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From: Andrew Usher on 22 Dec 2008 00:40
On Dec 21, 3:35 pm, Mark Thorson <nos...(a)sonic.net> wrote:

> > I know that wax is not amorphous, but how can it be entirely
> > crystalline? That was my question.
>
> It's not a perfect crystal. Most crystals aren't.

By 'perfect crystal' I don't mean free of defects, but having an
infinitely repeating unit cell, as classical crystallography assumes.

Inorganic solids, and pure organic substances form perfect crystals
in my sense.

> But the long hydrocarbon chains will line up with
> each other, forming imperfect crystals.

Right, that's what I conjectured.

> > > Some additives such as stearic acid are often used as hardeners
> > > for paraffin wax. Pure stearic acid solidifies into a large-grain
> > > polycrystalline solid.
>
> > So it wouldn't be so without additives?
>
> No, I confused you by giving you too much information.
> Pure paraffin is crystalline, but with finer grain
> than stearic acid.

Does the stearic acid enter solid solution, or does ir form a
separate crystalline phase? I know that if a monocarboxylic acid
were long enough, it would have to go in solution, and if it were
short enough, to separate, but I don't know where stearic acid
would fall.

Andrew Usher
From: Andrew Usher on 22 Dec 2008 00:47
On Dec 21, 1:27 pm, Edward Green <spamspamsp...(a)netzero.com> wrote:
> On Dec 20, 3:32 am, Andrew Usher <k_over_hb...(a)yahoo.com> wrote:
>
> > It came to me one day when observing paraffin wax freeze, that the wax
> > could not be a perfect crystal since it is not a pure substance.
> > Rather, it consists of countless isomers; yet, it seems to be
> > crystalline from its opacity
>
> Don't follow your reasoning. Many crystals are clear. Do organics
> tend to become opaque on crystalization as opposed to remaining clear
> on glassification?

Polycrystalline materials tend to be opaque or translucent, while
glasses are clear.

> > and apparent sharp melting and freezing.
>
> I can easily wave my hands around that. Mixture of random length
> (longish) carbon chains have a statistically sharp pseudo-
> crytalization of domains of matching chain fragments.

OK. So paraffin wax is just like polyethylene, except with a much
shorter chain length.

> > So we have a group of materials that can not form perfect crystals (or
> > segregate into solids that do), which are all organic, but there can
> > be made inorganic polymers that surely behave the same way. They are,
> > of course, thermodynamically unstable, as all organics are.
>
> Again don't follow your reasoning. What do you mean "all organics
> are" ... "thermodynamically unstable"? Do you mean they all are
> unstable wrt some other atmoic (i.e., chemcially different)
> arrangement of the the carbons and hetero-atoms?

Yes. The only stable C-H phases at 1 atm are H2, CH4, and graphite.

> Or do you mean that,
> as pseudo-crystals, they are thermodynamically unstable against an
> eventual segregation into crystals of similar species?

Entropic effects cause them to mix, so they should theoretically
separate at 0 K, but that could never be observed.

> > So can I set down a general rule, that the thermodynamically most
> > stable state of any aggregation of elements, will form one or more
> > crystalline phases, which form the whole bulk, at sufficiently low
> > temperatures?
>
> I doubt solid wax is more than partially ordered. Whether, assuming
> no chemical changes (fixed spectrum of chains lengths) this is
> unstable wrt segregation, I don't know.

My hypothesis says that any solid that does not form one or more
crystalline phases must be unstable, in the sense that there is some
other molecular arrangement with lower free energy.

Andrew Usher
From: Edward Green on 23 Dec 2008 21:02
On Dec 22, 12:47 am, Andrew Usher <k_over_hb...(a)yahoo.com> wrote:
> On Dec 21, 1:27 pm,EdwardGreen<spamspamsp...(a)netzero.com> wrote:
<...>
> >  What do you mean "all organics
> > are" ... "thermodynamically unstable"?  Do you mean they all are
> > unstable wrt some other atmoic (i.e., chemcially different)
> > arrangement of the the carbons and hetero-atoms?
>
> Yes. The only stable C-H phases at 1 atm are H2, CH4, and graphite.

