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.     

Contact angle hysteresis on regular pillar-like hydrophobic surfaces

A series of pillar-like patterned silicon wafers with different pillar sizes and spacing are fabricated by photolithography and further modified by a self-assembled fluorosilanated monolayer. The dynamic contact angles of water on these surfaces are carefully measured and found to be consistent with the theoretical predictions of the Cassie model and the Wenzel model. When a water drop is at the Wenzel state, its contact angle hysteresis increases along with an increase in the surface roughness. While the surface roughness is further raised beyond its transition roughness (from the Wenzel state to the Cassie state), the contact angle hysteresis (or receding contact angle) discontinuously drops (or jumps) to a lower (or higher) value. When a water drop is at the Cassie state, its contact angle hysteresis strongly depends on the solid fraction and has nothing to do with the surface roughness. Even for a superhydrophobic surface, the contact angle hysteresis may still exhibit a value as high as 41 degrees for the solid fraction of 0.563.     

Influence of DNA adsorption and DNA/cationic surfactant coadsorption on the interaction forces between hydrophobic surfaces

The forces between hydrophobic surfaces with physisorbed DNA are markedly and irreversibly altered by exposure to DNA/cetyltrimethylammonium bromide (CTAB) mixtures. In this colloidal probe atomic force microscopy study of the interactions between a hydrophobic polystyrene particle and an octadecyltrimethylethoxysilane-modified mica surface in sodium bromide solutions, we measure distinct changes in colloidal forces depending on the existence and state of an adsorbed layer of DNA or CTAB-DNA complexes. For bare hydrophobic surfaces, a monotonically attractive approach curve and very large adhesion are observed. When DNA is adsorbed at low bulk concentrations, a long-range repulsive force dominates the approach, but on retraction some adhesion persists and DNA bridging is clearly observed. When the DNA solution is replaced with a CTAB-DNA mixture at relative low CTAB concentration, the length scale of the repulsive force decreases, the adhesion due to hydrophobic interactions greatly decreases, and bridging events disappear. Finally, when the surface is rinsed with NaBr solution, the length scale of the repulsive interaction increases modestly, and only a very tiny adhesion remains. These pronounced changes in the force behavior are consistent with CTAB-induced DNA compaction accompanied by increased DNA adsorption, both of which are partially irreversible.     

Stability of self-assembled hydrophobic surfactant layers in water

Using contact angle measurements, surface force balance experiments, and AFM imaging, we have investigated the process of self-assembly of surfactants onto mica and the subsequent stability of those layers in pure water. In the case of cetyltrimethylammonium bromide (CTAB), the stability of a monolayer when immersed in pure water is found to be dependent on initial immersion time in surfactant, which is likely to be caused by an increase in the proportion of ion-exchange to ion-pair adsorption when incubated in surfactant for longer periods of time. Infinite dilution of the surfactant solution before withdrawal of the sample is found to have little effect on the stability of the resulting layer in pure water. The nature of the counterion is found to affect dramatically the stability of a self-assembled surfactant monolayer: cetyltrimethylammonium fluoride (CTAF) forms a layer that is much more stable in water than CTAB, which is likely to be due to faster and more complete ion-exchange with the mica surface for CTAF. Surface force balance experiments show that when the hydrophobic monolayer is immersed in pure water it does not simply dissolve into the water; instead it rearranges, possibly to patches of bilayer or hemimicelles. The time scale of this rearrangement agrees well with the time scale of the change from a hydrophobic to more hydrophilic surface observed using contact angle measurements. AFM imaging has also in some cases shown an evolution from an even monolayer to patches of bilayer.     

Cationic electrolyte adsorption layers on hydrophilic and hydrophobic surfaces

The capillary electrokinetics method (measurements of streaming potential and current in original and hydrophobized fused quartz capillaries with radii of 5–7 μm) is employed to study the formation of adsorption layers upon contact with solutions containing a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). It is shown that polyelectrolyte adsorption causes the charge reversal of both hydrophilic and hydrophobic surfaces, with a smaller amount of the substance being adsorbed on the hydrophobic than on the hydrophilic surface. The adsorption on both surfaces increases with the polymer solution concentration. The cationic polyelectrolyte adsorption on the pure quartz surface occurs mainly due to the electrostatic attraction, while, in the case of the hydrophobic surface, the contribution of hydrophobic interactions increases. The study of the layer deformability shows that, on the hydrophilic surfaces, the layer ages and its structure depends on the polymer solution concentration. On the modified surface, the deformation of even freshly formed layers is slight, which suggests that a denser layer is formed on the hydrophobic surface. In contrast to the hydrophilic surface, the polyelectrolyte is partly desorbed from the hydrophobic surface.     

