Practical 1: The Use of Wetting Agent in Suspension

Objective

To study the effect of different amount of tragacanth on the sedimentation rates of suspensions.

Introduction

Suspension is also a liquid system.Suspension refers to a two-phase system consisting of a finely divided solid dispersed (suspended) in a liquid (the dispersing medium). It consists of at least two phases and forms heterogeneous systems. The continuous or external phase is generally a liquid or a semi-solid, and the dispersed or internal phase is made up of particulate matter, known as continuous phase. The continuous phase is the solid material (usually the drug) which does not dissolve in the vehicle to any appreciable extent, but remains as solid particles which are distributed throughout the vehicle. Suspensions are intended for oral administration as sweetened, flavored formulations or for topical application where they are referred to as "lotions." Suspensions are also used as non-sweetened, non-flavored formulations for many parenteral routes of administration (e.g., intraocular, intranasal, IV, IM, ID, SC).

Suspensions possess certain advantages over other dosage forms. Moreover, drugs in suspension are chemically more stable than in solution. However, suspensions also possess some disadvantages relative to other dosage forms. The primary disadvantage is their physical instability; i.e., that they tend to settle over time leading to a lack of uniformity of dose. This can, however, be minimized by careful formulation and by shaking the suspension before each dose is delivered. Therefore, the best strategy is not to try to eliminate separation, but rather to decrease the rate of settling and to permit easy re-suspension of any settled particulate matter. One of the strategies is to use wetting agent like tragacanth to reduce surface tension. 

Materials

1mL graduated pipette, pipette bulb, weighing boat, mortar and pestle, 50mL graduated cylinder, 200mL graduated cylinder, 100mL beaker, Parafilm®, weighing boat, viscometer.

Apparatus

Chalk, tragacanth, concentrated peppermint water, double strength chloroform water, distilled water, syrup BP.

Methodology

1. A suspension of Pediatric Chalk Mixture (150 mL) is prepared according to the following formula:

Ingredient		Suspension
	A	B	C	D
Chalk (g)	3	3	3	-
Tragacanth (g)	0.0	0.1	0.3	0.5
Concentrated peppermint water (mL)	0.6	0.6	0.6	0.6
Syrup BP (mL)	15	15	15	15
Double Strength Chloroform water (mL)	75	75	75	75
Distilled water q.s. (mL)	150	150	150	150

2. 5 mL of the suspension is poured into a weighing boat and each formulation is labelled.  The texture, clarity and colour of each suspension is observed and compared.
3. The sedimentation rate of each suspension is determined. The suspension is shaken vigorously to make sure all of the particles are uniformly suspended, and the time is noted. The boundary between the sediment and the supernatant is observed and the time it takes for the boundary to pass each 10 mL graduation until the volume of sediment has reached 80 mL is recorded.
4. The gradated cylinder is set down on the lab bench, and the lab timer is started at this point.
5. For suspensions A to D, the sedimentation volume of the suspensions at t=0, 2, 5, 10, 15, and 30 min is recorded.
6. The obtained data is recorded in the following table 1:

7. The sedimentation volume ratio  is calculated using the following formula:

Sedimentation volume ratio = Hu/Ho

Hu: ultimate height of the sediment, i.e., the height of the sediment at a particular time

Ho: initial height of the total suspension

8. The ease of re-dispersibility of each formulation is determined after the last measurement. To do this:

                                i.            The Parafilm® is made sure is snugged on the mouth of the graduated cylinder, and the seal is re-enforced with gloved hand.

                              ii.            The number of inversions it takes to completely re-disperse the drug is counted.

                            iii.            Observations are recorded.

9. 95 mL of suspension is poured into a 100 mL beaker and the viscosity of the suspension is determined using viscometer at 100 rpm for 30 seconds.
10. The data is recorded in the following table 3:
11. Each suspension is poured into a plastic bottle. After storing all the suspensions for a period of 4 days, the ease of re-dispersion in each system and which system is most acceptable are determined.

Results

 Table 1: Sedimentation volume of suspensions at pre-determined time

Suspension	Sedimentation volume at pre-determined time                             (min)						Distinct boundary            (yes/no)
	0	2	5	10	15	30
A	150	100	60	30	20	17	yes
B	150	130	80	32	28	24	no
C	150	146	90	50	42	38	No
D	-	-	-	-	-	-	no

Table 2:  Time taken for the boundary between sediment and supernatant  to pass each 10ml graduation

Sedimentation volume   (mL)	Time taken for boundary to pass each 10 mL graduation until the volume of sediment reached 80 mL for each suspension (mins)
	A	B	C	D
150	0:00	0:00	0:00	-
140	0:30	1:24	2:00	-
130	1:20	1:41	3:15	-
120	1:37	2:24	5:21	-
110	2:22	2:42	6:40	-
100	2:49	3:30	7:20	-
90	2:55	4:00	8:33	-
80	3:02	6:00	9:36	-

