Chemical Properties of Direct Dental Materials W2 Flashcards
Provide the definition for an Acid
H+ concentration of greater than 1X10^-7M… LOW pH
Increases the concentration of H+ in a solution
Donates H+ through dissociation (HA -> A)
LOOSES the H+
Provide a definition of a Base
H+ concentration less than 1x10^-7….High pH
Raises pH by providing OH- (accepting H+)
GAINS the H+
pH equation
-log10 [H+]
What is the log scale in/increase by
1pH unit = 10 fold change in H+
What constitues a strong acid how does it differ to a weak acid?
Strong acid = Dissociates completely in aqueous solution (100% ionised in solution)
vs
Weak acid = majority of molecules remain undissociated ie. doesnt dissociate coompletely
Definition of a buffer
Solutions that resist changes in pH when small quantities of acid or base are added
How does a buffer work in acidic conditions?
Lots of H+ = the buffer takes on H+ to help the raise the pH of the solution
How does a buffer work in basic conditions?
Little/no H+ = the buffer donates H+ to help the lower the pH of the solution
Define Amphoteric
Substance acts as either an acid or a base
HA (acid)
H20 (base)
What are the conjugates
A- = conjugate base (has donated H+)
H30+ = conjugate acid (has accepted H+)
What is an example of an amphoteric molecule
An amino acid
In the presence of a base the amino group donates a H+ (acts as a acid)
In the presence of an acid the carboxyl group takes up a H+ (acts as a base)
What is the term for equilibrium of charge
zwitterion
Is water amphoteric, and if so what is donated/taken up in the presence of a base vs an acid
Yes, as it can act as an acid or a base
In the presence of an acid, H20 acts as a base ie. takes up H+ = H30+ (conjugate acid)
In the presence of a base H20 acts as an acid ie. donates H+ = OH- (conjugate base)
What is H30+ (hydronium ion) often shortened to?
H+
At equilibrium (25 degrees) water has equal concentrations of…
H30+ (H+) and OH-
What is Kw
Equalibrium constant for water (when at equilibrium!)
(dissociation constant - percentage of dissociation)
= [H30+]x[OH-]
= 10^-14 or pKw (14)
How do you calculate Kw when given the pH of solution and temperature
Reverse log!
First find [H+]
[H+] = 10^-pH
[H+] = 10^-7
The find [OH-]
[OH-] = [H+] = 10^-7 M at 25 degrees
Therefore Kw = (10^-7) x (10^-7) = 10^-14
What level of H+ and OH- constitute a neutral solution, acidic solution and basic solution?
Neutral solution: H+ = OH-
Acidic solution: H+ > OH-
Basic solution: H+ < OH-
pH + pOH = ?
14
Provide the definition for a conjugate base
Molecule that is left behind after the acid looses the H+
ie. A- (OH-)
Provide the definition for a conjugate acid
The molecule formed after the base gains a H+
ex. H30+ (H+)
Compare the terms ‘strength’ and ‘concentration’
Strength: proportion has reacted with water to produce ions (percentage of dissociation)
Concentration: how much of acid is dissolved in solution (dissolving in water)
Acid dissociation constant (Ka) is…
Equilibirum constant for acid dissociation (quantifies strength of acid) ie. how much dissociated
Products [H+] [A-]
_______ _______
Reactants [HA]
What happens to pH as the concentration of the acid/and or Ka, increases
pH decreaes because you are getting more dissociation of the acid
high Ka = low pH
Ka to pKa equation…
pKa = -log Ka
High Ka = Low pKa
The lower the pKa the more acidic the solution
What does Acid etching achieve?
Roughens tooth surface by local demineralisation of enamel and dentine = tags/pores so that resin can bond better
Usually 35% phosphoric acid for 15-30 seconds
Equation for percentage dissociation of a weak acid
% dissociation = A-
___ x 100
HA
What is Kb and the equation to figure it out
Equilibrium constant for basic dissociation reaction
- Extent of ionisation (percentage dissociation) of a weak base
= [BH+][OH-] Products
__________ ie. _______
[B] Reactants
pKb = -log Kb
Large Kb = strong base
ie. high level of dissociation = high pH (low pKb)
What are the symptoms of Xerostomia
Dry oral mucosa
Dry and fissured tongue
Bad breath
Generalised caries
Generalised plaque accumulation
What is Rheology
Study of deformation and flow of materials against variables: temp, pressure, time and applied stress
What is Viscosity
Resistance to flow or strain
What is setting time
the time taken for a material to reach its final set state
Increased viscosity = what type of flow?
High resistance to flow
What is wetting?