Reentrant wetting transition on surfactant solution surfaces

The wetting of polydimethylsiloxane oil drops on the surfaces of anionic surfactant sodium dodecylsulfate solutions is studied systematically by changing the bulk surfactant concentration. The wetting state changes from complete wetting to pseudopartial wetting at 0.3 cmc (critical micelle concentration) surfactant concentration and there is a reentrant transition back to complete wetting at 1.4 cmc. The measured free energy is consistent with the prediction of the wetting theory. The interaction potential minimum of the two surfaces of the oil film disappears at the reentrant point, which is speculated to be an effect of micelle formation in the solution.     

Effect of nanobubbles on friction forces between hydrophobic surfaces in water

Micro- and nanoscale combined hierarchical polymer structures were fabricated by UV-assisted capillary force lithography. The method is based on the sequential application of engraved polymer molds with a UV-curable resin of polyurethane acrylate (PUA) followed by surface treatment with a trichloro(1H,1H,2H,2H-perfluorooctyl) silane in vapor phase. Two distinct wetting states were observed on these dual-roughness structures. One is “Cassie–Wenzel state” where a water droplet forms heterogeneous contact with microstructures and homogeneous contact with nanostructures. The other is “Cassie–Cassie state” where a droplet makes heterogeneous contact both with micro- and nanostructures. A simple thermodynamic model was developed to explain static contact angle, hysteresis, and wetting transition on dual-roughness structures.     

Direct measurement of hydrophobic forces on cell surfaces using AFM

Although hydrophobic forces are of great relevance in biological systems, quantifying these forces on complex biosurfaces such as cell surfaces has been difficult owing to the lack of appropriate, ultrasensitive force probes. Here, chemical force microscopy (CFM) with hydrophobic tips was used to measure local hydrophobic forces on organic surfaces and on live bacteria. On organic surfaces, we found an excellent correlation between nanoscale CFM and macroscale wettability measurements, demonstrating the sensitivity of the method toward hydrophobicity and providing novel insight into the nature of hydrophobic forces. Then, we measured hydrophobic forces associated with mycolic acids on the surface of mycobacteria, supporting the notion that these hydrophobic compounds represent an important permeation barrier to drugs.     

Wetting transition on hydrophobic surfaces covered by polyelectrolyte brushes

We study the wetting by water of complex "hydrophobic-hydrophilic" surfaces made of a hydrophobic substrate covered by a hydrophilic polymer brush. Polystyrene (PS) substrates covered with polystyrene- block-poly(acrylic acid) PS- b-PAA diblock copolymer layers were fabricated by Langmuir-Schaefer depositions and analyzed by atomic force microscopy (AFM) and ellipsometry. On bare PS substrate, we measured advancing angles theta A = 93 +/- 1 degrees and receding angles theta R = 81 +/- 1 degrees . On PS covered with poorly anchored PS- b-PAA layers, we observed large contact angle hysteresis, theta A approximately 90 degrees and theta R approximately 0 degrees , that we attributed to nanometric scale dewetting of the PS- b-PAA layers. On well-anchored PS- b-PAA layers that form homogeneous PAA brushes, a wetting transition from partial to total wetting occurs versus the amount deposited: both theta A and theta R decrease close to zero. A model is proposed, based on the Young-Dupre equation, that takes into account the interfacial pressure of the brush Pi, which was determined experimentally, and the free energy of hydration of the polyelectrolyte monomers Delta G PAA (hyd), which is the only fitting parameter. With Delta G PAA (hyd) approximately -1300 J/mol, the model renders the wetting transition for all samples and explains why the wetting transition depends mainly on the average thickness of the brush and weakly on the length of PAA chains.     

Impact of surface forces on wetting of hierarchical surfaces and contact angle hysteresis

The influence of the long-range surface forces on the wetting of multi-scale partially wetted surfaces is discussed. The possibility of partial wetting is stipulated by a specific form of the Derjaguin isotherm. Equilibrium of a liquid meniscus inside a cylindrical capillary is used as a model. The interplay of capillary and disjoining pressures governs the equilibrium of the liquid in the nano- and micrometrically scaled pores constituting the relief of the surface. It is shown that capillaries with a radius smaller than a critical one will be completely filled by water, whereas the larger capillaries will be filled only partially. Thus, small capillaries will show the Wenzel type of wetting behavior, while the same liquid inside the large capillaries will promote the Cassie-Baxter type of wetting. Consideration of disjoining/conjoining pressure allows explaining of the “rose petal effect”, when a high apparent contact angle is accompanied with a high contact angle hysteresis.     

