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Fully sulfonated poly(styrenesulfonate) brushes were grown from the surface of colloidal silica particles and used to prepare stable trichloroethylene-in-water and heptane-in-water Pickering emulsions. These particles were highly charged and colloidally stable in water but could not be dispersed in trichloroethylene or heptane. Both two-phase (emulsion plus neat water) and three-phase (emulsion separating neat oil and water phases) systems were observed, with water-continuous emulsion phases in all cases. Emulsion phases containing as much as 83% (v/v) oil were stable for over six months. Poly(styrenesulfonate)-grafted particles were very efficient emulsifiers; stable emulsion phases were prepared when using as little as 0.04 wt% particles. The emulsifying effectiveness of the poly(styrenesulfonate)-grafted silica particles can be attributed to the hydrophobicity of the vinylic polymer backbone that makes this highly charged polyelectrolyte unusually surface active at the oil/water interface.

Covalent grafting of poly(ethylene glycol) (PEG) to pharmaceutical proteins, “PEGylation”, is becoming more commonplace due to improved therapeutic efficacy. As these conjugates encounter interfaces in manufacture, purification, and end use and adsorption to these interfaces may alter achievable production yields and in vivo efficacies, it is important to understand how PEGylation affects protein adsorption mechanisms. To this end, we have studied the adsorption of unmodified and PEGylated chicken egg lysozyme to silica, using optical reflectometry, total internal reflection fluorescence (TIRF) spectroscopy, and atomic force microscopy (AFM) under varying conditions of ionic strength and extent of PEG modification. PEGylation of lysozyme changes the shape of the adsorption isotherm and alters the preferred orientation of lysozyme on the surface. There is an abrupt transition in the isotherm from low to high surface excess concentrations that correlates with a change in orientation of mono-PEGylated conjugates lying with the long axis parallel to the silica surface to an orientation with the long axis oriented perpendicular to the surface. No sharp transition is observed in the adsorption isotherm for di-PEGylated lysozyme within the range of concentrations examined. The net effect of PEGylation is to decrease the number of protein molecules per unit area relative to the adsorption of unmodified lysozyme, even under conditions where the surface is densely packed with conjugates. This is due to the area sterically excluded by the PEG grafts. The other major effect of PEGylation is to make conjugate adsorption significantly less irreversible than unmodified lysozyme adsorption.

We have developed a pseudo-phase model to predict the self-assembly of nonionic surfactants on hydrophobic solid or fluid interfaces and in bulk solution. The uniqueness of this model is that it provides the relationship between molecular structure and self-assembly in solution and on interfaces. This model requires the input of minimal new experimental data. The remaining model parameters may be calculated on the basis of the surfactant molecular structure. The validity of the model has been established by comparing predictions with a wide array of experimental data for nonionic surfactant adsorption at the hydrophobic solid−water interface and at the air−water interface. The same model is then used to predict the self-assembly in bulk solution. The model predictions for critical aggregation concentration, aggregate shapes, and adsorption isotherms of various surfactants are in good agreement with the experimental data available in the literature.

The self-assembly of nonionic surfactants in bulk solution and on hydrophobic surfaces is driven by the same intermolecular interactions, yet their relationship is not clear. While there are abundant experimental and theoretical studies for self-assembly in bulk solution and at the air−water interface, there are only few systematic studies for hydrophobic solid−water interfaces. In this work, we have used optical reflectometry to measure adsorption isotherms of seven different nonionic alkyl polyethoxylate surfactants (CH3(CH2)I-1(OCH2CH2)JOH, referred to as CIEJ surfactants, with I = 10−14 and J = 3−8), on hydrophobic, chemically homogeneous self-assembled monolayers of octadecyltrichlorosilane. Systematic changes in the adsorption isotherms are observed for variations in the surfactant molecular structure. The maximum surface excess concentration decreases (and minimum area/molecule increases) with the square root of the number of ethoxylate units in the surfactant (J). The adsorption isotherms of all surfactants collapse onto the same curve when the bulk and surface excess concentrations are rescaled by the bulk critical aggregation concentration (CAC) and the maximum surface excess concentration. In an accompanying paper we compare these experimental results with the predictions of a unified model developed for self-assembly of nonionic surfactants in bulk solution and on interfaces.

