Krafts temperature

A temperature known as the Kraft temperature denotes the point above which the... 

Micelle formation below the Krafts temperature, perhaps as low as 25°C. 

An amphiphilic molecule has a single positively charged head group and is in solution with a concentration of 10 mM. If the critical micelle concentration of the molecule is 25 mM and the Krafts temperature is 25°C ... 

Miscelles from ionic surfactants can be formed only above a certain temperature called the Kraft temperature. 

Surfactant molecules in solution can form association colloids (called micelles) when the concentration of the surfactant is above a critical micelle concentration. This behavior only occurs above a given temperature, called the Kraft temperature. Below this temperature, the surfactant shows normal solubility behavior. In Fig. 14, a two-dimensional cut through a micelle, according to the most popular model, is shown. 

Explain why detergent does not clean very well below their Kraft temperatures. 

Amphiphilic surfactant molecules form spherical or nearly spherical aggregates called micelles, above a certain critical concentration, known as the critical micellar concentration (cmc) and above a critical temperature, called Kraft temperature [4,93]. The size of the micellar aggregates is usually 1-10 nm and the aggregation number, l.e., the number of surfactant molecules per micelle, ranges from 20 to 200. The structure of a typical cationic micelle is shown schematically in Fig. 

Quaternary ammonium surfactants (dodecyl-, tetradecyl-, and hex-adecyltrimethylammonium or didodecyldimethylammonium) with Tp anion were prepared and characterized also by measurement of their Kraft temperatures and critical aggregation concentrations. NMR spectroscopy suggested that the single-chain surfactants containing the dodecyl ammonium form small aggregates in water. The X-ray structure analysis of the dodecyl derivative was performed.192Azolium poly(l,2,4-triazolyl)borate salts melt at relatively low temperatures to give thermally stable ionic liquids. Some of them exhibit conductivities close to that of KC1 in MeCN/H20... 

FIGURE 10.9 Highly schematic examples of some association colloids, (a) Micelles and bilayers. (From E. Dickinson, D. J. McClements. Advances in Food Colloids. Blackie, 1995.) (b) Crystal, lamellar, and gel structures of simple surfactant water mixtures T is temperature, T Kraft temperature. (Modified from a figure by N. J. Krog.)... 

As the concentration increases, the amphiphilic molecules form micelles and then form columns. The columns are arranged into a hexatic array. As the concentration further increases, the system forms a laminar structure, i.e., a smectic liquid crystal phase. Sometimes, a cubic phase may appear between the micelle and hexatic phases. In fact, the micelles are packed to form a cubic phase. The three phases are all liquid crystal phases hexatic phase, laminar phase and cubic phase. As seen in the figure, the phase diagram of amphiphilic molecules actually depends on the temperature as well. Tk in Figure 1.15 is the Kraft temperature, below which the system is phase separated into crystal and water. 

Hydrophilic-lipophilic balance (HLB) For a surfactant, the balance of tail hydrophobicity to headgroup hydrophilic-ity that determines the cmc, aggregation number, Kraft temperature, and other properties of micelles. The HLB is a quahtatively useful concept that is hard to apply quantitatively. 

Figure 13.3 shows a simplified phase diagram of an aqueous surfactant system. The Kraft point is the temperature at which the surfactant solubility is equal to the critical micelle concentration (CMC). When the temperature is lower than the Kraft temperature, the surfactant exists in gel or crystal form in the solution. When the temperature is above the Kraft temperature and the surfactant concentration is higher than the CMC, the micelles are formed in the surfactant solution. The micelles are initially spherical in shape. With the increase in surfactant concentration or upon... 

Phase diagram for a typical soap-water system. The nearly vertical dashed line shows the minimum concentration for micelle formation. The line separating the crystal in water part from the liquid crystaUine parts is called Kraft temperature (Tr). Vertical bars and symbols at the top of the figure indicate hypothetical sequence of phases in binary amphiphile-solvent systems. Here a, b, c and d indicate intermediate phases (for example the bicontinuous cubic phase), L2 denotes the inverse micelle solution, H2 is the inverse hexagonal phase, L is the lamellar phase, H, is the normal hexagonal phase and Lj is the normal micelle phase. Note, this idealized sequence has never been observed entirely and the phase boundaries are rarely vertical. 

The Kraft point (T ) is the temperature at which the erne of a surfactant equals the solubility. This is an important point in a temperature-solubility phase diagram. Below the surfactant cannot fonn micelles. Above the solubility increases with increasing temperature due to micelle fonnation. has been shown to follow linear empirical relationships for ionic and nonionic surfactants. One found [25] to apply for various ionic surfactants is ... 

The amount of softwood xylan sorbed from alkati at kraft cooking temperatures (100—170°C) is proportional to the quantity of hemiceUulose present and inversely to the extent of branching (57). At neutral pH, the presence of carboxyl groups inhibits sorption compared to a control, but no difference is observed when ionization is suppressed (58). 

The aromatic rings of kraft lignins can be sulfonated to varying degrees with sodium sulfite at high temperatures (150—200°C) or sulfomethylated with formaldehyde and sulfite at low temperatures (<100° C). Oxidative sulfonation with oxygen and sulfite is also possible. 

The type of varnish used in the process depends on the kraft paper manufacturer and basis weight of the papers the machine, temperature, and control (scraper bars, squeeze roUs) used the method of cutting the paper to size the laminate being produced (post-forming or regular) and the press-cure cycle (see Laminated materials, plastic). 

PolysuWde Process. One modification to the kraft process being appHed commercially is the polysulfide process (38). Under alkaline conditions and relatively low temperature (100—120°C), polysulfides oxidize the active end group of the polysaccharide polymer to an alkaH-stable aldonic acid. This reaction, known for many years (39), was not produced on a commercial scale until the development of an efficient method for in situ generation of the polysulfide in kraft white Hquor. 

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