Colloid Chemistry - Università degli studi di Padova · Silvia Gross –Chimica dei Colloidi –...

Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica

Istituto di Scienze e Tecnologie Molecolari

ISTM-CNR, Università degli Studi di Padova

e-mail: [email protected]

Silvia Gross

La chimica moderna e la sua comunicazione

Dipartimento di Scienze Chimiche

Università degli Studi di Padova

e-mail: [email protected]

http://www.chimica.unipd.it/silvia.gross/

Silvia Gross

Colloid Chemistry


Critical Micelle Concentration (CMC)

Souce of the table:


Factors affecting the CMC

Hydrophobic group

The length of the hydrocarbon chain is a major factor determining the CMC. For a homologous

series of linear single-chain surfactants the CMC decreases logarithmically with carbon number.

The relationship usually fits the Klevens equation:

log10(CMC) = A- BnC

where A and B are constants for a particular homologous series and temperature, and nc is the

number of carbon atoms in the chain, CnH2n+2.

Constant A varies with the nature and number of hydrophilic groups

Constant B approximately equal to log102 (B≈0.29–0.30) for all paraffin chain salts having a single

ionic head group (i.e. reducing the CMC to approximately one-half per each additional --CH2--

group).

Alkyl chain branching and double bonds, aromatic groups or some other polar character in the

hydrophobic part produce noticeable changes in CMC.

In hydrocarbon surfactants, chain branching gives a higher CMC than a comparable straight-chain

surfactant.



Hydrophobic group


Hydrophilic Group

For surfactants with the same hydrocarbon chain, varying the hydrophile nature (i.e. from ionic to

non-ionic) has an important effect on the CMC values.

C12 hydrocarbon, the CMC with an ionic head group lies in the range of 1*10-3 mol/dm3

C12 non-ionic material exhibits a CMC in the range of 1*10-4 mol/dm3.

Counter-ion Effects

In ionic surfactants micelle formation is related to the interactions of solvent with the ionic head

group. Since electrostatic repulsions between ionic groups are greatest for complete ionisation, an

increase in the degree of ion binding will decrease the CMC.

For a given hydrophobic tail and anionic head group, the CMC decreases asLi+ > Na+ > K+ > Cs+ > N(CH3)

+ > Ca2+ ≈ Mg2+ (lytropic series)

For a given hydrophobic tail and cationic head group, the CMC decreases as F- > Cl- > Br- > I-

Monovalent → divalent → trivalent: sharp decrease in CMC

Difference can be (partially) ascribed to degree of hydration of the head group



Hydration enthalpy (DHhyd in kJ/mol)

Ion Ionic Radius (pm)

Group 1A(1)

Group 2A(2)

Group 7A(17)

Li+

Na+

K+

Rb+

Cs+

Mg2+

Ca2+

Sr2+

Ba2+

F-

Cl-

Br-

I-

76

102

138

152

167

-510

-410

-336

-315

-282

72

100

118

133

-1903

-1591

-1424

-431

181 -313

196 -284

220 -247

DHhydr (kJ/mol)


Critical Micelle Concentration (CMC)


Effect of added salt

The presence of an indifferent electrolyte causes a decrease in the CMC of most surfactants.

The greatest effect is found for ionic surfactants. The principal effect of the salt is to partially screen

the electrostatic repulsion between the head groups and so lower the CMC. For ionics, the effect of

adding electrolyte can be empirically quantified. Ci = concentration of electrolyte

Effect of temperature

The influence of temperature on micellisation is usually weak, reflecting subtle changes in bonding,

heat capacity and volume that accompany the transition. This is, however, quite a complex effect. It

was shown, for example, that the CMC of most ionic surfactants passes through a minimum as the

temperature is varied from 0°C to 70° C. The major effects of temperature are the Krafft and

cloud points. For polymeric surfactants strong effects of temperature on CMC are observed and it is

common to define a critical micelle temperature (CMT) for this class of surfactants.


log10(CMC) = -alog10Ci+ b


Temperature-dependence

The solubility of ionic surfactants increases gradually with increasing

temperature, but at a critical temperature there is a rapid increase of

solubility with further increase in temperature (Krafft Temperature) (micelle

formation becomes possible)

Solutions of nonionic surfactants of the ethoxylate type show special

behaviour with increasing temperature, namely ‘‘clouding’’ above a certain

critical temperature.

This is defined as the cloud point (CP), which depends on the surfactant

concentration and its composition.


Cloud point (CP) of surfactants

Temperature at which phase separation of the

surfactant takes place.

