Alexander Silberberg

Distribution of conformations and chain ends near the surface of a melt of linear flexible macromolecules

Abstract In a melt of linear flexible macromolecules the conformations adopted by the chains follow the ideal Kuhn random walk distribution, because of the extremely high, but uniform, local conformational attrition factor. Long-range exclusion effects are overwhelmed and pseudoideal conditions prevail. One may assume that at an interface with a melt all but the uppermost segment layers are still ideal in the above sense. This assumption leads to a method allowing the enumeration of conformations according to their end-to-end distance and the determination of the mean square end-toend distance. The result is that coils in the interfacial layer tend to be compressed. The extent of the surface zone of altered end-to-end distribution is scaled by the root mean square end-to-end distance of the coils in the bulk of the melt.

Structure and Function of Mucus

A glycoprotein building block is common to mammalian mucins. This structure is composed of several protein chains having the same sequence. The carbohydrate side chains, which constitute over three-quarters of the weight, coat only some two-thirds of the backbone chain. The bare protein chains are linked by disulphide bridges and can be digested away with trypsin. Either procedure rapidly solubilizes mucus and results in a structural unit of about 500 000 molecular weight. Mucus solubilizes spontaneously. The first size unit which reaches solution is about 15 X 10(6) molecular weight but continues to break down further. Mechanical agitation considerably speeds up this process. The gel-like character which is an essential feature of mucus--which cannot otherwise act as transport coupler--is thus a transient phenomenon. The problem of how such a structure can arise from the building blocks known to be available is discussed.

Irreversible adsorption of triple-helical soluble collagen monomers from solution to glass and other surfaces

The adsorption from various solutions of triple-helical soluble collagen monomers to solid surfaces was studied by labeling the collagen with 1251. Adsorption to glass, siliconized glass, and Teflon, from aqueous solutions at various pH and ionic strength, was determined at collagen concentrations from 2 to 25 μg/ml. Adsorption was shown to be irreversible and little dependent on pH and ionic strength but increasing enormously as the surface is made more hydrophobic. Surface denaturation of the collagen by heat results in a substantial loss of material. The kinetics of adsorption suggest that the adsorption process may be selective and that not all collagen molecules which reach the surface are immediately adsorbed. Checking these results with earlier measurements of adsorbed layer thickness, a model for collagen adsorption is proposed.

Multilayer adsorption of macromolecules

Abstract The adsorption of long-chain flexible macromolecules from dilute solution to a solid or other sharply defined substrate has a number of peculiar features. Most notable is the fact that not all polymer segments will seek to establish contact with the phase boundary. In one and the same macromolecule there will be some segments bonded to the surface and some sticking far into the solution. There is internal equilibrium and exchanges will occur between segments in free solution and segments attached to the surface. The segments in free solution can be seen to create a surface zone of fairly high polymer segment concentration. Since there is equilibrium, all macromolecules which have at least one segment in contact with the surface belong to the same class of adsorbed species. When, therefore, we speak of multilayer formation, we are defining a class of macromolecules which occupy locations close to the surface but which have not a single segment in contact with it. Such a concentrated surface zone would have to follow the ordinary rules of phase separation in polymer systems except that the presence of the first monolayer establishes a zone where the polymer concentration is maintained at a much higher level. The question is here investigated under what circumstances a preformed concentrated layer can attract the formation of a more extensive surface phase than is provided by the “monolayer,” even though the concentration of polymer in the bulk solution, from which adsorption is to occur, is below the critical concentration for phase separation and even though the solvent is better than critical. It is shown that multilayers are quite feasible within 20°C of the θ-point but the number of additional macromolecules has to be less than the number of those adsorbed.

Flow through gel-walled tubes

Abstract The phenomenon discovered by J. Lahav, N. Eliezer, and A. Silberberg (Biorheology 10, 595, 1973) that flow through gel-coated tubes is subject to considerably increased drag was further examined. Data were taken over a much wider range of flow and it was shown that the functional dependence of the loss of throughput on the Reynolds number is similar to that in incipient turbulent flow through rigid tubes indicating a comparable mechanism. With gel-walled tubes, however, the effects are observable at much lower Reynolds numbers. The parameter α, the shear strain induced by the flow in the gel mantle, correlates with the throughput data obtained with all gel-coated systems over the entire range. The functional interdependence of all these parameters has been derived. The impression is gained that oscillations in the gel wall induce a turbulent boundary layer while core flow remains laminar. As Reynolds number increases the “boundary layer” encompasses the entire tube.

