Monolayers negative adsorption

The Motomura analysis in Figure 2 shows the effects of monolayer compression. As expected, compression causes some ejection of surfactant, particularly at low surfactant concentrations. There even appears to be negative adsorption at 0.6 mmol kg" but while this result is plausible in a qualitative sense, the depth from which surfactant would need to be excluded is unacceptably great. 

Polysaccharides interfaced with water act as adsorbents on which surface accumulations of solute lower the interfacial tension. The polysaccharide-water interface is a dynamic site of competing forces. Water retains heat longer than most other solvents. The rate of accumulation of micromolecules and microions on the solid surface is directly proportional to their solution concentration and inversely proportional to temperature. As adsorbates, micromolecules and microions ordinarily adsorb to an equilibrium concentration in a monolayer (positive adsorption) process they desorb into the outer volume in a negative adsorption process. The adsorption-desorption response to temperature of macromolecules—including polysaccharides —is opposite that of micromolecules and microions. As adsorbate, polysaccharides undergo a nonequilibrium, multilayer accumulation of like macromolecules. 

In Fig. 5.21, from Dawson s paper, the uptake at X for the 250°C-outgassed sample is dose to the calculated value for a monolayer of water with a (H20) = 101 A. Point X has therefore been ascribed to a close-packed monolayer of water on a hydroxylated surface of rutile. The fact that the differential entropy of adsorption relative to the liquid state (calculated from the isosteric heat of adsorption) changes sharply from negative to positive values in this region with A s 0 at X was regarded as supporting evidence. ... 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

Monolayers of distearoylphosphatidylcholine spread on the water-1,2-dichloro-ethane interface were studied by Grandell et al. [52] in a novel type of Langmuir trough [53]. Isotherms of the lipid were measured at controlled potential difference across the interface. Electrocapillary curves derived from the isotherms agreed with those measured under the true thermodynamic equilibrium. Weak adsorption or a stable monolayer was found to be formed, when the potential of the aqueous phase was positive or negative respectively, with respect to the potential of the 1,2-dichloroethane phase [52]. This result... 

When the pressure of A is sufficiently low, its coverage is much less than one monolayer and 6a = Ka Pa to give first-order kinetics. As Pa increases and 6a approaches unity, the coverage stops increasing [Oa = Ka Pa/( 1 + Ka Pa)] so the reaction becomes zeroth order in P/i as 1, where Kr = kft. Finally if the product B is sufficiently strongly adsorbed to build up a sufficient coverage, then adsorbed B blocks the adsorption of A 6a = Ka Pa Kb Pb) and the rate becomes first order in Pa but negative order in the product B. 

Similar behavior of other aromatic disulfides and thiols on gold electrodes has been described based on the SERS experiments [167]. Adsorption of benzenethiol, benzenemethanethiol, p-cyanobenzenemethanethiol, diphenyl sulfide, and dibenzyl sulfide was studied on the roughened gold electrode. All these species adsorb dissociatively as the corresponding thiolates. Monolayers formed from symmetric disulfides were exactly like those formed from the corresponding thiols. These monolayers were stable in a wide potential window from -1-800 to —1000 mV (versus SCE), which was limited by the oxidation of the Au surface from the positive side and hydrogen evolution at —1000 to —1200 mV at the negative side. 

Fig. 1 Potential-dependent SHG response of a thin silver film (45 nm) in contact with a solution of 0.1 M NaCl04 (solid circles), 0.1 M NaCl04 -1-50 mM urea (solid squares), and 0.1 M sodium acetate - -5 mM lead acetate (open triangles). Most of the crystallites of the pc-Ag film were oriented with the (111) face parallel to the surface. The minimum of the curve occurs at around —0.75 V, which is close to the PZC for Ag(l 11) surface. Adsorption of urea causes a little shift in PZC and a significant decrease of the SHG intensity negative of PZC point, whereas the deposition of a monolayer of Pb causes a dramatic increase in the SHG intensity [6, 9].

Daujotis et al. [32] have described the use of electrochemical quartz microbalance for the quantitative studies on monolayer adsorption on working mercury electrodes. Mercury was deposited on Pt at negative potentials (—0.4 to —0.5 V versus AgjAgCljKClgat)- In order to avoid undesirable transformation of mercury into, for example, larger droplets, the thickness of mercury film could not exceed 20 nm. Then, the linear dependence of the frequency change on the added mass was achieved. Applicability of such an electrode for EQCM measurements has been demonstrated by performing electroreduction of Pb(II) and T1(I), as an example. 

---