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1、【精品文档】如有侵权，请联系网站删除，仅供学习与交流表面活性剂2.精品文档.increase becomes very rapid near the cloud point being because of the large increase in aggregation number. However, polar materials which are solubilized in the palisade layer of the micelle have very different behavior as the temp. rises. The amount of solubil
2、ized material generally goes through a maximum as the temperature is increased to the point of cloud, reflecting the thermal agitation of the surfactant molecules in the micelle and the dehydration and higher coiling of the polyoxyethylene chains, decreasing the space in the palisade layer.3. Micell
3、ar Catalysis The effect of micelles on organic reactions can be attributed to both electrostatic and hydrophobic interactions. Electrostatic interaction may affect the rate of a reaction either by its effect on the transition state of the reaction or by its effect on the concentrations of reactant i
4、n the vicinity of the reaction sites. Thus, a cationic micelle with its multiplicity of positively charged hydrophilic heads may catalyze the reaction between a nucleophilic anion and a neutral substrate by delocalizing the negative charge developed in the transition state of this reaction, thereby
5、decreasing the energy of activation of the reaction. It may also catalyze the reaction by increasing the conc. of nucleophilic anion at the micelle-water interface close to the reaction sites of the substrate. It is necessary for catalysis to accur that, a) the substrate should be solubilized by the
6、 micelle, and b) the locus of solubilization should be such that the reaction sites of the substrate should be accessible to the attacking reagent. The hydrophobic interactions will be of major important because they determine the extent and the locus of solubilization in the micelle. Surfactants ha
7、ve been used extensively for the enhancement or inhibition of industrially and biologically important free radical processes such as emulsion polymerization and the oxidation of hydrocarbons and unsaturate oils.In table 4-1, some data of solubilization of ethylbenzene in aqueous potassium soaps solu
8、tions at 25 are given. Table 4-1 Surfactant solution Moles of solubilizate per mole surfactant C7H15COOK 0.30M 0.004 0.48M 0.025 0.66M 0.048 0.83M 0.080 C9H19COOK 0.10M 0.014 0.23M 0.116 0.44M 0.154 0.50M 0.174 0.72M 0.202 C11H23COOK 0.042M 0.166 0.20M 0.318 0.50M 0.424 0.60M 0.452 0.86M 0.506 C13H2
9、7COOK 0.096M 0.563 0.24M 0.728 0.50M 0.855 0.57M 0.872 C15H31COOK 0.070M 1.06 0.15M 1.14 0.23M 1.32 0.29M 1.47Chapter 5. Reduction Of Surface And Interfacial Tension By Surfactants1. General Considerations Reduction of surface or interfacial tension is one of the most fundamental properties of surfa
10、ctants in solution. It depends directly on the replacement of molecules of the solvent at the interface by surfactant molecules, and therefore on the surface excess conc. of surfactants, as shown by the Gibbs equation:Where d is the value of change in the surface tension, i is the surface excess con
11、c. of component i, and di is the change in the chemical potential of component i. As has been discussed previously, the molecules at the surface of a liquid have potential energies greater than those of that in the interior of the liquid. And work must be expended to bring a molecule from the interi
12、or to the surface. The surface free energy per unit area, or surface tension, is a measure of this work. Surface tension is often conceptualized as a force per unit length at a right angle to the force required to pull the surface molecules apart in order to permit expansion of the surface, i.e. to
13、create new surface. At the interface between two phases, a and b, the interfacial free energy per unit area of the interface, i.e. the interfacial tension i is given by:Where a andb are the surface tensions of pure liquids a and b, respectively. ab is the a-b interaction energy per unit area across
14、the interface. If phase a is a gas, then, a is about zero andab is also very small, so small that could be neglected, thus, i =b . When the interaction between a and b is very strong that is equal to that between the same molecules a-a and b-b, i.e., In such case the interface disappears and the two
15、 phases merge spontaneously to form a single one. When a surfactant is added in a system of two immiscible phases, e.g. heptane/water, it will adsorb at the interface and orient itself there with the hydrophilic group toward water and the hydrophobic group toward the heptane. In this case the surfac
