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DETERMINATION OF THE CRITICAL MICELLE CONCENTRATION (cmc) OF SDS

DETERMINATION OF THE CRITICAL MICELLE CONCENTRATION (cmc) OF SDS. Surfactants are amphiphilic molecules that possess both hydrophobic and hydrophilic properties.

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DETERMINATION OF THE CRITICAL MICELLE CONCENTRATION (cmc) OF SDS

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  1. DETERMINATION OF THE CRITICAL MICELLE CONCENTRATION (cmc) OF SDS Surfactants are amphiphilic molecules that possess both hydrophobic and hydrophilic properties. A typical surfactant molecule consists of a long hydrocarbon ‘tail’ that dissolves in hydrocarbon and other non-polar solvents, and a hydrophilic ‘headgroup’ that dissolves in polar solvents (typically water).

  2. When a sufficient amount of Surf. is dissolved in water, several bulk solution properties are significantly changed, particularly the surface tension (which decreases) and the ability of the solution to solubilise hydrocarbons, (which increases). These changes do not occur until a minimum bulk Surf. concentration is reached. This concentration is called the critical micelle concentration (CMC). Several experiments, including light scattering and NMR, show that below the CMC, the surfactant exists mainly as solvated monomeric species, whereas above the CMC these monomers undergo self-assembly to form roughly spherical structures

  3. Below the CMC, the addition of surfactant to an aqueous solution causes an increase in the number of charge carriers and consequently, an increase in the conductivity. Above the CMC, further addition of surfactant increases the micelle concentration while the monomer concentration remains approximately constant (at the CMC level). Since a micelle is much larger than a Surf. monomer it diffuses more slowly through solution and so is a less efficient charge carrier. A plot of conductivity against surfactant concentration is, thus expected to show a break at the CMC

  4. Lo scopo di questa esperienza è di determinare la CMC di due tensioattivi ionici:Sodio Dodecil Solfato (SDS) e Dodecilpiridinio Cloruro Monoidrato (DPC). Per ogni tensioattivo vengono date due soluzioni a diversa concentrazione (0.05 M e 0.5 M), mentre quelle di partenza 110-4 M devono essere preparate per pesata. Si predispone il contenitore di misura con l’ancoretta e lo si dispone sull’agitatore magnetico, collegandolo al sistema termostatico e si fa circolare acqua a 25°C. Dopo aver controllato che la conducibilità dell’acqua milli-Q non superi i 2 mS, si dosano 60 ml di soluzione acquosa 110-4 M del tensioattivo nel reattore termostatato.

  5. Si chiude il contenitore e si introducono la cella per la misura di conducibilità e la sonda di temperatura. Quando la soluzione raggiunge la temperatura stabile di 25°C si iniziano le misure. Le aggiunte del tensioattivo vengono fatte mediante pipette automatiche da 20-200 ml e 200-1000 ml. Si lascia sotto agitazione e quando il valore è stabile si legge la conducibilità. Si aggiunge immediatamente una seconda quantità di tensioattivo e si procede come prima. In genere bastano 25-30 misure distribuite omogeneamente nel range 110-4 M < CTENS < 710-2 M.

  6. Nel preparare i calcoli, tenere conto dell’effetto di diluizione che si instaura ad ogni aggiunta di soluzione di tensioattivo. Durante le determinazioni a basse concentrazioni il valore può variare decisamente. Non ci si deve preoccupare, in quanto le variazioni hanno piccolo effetto sulla bontà dei risultati della retta che si determina.

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