chimia coloizilor si interfetelor
DESCRIPTION
Nanochimie, Surfactanti, determinarea tensiunii facialeTRANSCRIPT
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Cat de mare este “nano”?
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Nanochimia –chiar atat de noua?
• nanosisteme preparate in antichitate (negru de fum pentru cerneala – China)
• cercetari de nanotehnologie stiintifice – prepararea solului de Au – Rayleigh, 1876
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• Nanotehnologia implica studierea, manipularea, crearea si folosirea materialelor, dipozitivelor si sistemelor tipice cu dimensiuni mai mici de 100 nm
• Ostwald (1903) “Coloid = sistemul care contine entitati avand in cel putin o directie dimensiuni cuprinse intre 1 nm si 1 m”
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Sisteme nanostructurate
• 3D – nanoparticule cu diverse forme aprox. izotrope (nanosfere)
• 2D nanoparticule anizotrope nanotuburi, nanofire, nanocilindri)
• 1D – monostraturi, multistraturi, filme subtiri
• 0 D – aparent coerente (materiale poroase)
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Raportul nr. atomi de pe suprafata/nr.total atomi in nanoparticule (Ichinose si colab,
1992
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‘Nanotechnology and the Developing World, F. Salamanca-Buentello
et al, April 205,www.plosmedicine.org
• 10 nanotehnologii de top in rezolvarea problemelor considerante grave si stringente la nivel mondial de catre ONU
• Energy storage, production and conversion• Agricultural productivity enhancement• Water treatment and remediation• Disease diagnosis and screening• Drug delivery systems• Food processing and storage• Air pollution and remediation• Construction• Health monitoring• Vector and pest detection and control
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SURFACTANTI IN SOLUTIE Surfactant = substanta superficial activa
= substanta amfifila
= substanta tensioactiva
(surfactant, tenside, tensioactif)
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TIPURI DE SURFACTANTI
• Ionici – anionici
- cationici
- zwitterionici
• Neionici
(exemple)
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Proprietati coloidale specifice
1. AUTOASOCIEREA
2. ADSORBTIA LA INTERFETE
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n
Schema procesului de micelizare
!!!! MICELIZAREA ESTE SPECIFICA SURFACTANTILOR CU PARTE HIDROCARBONATA MAI MARE DE 12 ATOMI DE CARBON
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CONCENTRATIA CRITICA MICELARA CCM
CCM = concentratia minima de surfactant la care incep sa se formeze micele de asociatie
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Metode experimentale pentru determinarea CCM
• -masurarea proprietatilor functie de concentratie:
• - proprietati optice (metoda turbiditatii, metoda interferometrica)
• - proprietati electrice (metoda conductivitatii electrice – v. lab.)
• proprietati cinetico-moleculare (metoda osmotica)• proprietati superficiale (metoda tensiunii
superficiale)
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Tipuri de micele
micela directa micela inversa
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Forta motrice a fenomenului de autoasociere a surfactantilor in
mediu apos =EFECTUL HIDROFOB
(teoria legaturilor hidrofobe -Tanford)
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Marimi caracteristice ale micelelor
• numar de agregare
• sarcina micelelor
• masa micelara
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Agregate superioare
-Micele cilindrice
- Faze cristal lichid hexagonale
- Faze cristal lichid lamelare
- Faza cristal lichid cubice (cubozomi)
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Termodinamica micelizarii
• Micelizarea = proces spontan care are loc cu scaderea energiei libere a sistemului
= proces critic care incepe de la o anumita concentratie de surfactant
Gm0 = RT ln CCM
Gm0 = RT (2-p/n) ln CCM
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Solubilitatea surfactantilor
• Variatia solubilitatii surfactantilor cu temperatura functie de natura chimica a grupei polare
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SURFACTANTI IONICI
0
1 0
2 0
3 0
4 0
5 0
6 0
0 2 4 6 8 1 0 1 2
Temperatura
So
lub
ilita
tea
PK
PK
Punct Krafft sau temperatura Krafft
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SURFACTANTI NEIONICI
PC
Punct de ceata
(cloud point)
0
1 0
2 0
3 0
4 0
5 0
6 0
0 1 2 3 4 5 6 7 8 9 1 0
Temperatura
So
lub
ilita
tea
PC
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Solubilizarea in micele de surfactanti
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Aplicatii ale sistemelor micelare de surfactanti
- Industria cosmetica- Industria farmaceutica- Industria detergentilor*** Detergenta si procesele de curatare se
bazeaza pe fenomenul de solubilizare in agregate de surfactanti de tip micele sau agregate superioare
- Sinteze chimice in mediu neomogen- Procedee de curatare in restaurarea operelor
de arta
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Interfete
• Interfata = suprafata de separare intre fazele care compun un sistem coloidal
• Exemple de interfete :• Interfata lichid-gaz L/G• Interfata lichid-gaz L/L• Interfata lichid-gaz S/G• Interfata lichid-gaz S/L
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Tensiunea interfaciala
• TENSIUNEA INTERFACIALĂ (SUPERFICIALĂ)
• = lucrul mecanic necesar pentru a mări izoterm şi reversibil interfaţa de separaţie dintre faze cu o unitate.
• [] = erg/cm2 = mJ/m2
• = forţa care acţionează pe unitatea de lungime a perimetrului de separaţie a fazelor, orientată în sensul micşorării interfeţei.
• [] = dyne/cm = mN/m
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• Tensiunea interfacială este rezultanta forţelor intermoleculare de coeziune necompensate ce se manifestă prin interfaţa dintre două faze ca urmare a manifestării forţelor fizice de tip Van der Waals.
L
G
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•
lF
L
AFLW
l
F
lL
FL
A
W
22
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• Interacţiile prin forţe fizice care contribuie la valoarea tensiunii interfaciale
• forţe de dispersie( London) • forţe de tip dipol-dipol (Keesom) • forţe de inducţie (Debye) • legături de hidrogen (interacţii de tip acid-bază
Lewis sau electron acceptor/electron donor)
• Componentele tensiunii interfaciale d (de dispersie) p (polară)
pd
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Variatia tensiunii superficiale cu temperatura
• Ec van der Waals Guggenheim
• Ec Cinni pt apa
1)1(k0k
cT
T
23)10*2885,0(1396,0668,75 tt
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• Ec Fowkes pentru tensiunea interfaciala L/L
• Regula Antonov
2
1
212112 2 dd
)()(/ AUUAAU
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• Din cauza necompensării interacţiunilor prin forţe fizice, moleculele superficiale se comportă ca o peliculă elastică care tinde să micşoreze interfaţa dintre două medii.
• Tensiunea interfacială se manifestă prin existenţa unei presiuni suplimentare la o interfaţă curbă faţă de o interfaţă plană, această diferenţă de presiune se numeşte presiune capilară (pc) şi este dată de ecuaţia Laplace:
R
2pp c
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• Metoda Wilhelmy (metoda smulgerii plăcuţei)• Metoda du Nöuy (metoda smulgerii inelului)• Metoda presiunii maxime a bulelor de gaz• Metoda volumului picăturii• Metoda ascensiunii capilare • Metoda picăturii rotitoare• Metoda meniscului rotitor• Metoda picăturii suspendate• Metoda determinării unghiului de contact
Metode de determinare a tensiunii interfaciale