The dependence of polyelectrolyte multilayer thickness on salt concentration, salt type, solvent quality, deposition time, and polymer concentration is evaluated. Polymers are deposited on spinning silicon wafers. For the strong polycation/polyanion pair studied, film thickness is approximately proportional to the number of layers and the salt concentration. The irreversibility of overall molecule adsorption is indicated by the lack of exchange of surface (radiolabeled) for solution polymer. The hydrophobic nature of the driving force for polymer sorption is illustrated by the choice of salt counterion or solvent. Analyzed within the framework of ion exchange, the net energy of ion pair formation is not high, at most a few kT. Salt, competing with polymer segments for the surface, permits localized rearrangements. In the mechanism proposed, excess polymer is accommodated within several layers, rather than in one layer of loops and tails. Steric barriers coupled with slow conformational changes are responsible for long-term polymer adsorption. Considering the disorder and interpenetration, multilayer buildup has much in common with solution phase or coprecipitated polyelectrolyte complexes. Surface hydrophobicity can be enhanced using fluorinated surfactants as counterions.

Coating surfaces with thin or thick films of zwitterionic material is an effective way to reduce or eliminate nonspecific adsorption to the solid/liquid interface. This review tracks the various approaches to zwitteration, such as monolayer assemblies and polymeric brush coatings, on micro- to macroscopic surfaces. A critical summary of the mechanisms responsible for antifouling shows how zwitterions are ideally suited to this task.

Stoichiometric polyelectrolyte complexes (PECs) of the strong polyelectrolytes poly­(styrenesulfonate) (PSS) and poly­(diallyldimethylammonium) (PDADMA) were dissociated and dissolved in aqueous KBr. Water was added to dilute the salt, allowing polyelectrolytes to reassociate. After appropriate equilibration, these mixtures yielded compositions spanning complexes (solid) to coacervates (elastic liquid) to dissolved solutions with increasing [KBr]. These compositions were defined by a ternary polymer/water/salt phase diagram. For coacervates, transient microphase separation could be induced by a small departure from equilibration temperature. A boundary between complex and coacervate states was defined by the crossover point between loss and storage modulus. Salt ions within the complex/coacervate were identified as either ion paired with polyelectrolytes (“doping”) or unassociated. The fraction of ion pair cross-links between polyelectrolytes as a function of KBr concentration was used to account for viscosity using a model of “sticky” reptation.

Alkanethiols bearing radiolabeled (35S) head groups have been incorporated into self-assembled monolayers (SAMs) on a variety of substrates to determine coverage, thermaland photostability, and surface roughness. Spontaneous desorption of octadecanethiol SAMs immersed in solvent under ambient conditions is observed for all substrate/solvent combinations, including gold, silver, platinum, and copper and water, ethanol, tetrahydrofuran and hexane. A quantitative analysis of desorption suggests pseudo-first-order kinetics with rate constants in the range of 10-5 s-1 for THF. Evidence for multilayering on copper is found. Molecular hydrogen is not involved in the rate-determining step for desorption. Self-exchange of surface-labeled thiol with solution unlabeled thiol is also described by first-order kinetics. Both desorption and self-exchange experiments yield residual thiols at the surface which cannot be exchanged, and which are presumed to result from stronger binding at defect sites. Similarities in kinetics for exchange and desorption point toward a common mechanism for surface detachment, postulated to be a rate-limiting desorption step as a disulfide. Estimates for free energies of adsorption of a thiol generating molecular hydrogen and adsorption of a disulfide yield -5.5 and -24 kcal mol-1, respectively. In conjuction with recent evidence that thiols are actually adsorbed as disulfides, RSSRAu2(S), the desorption is represented by RSSRAua® -* RSSR + 2Au(S) (slow). For exchange this is followed by 2RSH + 2Au(S) -RSSRAu2(S> + H2 (fast), with some contribution from direct thiol/disulfide interchange for high concentrations of solution thiol, viz: RS*SRAu2(S) + RSH -* RSSRAu2(S) + R*SH. On clean gold surfaces, adsorption is shown to be diffusion limited. Finally, strategies for enhancing the stability of SAM monolayers made from thiols are discussed.

The ion and polymer content of polyelectrolyte multilayers constructed via layer-by-layer deposition have been directly probed using radioanalytical methods. Multilayers were fabricated using salt-containing or salt-free solutions. Charges on polyions quantitatively balance each other. As-deposited multilayers contain no salt ions within a limit of detection of a few ppm. All excess charge, which is reversed on each deposition step, resides at the surface. Surface charge controls the amount of polymer deposited and represents, on average, one-half of the charge within a single molecular layer. Internal charge can be regulated following deposition if one of the polyelectrolytes employed is redox-active, such as a polyviologen. Under electrochemical reduction, bulk charge compensation in a polyviologen/poly(styrene sulfonate) multilayer is preserved mainly by cation influx. Residual salt ions accumulate as conformational changes occur during repeated electrochemical cycling. When a thermally labile precursor to poly(p-phenylene vinylene) is incorporated as polycation, salt cation uptake is observed when positive charge is thermally eliminated from the multilayer. Evidence for disruption of this structure is observed when the charge density on one of the constituents approaches zero. For typical deposition times, usually up to 1 h, polymer deposition is kinetically irreversible, and the top layer is not stripped from the surface on exposure to its oppositely charged counterpart. These results provide further confirmation of extensive interpenetration and disorder as well as limited mobility within polyelectrolyte multilayers.