Comparing surface and bulk

Fig. 1. Schematic comparing surface and bulk erosion. In surface erosion (top), water does not penetrate far into the bulk, but hydrolyzes functional groups on the surface. The resulting monomers dissolve and diffuse away from the device. In bulk erosion (bottom), water penetrates into the bulk, polymer may dissolve, and is ultimately hydrolyzed into monomer.

Jenett, H., Bubert, H., and Grallath, E., Comparative surface and bulk analysis of oxygen in Si3Na powders, Fresenius Z. Anal. Chem., 333, 502 (1989). 

Daley, C.R., Fakhraai, Z., Ediger, M.D., Forrest, J.A. Comparing surface and bulk flow of a molecular glass former. Soft Matter 8(7), 2206-2212 (2012)... 

If we assume that the rates of adsorption and desorption are both large compared with the surface migration rate, the surface and bulk concentrations of each species will be almost in equlibriura, and hence will be... 

According to Table 3, the pair interactions converge to their bulk values, but the differences between their surface and bulk values are quite pronounced, larger than in the non-selfconsistent theory. In the model I, the surface value of the first nearest neighbor pair interaction is -0.34 mRy, to be compared with 6.28 mRy found for model... 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

Evidence for a marked difference between the surface and bulk compositions of dilute copper-nickel alloys has been reported recently by a number of investigators (82, 87-90). Much of the experimental evidence comes from hydrogen adsorption data (74, 82, 87, 90). The conclusions of van der Plank and Sachtler were based on the premise that nickel chemisorbs hydrogen while copper does not (82, 87). The total adsorption of hydrogen at room temperature was taken as a measure of the amount of nickel in the surface. However, in hydrogen adsorption studies on the catalysts used to obtain the catalytic results in Fig. 6, the amount of adsorption on the copper catalyst, while small compared to the adsorption on nickel, is not negligible (74) However, the amount of strongly adsorbed... 

Erosion is typically characterized by either occurring on the surface or in the bulk. Surface erosion is controlled by the chemical reaction and/or dissolution kinetics, while bulk erosion is controlled by diffusion and transport processes such as polymer swelling, diffusion of water through the polymer matrix, and the diffusion of degradation products from the swollen polymer matrix. The processes of surface and bulk erosion are compared schematically in Fig. 1. These two processes are idealized descriptions. In real systems, the tendency towards surface versus bulk erosion behavior is a function of the particular chemistry and device geometry (Tamada and Langer, 1993). Surface erosion may permit the... 

Finally, Rong and coworkers discuss the roll of surface oxygen on the MCS process75. Rong employed a lab-scale stirred bed reactor and then applied XPS to analyze the silicon samples before and after the reaction. The reactivity of silicon depended on the initial thickness of the native oxide on the silicon. After the reaction the surfaces of all of the samples were mostly covered with Si02. There was no observed correlation between the surface and bulk O content. XPS analysis showed the presence of Al, Ca and Ti impurities in some samples. Titanium on the surface appeared to increase the reactivity, whereas Ca decreased the selectivity of Di formation. Addition of ZnO to the silicon before CuCl improved reactivity and also decreased the induction period of the reaction. XPS studies of samples prepared in this manner exhibited a lower Zn surface concentration compared to the samples where CuCl, Si and ZnO were mixed together. 

Figure 27a presents surface parameter (-%s) plotted vs bulk parameter % for four blend pairs (dx1/hx2, hx1/dx2) (different symbols correspond to different blend pairs). In turn, Fig. 27b compares the change in surface and bulk parameters caused by the exchange of the blend component stained by deuterium. While Fig. 27b reveals that the change in %s upon isotope swapping seems to be related with similar change in %, Fig. 27a shows that some relations between absolute values of %s and % may exist only within each blend pair Xj/x2. 

The surface atomic ratios of n(Si)/n(Ti) of the sol-gel titania/silicas increase with increasing Si/Ti ratio of 1 to 5. However, those of the precipitated titania/silicas are almost constant and their values are low. These results mean that the distribution of titanium and silicon around the surface of catalyst depends on the preparation method. Therefore, there is a good correlation between the surface and bulk Si/Ti ratios in the sol-gel catalyst. This suggests that Si and Ti components are homogeneously dispersed at both the surface and inside of the sol-gel titania/silica. On the other hand, the content of titanium at the surface of precipitated titania/silica was much higher compared to the corresponding sol-gel titania/silica and did not depend on the bulk titanium content. These results mean that the surface of precipitated titania/silica was covered with titania as is expected from the preparation procedure. 

This process is schematically shown in Fig. 8 where it has been analyzed in terms of the movement of two fronts, Vl5 the movement of a hydrating front and V2, the movement of an erosion front [29]. Clearly, the ultimate behavior of a device will be determined by the relative movement of these two fronts. If Vi > V2, the thickness of the reaction zone will gradually increase and at some time, the matrix will be completely permeated by water. At that point, all ortho ester linkages will hydrolyze at comparable rates and bulk hydrolysis will take place. However, if Vx = V2, then hydrolysis is confined to the surface layers and only surface hydrolysis will take place. In this latter case, rate of polymer erosion will be completely determined by the rate at which water intrudes into the polymer. 

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