Expanded state

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

A type of physical stabili2ation process, unique for poly(vinyl chloride) resias, is the fusion of a dispersion of plastisol resia ia a plastici2er. The viscosity of a resia—plastici2er dispersioa shows a sharp iacrease at the fusioa temperature. Ia such a system expansioa can take place at a temperature corresponding to the low viscosity the temperature can then be raised to iacrease viscosity and stabili2e the expanded state. 

CellgeometTy is governed predominantly by the final foam density and the external forces exerted on the cellular stmcture prior to its stabilization in the expanded state. In a foam prepared without such external forces, the cells tend to be spherical or ellipsoidal at gas volumes less than 70—80% of the total volume, and they tend toward the shape of packed regular dodecahedra at greater gas volumes. These shapes have been shown to be consistent with surface chemistry arguments (144,146,147). Photographs of actual foam cells (Fig. 2) show a broad range of variations in shape. 

Robinson, I.K., Harrison, S.C. Structure of the expanded state of tomato bushy stunt virus. Nature 297 ... 

Step 4 Determine internal energy in expanded state, Uy... 

The specific internal energy of the fluid in the expanded state U2 can be determined as follows If a thermodynamic graph is used, assume an isentropic expansion (entropy s is constant) to atmospheric pressure po- Therefore, follow the constant-entropy line from the initial state to Po- Read h- and V2 at this point, and calculate the specific internal energy U2-... 

When a component of interest is considerably surface active, its adsorbed amount is high even when its bulk concentration is low. The second terms on the right-hand side of Eqs. (4)-(6) are then small and the relative surface excesses are simply taken as the surface excesses, which, in turn, may be seen as the surface concentration. For example, dilaur-oylphosphatidylcholine forms a saturated monolayer in the liquid-expanded state at the nitrobenzene-water interface when its concentration in nitrobenzene is 10 moldm [30]. Then the experimentally obtained value, 1.76 x 10 °molcm, can be considered to be the surface concentration. 

Kakiuchi et al. [75] used the capacitance measurements to study the adsorption of dilauroylphosphatidylcholine at the ideally polarized water-nitrobenzene interface, as an alternative approach to the surface tension measurements for the same system [51]. In the potential range, where the aqueous phase had a negative potential with respect to the nitrobenzene phase, the interfacial capacity was found to decrease with the increasing phospholipid concentration in the organic solvent phase (Fig. 11). The saturated mono-layer in the liquid-expanded state was formed at the phospholipid concentration exceeding 20 /amol dm, with an area of 0.73 nm occupied by a single molecule. The adsorption was described by the Frumkin isotherm. 

Phospholipid monolayers in liquid expanded state are likely to modify mostly the interfacial concentrations, and this electrostatic effect can be described by Eq. (6). Taking a 1/2, in agreement with most experimental results, and introducing the approximation A 02 = 0, Eq. (6) simplifies to [59]... 

The effect of phospholipid monolayers on the rate of charge transfer has been the subject of several experimental studies, but still there is a need for additional experimental evidence. For large molecular areas, the effect on the rate of ion transfer seems to be negligible [5]. An increasing surface concentration of lipids leads to liquid expanded states where the electrostatic effects are noticeable. An enhanced rate of ion transfer across monolayers of pure phospholipids has then been observed both for the cases of tracer [11,12] and supporting electrolyte ion transfer [13,17]. Finally, the blocking effect is dominant in liquid condensed monolayers [15]. 

Fig. 3. Swollen temperature- and pH-sensitive hydrogels may exhibit an abrupt change from the expanded (left) to the collapsed (syneresed) state (center) and then back to the expanded state (right) as temperature and pH change.

Hysteresis was generally observed in the compression-expansion cycles of the force-area isotherms, indicating that the timescale for relaxation of the fully compressed film back to its expanded state was slower than the movement of the barrier of the Langmuir trough. Our studies, like many others, imply that monolayers are metastable and that reversible thermodynamics can only be applied to their analysis with caution. 

Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers have shown efficacy in preventing the clinical progression of renal disease in patients with type 2 DM. Diuretics are frequently necessary due to volume-expanded states and are recommended second-line therapy. 

Serum sodium concentration is generally maintained by an increase in fractional excretion of sodium, resulting in a volume-expanded state. The most common manifestation of increased intravascular volume is systemic hypertension. 

Influence of subphase temperature, pH, and molecular structure of the lipids on their phase behavior can easily be studied by means of this method. The effect of chain length and structure of polymerizable and natural lecithins is illustrated in Figure 5. At 30°C distearoyllecithin is still fully in the condensed state (33), whereas butadiene lecithin (4), which carries the same numEer of C-atoms per alkyl chain, is already completely in the expanded state (34). Although diacetylene lecithin (6) bears 26 C-atoms per chain, it forms both an expanded and a condensed phase at 30°C. The reason for these marked differences is the disturbance of the packing of the hydrophobic side chains by the double and triple bonds of the polymerizable lipids. At 2°C, however, all three lecithins are in the condensed state. Chapman (27) reports about the surface pressure area isotherms of two homologs of (6) containing 23 and 25 C-atoms per chain. These compounds exhibit expanded phases even at subphase temperatures as low as 7°C. 