The coadsorption of a positively charged polyelectrolyte (with 10% of the segments carrying a permanent positive charge, AM-MAPTAC-10) and an anionic surfactant (sodium dodecyl sulfate, SDS) on silica and glass surfaces has been investigated using optical reflectometry and a noninterferometric surface force technique. This is a selective coadsorption system in the sense that the polyelectrolyte does adsorb to the surface in the absence of surfactant, whereas the surfactant does not adsorb in the absence of polyelectrolyte. It is found that the total adsorbed amount goes through a maximum when the SDS concentration is increased. Maximum adsorption is found when the polyelectrolyte/surfactant complexes formed in bulk solution are close to the charge neutralization point. Some adsorption does occur also when SDS is present in significant excess. The force measured between AM-MAPTAC-10-coated surfaces on approach in the absence of SDS is dominated at long range by an electrostatic double-layer force. Yet, layers formed by coadsorption from solutions containing both polyelectrolyte and surfactant generate long-range forces of an electrosteric nature. On separation, adhesive interactions are found only when the adsorbed amount is low, i.e., in the absence of SDS and in a large excess of SDS. The final state of the adsorbed layer is found to be nonhysteretic, i.e., independent of the history of the system. The conditions for formation of long-lived trapped adsorption states from mixed polymer−surfactant solutions are discussed.

The carryover of surfactant ahead of contact lines on hydrophilic surfaces results in unusual wetting behavior of surfactant solutions, such as the autophobic effect. This carryover of surfactant ahead of the contact line is due to the strong interactions between the hydrophilic surface and the headgroup of ionic and nonionic surfactants. However, in the case of a nonionic surfactant and a hydrophobic surface, the possibility of self-assembly of surfactant ahead of the contact line has been neglected because of the absence of any obvious strong interactions and any unusual wetting behavior. In this paper, we provide evidence of carryover of a nonionic surfactant (octaethyleneoxide monododecyl ether, C12E8) ahead (i.e., on the solid−vapor, SV, side) of advancing and receding contact lines on hydrophobic self-assembled monolayers of octadecyltrichlorosilane. This system of a nonionic surfactant and hydrophobic surface was thought to be an unlikely system to show the presence of surfactant ahead of the contact line. However, the presence of a region with higher wettablity ahead of advancing and receding contact lines in condensation figure experiments provides direct evidence of the self-assembly and carryover of nonionic surfactant ahead of the contact line even on a hydrophobic surface. More evidence of this carryover is provided by comparing advancing and receding contact-angle measurements, along with independently determined solid−liquid (SL) and liquid−vapor (LV) surface tensions. These measurements show that the local SV surface tension increases with an increasing bulk surfactant concentration. As a result of this increase in the SV surface tension on a hydrophobic surface, the contact angle is lower than expected from the reduction in the SL and LV surface tensions alone. This phenomenon is “the autophilic effect”, in contrast with the autophobic effect that results in a larger contact angle. Our results also invalidate the assumption of no surfactant ahead of the contact line, a basis commonly used to estimate adsorption on SL interfaces from contact-angle data.