Wetting is used by bonding mechanisms that have adhesive properties
- Wet adherant surface ex. Tooth
What does the contact angle refer to?
The ability of a material to wet adherant surface (measure of wetting)
- Low contact angle (less than 90 degrees) corresponds= GOOD wetting ie. high wettability or hydrophilicity
- High contact angle (more than 90 degrees) = BAD wetting ie. globule/hydrophobicity
What surface energy is good for adhesion
Tooth surface needs to have a higher surface energy than the adhesive (resin)
Why do we etch the surface of the tooth
Increases the surface energy of the tooth therefore optimising the chance for chemical bonding and mechanical interlocking
What condition is associated with low blood pH
Metabolic acidosis
Symptoms: high blood pressure, kidney disease, diabetes
What condition is associated with high blood pH
Metabolic alkalosis
Associated with adrenal disease or alcohol abuse
What is the normal pH range for Saliva
pH 6.2-7.6
What effects the pH of Saliva
Food and Drink
Bacteria in the oral cavity
Age
At what pH does demineralisation begin?
pH 5.5 = Enamel breakdown, Dentine exposure, discomfort
What is a Buffer
Solutions that maintain a pH when a small amount of acid or base is added.
ie. resistant to changes in pH
What are the the three systems that buffer saliva pH
- Carbonic acid/Bicarbonate
- Phosphate buffer
- Protein buffer
What is the major buffering system for STIMULATED saliva
Bicarbonate (carbonic acid)
How does the bicarbonate buffering system work?
Stimulated salvia (during meal)
= increased saliva
= increased bicarbonate concentration
bicarbonate combines with acid (H+ ions) = carbonic acid
as carbonic acid is a weak acid it dissociates into water and Co2.
All done in the presence of carbonic anhydrase VI (secreted by serous acinar cells) which releases Co2 as a gas.
What system is refered to as ‘phase buffering’
bicarbonate buffering system
What happens with the bicarbonate buffering system in basic conditions?
Strong base (OH-) react with the carbonic acid
= bicarbonate and H+ ions
= reducing the potential for an increase in pH
What is the major buffering system for UNSTIMULATED saliva
Phosphate buffer
Why is the phosphate buffering system limited?
pKa ranges from 6.8-7.2 (similar to the pH of the saliva)
Low phosphate avaiability/concentration in the oral cavity during eating
= effectiveness limited
At rest (not eating) concentration of phosphate is higher
How does the phosphate buffering system work?
When acid is present it combines with monohydrogen phosphate (weak base) = dihydrogen phosphate (conjugate base)
If conditions become basic dihydrogen phosphate combines with OH- to form monohydrogen phosphate and water
How does the protein buffering system work?
Nearly all proteins can function as buffers because they are made of amino acids (amphoteric), they can bind H+ or donate H+
Example: Salivary proteome proteins like Amylase, mucin, IgA, albumin
At what pH does the protein buffer work?
buffering below pH 5
Indications of a unblanced saliva pH
Bad breath, temperature sensitivity, tooth cavities
What are dental cements used for?
Retention of restorations (crows ect.) or ortho bands
Pulp protection
Root canal sealers
Describe/List the components of a Zinc Phosphate cement
Hint: 2 major components
- Powder component (base)
ZnO - Liquid component (acid)
H3PO4
What happens when the powder and liquid components of a Zinc Phosphate cement are mixed
Acid attacks and dissolves the outer layer (ZnO - base)
= release Zn2+ ions into liquid
= formation of Zn3PO4 + H2O (hydrated zinc phosphate)
What happens to the pH of Zinc Phosphate cement
pH goes from 1.6 at 2 minutes
to
6-7 after 34 hours
What is the action of local anaesthetic?
interupts neural conduction by blocking Na2+ gated channels ie. inhibiting influx of Na2+ = no AP
What happens from the time a local anaesthetic is injected to its onset and how does this depend on pH
Local anestetic stable in solution (time of injection): quaternary, water soluble
- Cant penetrate neural membrane
When exposed to pH 7.4: tertiary, lipid soluble
- Diffuses through neural membrane = onset
What is the pKa for local anaesthetics?
Ionisation constant:
Predicts the proportion of molecules that are lipid soluble vs water soluble form (50% of each form)
Local anesthetic isnt working on a patient with an infection… why?
Infection = acidic environment
= favours quaternary water soluble form
= fewer molecules penetrating the nerve
= no onset
Why are intermolecular forces critical for dental materials
Intermolecular forces affect flow, viscosity and surface wetting which are critical parameteres to material performance
What does Hydrogen bonding involve?