Distribution of segments near the surface of a melt of linear flexible macromolecules: Effect on surface tension

Abstract The concept of configuration swap introduced previously (A. Silberberg, J. Colloid Interface Sci. 90, 86 (1982)) gives a method for constructing the conformations of a flexible polymer chain in the melt near a surface. This has now been used to compute the distribution of centers of mass and the distribution of segments around them in the surface zone. The constraints imposed by the surface correspond to a reduction in entropy and a positive contribution to the temperature coefficient of surface tension. This effect provides a small upward shift and can be used to bring the prediction for the surface tension, by C. I. Poser and I. C. Sanchez (based on a gradient of density through the surface zone optimized by the Cahn-Hilliard approach, J. Colloid Interface Sci. 69, 539 (1979)), into full agreement with experiment.

Gas composition and pressure in the middle ear: A model for the physiological steady state

The gas contents of the physiological middle ear periodically cycle through two phases in steady state. During phase I, the eustachian tube is shut and the middle ear gas space is effectively closed. Gas is absorbed or produced at the mucosal surface, and the total pressure changes correspondingly. During phase II, which is of very short duration, the eustachian tube opens, a bolus of gas passes between the middle ear and the nasopharynx, and the total pressure in the middle ear rapidly adjusts to that in the nasopharynx. Since nasopharyngeal pressure fluctuates in time, so does the pressure in the middle ear. The effect of these pressure changes is to produce a level of ventilation in the middle ear, which depends on a combination of three parameters: the volume of the middle ear, multiplied by the mean amplitude of pressure variations in the nasopharynx, divided by the mean elapsed time between successive eustachian tube openings. Assuming steady-state conditions, the composition of middle ear gas can be computed and is predicted to range from PN2 = 621, PO2 = 46, PCO2 = 46, PH2O = 47 mm Hg in the case when nasopharynx fluctuations are small, to a match with nasopharyngeal gas composition, when the fluctuations are large.

Theoretic Analysis of Middle Ear Gas Composition under Conditions of Nonphysiologic Ventilation

As gas flows in and out of the nasopharynx, the pressure in that region fluctuates. It drops below or rises above atmospheric pressure, which is itself not constant but is subject to changes in altitude and weather. Such pressure changes in the nasopharynx produce a pumping of gas into and out of the middle ear. The net amount of middle ear gas transferred from or to the nasopharynx will, component for component, in steady state exactly equal the amount of middle ear gas transferred to or from the microcirculation by means of diffusional absorption by (or release from) the mucosa. In the case of a permanently patulous eustachian tube, a single parameter, characteristic of the rate of ventilation through the open eustachian tube, is found to determine the gas composition in the middle ear, whereas in the case of a middle ear ventilated by tympanostomy, two rate-of-ventilation parameters, one for gas flow through the ventilation tube and one for flow through a periodically open eustachian tube, determine the steady state gas composition. A high rate of ventilation favors absorption of oxygen and venting of carbon dioxide from the middle ear in both cases.

Function and properties of epithelial mucus.

In the case of ciliated epithelia over which transport of particulate matter takes place, mucus has been shown to fulfill the role of mechanical coupler (Sade, et al., 1970). Whereas ciliary beat provides the source of motion, the fresh secretion of mucus is essential for converting this motion into the movement of particles over the epithelial surface. For example, on the palate of the frog, the mucociliary system provides the means of transporting food particles from the mouth to the gullet. When such a palate is excised it continues to transport particles, but repeated loading of the palate, say by little steel balls, gradually exhausts it of its mucus. Particle velocity drops and eventually reaches zero even though ciliary beat continues. The palate epithelium, locally has become depleted of its mucus which is found accumulated along the cut edge. When some of this mucus is returned to the epithelium, transport of the steel balls is resumed at the original velocity. Such a biological preparation can be used, therefore, to test whether other systems can be applied to the epithelium and substitute for mucus. It can also answer the question of what properties they must possess to provide this function.

Interaction of lipid multilayers with water

Abstract Lipid multilayers built by the monolayer transfer method of Blodgett and Langmuir have often been considered the equivalent of two-dimensional crystals. Their stability in air seems to confirm this view. When such multilayers are exposed to water, however, water molecules penetrate the outer structure and in equilibrium establish a concentration profile which is high at the surface and then, monotonously, drops to zero some six monolayers down. This result was established using transfer of label from tritiated water and changes in fluorescence of dye molecules embedded at fixed levels in the multilayer.