In the context of this discussion (solidification of paraffin), this
may be only somewhat more relevant than all nuclear species being
unstable wrt transformation to iron, IIRC. Part of thermodynamics is
deciding artfully what to ignore.

> > Or do you mean that,
> > as pseudo-crystals, they are thermodynamically unstable against an
> > eventual segregation into crystals of similar species?
>
> Entropic effects cause them to mix, so they should theoretically
> separate at 0 K, but that could never be observed.

Entropy may always cause _some_ mixing, but it's possible that even at
RT the true thermodynamic equilibrium consists of segregated phases of
various chain lengths, with minor intermixing. After all, we have, in
general, multiple phases in mixtures at room temperature: that's some
kind of compromise between entropy of mixing and ... just to make a
rounded phrase ... enthalpy of mixing.

> > > So can I set down a general rule, that the thermodynamically most
> > > stable state of any aggregation of elements, will form one or more
> > > crystalline phases, which form the whole bulk, at sufficiently low
> > > temperatures?
>
> > I doubt solid wax is more than partially ordered.  Whether, assuming
> > no chemical changes (fixed spectrum of chains lengths) this is
> > unstable wrt segregation, I don't know.
>
> My hypothesis says that any solid that does not form one or more
> crystalline phases must be unstable, in the sense that there is some
> other molecular arrangement with lower free energy.

I wonder. I'd have to say "no opinion" for now. Just for the sake of
argument, why couldn't a partially ordered pseudo-crystal be the true
equilibrium?

I see now your original hypothesis was "at sufficiently low
temperatures"... that makes it weaker (hence more plausible). At the
limit of absolute zero we are guaranteed a lowest energy state as the
thermodynamic equilibrium state: note, just to be cute, this doesn't
immediately exclude mixing or disorder -- so long as the mixed or
disordered state happened to be lower (or at least as low) in energy
than any fully ordered state. Do you know any reason this possibility
is excluded?
From: Andrew Usher on 24 Dec 2008 03:28
On Dec 23, 8:02 pm, Edward Green <spamspamsp...(a)netzero.com> wrote:

> > Yes. The only stable C-H phases at 1 atm are H2, CH4, and graphite.
>
> In the context of this discussion (solidification of paraffin), this
> may be only somewhat more relevant than all nuclear species being
> unstable wrt transformation to iron, IIRC. Part of thermodynamics is
> deciding artfully what to ignore.

I only mention it because it was behind my final conjecture. It's also
the
fundamental difference between 'organic' and 'inorganic' chemistry.

> > > Or do you mean that,
> > > as pseudo-crystals, they are thermodynamically unstable against an
> > > eventual segregation into crystals of similar species?
>
> > Entropic effects cause them to mix, so they should theoretically
> > separate at 0 K, but that could never be observed.
>
> Entropy may always cause _some_ mixing, but it's possible that even at
> RT the true thermodynamic equilibrium consists of segregated phases of
> various chain lengths, with minor intermixing. After all, we have, in
> general, multiple phases in mixtures at room temperature: that's some
> kind of compromise between entropy of mixing and ... just to make a
> rounded phrase ... enthalpy of mixing.

Yes, that's how it works. But the enthalpic effects here are very
small.

> > My hypothesis says that any solid that does not form one or more
> > crystalline phases must be unstable, in the sense that there is some
> > other molecular arrangement with lower free energy.
>
> I wonder. I'd have to say "no opinion" for now. Just for the sake of
> argument, why couldn't a partially ordered pseudo-crystal be the true
> equilibrium?

Actually, I meant stable in the first sense when I said this. That is,
hydrocarbons except methane would count as unstable.