Tuning nanoscopic water layers on hydrophobic and hydrophilic surfaces with laser light

The evolutional function of ordered interfacial water near solid surfaces was postulated by Szent-Gy?rgyi: "Life actually, may have started with building these water structures." Here we report their tunability with laser light on both hydrophobic and hydrophilic surfaces. On the former, the light caused their depletion--on the latter, an increase in fluidity--as measured by atomic force acoustic microscopy. Interfacial water layers play a key role in cellular recognition. Their tunability promises to revolutionize various fields in biomedical engineering and life sciences.     

Driving forces of phase transitions in surfactant and lipid systems

In aqueous surfactant and lipid systems, different liquid crystalline phases are formed at different temperatures and water contents. The "natural" phase sequence implies that phases with higher curvature are formed at higher water contents. On the other hand, there are exceptions to this rule, such as the monoolein/water system. In this system an anomalous transition from lamellar to reverse cubic phase upon addition of water is observed. The calorimetric data presented here show that the hydration-induced transitions to phases with higher curvature are driven by enthalpy, while the transitions to phases with lower curvature are driven by entropy. It is shown that the driving forces of phase transitions can be determined from the appearance of the phase diagram using the approach based on van der Waals differential equation. From this approach it follows that the slope of the phase boundary should be positive with respect to water content if the phase diagram obeys the "natural" phase sequence. The increase of entropy, which drives the anomalous phase transitions, arises from the increase of disorder of the hydrocarbon chains.     

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.     

Simultaneous measurement of structural and hydrodynamic forces between colloidal surfaces in complex fluids

An AFM study was performed to measure the effect of approach/retraction speed on the interaction force between a colloidal particle and a flat substrate in aqueous solutions containing silica nanospheres at concentrations of 4.5 and 6.5 vol.%. The total force consisted of contributions of electrostatic, depletion, structural and hydrodynamic forces. The hydrodynamic component of the force could be isolated by comparing the force profiles measured upon approach and retraction. It was found that when the hydrodynamic component was subtracted from the total force, the resulting force profiles measured at scan speeds of 80, 800, 2400, 4800 and 11,200 nm/s all overlaid, indicating that the surface forces (electrostatic, depletion and structural) were not affected by the scan speed. This result was further supported by an approximation of the rates of viscous and diffusive motion in the gap region. In addition, the variation of the hydrodynamic force with particle/plate separation distance agreed relatively well with a prediction made using the mobility correction factor developed for simple fluids, suggesting that the nanoparaticles do not alter the flow in the lubrication layer at these concentrations.     

Ion induced lamellar-lamellar phase transition in charged surfactant systems

We propose a model for the liquid-liquid (L(alpha)-->L(alpha(') )) phase transition observed in osmotic pressure measurements of certain charged lamellae-forming amphiphiles. The model free energy combines mean-field electrostatic and phenomenological nonelectrostatic interactions, while the number of dissociated counterions is treated as a variable degree of freedom that is determined self-consistently. The model, therefore, joins two well-known theories: the Poisson-Boltzmann theory for ionic solutions between charged lamellae and the Langmuir-Frumkin-Davies adsorption isotherm modified to account for charged adsorbing species. Minimizing the appropriate free energy for each interlamellar spacing, we find the ionic density profiles and the resulting osmotic pressure. While in the simple Poisson-Boltzmann theory the osmotic pressure isotherms are always smooth, we observe a discontinuous liquid-liquid phase transition when the Poisson-Boltzmann theory is self-consistently augmented by the Langmuir-Frumkin-Davies adsorption. This phase transition depends on the area per amphiphilic head group, as well as on nonelectrostatic interactions of the counterions with the lamellae and interactions between counterion-bound and counterion-dissociated surfactants. Coupling the lateral phase transition in the bilayer plane with electrostatic interactions in the bulk, our results offer a qualitative explanation for the existence of the L(alpha)-->L(alpha(') ) phase transition of didodecyldimethylammonium bromide (DDABr), but the transition's apparent absence for the chloride and the iodide homologs. More quantitative comparisons with experiment require better understanding of the microscopic basis of the phenomenological model parameters.     