We use total internal reflection fluorescence (TIRF), streaming current, and optical reflectometry measurements, under a variety of solution conditions, to examine the mechanism of the chicken egg lysozyme reorientation on silica surfaces, originally proposed by Robeson and Tilton (Robeson, J. L.; Tilton, R. D. Langmuir 1996, 12, 6104). The TIRF data suggest that a two-stage reorientation occurs in the lysozyme layer during adsorption. The first stage involves a reorientation to optimize lysozyme interaction with the charged surface and to reduce lateral repulsions between adsorbed protein molecules. This stage occurs when the adsorbed layer reaches a critical surface coverage and indicates the importance of lateral interactions between adsorbed proteins. Moreover, the reorientation rate depends on both the bulk protein concentration and the fluid wall shear rate, indicating that incoming protein molecules from the bulk solution also participate in the reorientation. The preferred orientation puts the active site face toward the solution and the most positively charged amino acid patch into contact with the negative silica surface. A second stage of slow restructuring occurs as the adsorbed layer coverage asymptotically approaches saturation and corresponds to reorientation of the protein molecules that fill surface vacancies made accessible by the first reorientation. Consistent with this proposed two-stage reorientation mechanism, streaming current measurements indicate that adsorbed lysozyme initially causes a significant interfacial potential reversal, followed by a slow relaxation to less positive ζ-potentials as the lysozyme optimizes its orientation. Accordingly, the desorbable fraction of the adsorbed layer after reorientation is significantly smaller than that before reorientation.

By adsorbing polymers in the form of polymer/surfactant complexes, it is possible to control the nonequilibrium structure of adsorbed polymer layers. At moderate NaCl concentrations, the extent of poly(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide) triblock copolymer adsorption to silica surfaces is increased by a sequential sodium dodecyl sulfate (SDS) complex coadsorption procedure, when compared to the extent of adsorption in the absence of surfactant. The complex adsorption procedure entails polymer/surfactant complex formation and coadsorption at high SDS concentrations, followed by SDS dilution and eventually complete SDS removal, while maintaining a constant polymer concentration in solution. The amount of polymer that remains adsorbed after the complex adsorption procedure is nearly 40% greater than the amount that ordinarily adsorbs for the same polymer concentration in the absence of surfactant. We use independent atomic force microscopy and streaming current techniques to measure the polymer layer thicknesses. Both techniques indicate that polymer layers that were produced by the complex adsorption procedure followed by thorough rinsing in surfactant-free polymer solutions are significantly denser than the ordinary layers produced by adsorption in the absence of surfactant. Layers produced by complex adsorption contain more mass per unit area but are only approximately 70% as thick as the ordinary layers. The differences between the two adsorption procedures persist even during continued bathing of the layers in identical surfactant-free polymer solutions for 6 h. Polymer/surfactant complex adsorption thereby provides a tool for guiding adsorbed polymer layers into persistent nonequilibrium structures that might not be accessible by ordinary adsorption procedures.

We examined coadsorption from mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) with either hydroxypropyl cellulose (HPC) or hydrophobically modified hydroxyethyl cellulose (hmHEC). Coadsorption to nonselective, hydrophobic poly(dimethylsiloxane) (PDMS) surfaces was compared with coadsorption to negatively charged silica surfaces that were selective for the polymers. Optical reflectometry provided the total extent of adsorption as well as the adsorption kinetics. We referenced SDS concentrations against the critical association concentration (cac) and saturation concentration (csat) for solution-phase polymer/surfactant binding as determined by pyrene solubilization and fluorescence spectroscopy methods. Depending on the bulk SDS concentration, the total adsorbed mass could be either increased or decreased compared to the adsorbed mass attained from SDS-free polymer solutions. Above the cac, the total adsorbed amount decreased with increasing SDS concentration, for either polymer on either the silica or the PDMS surface. At high SDS concentrations with silica surfaces, adsorption of either polymer was completely prevented, leaving the surface bare, but the threshold SDS concentrations for this effect were well above the csat for the particular polymer. On the hydrophobic PDMS surface, only SDS adsorbed when its concentration exceeded the ordinary critical micelle concentration (measured in the absence of polymer). On either surface and at any SDS concentration where some polymer did adsorb, the total extent of adsorption was greater with the more hydrophobic hmHEC polymer than with the HPC polymer.

