Li83

in which e denotes charge of electron, n number of ions in 1 mL, y valence of ions, e dielectric constant of the medium, k Boltzman constant, and T temperature in K.

The dependence of electrophoretic mobility of poly(e-caprolactone) and po-ly(l,l-lactide) microspheres on pH of suspending medium is shown in Fig. 22.

FIG. 22 Dependence of electrophretic mobilities of poly(e-caprolactone) and poly(l,l-lactide) microspheres on pH. (From Ref. 65.)

FIG. 22 Dependence of electrophretic mobilities of poly(e-caprolactone) and poly(l,l-lactide) microspheres on pH. (From Ref. 65.)

Stabilization of microspheres with Triton X-405 does not introduce any additional charge (molecules of this surfactant do not contain any ionic or ionophoric groups). Thus, electrophoretic mobility of microspheres in this system should depend solely on electrostatic charge due to the presence of carboxylic groups resulting from polyester chain scission during hydrolysis. Indeed, according to plots in Fig. 22, the isoelectric point for poly(e-caprolactone) and poly(l,l-lac-tide) microspheres is at pH 2.9 and 4.4, respectively. It is also known that typical carboxylic acids have pKa ranging from 4 to 6 [67] and therefore in a range of pH from 2 to 4 these acids are ionized in less than 1%.

The electrophoretic mobility of microspheres stabilized with ionic surfactants (SDS and ASB) was negative in the pH range used in the studies (from pH 3 to 11). This was due to -SO- and —OSO- groups (more acidic than carboxyl groups) introduced into surface layer by surfactants.

It is worth noting that for pH > 7 for all types of poly(e-caprolactone) and poly(l,l-lactide) microsphere suspensions the electrostatic mobility reached plateau. This means that under these conditions all carboxyl, sulfate, and sulfite groups in particle surface layers were fully ionized. Values of electrostatic charge densities calculated from these maximal (with respect to their absolute

TABLE 6 Surface Charge Density for Poly(e-caprolactone) and Poly(l,l-lactide) Microspheres (Stabilized with Triton X-405, SDS, and ASB Surfactants) Calculated from Microsphere Electrophoretic Mobilities at Their Maximal Absolute Values

Stabilizing

108

C •

107

Microspheres

surfactant

s )

C (|lC

cm )

(mol

m-2)

Poly(e-caprolactone)

Triton X-405

-0.72 :

0.04

-2.5

0.1

-2.6

± 0.1

SDS

-1.70:

0.07

-5.9

0.2

-6.1

± 0.2

ASB

-2.5 :

0.1

-8.7

0.4

-8.9

± 0.4

Poly(l,l-lactide)

Triton X-405

-1.09 ±

-3.8:

0.2

-3.9

± 0.2

SDS

-1.77

0.09

-6.2:

0.3

-6.4

± 0.3

ASB

-2.05

t 0.08

-7.2

0.3

-7.4

± 0.3

values) electrophoretic mobilities are given in Table 6. The stability of microsphere suspensions stabilized not only due to steric but also due to ionic interactions could be affected not only by pH but by ionic strength as well. Destabiliza-tion of microsphere suspensions was monitored visually by observation of a drop of suspension on a black plate used for the diagnostic latex tests. Drops of stable suspension look homogeneous whereas loss of stability is manifested by formation of particle aggregates. It has been found [65] that poly(e-caprolac-tone) microspheres formed stable suspensions in buffers within pH range from 3 to 11 and for ionic strength from 10-3 to 1 mol/L, regardless of the nature of the surfactant (Triton X-405, SDS, ASB). In the case of the larger poly(l,l-lactide) microspheres the stability of suspensions was dependent on pH and on the ionic strength of the medium as well on the nature of surfactant. ASB stabilized suspension of poly(l,l-lactide) microspheres effectively in the full pH range (from 3 to 11) and ionic strength (from 10-3 to 1 mol/L). Stabilization of poly(l,l-lactide) microspheres with Triton X-405 and SDS was dependent on pH and ionic strength in the same manner (cf. Fig. 23; destabilization occurred for higher values of ionic strength and lower values of pH).

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