In the case of Pluronics, dehydration of the

polyoxyethylene chains is followed by more tight

coiling (=avvolgimento)

Electrolytes lower the CP.

Maximum surface activity close to CP.

Typically referred to 1% wt. solution

Above the cloud point, the system

consists of an almost micelle-free dilute solution at a

concentration equal to its CMC at that temperature,

and a surfactant-rich phase.

ASTM Norm: https://www.youtube.com/watch?v=6pCNB4OjpYg

https://www.youtube.com/watch?v=hIh95xS85y4American Society for Testing and Materials International

https://www.youtube.com/watch?v=6pCNB4OjpYg


Cloud point (CP) of surfactants

CP increases with increasing EO number of chains.

CP decreases with increasing electrolyte concentration (for most

electrolytes).

Origin:

With rising temperature, the PEO chain becomes dehydrated (breaking of

hydrogen bonds) and, at the CP, the dehydrated micelles aggregate, which

is probably the origin of the clouding phenomenon.


Krafft temperature of surfactants

As for most solutes in water, increasing temperature produces an increase

in solubility.

However, for surfactants, which are initially insoluble, there is often a

temperature at which the solubility suddenly increases very

dramatically.

This is known as the Krafft point or Krafft temperature, TK, and is defined

as the intersection of the solubility and the CMC curves, i.e. it is the

temperature at which the solubility of the monomeric surfactant is

equivalent to its CMC at the same temperature.

Below TK surfactant monomers only exist in equilibrium with the hydrated

crystalline phase, and above TK micelles are formed providing much

greater surfactant solubility.


Krafft point (IUPAC)

Krafft point

The temperature (more precisely, narrow temperature range) above

which the solubility of a surfactant rises sharply. At this temperature

the solubility of the surfactant becomes equal to the critical micelle

concentration. It is best determined by locating the abrupt change in

slope of a graph of the logarithm of the solubility against T or 1/T

https://goldbook.iupac.org/html/K/K03415.html


Krafft point (IUPAC)


Thermodynamics of micellization

DG is large and negative

DH is positive, indicating that the process is slightly endothermic

TDS is large and positive → in micellization process there is a net

increase in entropy


standard free energy of micellization increasingly negative as the chain length increases.

→ the c.m.c. decreases with increasing alkyl chain length (remind: log10(CMC) = A- BnC)

DHbecomes less positive and TDS becomes more positive with increasing surfactant chain length.

→the large negative free energy of micellization is made up of a small positive enthalpy (which

decreases slightly with increasing chain surfactant length) and a large positive entropy term TDS, which

becomes more positive as the chain is lengthened: why?



Entropy change during micellization



micelle formation must be predominantly

an entropy driven process

1.DG0 for micellization are negative: spontaneous

process

2.DH0 for micellization are negative/positive

3.DS0 for micellization are positive


𝚫𝑮𝒎𝒊𝒄𝟎 = 𝚫𝑯𝒎𝒊𝒄

𝟎 - T𝚫𝑺𝒎𝒊𝒄𝟎

nS ↔ Sn

𝚫𝑺𝒎𝒊𝒄𝟎 < 0 would be expected:

number of independent kinetic units

decreases upon micellization

n 1



Entropy change during micellization

nS ↔ Snn = degree of aggregation

number on independent kinetic units decreases → DS < 0 (if we consider configurational entropy)

The system water/surfactant has to be considered. Extensive and dynamic hydrogen bonding

→ water must experience an increase in entropy to account for DSmic > 0

loose network of tetrahedra bound at the corners

thermal fluctuactions: various part continuously break and reform in

liquid water: relative “disorder”


Hydrophobic effect

when alkyl groups of free surfactant

molecules are surrounded by water, the H2O

molecules form clathrate cavities (i.e.

stoichiometric crystalline solids in which

water forms cages around solutes).

→ predominant effect of the hydrocarbon

part of the molecule is to increase the

degree of structure in the immediately

surrounding water. It acts as “nucleation

site” for water network formation around

cavity.

During the formation of micelles, the reverse

process occurs: as lyophobic residues

aggregate, the highly structured water

around each chain collapses back to

ordinary (less ordered) bulk water thereby

accounting for the large overall gain in

entropy, DSmic. DHmic small (enhanced H-

bond)


Hydrophobic effect


Shape and size of micelles

Micelles are typically (but not always) spherical, with a radius

approximately equal to the chain length of R (for ionic

surfactants), and an aggregation number (n) of 50–100.