16、tant molecules replace the water and heptane molecules at the original interface. The interaction across the interface is now between the hydrophilic groups of the surfactant and the water molecules on one side of the interface and between the hydrophobic groups of the surfactant and the heptane mol
17、ecules on the other side. Since the interactions are much stronger than the original interaction between the highly dissimilar heptane and water molecules, the interfacial tension is now reduced markedly by the presence of the surfactant. Thus we could say that the functions of the hydrophobic group
18、 in such a case are: a) to produce spontaneous adsorption of the surfactant molecule at the interface; b) to increase the interaction across the interface between the surfactant molecules and the molecules in the adjacent phase. And the function of the hydrophilic group is to provide strong interact
19、ion between the surfactant molecules at the interface and the solvent molecules. In general, food surface or interfacial tension reduction is shown only by those tensides that have an appreciable but limited solubility in the system under the conditions of use.2. Efficiency in the surface or interfa
20、cial tension reduction1. A suitable measure for the efficiency of a surfactant should be the ratio, Ci /C2, where Ci is the conc. of surfactant at the interface and C2 is the bulk conc. of the surfactant in equilibrium with the surface conc., both are expressed in moles per liter. Ci is then related
21、 to its surface excess conc. , where is in moles per cm2. Thus, Ci=1000/d+ C2 , where d is the thickness of the interfacial region, in cm. In the case of surfactants, is about 1 to 510-10 moles/cm2, while d is 5010-8 cm or less, C2=0.01M or less. Thus, 2. As has been shown previously, the value of i
22、s close to its maximum when the surface tension has been reduced by 20 dynes/cm, and most surfactant molecules are lying slightly tilted to the interface. It is assumed that the thickness, d, of the interface region is determined by the height of the surfactant molecules normal to the interface, the
23、n d is inversely proportional to the surface area per adsorbed molecule, a2, a larger a2 indicates a smaller angle of the surfactant molecule with respect to the interface. Since That is, /d may be considered to be a constant, and This means that the bulk conc. of the surfactant, C2, needed to produ
24、ce a 20 dynes/cm reduction in surface tension is a measure of the efficiency of surface tension reduction by the surfactant.3. Since Ci/C2=exp(-Gtr/RT), (see also chap.2), where Gtr is the free energy change of transferring a surfactant molecule from the interior of the liquid phase to the interface
25、, at =20 dynes/cm, log Ci/C2=log(K1/C2)=20= -Gtr/2.303RT, i.e., This is similar to that of the acidity of a solution, pH, and it can thus be written as pC20 simply. The pC20 has been used as a measure of the efficiency of adsorption at the L/L and L/G interfaces. And it can thus be also used as a me
26、asure of the efficiency of the reduction of surface tension by surfactants.1. For a straight chain surfactant, CH3(CH2) nW, where W is the hydrophilic group, Gtr, can be broken into the free energy changes associated with the transfer of the groups: Gtr (-CH3), Gtr (-CH2-) and Gtr (-W) from the inte
27、rior of the liquid phase to the interface at =20:Where m is the total number of C-atoms in the surfactant molecule, and K is a constant being equal to Gtr (-CH3)-Gtr (-CH2-). Thus, For a homologous series of straight-chain surfactants with the same -W at the same temperature, and under conditions wh
28、ere Gtr (-W) is independent of the length of the hydrophobic group,Where Ktr is a constant relating mainly to the energy change involving the transfer of -W form the interior of the liquid phase to the interface, Ktr is generally negative.5. In aqueous solution, the efficiency increases with increas
29、es in the hydrophobic character of the surfactant. Eq. (5-11) indicates that the pC20 is a linear function of the number of the C-atoms, m, in a straight-chain hydrophobic group. This is valid for several homologous series of anionic, cationic and nonionic surfactants.6. As to the effects on the eff
30、iciency of the interfacial tension reduction, we summarize briefly as follows:l A phenyl group in the hydrophobic chain has a equivalent length of about 3.5 CH2- groups in a straight chain with one terminal hydrophilic group.l Replacement of a straight chain by branched or unsaturated one containing
