Effect of surfactants on wetting of super-hydrophobic surfaces

The effect of surfactants on wetting behavior of super-hydrophobic surfaces was investigated. Super-hydrophobic surfaces were prepared of alkylketene dimer (AKD) by casting the AKD melt in a specially designed mold. Time-dependent studies were carried out, using the axisymmetric drop shape analysis method for contact angle measurement of pure water on AKD surfaces. The results show that both advancing and receding contact angles of water on the AKD surfaces increase over time ( approximately 3 days) and reach the values of about 164 and 147 degrees , respectively. The increase of contact angles is due to the development of a prickly structure on the surface (verified by scanning electron microscopy), which is responsible for its super-hydrophobicity. Aqueous solutions of sodium acetate, sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, and n-decanoyl-n-methylglucamine were used to investigate the wetting of AKD surfaces. Advancing and receding contact angles for various concentrations of different surfactant solutions were measured. The contact angle results were compared to those of a number of pure liquids with surface tensions similar to those of surfactant solutions. It was found that although the surface tensions of pure liquids and surfactant solutions at high concentrations are similar, the contact angles are very different. Furthermore, the usual behavior of super-hydrophobic surfaces that turn super-hydrophilic when the intrinsic contact angle of liquid on a smooth surface (of identical material) is below 90 degrees was not observed in the presence of surfactants. The difference in the results for pure liquids and surfactant solutions is explained using an adsorption hypothesis.     

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.     

Drying dissipative structures of nonionic surfactants in aqueous solution

Macroscopic and microscopic dissipative structural patterns form in the course of drying a series of aqueous solutions of polyoxyethylenealkyl ethers. The shift from the single round hill with accumulated surfactant molecules to the broad ring patterns of the hill in a macroscopic scale occurs as the HLB (hydrophile-liophile balance) of the surfactant molecules increases. The patterns correlate intimately with the HLB values of the surfactants. Microscopic patterns of small blocks, starlike patterns, and branched strings are formed. The size and shape of the surfactant molecules themselves influence the drying patterns in part. The pattern area and the time to dryness have been discussed as a function of surfactant concentration and HLB of the surfactants. The convection flow of water accompanying the surfactant molecules, the change in the contact angles at the drying frontier between solution and substrate in the course of dryness, and interactions among the surfactants and substrate are important for the macroscopic pattern formation. Microscopic patterns are determined in part by the shape and size of the molecules, translational Brownian movement of the surfactant molecules, and the electrostatic and hydrophobic interactions between surfactants and/or between the surfactant and substrate in the course of solidification.     

Thin wetting films from aqueous solutions of surfactants and phospholipid dispersions

The microscopic thin wetting film method was used to study the stability of wetting films from aqueous solution of surfactants and phospholipid dispersions on a solid surface. In the case of tetradecyltrimethylammonium bromide (C(14)TAB) films the experimental data for the receding contact angle, film lifetime, surface potential at the vapor/solution and solution/silica interface were used to analyze the stability of the studied films. It is shown that with increasing C(14)TAB concentration charge reversal occurs at both (vapor/solution and solution/silica) interfaces, which affects the thin-film stability. The spontaneous rupture of the thin aqueous film was interpreted in terms of the earlier proposed heterocoagulation mechanism. The presence of the mixed cationic/anionic surfactants was found to lower contact angles and suppresses the thin aqueous film rupture, thus inducing longer film lifetime, as compared to the pure amine system. In the case of mixed surfactants hetero-coagulation could arise through the formation of ionic surfactant complexes. The influence of the melting phase-transition temperature T(c) of the dimyristoylphosphatiddylcholine (DMPC) on the stability of thin films from dispersions of DMPC small unilamellar vesicles on a silica surface was studied by measuring the film lifetime and the TPC expansion rate. The stability of thin wetting films formed from dispersions of DMPC small unilamellar vesicles was investigated by the microinterferometric method. The formation of wetting films from diluted dispersions of DMPC multilamellar vesicles was studied in the temperature range 25-32 degrees C. The stability of thin film of lipid vesicles was explained on the basis of hydrophobic interactions. The results obtained show that the stability of wetting films from aqueous solutions of single cationic and mixed cationic-anionic surfactants has electrostatic origin, whereas the stability of the phospholipid film is due to hydrophobic interaction.     