Central core: hydrocarbon (expulsion of hydorophobic tail

from the polar medium is an important driving force:

hydrophobic effect)

Further shapes:

rod-shaped micelles

lamellar micelles



http://pubs.rsc.org/en/content/articlelanding/2008/pp/b717475f/unauth#!divAbstract

The cationic and anionic

regions of the micelle are

depicted by open and filled

circles, respectively.

Covalently bonded donor

(δ+) and acceptor (δ−) part

of the probe are indicated

by blank and shaded

squares, respectively.



Critical Packing Parameter (CPP)

For a surfactant with a tail volume, V, a head area A and a tail length l a

simple dimensionless number, the CPP, can be calculated:

CPP=V/(A*l)

If V ~ A·l then the surfactant is fairly symmetrical (CPP~1) and it's quite

easy for it to pack into cubic or simple Lα phases. If the head is very large

(e.g. a bulky ethoxylate) and the tail volume is fairly small then CPP<1/3

and it's easy to pack the surfactant into familiar o/w micelles. In between

these two cases (1/3<CPP<1/2) hexagonal packing becomes possible. At

the other extreme with a very bulky tail (V is large) and a small head

and/or short tail then CPP>1 and any attempt to pack the surfactant

molecules together is going to have difficulties and the phases are quite

complex - or you have reverse (w/o) micelles.

Souce of definition: https://www.stevenabbott.co.uk/practical-

surfactants/cpp.php


Solubilization (referred to surfactants)

Above the surfactants' CMC, the solubility of (water)

poorly-soluble substances increases dramatically

due to the formation of surfactant micelles →

solubilization

surfactants → solubilizer

solutes→ solubilizates

Result: thermodynamic stable isotropic solution

Surfactants with HLB values 15~18 are the best

solubilizing agents.

The commonly used solubilizers: Tweens


Solubilization

The mechanism for solubilization

Solubilization is the process of incorporation of the solubilizate into or onto

the micelles.


Solubilization

Above a certain limit: phase separation of the solubilizate

Solubilizate may affect the value of CMC

Localization: solubilizate from polar water to unpolar hydrocarbon (gradient

of polarity): relevant for drug delivery


Micellar solubilization of drugs

J Pharm Pharmaceut Sci 8(2):147-163, 200

At surfactant concentrations above the cmc the solubility increases linearly with

the concentration of surfactant, indicating that solubilization is related to micellization


Thermodynamics of solubilization

Partitioning of a substance between micelle and aqueous phase according to a partitioning coefficient P:

Molar solubilization capacity: number of moles of the solute which can be solubilized by one mole of

micellar surfactant.

Stot = total solute solubility

Sw = solute solubility in water

Cs = molar concentration of surfactant in solution

CCMC = CMC

∆𝐺𝑠0 = −𝑅𝑇 ln𝑃

𝜒 =𝑆𝑡𝑜𝑡 − 𝑆𝑊𝐶𝑠 − 𝐶𝐶𝑀𝐶

𝑃 =𝑆𝑡𝑜𝑡 − 𝑆𝑊

𝑆𝑊𝑃 =

𝜒 𝐶𝑠 − 𝐶𝐶𝑀𝐶𝑆𝑊



Possible loci of solubilization of drugs in surfactant micelles, depending on drug hydrophobicity.

There is a gradient of polarity inside the micelle

Black bold lines (—) : drug at different sites in the micelle. The black circles represent the surfactant heads, the black

bold curved lines represent surfactant heads consisting of PEO, and the light black curved lines represent the

surfactant tails.

(1) hydrophilic drugs can be adsorbed on the surface of the micelle

(2) drugs with intermediate solubility should be located in intermediate positions within the micelle such as between

the hydrophilic head groups of PEO micelles

(3) in the palisade layer between the hydrophilic groups and the first few carbon atoms of the hydrophobic group, that

is the outer core

(4) completely insoluble hydrophobic drugs may be located in the inner core of the micelle

Micellar solubilization is an effective method for dissolving hydrophobic

drugs in aqueous environments



J Pharm Pharmaceut Sci 8(2):147-163, 200

The lower is the cmc value of a given surfactant, the more stable are the micelles. This is especially

important from the pharmacological point of view, since upon dilution with a large volume of the blood,

considering intravenous administration, only micelles of surfactants with low cmc value still exist, while

micelles from surfactants with high cmc value may dissociate into monomers and their content may

precipitate in the blood.

RSC Adv., 2014,4, 8165-8176



https://www.sciencedirect.com/science/article/pii/S0378517317308517



RSC Adv., 2014,4, 17028-17038

Colloid Chemistry - Università degli studi di Padova · Silvia Gross –Chimica dei Colloidi –...

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