31、 the same number of C-atoms decreases the efficiency.l Carbon atoms on the side chain have about 2/3 the equivalent length of that in the straight chain.l When the hydrophilic group is not terminally located the hydrophobic chain acts as if it were branched. Carbon atoms on the shorter portion have
32、about 2/3 the effect of that in the longer portion. l Carbon atoms in a straight long chain with two terminal hydrophilic groups on both ends of it have about 1/2 the -CH2- group in that with only one terminal hydrophilic group.l In such compounds with structures of R-(OC2H4)nOSO3-Na+ where n=13, or
33、 RCONH(C2H4O) 2H, the 1st -OC2H4- group has equivalent length about 2.5 -CH2-groups in a straight chain. The additional -OC2H4- groups have little or no effect on the reduction of surface tension.l The replacement of hydrocarbon chain by a fluorocarbon-based hydrophobic one causes a very large incre
34、ase in the efficiency.l A change in the sign of the charge of a univalent ionic head produces little effect on the efficiency. But the replacement of the counter-ion by one that is more tightly bound increases the efficiency.l The replacement of an ionic head by a nonionic one or the addition of neu
35、tral electrolyte to ionic solution results in a large increases in the value of pC20.7. Some values of Gtr (-CH2-) and Ktr of surfactant solutions with and without presence of electrolyte are listed in tables 5-1 and 5-2. Table 5-1 Values of Gtr(-CH2-) and ktr for ionics in water in the absence of a
36、dditional electrolyte. Surfactant Temp.() Gtr(-CH2-) ktrRSO-4Na+ or K+ 25 -0.70RT -1.12RSO-4Na+ or K+ 60 -0.67RT -1.26RC6H4SO-4Na+ 70 -0.65RT -1.27RC5H5 N+Br- 30 -0.68RT -1.27RSO-4Na+(heptane/water) 50 -0.66RT -0.74 Tables 5-2 Values of Gtr(-CH2-) and ktr for ionics in solution of constant ionic str
37、ength and for nonionic. Surfactant Temp. () Gtr(-CH2-) ktrRN(CH3) 3+ Cl- 25 -0.76RT -0.29I=0.1M(NaCl) R(OC2H4) 6OH 25 -0.99RT -0.08 By comparing the values of Gtr(-CH2-) and ktr listed in tables 5-1 and 5-2, it can be seen that:a) For Gtr(-CH2-): The smaller values in table 5-1 than that in table 5-
38、2 reflect the higher position in the interface of the hydrophobic groups of the compounds in table 5-2; ionics at the 0.1M NaCl/air interface are more closely packed than at the water/air interface in the absence of electrolyte (fig.5-1). a. I=0 b. I=0.1M Fig. 5-1 Effect of electrolyte on the packin
39、g and the position of surfactant molecules at interfacesFor the polyoxtethylenated nonionics, the hydrophobic groups are some what elevated above the water/air interface by the EO groups(fig.5-2).b) For ktr: The smaller values of ktr in table 5-2,compared to those in table 5-1,reflect the smaller wo
40、rk involved in transferring the -w from the bulk phase to the interface. The electrolyte causes a compression of the electrical double layer of the ionics, reflecting a decrease in the repulsion between the ionic heads in the oriented layer at the interface.For nonionics, the electrical work term is
41、 missing, bring the absolute value of ktr down almost to Zero(0.08).Fig. 5-2 An exaggerated sketch of the arrangement of nonionic surfactant molecules at water/air interfacec) TheGtr(-CH2-) values of (0.65-0.70)RT in table 5-1 mean that, if the hydrophobic group of anionic surfactant is increased by
42、 two CH2- units, the pC20 will be increased by 0.56-0.60, and the same 20 dynes/cm reduction of surface tension can be obtained with a C2 only the 25-30% of that of the original surfactant.8. For polyoxyethylenated nonionics containing the same hydorphobic group, the pC20 appears to show a linear re
43、lationship to the number of EO groups, n, in the molecule in the range of n=730: Where Atr and Btr are constants. The value of Atr should be determined mainly by the free energy change of transferring the hydrophobic group from the interior of the liquid phase to the interface at =20 dynes/cm, and i
44、t should be about the value of m(0.99RT/2.3RT). (see eq.5-10 or 5-11 and table 5-2) The value of Btr reflects the free energy change of transferring an OE group from the interior of the bulk phase to the interface. These values of Atr and Btr for polyoxyethylenated nonionics are listed in table 5-3.Table 5-3. Surfactant Temp.() Atr B trC12H25(OC2H4) xOH 23 +5.92 -0.083 55 +5.68 -0.010p-t-C8H17 C6H4 (OC2H4) xOH 25 +5.41 -0.069C9H19 C6H4 (OC2H4) xOH 25