Effect of anionic surfactant and short-chain alcohol mixtures on adsorption at quartz/water and water/air interfaces and the wettability of quartz

Measurements of the advancing contact angles for aqueous solutions of sodium dodecyl sulfate (SDDS) or sodium hexadecyl sulfonate (SHS) in mixtures with methanol, ethanol, or propanol on a quartz surface were carried out. On the basis of the obtained results and Young and Gibbs equations the critical surface tension of quartz wetting, the composition of the surface layer at the quartz-water interface, and the activity coefficients of the anionic surfactants and alcohols in this layer as well as the work of adhesion of aqueous solutions of anionic surfactant and alcohol mixtures to the quartz surface were determined. The analysis of the contact angle data showed that the wettability of quartz changed visibly only in the range of alcohol and anionic surfactant concentration at which these surface-active agents were present in the solution in the monomeric form. The analysis also showed that there was a linear dependence between the adhesion and the surface tension of aqueous solutions of anionic surfactant and alcohol mixtures. This dependence can be described by linear equations for which the constants depend on the anionic surfactant and alcohol concentrations. The slope of all linear dependence between adhesion and surface tension was positive. The critical surface tension of quartz wetting determined from this dependence by extrapolating the adhesion tension to the value equal to the surface tension (for contact angle equal zero) depends on the assumption whether the concentration of anionic surfactant or alcohol was constant. Its average value is equal to 29.95mN/m and it is considerably lower than the quartz surface tension. The positive slope of the adhesion-surface tension curves was explained by the possibility of the presence of liquid vapor film beyond the solution drop which settled on the quartz surface and the adsorption of surface-active agents at the quartz/monolayer water film-water interface. This conclusion was confirmed by the work of adhesion of aqueous solutions of anionic surfactants and short-chain alcohol mixtures to the quartz surface determined on the basis of the contact angle data and molar fraction of anionic surfactants and alcohols and their activity coefficient in the surface layer.     

熔融灰渣侵蚀耐火材料的界面特性研究

研究了碱酸比不同的3种熔融煤灰(A、J1和J2)在莫来石和刚玉两种耐火材料上的界面特性。结果表明,A对莫来石和刚玉都有很好的润湿性,而J1和J2对这两种耐火材料则经历由不完全润湿到润湿的过程。熔渣与耐火材料(S-R)界面宽度随着接触角减小而增大。从灰渣开始熔融至1 520 ℃,熔融煤灰J1在耐火材料表面上的接触角随着倾角的增加而减小;当倾角为20°时,其接触角突然增大。SEM-EDS分析表明,由于界面反应,A比J1更容易渗透进入耐火材料内部。对于碱酸比不同的煤灰,渗透和酸碱反应是熔渣对耐火材料腐蚀的关键。     

Determination of the critical surface tension of wetting of minerals treated with surfactants by shear flocculation approach

This paper contributes the shear flocculation method as a new approach to determine the critical surface tension of wetting of minerals treated with surfactants. This newly developed approach is based on the decrease of the shear flocculation of the mineral suspension, with decreasing of the surface tension of the liquids used. The solution surface tension value at which shear flocculation does not occur can be defined as the critical surface tension of wetting (gamma c) of the mineral. By using the shear flocculation method, the critical surface tensions of wetting (gamma c) for calcite and barite minerals, treated with surfactants, were obtained as 30.9 and 35.0 mN/m, respectively. These values are in good agreement with data reported previously on the same minerals obtained by the contact angle measurement and flotation methods. The chemical agents used for the treatment of calcite and barite particles were sodium oleate and sodium dodecyl sulfate, respectively.     

