临界胶束中浓度检测方案(表面张力仪)

智能文字提取功能测试中

-dataphysics-dataphysics- DataPhysics Instruments GmbH RaiffeisenstraBe 34 D-70794Filderstadt phone ++49 (0)711770556-0 fax++49 (0)711-770556-99 e-mail info@dataphysics.de internet http：//www.dataphysics.de Application Note 11 Determination of Critical Micelle Concentration with DataPhysics DCAT Series Determinationn of criticalmicellee concentration(CMC) of Triton X-100 in aqueous solutions anddetermination of the ethoxylation grade of theTriton X-100 mixture. Problem Determination of the critical micelle concentration(CMC) of Triton X-100 in aqueous solutions with theDataPhysics DCAT combined with the liquid dispenseunit (LDU). The literature values of the CMC of Triton X-100differ in the range of 0.24 to 0.433 mmol/l. Thereason for this variance is that the exact molecularstructure of available Triton X100 differs， accordingto side chain configuration (due to the extent ofethoxylation).As an averagethe valueofethoxylationis dedscribedas9.55.. ItIthhasbeensuggested that the relationship between CMC andthe extent of ethoxylation could be investigated.With the work of Li et al. it was demonstrated that itis possible to determine the extent of ethoxylationwithin a Triton X-100 mixture， by a study of CMC. M. Li， et al. (2000)： Small Variations in the Composition and properties of Triton X-100； J. Coll. & Int. Sci.(230)； 135-139 S. Ledakowicz， et al. (1997)： Critical micelle concentration of nonionic detergents； Tenside Suf.Det. 34； 190-194 Method Surfactants， such as Triton X-100， are compoundswith hydrophilic and hydrophobic constituents. Inaqueous solutions the hydrophilic head of thesurfactant interacts with the water molecules. When surfactants are brought into an aqueoussolution， a higher concentration of surfactants isfound at the surface than in the bulk. This decreasesthe surface tension of the solution， as a function ofthe total surfactant concentration. Once the surfaceof the solution is saturated with surfactantmolecules (see fig. 1)， no more molecules canjoinn the surfacelayer， irrespective ofthe concentration of surfactant reached. This meansthat no further decrease in surface tensionisdetected. Beyond this point agglomerates， calledmicelles， are formed within the bulk solution. Thispoint is termed “the Critical Micelle Concentration”. Fig. 1： Dependence of the surface tension on thesurfactantconcentration：；ttheeccriticalalmicelleconcentration and the formation of micelles areindicated In order to determine the CMC of a surfactant withinan aqueous solution the Wilhelmy plate or Du Nouyring method is used with the DCAT， in combinationwith the LDU. As the concentration of surfactant isincreased， by software control of the LDU， it isdispersed via a“flea” driven by an electromagneticstirrer. The temperature with the sample vessel isalso controlled. In practice and in contrast to the ideal case describedin fig. 1.， during real measurements the curveproduced (surface tension VS. surfactantconcentration) rarely consists of two straight lineswhich meet in a clearly defined point. During thesereal case studies it is likely that our surfactantcontains contaminants or is， in fact， a mixture ofsurfactants. A typical curve of the dependence ofsurface tension on the concentration of a surfactant.with traces of contaminants， is displayed in fig.2. Fig.2：Dependence of the surface tension concentration in the presence of contaminants In most cases the contaminant is expected to be moresurface active that the pure surfactant. This drives thesurface tension (temporarily) below the value， observedin the curve typical for the pure surfactant， in the vicinityof the CMC. This is due to the contaminant occupyingthe surface in preference to the pure surfactant， atlower concentrations.As the experiment continues andmore surfactant (mixture) is added the contaminant isabsorbed in the forming micelles allowing the surfacetension to stabilize at the value expected of the puresurfactant. When observed； this process， compensatingfor the dip below the value expected， results in anincrease in surface tension， as the contaminant movesaway from the surface to be replaced with puresurfactant molecules. Procedure To determine the CMC of Triton X-100， with theDataPhysics DCAT andLDU， the surfactant wasdispersed in water (1：100 v/v). All tubes and thesyringe of the LDU were flushed， with water， toensure that there was no contamination during thedispensing process. The LDU system has to calibrated， be free of airbubbles and then charged with the Triton X-100solution. A test/sample vessel was filled with 100 ml of purewater anddaa. magnetic“flea” added. Betweenmeasurements (surface tension determination withthe Wilhelmy plate) a known volume of Triton X-100solution was added and the mixture stirred for 40 s. All elements of the method： dispense steps， stirringtime， rest time and surface tension evaluation arecontrolled through the SCAT 33 software. In order to determine the concentration regionwhere the CMC will be found， the first project is carried out over a wide concentration range (1.0E-5 to2.0E-2mol/l). A second， more precise，evaluation is then carried out(1.0E-4 to 1.0E-3mol/l) once an estimate of the CMChas been determined. In both procedures， a 20 step，logarithmic function，surfactant solution addition was carried out (see. fig Fig. 3： CMC determination of Triton X-100. The 20 measurement steps were set as a logarithmic function ofthe concentration. Results The result of the CMC measurements of Triton X-100was 0.4 mmol/l. The surface tension of the saturated solution was30.6 mN/m This data tells us that the effective ethoxylationvalue of the Triton X-100 used here is near 10.0. Summary The DCAT， in combination with the LDU， provides aconvenient and reproducible way to determine thecritical micelle concentration ofTriton X-100. Secondly， the measurements provide an easy andcheap way to determine the effective molecularstructure of the Triton X-100 mixture， withoutexpensive NMR-measurements.