> I see now your original hypothesis was "at sufficiently low
> temperatures"... that makes it weaker (hence more plausible). At the
> limit of absolute zero we are guaranteed a lowest energy state as the
> thermodynamic equilibrium state: note, just to be cute, this doesn't
> immediately exclude mixing or disorder -- so long as the mixed or
> disordered state happened to be lower (or at least as low) in energy
> than any fully ordered state. Do you know any reason this possibility
> is excluded?

No, I don't. It's just that in every case we know, the crystalline
state
has lower energy, so I make the generalisation.

Andrew Usher
From: Edward Green on 25 Dec 2008 18:41
On Dec 24, 3:28 am, Andrew Usher <k_over_hb...(a)yahoo.com> wrote:
> On Dec 23, 8:02 pm,EdwardGreen<spamspamsp...(a)netzero.com> wrote:
>
> > > Yes. The only stable C-H phases at 1 atm are H2, CH4, and graphite.
>
> > In the context of this discussion (solidification of paraffin), this
> > may be only somewhat more relevant than all nuclear species being
> > unstable wrt transformation to iron, IIRC.  Part of thermodynamics is
> > deciding artfully what to ignore.
>
> I only mention it because it was behind my final conjecture. It's also
> the
> fundamental difference between 'organic' and 'inorganic' chemistry.

But is the tendency of paraffin to decompose into graphite and methane
(?) important for your observation? I haven't noticed that even when
I find very old candles that they've evaporated, leaving only a stick
of graphite with a wick in it.

<...>

> > Entropy may always cause _some_ mixing, but it's possible that even at
> > RT the true thermodynamic equilibrium consists of segregated phases of
> > various chain lengths, with minor intermixing.  After all, we have, in
> > general, multiple phases in mixtures at room temperature: that's some
> > kind of compromise between entropy of mixing and ... just to make a
> > rounded phrase ... enthalpy of mixing.
>
> Yes, that's how it works. But the enthalpic effects here are very
> small.

Which, for what it's worth, would favor mixing, and increase the
probability that the mixed state is actually at or near equilibrium.

> > > My hypothesis says that any solid that does not form one or more
> > > crystalline phases must be unstable, in the sense that there is some
> > > other molecular arrangement with lower free energy.
>
> > I wonder. I'd have to say "no opinion" for now.  Just for the sake of
> > argument, why couldn't a partially ordered pseudo-crystal be the true
> > equilibrium?
>
> Actually, I meant stable in the first sense when I said this. That is,
> hydrocarbons except methane would count as unstable.

Well, I still think you're mixing apples and oranges here. At first
the question was about the stability and thermodynamic equilibrium of
solidified paraffin. I think that's at least a somewhat interesting
question, for which the obvious simplifying assumption would be "the
chains are all stable". Saying the solid paraffin is unstable because
the molecules are unstable is like saying a perfect crystal of a
single isomer of hexane is unstable at any temperature because hexane
is unstable wrt decomposition into graphite and methane. Why even
bring up paraffin then, if you are making a blanket statement about
organics?

> > I see now your original hypothesis was "at sufficiently low
> > temperatures"... that makes it weaker (hence more plausible).  At the
> > limit of absolute zero we are guaranteed a lowest energy state as the
> > thermodynamic equilibrium state: note, just to be cute, this doesn't
> > immediately exclude mixing or disorder -- so long as the mixed or
> > disordered state happened to be lower (or at least as low) in energy
> > than any fully ordered state.  Do you know any reason this possibility
> > is excluded?
>
> No, I don't. It's just that in every case we know, the crystalline
> state
> has lower energy, so I make the generalisation.

Heh... if we _did_ find a system with an equilibrium disordered phase
at 0K we would violate one of the "laws of thermodynamics" -- not one
of the main ones, mind you, but a kind of add on law which essentially
says "that doesn't happen". Well, possibly (it's the third law). I'm
not really clear on this. I remember the statement as involving
crystallization at 0K, but that's not the one I find immediately.
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