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.     

Three-phase contact parameters measurements for silica-mixed cationic–anionic surfactant systems

The stability and interactions in thin wetting films between the silica surface and air bubble containing (a) straight chain C₁₀ amine and (b) cationic/anionic surfactant mixture of a straight chain C₁₀ amine with sodium C₈, C₁₀ and (straight chain) C₁₂ sulfonates, were studied using the microscopic thin wetting film method developed by Platikanov [D. Platikanov, J. Phys. Chem. 68 (1964) 3619]. Film lifetimes, three-phase contact (TPC) expansion rate, receding contact angles and surface tension were measured. The presence of the mixed cationic/anionic surfactants was found to lessen contact angles and suppresses the thin aqueous film rupture, thus inducing longer film lifetime, as compared to the pure amine system. In the case of mixed surfactants heterocoagulation could arise through the formation of positively charged interfacial complexes. Mixed solution of cationic and anionic surfactants shows synergistic lowering in surface tension. The formation of the interfacial complex at the air/solution interface was confirmed by surface tension data. It was also shown, that the chain length compatibility between the anionic and cationic surfactants system controls the strength of the interfacial complex. The observed phenomena were discussed in terms of the electrostatic heterocoagulation theory, where the interactions can be attractive or repulsive depending on the different surface activity and charge of the respective surfactants at the two interfaces.     

Control over wettability of polyethylene glycol surfaces using capillary lithography

We demonstrate that wettability of poly(ethylene glycol) (PEG) surfaces can be controlled using nanostructures with various geometrical features. Capillary lithography was used to fabricate PEG nanostructures using a new ultraviolet (UV) curable mold consisting of functionalized polyurethane with acrylate group (MINS101m, Minuta Tech.). Two distinct wetting states were observed depending of the height of nanostructures. At relatively lower heights (< 300 nm for 150 nm pillars with 500 nm spacing), the initial contact angle of water was less than 80 degrees and the water droplet easily invaded into the surface grooves, leading to a reduced contact angle at equilibrium (Wenzel state). At relatively higher heights (> 400 nm for 150 nm pillars with 500 nm spacing), on the other hand, the nanostructured PEG surface showed hydrophobic nature and no significant change in contact angle was observed with time (Cassie state). The presence of two wetting states was also confirmed by dynamic wetting properties and contact-angle hysteresis. The wetting transition from hydrophilic (bare PEG surface) to hydrophobic (PEG nanostructures) was described by the Cassie-Baxter equation assuming that enhanced hydrophobicity is due to the heterogeneous wetting mediated by an air pocket on the surface. The measured contact angles in the Cassie state were increased with increasing air fraction, in agreement with the theoretical prediction.     

The contact angles of surfactant solution on the quartz surface

Summary The contact angles formed on the quartz surface by 0,1 N NaCl aqueous solution at different pH were measured. The effect of surfactants such as anion-active sodium dodecyl sulphate (NaDS) and cation-active cetyltrimethylammonium bromide (CTAB) was investigated. The results are interpreted in terms of the Frumkin-Derjaguin theory of wetting.With 4 figures and 1 table     

Silicon-modified carbohydrate surfactants. VII: Impact of different silicon substructures on the wetting behaviour of carbohydrate surfactants on low-energy surfaces — distance decay of donor–acceptor forces

The wetting behaviour of carbohydrate surfactants bearing siloxane, carbosilane, polysilane or silane moieties has been investigated. By static surface tension (γ_lv, σ) and wetting tension (γ_sv−γ_sl, α) measurements on a non-polar perfluorinated surface (FEP®), the contact angles of aqueous surfactant solutions above the critical micelle formation concentration (cmc) were determined. Surface tension and wetting tension react independently on defined changes in the chemical structure of the surfactant molecules. Siloxane surfactants reduce the surface tension most effectively, whereas for a neopentyl-substituted silane derivative the lowest solid/liquid interfacial tension was found. The data for isomeric siloxanes, carbosilanes and silanes suggest that donor–acceptor forces at solid interfaces have a maximum range of about 4.5 Å. © 1998 John Wiley & Sons, Ltd.     