Table 3: Number of inversions needed to redisperse the drug

Suspension	Number of Inversion needed
A	4
B	2
C	1
D	-

Table 4: Viscosity of suspension

Suspension	Reading	Viscosity (cP)	Mean	SD
A	1	16.5	14.8	0.77
	2	15.9
	3	12.0
B	1	12.3	16.7	3.96
	2	15.9
	3	21.9
C	1	20.7	17.0	2.75
	2	14.1
	3	16.2
D	1	17.4	17.5	2.33
	2	20.4
	3	14.7

Table 5: Physical appearances of all suspensions produced.

Suspension	Amount of tragacanth (g)	Texture	Clarity and appearance	Colour of suspension
A	0.0	Less viscous so easy to sediment and re-disperse, more watery, rough texture	Moderately opaque, less cloudy, has distinct boundary layer	Less milky
B	0.1	Viscous, easy to sediment and re-disperse	Opaque,  cloudy	Milky white
C	0.3	Viscous, less easier to sediment and re-disperse	Opaque, more cloudy	Milky white
D	0.5	Highly viscous, smooth texture	Translucent, not cloudy	Brownish colour

Discussion

1.     Compare and discuss physical appearances of all the suspensions produced.

The varying the amount of tragacanth gives different physical appearances of suspension in those formulations. Tragacanth is a suspending agent which decrease interfacial tension between solid particle and liquid from continuous phase.  As the amount of tragacanth added to the formulation is increased, it increases the viscosity of suspension and thus slowing down the sedimentation of particles within suspension in accordance with Stokes Law. It also helps prevent caking at the bottom besides to facilitate redistribution of a suspension on shaking and provides ‘structure’ to the medium in order to support the suspensoid. Besides, the formulation will be more opaque and cloudy. This is because the suspension stays disperse in the medium for a longer time compared to the formulation without Tragacanth. This will allow the pharmaceutical product to be withdrawn from the container to be consumed by the consumer.

When amount of the tragacanth is zero, the clarity of the solution is very clear due to the lesser amount of particles dispersed in liquid medium. The product appears to be separated in two layers with clear boundary whereby the dispersed phase seems to sediment at the bottom of the container. However, with suspending agents, it will cause disperse particles to suspense in the medium for longer duration and homogenous white chalky colour can be directly observed. The suspension is said to be opaque and cloudy.

To conclude, the texture of the suspension become more concentrated which directly proportional to the amount of tragacanth that had been added.

2.    Plot Hu/Ho vs. time for each of the suspensions (Table 1) Discuss the findings.

Time (min)	Sedimentation volume ratio Hu/Ho
	A	B	C	D
0	0	0	0	-
2	0.66	0.86	0.97	-
5	0.40	0.53	0.60	-
10	0.20	0.21	0.33	-
15	0.13	0.19	0.28	-
30	0.11	0.16	0.25	-

From this graph, we can conclude that as the amount of tragacanth increases, the lower the sediment formed and time for sedimentation to occur is longer. In this experiment, as the time increases, the sedimentation start to occur due to the inter-particular attractive forces are stronger than the repulsive forces of the tragacanth powder. Suspension which contain more tragacanth is more stable as tragacanth act as the suspending agent.

For suspension A, it contain less tragacanth than B and C. Thus, it takes short time to form sediment. The amount of tragacanth in C is more than in B which is 0.5g of tragacanth. Thus, it takes longer time to form sediment than B. For suspension D, it does not contain chalk eventhough it contains 0.5g of tragacanth. So, there is no sedimentation occur because there is no reaction between the chalk and the tragacanth.

3.     Briefly explain the principle of analysis using viscometer. Plot the viscosity vs tragacanth content (Table 4). Discuss the findings.

A viscometer is an instrument used to measure the viscosity of a fluid. Viscometers only measure under one flow condition. There are different types of viscometer which are rotational viscometer, vibrational viscometer, oscillating viscometer, falling piston viscometer and falling sphere viscometer. The one that is being used in this experiment is rotational viscometer.

Rotational viscometer gathers data on a material’s viscosity behavior under different conditions. The rotational viscometer basically consists of two parts which are a head unit with a motor and a spindle that is driven by the motor. The viscosity is determined by measuring the resistance of a spindle rotating in the sample. Viscosity can be thought of similar to a coefficient of friction. The viscosity of the fluid produces drag, which means more force is required to produce motion. In an ideal situation with one plate stationary, one plate moving, and a film of fluid between them the force required to cause motion is:

F = (mu) (A) (u / y)

Where F is the force, mu is dynamic viscosity, A is the area of the plates, u is the speed, and y is the distance between plates. If we rearrange the equation,

mu = (F*y)/(A*u)

A rotational viscometer uses this principle, and this equation (modified for rotation) to calculate a fluid's viscosity. A probe consisting of a fixed element and a rotating element (like the plates in the ideal example) are placed in the fluid. The moving probe is turned at a constant speed, equivalent to u in the equation. There is a sensor in the viscometer to read the amount of torque that the fluid is pushing back with. It takes all of these parameters and crunches a version of the equation above to provide viscosity measurement.