We studied the reversibility of coadsorption from mixtures of the anionic surfactant sodium dodecyl sulfate with either hydroxypropyl cellulose (HPC) or hydrophobically modified hydroxyethyl cellulose (hmHEC) using optical reflectometry. The coadsorption to nonselective hydrophobic poly(dimethylsiloxane) (PDMS) surfaces was compared with coadsorption to negatively charged silica surfaces that were selective for polymer adsorption. The surface selectivity determines the reversibility of coadsorption with respect to changes in the solution sodium dodecyl sulfate (SDS) concentration. On the selective silica surface, an adsorbed layer becomes kinetically trapped in path-dependent states because SDS is electrostatically repelled from the negatively charged surface and is therefore unable to solubilize the polymer/surface contacts. On the nonselective PDMS surface, coadsorption in the HPC/SDS system is reversible, and although some irreversibility persists in the hmHEC/SDS system, the severity of the kinetic trapping effect is greatly reduced compared with that of the same system on silica. The decreased kinetic trapping effects are attributed to surfactant adsorption to the hydrophobic PDMS surface. Finally, a streaming current technique was used to measure the electrokinetic thickness of kinetically trapped polymer layers that were formed on silica by coadsorption with SDS, followed by rinsing with SDS-free polymer solution. The layer thickness was history-dependent:  despite prolonged exposure to a constant concentration polymer solution, the adsorbed layer thickness depended on the SDS concentration that existed during the initial coadsorption stage.

Surfactant solutions exhibit a wide variety of wetting and dewetting behaviors on high energy surfaces. These behaviors are driven by surfactant self-assemblies at the moving contact line. To probe these self-assemblies, we study the structure of C12En (1 ≤ n ≤ 8) surfactants at the three interfaces near a contact line receding across a hydrophilic surface. We determine the area per molecule adsorbed at the solid/liquid (SL) and liquid/vapor (LV) interfaces and the structural details of the monolayer deposited to the solid/vapor (SV) interface at the receding contact line. For all n, the monolayer deposited at the SV interface is substantially less dense than the amount delivered to it by the LV interface, with the SL interface making a small-to-negligible contribution to the deposited monolayer. A dividing streamline must exist in the bulk, along which surfactant from the LV interface is returned to solution. For n ≥ 6 the ethylene oxide headgroups begin to behave like poly(ethylene oxide) (PEO) polymer at the SL interface. At the LV interface the area per molecule increases monotonically with n, suggesting increasing disorder in the headgroup region. The deposited monolayer at the SV interface shows a more complicated, nonmonotonic dependence on n. Processes at the receding contact line and the structure of the deposited monolayer show marked transitions at n = 3, indicating a significant interaction between headgroup and substrate for n > 3.

Interfacial properties can be tuned by exploiting polymer/surfactant interactions. We find that coadsorption of the anionic surfactant sodium dodecyl sulfate (SDS) and the amphiphilic triblock copolymer poly(ethylene oxide-b-propylene oxide-b-ethylene oxide), Pluronic F108, to silica is extremely sensitive to SDS concentration and ionic strength. First, using a pyrene solubilization assay we identify the surfactant concentration regimes where different F108/SDS aggregates form in bulk solution at several ionic strengths. We then measure the total surface excess concentration of coadsorbing F108 and SDS using optical reflectometry. Above the critical aggregation concentration where F108/SDS aggregates form, the coadsorbed amount decreases with increasing surfactant concentration until an SDS concentration is reached at which adsorption is prevented entirely. Furthermore, although adsorbed layers containing only F108 are irreversibly adsorbed, F108/SDS layers are reversibly adsorbed. These results suggest that F108 is “shuttling” the normally nonadsorbing SDS to the silica surface. At high ionic strength, we find that sequential coadsorption followed by removal of SDS from the adsorbed layer results in an enhanced adsorbed amount of F108 (compared to direct adsorption of F108 in the absence of SDS). Thus, surfactant-free F108 layers can be “sculpted” into a different conformation by sequential processing with SDS. Finally, scaling of our coadsorption data with the bulk binding transitions (onset of cooperative binding and saturation of the polymer) indicates that changes in adsorbed amount occur at SDS concentrations both below where aggregates form and above the point where the polymer is saturated in the bulk.