Wettability of a polytetrafluoroethylene surface by an aqueous solution of two nonionic surfactant mixtures

Measurements of the advancing contact angle (theta) were carried out for an aqueous solution of p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol)s (Triton X-100 (TX100) and Triton X-165 (TX165) mixtures) on polytetrafluoroethylene (PTFE). The obtained results indicate that the wettability of PTFE depends on the concentration and composition of the surfactant mixture. The minimum of the dependence between the contact angle and composition of the mixtures for PTFE for each concentration at a monomer mole fraction of TX100, alpha = 0.8, points to synergism in the wettability of PTFE. This effect was confirmed by the negative values of interaction parameters calculated on the basis of the contact angle and by the Rosen approach. In contrast to Zisman, there was no linear dependence between cos theta and the surface tension of an aqueous solution of TX100 and TX165 mixtures for all studied systems, but a linear dependence existed between the adhesional tension and surface tension for PTFE over the whole concentration range, the slope of which was -1, indicating that the surface excess of the surfactant concentration at the PTFE-solution interface was the same as that at the solution-air interface for a given bulk concentration. Similar values of monomer mole fractions of the surfactants at water-air and PTFE-water interfaces calculated on the basis of the surface tension and contact angles showed that adsorption at these two interfaces was the same. It was also found that the work of adhesion of an aqueous solution of surfactants to the PTFE surface did not depend on the type of surfactant and its concentration. This means that for the studied systems the interaction across the PTFE-solution interface was constant and was largely of Lifshitz-van der Waals type. On the basis of the surface tension of PTFE, the Young equation, and the thermodynamic analysis of the adhesion work of an aqueous solution of surfactant to the polymer surface, it was found that in the case of PTFE the changes in the contact angle as a function of the mixture concentration of two nonionic surfactants resulted only from changes in the polar component of the solution surface tension.     

The wetting behavior of grafted hydrophilic acrylic monomers

The wetting behavior of vapor phase photografted hydrophilic acrylic monomers was evaluated by the three most commonly employed techniques, i.e., the captive bubble, the sessile drop, and the Wilhelmy plate technique. The measured contact angles and the overall wetting behavior were discussed in light of the non-ideal nature of these surfaces.It was found that the peculiar nature of hydrophilic grafted surfaces is carefully reflected in the experimentally measurable contact angles. While in the case of the captive bubble the hydrophilic and rough nature of these coatings prevent the bubble-surface contact, in the case of the sessile drop the measured contact angles follow the behavior predicted by contact angle hysteresis theories. Wilhelmy plate measurements, performed as sequential scanning loops, show velocity-dependent effects which are linked to the composition, morphology and mobility of the grafted surfaces.     

Effect of various treatment and glazing (coating) techniques on the roughness and wettability of ceramic dental restorative surfaces

Surface treatment procedures such as grinding and polishing are needed to provide the ceramic dental restorative materials with proper fitting and occlusion. The treated surfaces are customarily glazed to improve the strength and smoothness. Though smoothness and wetting of the dental surfaces are important to minimize bacterial plaque retention, influence of the surface treatment and glazing procedures on the final surface roughness and its correlation to wettability are overlooked.

In this work, effect of various treatment (diamond fraising, stoning, sanding and aluminum oxide and rubber polishing) and glazing (auto and overglazing) techniques on the final roughness and the resulting wettability of dental ceramic surfaces were investigated using scanning electron microscopy (SEM) observations and atomic force microscopy (AFM) scans, 75 scans per sample. The surfaces were characterized and assigned an average roughness measure, R_a. The wettability of the same surfaces was evaluated using micro-contact angle measurements (25 micro-bubbles placed on a grid on each surface) to correlate the final surface roughness and wettability.

