Aqueous solutions friction coefficient

The value of ( used here corresponds to 10 3 of its value in room temperature aqueous solutions. [80] On the one hand, using such weak friction improves the sampling efficiency in the simulations and does not affect equilibrium structural properties. On the other hand, the dynamical properties that we observe may be different from those probed by SM-FRET techniques, which would not be able to resolve conformational dynamics on such fast time scales. Thus, the relevance of the following analysis of dynamical properties relies on the assumption that increasing the friction will not significantly alter our main conclusions. It is interesting to note in this context that the folding mechanism in similar models has been observed to be relatively insensitive to the value of the friction coefficient. [81]... 

We can express the phenomenological coefficients in terms of the frictional forces assuming that for a steady-state flow, the thermodynamic forces Xare counterbalanced by a sum of suitable frictional forces F. Thus, for a solute in an aqueous solution, we have... 

Table 1-1 lists the magnitudes of diffusion coefficients for various solutes in water at 25°C.6 For ions and other small molecules, Dfs in aqueous solutions are approximately 10-9 m2 s-1. Because proteins have higher relative molecular masses (i.e., higher molecular weights)7 than the small solutes, their diffusion coefficients are lower (Table 1-1). Also, because of the greater frictional interaction between water molecules and fibrous proteins than with the more compact globular ones, fibrous proteins often have diffusion coefficients that are approximately half of those of globular proteins of the same molecular weight.

The diffusion coefficient depends upon the ease with which the solute molecules can move. In biology we are mainly concerned with aqueous systems, and the diffusion coefficient of a solute in aqueous solution is a measure of how readily a solute molecule can push aside its neighboring water molecules and move into another position. An important aspect of the theory of diffusion is how the magnitudes of the frictional coefficients f and hence of the diffusion coefficients D (see (11.63)), depend upon the properties of the solute and solvent molecules. 

To evaluate the lubricating properties of PLL-g-dex copolymer solutions and compare them with other standard aqueous solutions, including HEPES buffer solution, dextran solutions (dex(5.2)), as well as PLL(20)-g-PEG(5) solutions, coefficient of friction (fi) versus speed plots were acquired under both sliding and mixed sliding/rolling conditions using a pin-on-disk tribometer and MTM, respectively. 

The same equation (2) was also used by Spikes et al. to model the classical friction coefficient in the presence of electrochemical effects between two metal oxide surfaces in aqueous solutions onamacroscopic scale [29,30] and by Lin et al. to model force interactions between tip and surface in an electrolyte for larger separations [18]. 

FIGURE 17.18 Changes in the coefficient of friction of aqueous solutions of oxyethylated alcohols as a function of concentration at a load of 2 kN. (Data obtained using tester T02.)... 

Tribological test results for aqueous solutions of ethoxylated sorbitan esters have been presented in a publication [48], It has been demonstrated that addition to water of such esters as ethoxylated sorbitan monolaurate, ethoxylated sorbitan monostearate, or ethoxylated sorbitan monooleate results in a significant reduction in the coefficient of friction and wear and prevents seizure of mating friction-couple elements. It has been found that the concentration of the additive in water, and not the compound s structure, has the decisive effect on the improvanent in tribological characteristics [48],... 

The tests were conducted on aqueous solutions of SML/ESMIS mixtures at the concentrations of 0.01%, 0.1%, 1%, and 4 wt%. A T-11 tribometer (pin-on-disc) produced by ITeE Poland (Institute for Sustainable Technologies, National Research Institute, Radom, Poland) was used in the tests. The pins used were 3.0 mm in diameter and were made of LH15 steel or polyamide-6, while the discs were 25 mm in diameter and were made of polyamide-6. Before the tests, all components of the friction couples were thoroughly chemically cleaned. An ultrasonic cleaner was employed. The steel elements were cleaned in n-hexane, acetone, ethyl alcohol, and distilled water. The polyamide-6 elements were cleaned in ethyl alcohol and distilled water. After the cleaning process all friction-couple components were dried (50°C, 30 min). The time of the test run was 900 s, the friction-couple load was 10 N, and the sliding velocity was 0.1 m/s. The coefficient of friction was calculated from friction force measurements using... 

Figure 18.10 presents examples of dependences of friction coefficient changes as a function of time for water and 0.01% aqueous solutions of SML/ESMIS mixtures. The highest friction coefficient values were observed for the tests in which pure water was the lubricant. Over the test duration, the coefficient of friction increased from a value of 0.2 to about 0.3. Over the course of measurement, in the case of all the solutions containing the SML/ESMIS mixtures, only a slight reduction in resistance to motion occurred, and no changes in friction coefficient values were observed. However, the compositions analyzed differed in their ability to reduce p. A set of mean friction coefficient values from the whole 900-s test can be seen in fig. 18.11. 

FIG U RE 18.10 Examples of dependences of friction coefficient changes on time for a steel-pin-polyamide-disc pair for water and 0.01% aqueous solutions of SML/ESMIS mixtures. The following SML/ESMIS ratios were investigated 0 10,1 9, 3 7,5 5, and 7 3. 

A relatively high scatter in mean values of p was observed when 1% aqueous solutions of SML/ESMIS mixtures were examined (fig. 18.13). A friction coefficient value of 0.22 was obtained for the composition containing only ESMIS (0 10). Then the p value decreased to 0.2 for a 1 9 mixture. The lowest resistance to motion (p = 0.1) was observed for the 3 7 ratio. The solutions of the other two mixtures with 5 5 and 7 3 SML/ESMIS ratios exhibited higher resistances to motion the p values obtained equaled 0.16 and 0.22, respectively. 

Figure 18.14 shows the friction coefficient values obtained in tests carried out in the presence of 4% aqueous solutions of SML/ESMIS mixtures. No significant differences in friction coefficient values were observed for solutions of all the mixtures, the values ranged from 0.16 to 0.18. 

Figure 18.15 shows examples of dependences of p on time for water and 0.01% aqueous solutions of SML/ESMIS mixtures. In the case of water, during the conrse of the test the coefficient of friction increased slightly from the value of 0.12 to 0.16. The dependences obtained for solutions of all mixtures were similar the p values practically did not change dnring the course of the test. 

The influence of aqueous solutions of SML/ESMIS mixtures on the coefficients of friction, wear, and temperature in the friction pair was evaluated under the following conditions rotational speed of the spindle, 200 20 rpm constant applied load, 3.0 kN (in the case of water, the tests were conducted at the load of 2.0 kN) test duration, 900 s. Data were automatically acquired every second. 

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