adsorption of multiblock copolymers onto a chemically heterogeneous surface: a model of pattern recognition

We present a statistical mechanical model, which is used to investigate the adsorption behavior of two-letter (AB) copolymers on chemically heterogeneous surfaces. The surfaces with regularly distributed stripes of two types (A and B) and periodic multiblock copolymers (Al)B(l))(x) are studied. It is assumed that A(B)-type segments selectively adsorb onto A(B)-type stripes. It is shown that the adsorption strongly depends on the copolymer sequence distribution and the arrangement of selectively adsorbing regions on the surface. The polymer-surface binding proceeds as a two-step process. At the first step, the copolymer having short blocks adsorbs onto the surface as an effective homopolymer, which does not feel chemical pattern. At the second step, when the polymer-surface attraction is sufficiently strong, the adsorbed chain adjusts its equilibrium conformation to reach the perfect bound state, thereby demonstrating ability for pattern recognition. The key element of this mechanism is the redistribution of strongly adsorbed copolymer diblocks A(l)B(l), which behave as surfactants, between multiple AB interfaces separating A and B stripes on the adsorbing surface. Such redistribution is accompanied by a well-pronounced decrease in the system entropy. We have found that marked pattern recognition is possible for copolymers with relatively short blocks at high polymer/surface affinities, beyond the adsorption threshold.