Coadsorbing poly-l-lysine hydrobromide inhibits hexadecyltrimethylammonium bromide adsorption to silica surfaces. Using optical reflectometry to measure surface excess concentrations, we find that this inhibition depends on the concentration of added 1:1 electrolyte, both quantitatively and qualitatively. In the absence of added salt, the polyelectrolyte causes a rather uniform decrease in the extent of surfactant adsorption, and the shape of the surfactant coadsorption isotherm is qualitatively similar to the isotherm for adsorption in the absence of polyelectrolyte. In particular, the surfactant concentration marking the onset of cooperative adsorption, where admicelles are formed at bulk concentrations below the critical micelle concentration, is unchanged by the presence of the polyelectrolyte. In contrast, when the surfactant coadsorbs in the presence of 10 mM KBr, the polyelectrolyte eliminates this adsorption regime altogether. Admicelles do not form until the bulk surfactant concentration exceeds the critical micelle concentration. Upon addition of the 1:1 electrolyte, the inhibition mechanism changes from a simple competition for available surface area to a more profound disruption of surfactant interfacial self-assembly. By comparing a low molecular weight oligolysine with three higher molecular weight polylysine samples, we find that this change in inhibition mechanism can be traced to the effect of salt on the relative adsorption energies of the surfactant and the polyelectrolyte, but kinetically trapped or frustrated states exert a large influence on the composition of the mixed adsorbed layer in the case of higher molecular weight polyelectrolytes.

Micellar aggregation numbers for ionic surfactants are sensitive to both the ionic strength and the type of counterion present. The current study considers the effects of electrolyte conditions on solubilization of pyrene, a polycyclic aromatic hydrocarbon, in micellar dodecyl sulfate surfactant solutions. We use ultraviolet−visible spectrophometry to measure the pyrene solubilizing powers, and we use an excimer fluorescence technique both to measure the micellar aggregation numbers in the presence of pyrene solubilizates and to measure the pyrene solubilization capacities. The aggregation number increases with increasing concentration of background electrolyte. When considering different types of counterions (NH4⁺, Na⁺, and Li⁺), the aggregation number increases with increasing counterion binding affinity. We estimate the latter by electrical conductivity measurements. In all cases of varying electrolyte conditions examined, the solubilization capacity increases siginficantly with increasing aggregation numbers, and the solubilizing power is therefore only weakly dependent on the electrolyte conditions.

Combining optical reflectometry and atomic force microscopy (AFM), we have studied the effects of the surfactant counterion on the adsorption isotherms, kinetics, and layer structure for cationic hexadecyltrimethylammonium (C16TA⁺) surfactants on negatively charged silica surfaces. The adsorption kinetics suggest that the adsorption mechanism changes at the critical micelle concentration (cmc). A change in mechanism is also suggested by differences observed in the state of interfacial self-assembly on either side of the cmc. Above the cmc, increasing the binding affinity of the counterion (from chloride to bromide) increased the surface excess concentration by approximately 60% and changed the structure of the adsorbed surfactant layer from aggregates with circular projections to wormlike micelles. The addition of 10 mM KCl or KBr increased the surfactant surface excess concentration for both counterions. Below the cmc, the counterion has only a small effect on the structure of the adsorbed layer, and the isotherms are similar, provided the surfactant concentration is scaled by the appropriate cmc. By quantitatively analyzing the AFM images and comparing this to the surface excess concentration measured by reflectometry, we determined that surfactants pack differently in adsorbed aggregates than they do in aggregates formed by self-assembly in solution. Finally, we show that an impurity present in poly(vinyl chloride) tubing explains anomalous adsorption behavior previously reported for C16TAB on silica.