Strong dipole-dipole force involving F,O,N lone pairs interacting with a H also bonded to a F,O,N.
ie. H atom bonded covalently to a F,O,N (DONOR ATOM) - makes delta positive and therefore connected to…
a F,O,N (ACCEPTOR ATOM)
In hydrogen bonding what is the Donor atom?
Electron withdrawing group ex. H-F, H-O or H-N
Donor = H going away from that atom
In hydrogen bonding what is the Acceptor atom?
Avaiable lone pair on the acceptor atom ex. F,O,N - interacts with the slightly positive H atom
Acceptor = lone pairs!
Draw a Hydrogen bond example
H20 (acceptor) bonded to H of another H20 (donor)
Rank the relative strengths of the intermolecular forces
Weakest
- London Dispersion forces
- Dipole Dipole
- Hydrogen Bonding
Strongest
Surface energy (tension) definition
Energy required to overcome intermolecular forces to create a new surface in a liquid
Associated with a liquid or solid surface
- Arises because molecules at the surface are not attracted equally in all directions to molecules around them, at the surface they are in a higher energy state (not as stable) as tthose below them
Good Surface Wetting involves what energy states
Low surface energy liquid will wet a high energy surface solid
= achieves optimal bond strength
Rheology definition
Study of the deformation and flow characteristics of matter against variables (temperature, pressure time and applied stresses)
Viscosity definiton
Measure of the fluids resistance to flow
ie. Internal friction of molecules (how easy molecules move past each other)
What effects Viscosity
- molecule size
- predominent intermolecular forces
Will a large hydrocarbon have a greater or lower viscosity and surface area
- high viscosity/surface energy (bigger the molecule the harder to slip over eachother and greater dispersion forces)
ie. Increase size, increasing dispersion forces = increased viscosity AND surface energy
Why does falling water take the form of a droplet
liquids (such as water) can change their shape to minimise their surface energy
What effects surface energy?
- Intermolecular forces
- Functional groups
- Crystal planes presenting at the surface
What does cleaning a surface do in relation to surface energy?
cleaning the contaminated surface raises surface energy of the surface (you want as high of a surface energy as possible)
Does metallic bonding, H bonding dipole or dispersion forces have the highest surface energy
Metallic = highest
H-Bonding
Dipole
Dispersion = lowest
How does viscosity and surface energy relate
As surface energy increases, so does viscosity (usually)
What effects Wetting?
- surface energy of the solid and liquid
- rheology (flow) of the liquid/suspension
What contact angle is relevant for ‘perfect wetting’
0 degrees = perfect wetting
What does contamination do to a surface
lowers solid surface energy (decreasing wettabillitty)
How can you raise surface energy
cleaning the biofilm/pellicle off enamel raises surface energy and therefore wettability
How can you lower surface energy
By using surfactants in liquids (surface active agents) = lower surface energy by disturbing the molecules at the surface
Hydrophobic tails congregate and align at the surface of the liquid and heads buried in the water = lowering surface energy with increasing concentration
What is an example of a surfactant
Sodium lauryl sulphate (SLS)
What happens at very high concentrations of surfactants in liquids
FA’s self assemble into micelles
What is capillary action?
The spontaneous rising of a liquid in a narrow tube as a result of high cohesive (intermolecular - H BONDING) and high adhesive (liquid/wall - Surface energy) forces
How is capillary action relevant to dentistry
Capillary action of saliva occurs in crevices around/between teeth
Denture retention is assisted by adhesvie action and capillary action
How is spectroscopy used in the dental context
1) Digital shade guide (spectrophotometer)
2) Vein visualisation (aka Vein mapping)
3) Intra Oral Scanners
What is the most common technique of shade matching
Visual shade matching
What are the disadvantages of Visual shade matching
- Conditions of light, experience, age, eye fatigue, colour blindness effects the shade choice
How do you determine how acidic or basic a molecule is
1) electronegativity
2) size of atom
3) resonance
How does electronegativity effect acidity
The more electronegative a element is the more it creates a dipole (ie. a more positive H) = more acidic
How does atom size effect acidity
The larger the atom the more the electrons move around and there is greater distance between ion and H+ ie. long bond length = H+ more likely to dissociate = more acidic
smaller molecules its harder to pull off atoms
How does resonance effect acidity
Resoance: different bond orientations/confirmations
= stabilises conjugate base
Stronger acids have stable conjugate bases!
How does the trend of acidity move when you go towards the right of the periodic table (across periods) and down groups
Stronger acid going to right across period (relating to electronegativity)
Stronger acid going down a group (relating to size of atom)
How many steps are involved in salvia testing
5 steps
First 3 on unstimulated saliva
Last 2 on stimulated saliva