From these descriptions, it is seen that the films may, under given experimental conditions, show three first-order transition states, such as (i) transition from the gaseous film to the liquid-expanded (Lex), (ii) transition from the liquid-expanded (Lex) to the liquid-condensed (Lco), and (iii) from Lex or Lco to the solid state if the temperature is below the transition temperature. The temperature above which no expanded state is observed has been found to be related to the melting point of the lipid monolayer. 

The apparent similarity of the F, A curves of myristic acid to the F, V curves for an easily liquefiable gas such as carbon dioxide has led Adam to suggest that the molecules in the expanded films of these substances are in a similat state to those of say carbon dioxide in the vapour state. He adduces as further argument in support of this hypothesis, the much greater coefficient of compression of expanded films. Again it is evident that the molecules are not lying down flat in the expanded state, for the increased area even under low compressions is only 50 A., whilst for stearic acid of length 21 A. and of cross-section 5 2 A. a horizontal molecule would occupy at least 109 A. of area at the same time the area per molecule decreases somewhat as the length of the hydrocarbon chain increases. 

The interfacial tension of mixed adsorbed films of 1-octadecanol and dodecylammonium chloride has been measured as a function of temperature at various bulk concentrations under atmospheric pressure. The transition interfacial pressure of 1-octadecanol film has been observed to increase with the addition of dodecylammonium chloride and then to disappear. The interfacial pressure vs mean area per adsorbed molecule curves have been illustrated at a constant mole fraction of adsorbed molecules. With the aid of the thermodynamic treatment developed previously, we find that the mutual interaction between 1-octadecanol and dodecylammonium chloride molecules in the expanded state is similar in magnitude to the interaction between the scime kind of film-forming molecules. 

Figure 1 shows the plots of the interfacial tension against temperature at various values for = 7.54 mmol kg". All the curves except for the concentrated solution of dodecylammonium chloride have a break point which represents the phase transition from the condensed to expanded state. The phase transition temperature lowers with increasing and disappears above a certain ni2 value. 

Condensed monolayer films of pure 6 polymerized rapidly, as did mixed 6/DSPE films of up to 75% DSPE, provided the monolayers were in the condensed state [33], In the liquid-expanded state, polymerization did not occur. In the condensed state, lateral diffusion of individual lipids within the monolayer is severely restricted compared to the liquid-like state. This precludes initiation of polymerization by diffusive encounter between excited-state and ground-state diacetylene lipids. In order for polymerization to occur in the condensed state, the film must be separated into domains consisting of either pure 6 or pure DSPE. A demonstration that the rates of photopolymerization for pure 6 and mixed 6/DSPE monolayers are equal would be a more stringent test for separate domains of the lipids, but no kinetic data have been reported for this system. 

The precise structural details of the liquid-expanded state at the molecular level are not fully understood, but several generalizations do appear to be justified. The value of s several times the actual cross-sectional area of the amphipathic molecule, when oriented perpendicular to the surface. At the same time the area per molecule is considerably less than... 

For the condensed LC and S states a molecular interpretation is again possible. In both the values of o° are close to actual molecular cross sections when the molecules are oriented perpendicular to the surface. The difference between these two regions seems to involve the polar part of the molecule more than the hydrocarbon chain, which was more important for the more expanded states. The difference between and o c may involve a more efficient packing of the heads or the formation of fairly specific lateral interactions through hydrogen bonds, for example. The values of o° that are observed for monolayers of saturated w-alkyl compounds are only slightly larger than the close-packed cross sections obtained for these compounds in the bulk solid state by x-ray diffraction. 

With our model free energy, we can readily examine small fluctuations around homogeneous states. First let us suppose an isotropically expanded state under zero osmotic pressure IT, + njon 4- Tlel = 0. The deformed position X is composed of the average x = (xj, x2, x3) = ax°, a being the elongation ratio, and the deviation u,... 

At x > xcr, the network is in the very expanded state. The size of the chain between two junction points, R, is proportional to m R = aR0 ma/a /2 as it is for fully stretched chain. The reason for such an essential expansion is the osmotic pressure of counter ions which originates from their translational entropy. Trom the entropy consideration counter ions would like to leave the network, however, this is forbidden due to the condition of total electroneutrality. This effect was for the first time described in Ref. [7]. 

The photoelastic behavior of nonionized PAAm network and ionized P(AAm/MNa) network prepared by the copolymerization of AAm with MNa ( MNa = 0.05) was investigated in water-acetone mixtures [31]. For a pure PAAm network, the dependences of all photoelastic functions (see Eqs. (15) and (16)), i.e. modulus G, strain-optical function A and stress-optical coefficient C, on the acetone concentration in the mixtures are continuous (Fig. 17). At ac = 54 vol %, the ionized network undergoes a transition which gives rise to jumpwise change in G, A and C also the refractive index of the gel n8 changes discontinuously. While in the collapsed state the optical functions A and C are negative, in the expanded state they are positive. 

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