We examine the effect of poly(ethylene glycol) (PEG) on pyrene solubilization behaviors in aqueous sodium dodecyl sulfate (SDS) solutions. These solutions display strong polymer−surfactant complexation. Following the definitions of Ikeda and Maruyama (J. Colloid Interface Sci. 1994, 166, 1) we distinguish between the macroscopic solubilizing power and the microscopic solubilization capacity. With pyrene as a model solubilizate, we use ultraviolet absorbance spectrophotometry to measure solubilizing powers. We use excimer fluorescence spectroscopy to identify polymer−surfactant binding transitions and to measure the aggregation numbers of free SDS micelles and of PEG-bound SDS aggregates that contain solubilized pyrene in order to calculate solubilization capacities. The solubilization capacity and solubilizing power of free SDS micelles both increase with increasing aggregation number, when the aggregation number is increased by increasing ionic strength. The solubilization capacity is approximately 3 times more sensitive than the solubilizing power to a change in aggregation number. For a given value of the ionic strength, the aggregation number of a PEG-bound SDS aggregate is approximately 50−60% smaller than that of a free micelle, while its solubilization capacity is within approximately 20% of that of a free micelle. As a result, PEG increases the macroscopic solubilizing power at all SDS concentrations above the critical aggregation concentration by virtue of the greater number of distinct surfactant aggregates formed for a given SDS concentration in the presence of PEG. Compared to free micelles, the solubilizing power of PEG-bound SDS aggregates is significantly more sensitive to ionic strength.

Total internal reflection microscopy was used to measure the total interaction between a 6 μm glass sphere and a glass plate, separated by an aqueous solution containing 0.1−1.0 mM of KBr, when both surfaces are saturated with physisorbed polylysine. When the excess polylysine is completely removed from the solution, the sphere fluctuates around the secondary potential-energy minimum formed between double-layer repulsion and gravitational attraction. Subtracting gravity leaves a contribution from double-layer repulsion which decays exponentially with distance; the decay length is virtually identical to the Debye length calculated for each ionic strength. However, the presence of as little as 10 ppm of unadsorbed 26 kDa polylysine (rod length of 45 nm) causes a measurable attraction, although the most probable separation distance without polymer (150 nm) is much larger than the size of the macromolecule. Increases in the attraction with unadsorbed polymer concentration and decreases in the attraction with increasing KBr concentration correlate with the calculated osmotic pressure for two different molecular weights of polylysine, indicating that the attraction arises from the depletion of the polyelectrolyte from the gap between the sphere and the plate.

We used scanning angle reflectometry to measure the adsorption isotherm, adsorption kinetics, and desorption kinetics for cetyltrimethylammonium bromide (CTAB) surfactants on negatively charged silica surfaces. The initial adsorption rate increased with increasing CTAB concentrations between approximately 0.2 × cmc and 10 × cmc, displaying a discontinuous increase at the critical micelle concentration. The initial desorption rate was a monotonically increasing function of the bulk concentration of the surfactant solution from which the adsorbed layer was formed, both above and below the cmc. Combining equilibrium and kinetic information, we conclude that the adsorption mechanism and the structure of the adsorbed layer both change abruptly at the cmc. Below the cmc, monomeric surfactants adsorb to an extent that is consistent with a defective bilayer structure. Above the cmc, micelles adsorb directly to the surface, to an extent that is consistent with a close-packed monolayer of micelles. The adsorption rate was apparently limited by slow rearrangements within the adsorbed layer. CTAB adsorption was significantly hindered by coadsorption with polylysine, in terms of both the rate and extent of adsorption. The effect of polylysine on CTAB adsorption was very sensitive to the ionic strength and the order in which the surfactant and the polyelectrolyte were exposed to the surface. Different pathways to the same final bulk solution composition produced much different adsorption results. This demonstrates that coadsorption of CTAB and polylysine is inherently a nonequilibrium process dominated by kinetic traps. Although it had an overall hindering effect, coadsorption with polylysine did not alter the basic difference in CTAB adsorption mechanisms above and below the cmc.

