Adlayer thickness

Fig. 9 Adlayer thickness sensitivity against adlayer thickness for the conventional and reverse symmetry waveguides considered in Fig. 7. The adlayer RI equals 1.5...

Fig. 1. (a) A chemical structure of a 2.5th generation carboxylic acid-terminated poly(amido amine) (PAMAM) dendrimer. (b) Transmission surface enhanced infrared absorption spectra (SEIRAS) of dendrimer adlayers prepared at 30 min adsorption from aqueous solutions (0.01 wt.%) of a dendrimer at different pHs. Numerical values are pHs of the solutions, (c) Adsorption-desorption profiles as a function of time at different pHs and adlayer thicknesses at adsorption and desorption equilibrium as a function of pH for aqueous solutions (0.1 wt.%) of the dendrimer. The symbols, j and J, in the top figure denote start of adsorption and desorption, respectively. In the bottom figure, filled circle and opened square denote adlayer thicknesses at adsorption and desorption equilibrium, respectively. The dark tie denotes the calculated dendrimer size width. A solid curve is drawn to be visual, (d) Schematic illustration of dendrimers adsorbed at different pHs. Reprinted with permission from Ref. [69], 2006, American Scientific Publishers. 

These results imply that for the steric stabilization of Si02 particles, polymers with substantially lower molecular weight, corresponding to a thiimer stabilizing layer, are required than for AI2O3. The molecular weight Mw of an adsorbed homopolymer can be approximately related to the adlayer thickness by [17]... 

On the other hand, the adlayer thickness has also an influence on the viscosity of the dispersion. It is generally known that the viseosity 7 of a particle dispersion depends on its solids volume fraction O. The particles start to form a network as d> is increased, which leads to an increase in relative viscosity rjr as described in the modified Krieger-Dougherty equation... 

Figure 7. Volume fraction of AI2O3 nanoparticles resulting in a relative viscosity ijr of 1000 in dependence of adlayer thickness. Image reproduced from Ref. [58] copyright 2007 American Chemical Society.

Einarson and Berg (1993) have attempted to explain the data on flocculation kinetics of latex particles with a block copolymer adsorbed on them. The polymer was polyethylene oxide (PEO)/polypropylene oxide (PPO). PPO is water insoluble and forms the part that adsorbs on the latex PEO forms streaming tails into water. Some charge effects remain after the polymer adsorption. The total potential is DLVO plus elastic plus osmotic effects. After fitting the model to the experimental data, they were able to calculate the value of 6, which they called the adlayer thickness. Their data on the stability ratio of latex with and without the polymer and as a fimction of NaCl concentration are shown in Figure 3.23. Note that the polymer stabilizes the colloid by almost one order of magnimde in NaQ concentration. That is, polymers may be necessary to maintain stability in aqueous media containing substantial electrolyte. 

Table 1. Apparent Normalized Atomic Concentration (Determined witii XPS, Analysis Depth 10 nm), Adlayer Thickness (VASE), and Static Contact Angle for Homogenous Monolayers Assembled from PFAND (440 in 1 2 Propanol/H20) and ND (500 jtiM in Pnre H2O) Solutions...

Table 1 shows the surface-chemical composition (atom %), adlayer thickness, and static contact angle for different layers... 

VASE measurements reveal a thickness of 1.3—1.4 nm for densely packed monolayers of PFAND. The difference between molecular length (Rs 2.1 nm) and measured adlayer thickness is attributed to three considerations (i) relaxed PFAND molecules are most likely not straight but helical and bent as in the case of fluorinated alkyl thiols, (ii) adsorbed PFAND molecules, as fluorinated alkyls, might be tilted with respect to the surface normal by 30°-35° to optimize packing and intermolecular interactions, and (iii) VASE-determined thickness is systematically underestimated by the layer model that was employed, because of adventitious carbon/contamination, which is constantly replaced by PFAND during adsorption, a process referred to as self-cleaning by Buck et al. for thiols on gold. Comparison of C Is XP spectra allows an estimation of the replacement of surface contamination the amount of contamination on clean TiO surfaces is equivalent to a fraction (20-25%) of what is... 

A Variable Angle Spectroscopic EUipsometer of the type M200-F (J.A. Woollam Co. Inc., Lincoln, USA) with a spectral range from 245 to 995 nm was used to determine the thickness of the adsorbed polymer layers. Measurements were performed in ambient at three different angles (65, 70, and 75° with respect to the surface normal). For each polymer adlayer, i.e. Sil-PEG (from toluene), Sil-PEG (from acidic aqueous solution), and PLL-g-PEG (from aqueous HEPES buffer), five samples were prepared to obtain statistical data. The measurements were fitted with multilayer models using WVASE32 analysis software. The analysis of optical constants was based on a bulk silicon/ SiOj, layer, fitted in accordance with the Jellison model. After adsorption of the molecules, the adlayer thickness was determined using a Cauchy model A = 1.45, B = 0.01, C = 0). 

Table 1 shows the results of thickness measurements of the three polymer adlayers by ELM Sil-PEG from toluene, Sil-PEG from acidic aqueous solution, and PLL-g-PEG from HEPES buffer. The highest adlayer thickness was obtained from Sil-PEG adsorbed from toluene (2.21 0.25 nm), followed by PLL-g-PEG (1.41 0.05 nm) adsorbed from aqueous HEPES buffer... 

Eunctionalized PLL-g-PEG gradients were prepared in two subsequent immersion steps, as described above. Figure 7 displays the adlayer thickness after the different steps of the protocol as a function of the position on the gradient. A one-component PLL-g-PEG/PEG-biotin gradient (Fig. 7, step 1) was backfilled with unmodified PLL-g-PEG, which gradually filled up the empty sites on the surface (Fig. 7, step... 

Consider a substratum (su) and an adlayer (ad) of thickness t covering the fraction (p of substratum surface (Fig. 23c) the adlayer thickness is considered to be the same on all the covered areas. Expressions below are worked out in the case where proteins would be adsorbed on a fluorinated polymer. 

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