The results show that overglazing prevails over surface irregularities from different treatment procedures and provides homegeneously smooth surfaces with mean R_a < 10 nm. It also produces uniformly wetted surfaces with low contact angles around 20°. The autoglazed surfaces are less smooth (mean R_a around 50 nm) and displays sporadic topographic irregularities. They display larger and less uniform contact angles ranging between 35° and 50°. The results suggest that overglazing should be preferred after surface treatment to obtain a smooth and well-wetted dental ceramic surface.     

Wetting of real solid surfaces: new glance on well-known problems

The wetting of solid surfaces is treated. The Young and receding contact angles are experimentally unattainable values for the majority of solid surfaces. Actually, we always observe the apparent contact angle. This makes the characterization of wetting of real surfaces problematic. It is proposed in this paper to characterize wetting of real surfaces with the advancing contact angle and the minimum work of adhesion calculated according to the Dupre equation. The advancing contact angle, which depends slightly on the experimental technique used for its measurement, corresponds to the maximal solid/liquid surface tension and correspondingly to the minimal work of adhesion, calculated according to the Dupre equation.     

Wettability of polymeric solids by ternary mixtures composed of hydrocarbon and fluorocarbon nonionic surfactants

Contact angle (θ) measurements on poly(tetrafluoroethylene) (PTFE) and polymethyl methacrylate (PMMA) surface were carried out for the systems containing ternary mixtures of surfactants composed of: p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycols), Triton X-100 (TX100), Triton X-165 (TX165) and Triton X-114 (TX114), and fluorocarbon surfactants, Zonyl FSN100 (FSN100) and Zonyl FSO100 (FSO100). The aqueous solutions of ternary surfactant mixtures were prepared by adding TX114, FSN100 or FSO100 to binary mixtures of TX100+TX165, where the synergistic effect in the reduction of the surface tension of water (γ(LV)) was determined. From the obtained contact angle values, the relationships between cosθ, the adhesion tension and surface tension of solutions, cosθ and the reciprocal of the surface tension were determined. On the basis of these relationships, the correlation between the critical surface tension of PTFE and PMMA wetting and the surface tension of these polymers as well as the work of adhesion of aqueous solutions of ternary surfactant mixtures to PTFE and PMMA surface were discussed. The critical surface tension of PTFE and PMMA wetting, γ(C), determined from the contact angle measurements of aqueous solutions of surfactants including FSN100 or FSO100 was also discussed in the light of the surface tension changes of PTFE and PMMA under the influence of film formation by fluorocarbon surfactants on the surface of these polymers. The γ(C) values of the studied polymeric solids were found to be different for the mixtures composed of hydrocarbon surfactants in comparison with those of hydrocarbon and fluorocarbon surfactants. In the solutions containing fluorocarbon surfactants, the γ(C) values were different taking into account the contact angle in the range of FSN100 and FSO100 concentration corresponding to their unsaturated monolayer at water-air interface or to that saturated.     

Surface Forces in Wetting Films

Reviewed are theoretical and experimental results of Russian and foreign researchers of the surface forces acting in asymmetric system, wetting films contacting with the condensed phase on the one side and with the gaseous phase or own vapor on the other side. Wetting films play an important role in such phenomena as wetting and spreading, flotation, polymolecular adsorption, evaporation of liquids from porous bodies, and capillary condensation. Disjoining pressure isotherms of wetting films determine the wetting conditions and the values of forming contact angles. The mechanisms of film stability, rupture, and coalescence are discussed based on the theory of surface forces. The properties of the films of nonpolar liquids and liquid mixtures; polymolecular adsorption films of polar liquids; as well as wetting films of aqueous electrolyte solutions, surfactants and polymers are analyzed systematically.     

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.