Although amphiphilic polystyrene−poly(ethylene oxide) diblock copolymers (PS−PEO) adsorb from aqueous solutions to hydrophobic surfaces, they are unable to form an end-anchored brush. Four independent techniques&sbd;scanning angle reflectometry, hydrodynamic layer thickness measurements, streaming potential measurements, and total internal reflection microscopy&sbd;indicate that PS−PEO adsorption is indistinguishable from PEO homopolymer adsorption. The surface concentrations attained by three PS−PEO diblocks are independent of molecular weight (between 67 000 and 479 000) and are indistinguishable from PEO (420 000 molecular weight) surface concentrations. Hydrodynamic layer thicknesses and electrokinetic layer thicknesses are on the order of only a few nanometers for both PEO and PS−PEO diblocks, more characteristic of a homopolymer layer than an extended brush. Furthermore, adsorbed PS−PEO imparts no detectable steric repulsion to the energy of interaction between a Brownian particle and a wall. Apparently, brush formation is prevented because the surface affinity of the large water-soluble PEO block presents a large kinetic barrier to its being completely displaced from the surface by the insoluble PS block.

Coadsorption of polymers and surfactants is a poorly understood process that occurs in a variety of complex fluid applications. In single-component solutions, the cationic polyelectrolyte polylysine and the cationic surfactant cetyltrimethylammonium bromide (CTAB) both adsorb to negatively charged silica surfaces. Here we use scanning angle reflectometry to contrast adsorption from single-component solutions with a sequential adsorption process and a coadsorption process. When adsorbed from single-component solutions, polylysine adsorbs irreversibly, whereas CTAB adsorption is reversible. In the sequential adsorption case, CTAB neither displaces nor adsorbs to preadsorbed polylysine layers. When solutions contain both CTAB and polylysine, they coadsorb to form mixed layers. Mixed layer formation is indicated by a dramatic alteration of the kinetics and reversibility of adsorption compared to either single-component case. The amounts of CTAB and polylysine adsorbed in the mixed layers are both similar to the amounts adsorbed from the respective single-component solution.

We used scanning angle reflectometry to directly measure the extent of water-soluble polymer adsorption to insoluble dipalmitoylphosphatidylcholine monolayers at the air−water interface. Although high molecular weight polystyrene-b-poly(ethylene oxide) (PS−PEO) diblock copolymers are kinetically prevented from forming adsorbed brushes on solid surfaces, they did manage to adopt a more extended or brushy conformation at the fluid interfaces examined here. For both PS−PEO and PEO homopolymer, the ability of the adsorbing polymer to drive the liquid expanded-to-liquid condensed monolayer phase transition facilitated monolayer penetration. The significance of this excluded area sink effect in the monolayer penetration mechanism depended on the polymer architecture. PS−PEO was a more effective monolayer penetrator than PEO homopolymer owing to its ability to increase the PEO block extension when penetrating denser monolayers. Thus, both polymer architecture and excluded area effects determine the effectiveness of monolayer penetration.

By conjugating proteins with a common pH-sensitive fluorescent label, fluorescein isothiocyanate (FITC), and controlling the ionic strength, we provide a means to decrease the characteristic length scale of the total internal reflection fluorescence (TIRF) technique by two orders of magnitude. The usual characteristic length scale for TIRF is an optical length, specifically the evanescent wave penetration depth (on the order of 100 nm). In our experiments the penetration depth is replaced by the Debye screening length as the characteristic length scale. This is readily controlled to match the dimensions of an adsorbed protein layer (on the order of 1 nm). We achieve this length scale reduction by coupling the well-known pH-sensitivity of fluorescence emission by FITC-labeled proteins with the variation of electrostatic potential near a negatively charged surface. Using this fine-resolution TIRF capability in combination with scanning angle reflectometry, we find that lateral repulsions induce a dramatic reconfiguration of adsorbed lysozyme layers on negatively charged silica surfaces. This occurs as the surface concentration approaches the jamming limit for random sequential adsorption. The reconfiguration evidently optimizes electrostatic interactions in the adsorbed layer and decreases the effective excluded area per lysozyme. The decrease in effective excluded area allows adsorption to continue beyond the jamming limit to ultimately attain a hexagonal close packed monolayer of horizontally oriented lysozyme molecules. The adsorption kinetics switch abruptly from being transport-limited to surface-limited after the reconfiguration.