Tragacanth is a suspending agent that is used to suspend the chalk powder in this experiment. As we can see, if more amount of tragacanth is used, the viscosity of the suspension will increase. From the graph plotted, when there is no tragacanth in the suspension, the average viscosity of the suspension formulation is 14.8 cP meanwhile when we put 0.5 g of tragacanth in a different formulation, the average viscosity of the formulation is higher which is 17.5 cP. Thus, suspension formulation A with 0.0 g tragacanth used has the lowest viscosity while formulation E with 0.5 g of tragacanth used has the highest viscosity. As a conclusion, the viscosity of the suspension is proportional to the amount of suspending agent added into the system.

4.    After storing the suspensions for a period of 4 days, determine the ease of redispersion of each.

After 4 days of storing, suspension A is the easiest to redisperse because it does not contain tragacanth, then it is followed by suspension B and suspension C. Higher amount of Tragacanth powder in suspension will make the suspension difficult to be redispersed. 

5.    Based on all the observations, which product would be considered to be most acceptable? Explain.

Tragacanth acts as a suspending agent. Hence, the more tragacanth powder will form less sediment and less flocculation. At the same time, the more tragacanth powder will produce suspension which is stable, smooth and cloudy. In this experiment, suspension C is the most stable suspension because it has more tragacanth and more sendiment is formed. Also, the results show that suspension C takes longer time for sendimentation to occur.

6.    Briefly explain the function of each excipients used in the suspension formulation. Explain the influence of tragacanth on the physical characteristics and stability of a suspension.

Chalk	Active ingredient which acts as absorbent and antacid for treatment of diarrhea.
Tragacanth	Suspending agent to suspend and stabilise the chalk in the suspension by decreasing interfacial tension between solid particle and liquid from continuous phase.
Concentrated Peppermint Water	Flavouring agent  to increase patient acceptance
Syrup BP	Sweetening agent to make more palatable Co-solvent and thickening agent to make suspension more viscous and decrease sedimentation rate for easier re-dispersing ability and prevent crystallisation
Double Strength Chloroform Water	Preservative, flavouring agent and solvent
Distilled Water	Diluents in suspension Acts as vehicle of mixture carrying pharmaceutical active ingredients to patient’s body

Tragacanth functions as the suspending and thickening agent, which increases the viscosity of the suspension or any aqueous vehicle thus slowing down the rate of sedimentation of the suspended particles as per Stoke’s law. As in this experiment, chalk is non diffusible particles so by adding the tragacanth it will help to make the chalk remain dispersed for longer time throughout the suspension. Thereby, no distinct layers are present. It also appears more cloudy and smooth.

Too high in viscosity of a suspension will result in difficulty to pour the suspension out of the mouth bottle, therefore we also have to maintain the viscosity of the suspension at optimum range to provide its good flow.

Tragacanth changes the conformation on the surface and transforms flocculated particles into deflocculated particles spontaneously when changing of pH occurs. If pH changes, the stability of suspension also occurs due to presence of steric repulsion force.  It will form a film layer, as well as coat around particle and decrease the inter-particle attraction.

The stability and physical characteristics of suspension depends on the different amount of tragacanth used in formulation. As the amount of tragacanth used increased, the suspension will become more viscous and stable against coagulation.

Conclusion

The amount of Tragacanth used in suspension affects the rate of sendimentation and the height of sendiment formed. The higher the amount of Tragacanth powder, the lower the rate and height of sendimentation. Besides, Tragacanth powder also increases the viscosity of suspension formulation. The suspension with higher amount of Tragacanth is the more stable suspension. However, too much Tragacanth powder will cause the suspension more difficult to be redispersed. Hence, the amount of Tragacanth in suspension formulation should be moderate.

References:

1.  Michael E.Aulton, Aulton’s Pharmaceutics The Design And Manufacture of Medicines, third edition, 2007, Churchill Livingstone Elsevier

2.  Patrick J. Sinko, Phd, Martin’s Physical Pharmacy and Pharmaceutical Sciences, Sixth Edition, 2006, Wolters Kulwer.

3.  John F. Marriott, Pharmaceutical Compounding And Dispensing, 2006, Pharmaceutical Press Sdn Bhd