Isotropic-cholesteric phase transition in colloidal suspensions of filamentous bacteriophage fd

The co-existence concentrations of the isotropic and cholesteric liquid crystalline phases of the semi-flexible rod-like virus fd in aqueous suspension were measured as a function of ionic strength at room temperature. At several ionic strengths the magnetic-field-induced birefringence, which is proportional to the number of particles in a correlation volume N_corr, was measured for fd concentrations spanning the entire isotropic region. From this data the limiting concentration of stability (spinodal) of the isotropic phase, ρ*, was obtained. The co-existence concentrations and ρ* versus ionic strength compare well with predictions based on the theory of Khokhlov and Semenov, modified to include the effects of charge. A theoretical expression for the magnetic birefringence of persistent polymers was derived and agreed well with the data with the exception that N_corr at the isotropic to liquid crystal transition was smaller than predicted.     

Direct measurements of the effects of salt and surfactant on interaction forces between colloidal particles at water-oil interfaces

The forces between colloidal particles at a decane-water interface, in the presence of low concentrations of a monovalent salt (NaCl) and the surfactant sodium dodecyl sulfate (SDS) in the aqueous subphase, have been studied using laser tweezers. In the absence of electrolyte and surfactant, particle interactions exhibit a long-range repulsion, yet the variation of the interaction for different particle pairs is found to be considerable. Averaging over several particle pairs was hence found to be necessary to obtain a reliable assessment of the effects of salt and surfactant. It has previously been suggested that the repulsion is consistent with electrostatic interactions between a small number of dissociated charges in the oil phase, leading to a decay with distance to the power -4 and an absence of any effect of electrolyte concentration. However, the present work demonstrates that increasing the electrolyte concentration does yield, on average, a reduction of the magnitude of the interaction force with electrolyte concentration. This implies that charges on the water side also contribute significantly to the electrostatic interactions. An increase in the concentration of SDS leads to a similar decrease of the interaction force. Moreover, the repulsion at fixed SDS concentrations decreases over longer times. Finally, measurements of three-body interactions provide insight into the anisotropic nature of the interactions. The unique time-dependent and anisotropic interactions between particles at the oil-water interface allow tailoring of the aggregation kinetics and structure of the suspension structure.     

Correlation of surfactant/polymer phase behavior with adsorption on target surfaces

In this contribution, the phase behavior of a surfactant/polymer mixed system is related to the adsorption of a complex derived from the mixture onto a target surface. The phase map for the system sodium dodecyl sulfate (SDS, a model anionic surfactant)/pDMDAAC (poly(dimethyl diallyl ammonium chloride), a cationic polymer) shows behavior very typical of surfactant/oppositely charged polyelectrolyte mixtures. The predominant feature is a broad, two-phase region in the phase map which lies asymmetrically around the 1:1 stoichiometry of surfactant charge groups to polymer charge units. The overall controlling principle driving the phase separation is charge compensation. Excess of polymer yields an isotropic solution, as does a great excess of surfactant (termed resolubilization). The phase separating in the SDS/pDMDAAC system is characterized by a positive zeta-potential when the polymer is in excess and a negative zeta-potential when the surfactant is in excess. The surface charge properties of the precipitated phases are essentially identical to those of target particles (ground borosilicate glass) dispersed at the same approximate position in the phase map, suggesting that the surfactant/polymer complex at the precipitation boundary is the same as that adsorbing onto the pigment particle. This conclusion is confirmed by depletion studies which allow the polymer adsorption density to be determined. For polymer-rich systems, essentially all of the surfactant adsorbs along with the polymer via a high-affinity isotherm with a plateau coverage of about 0.8 mg polymer/m (2). Surfactant-rich systems adsorb with a similar affinity, despite the mismatch of the complex charge matching that of the particle surface. Once adsorbed, these complexes are not readily removed by rinsing, though complexes adsorbed from SDS-rich systems will lose excess surfactant upon extreme dilution. Over a wide range of surfactant-rich compositions, from 1:1 stoichiometry out toward the resolubilization zone, a chemical analysis reveals that the surfactant/polymer precipitate species consists of a 1:1 charge complex with the addition of about 0.25 mol of surfactant/mol of complex. Resolubilization of these sparingly soluble species is achieved simply by dilution to below their solubility limit.     

Highly hydrophobic surfaces with rose petal-effect based on nanocellulose films coated by nanostructured CuI layers

Cellulose - Inexpensive biodegradable and biocompatible materials, which enable a combination of high hydrophobicity with good adhesion of water droplets, as on rose petals, are in demand for...